- Intro
- EDITORIAL--Critic Critique
- CLASSIC CIRCUITRY--Grommes Hi-Fidelity Model 212 Preamp, Courtesy of Tom Tutay
- AUDIO AIDS
- EQUIPMENT REVIEWS: The Ramsey BS-601 Oscilloscope, Reviewed by Benjamin L. Poehland
- TROUBLESHOOTING: Professional vs. Amateur Audiophiles, BY Kirk Vistain
- BOOK REVIEWS: New Penguin Guide Reviewed by Patrick J. Amer
- JUST LOOKING
- CORRECTIONS
- LETTERS
Intro
Bill Ruck has taken time out of his busy schedule as chief engineer for KFOG in San Francisco to survey the growing noise levels in our power lines and in the air. His first article on this problem leads off this issue.
If your CD player doesn't sound just right, perhaps it is because economy-minded manufacturers of some early models included only one D-to-A converter, which resulted in an 11 usec phase difference. Arny Krueger, who has become well-informed about compact disk hardware, not only diagnosed the problem, but also designed a clever corrective device.
Ben Poehland contributes two articles this time. The first is a digital white/pink noise source , and the second (see below on this page) is a comprehensive report on a remarkably low-cost oscilloscope that will make every audio experimenter's mouth water .
Gary Galo is the most knowledgeable man I have ever met on the history of recorded music. He collects old recordings with particular reference to conductors and performance. For those who collect old 78s, his modification of the classic McIntosh C-8 preamp will be a godsend--and a rich source of basic information on another entrancing byway of this avocation.
Also take special note of Bill Sommerwerck's “Ambisonic Update” box on the Minim decoders , the only Ambisonic decoders currently available in the US.
Next time, we will have more from Bill Ruck on airborne noise remedies, a vacuum tube preamplifier with regulated supplies from J.J. Curcio and an add-on digital readout for Heath's IG-1275 sweep generator. Bill Cruce, Jim Boak and Pat Amer also team up to conduct listening tests on ten preamps in “The Great Preamp Shootout.”
Just Looking
The Direct-to-Tape Recording Company now offers its complete catalog of more than 50 recordings in the Beta HiFi Audio format. The tapes are custom recorded with up to five customer-specified recordings on each tape. These tapes are intended for audio playback only and contain no video information. Some of the recordings are licensed from Centaur, Titanic and AFKA, while the majority were produced by DTR itself. The DTR recordings were made with only two microphones and without limiting, equalization or compression.
DTR is also offering special introductory tapes. Organ-1/BHF includes three pipe organ recordings, while Listening 1/BHF has three recordings of popular/light classical music, including rags and Beatles music performed on the harpsichord and the American Brass Quintet performing popular music of the mid-1800s. Orchestra-1/BHF includes performances of “La Mer ” and “Daphnis and Chloe' by the London Symphony Orchestra and performances of Haydn symphonies by Monadnock Music. No title substitutions may be made on these tapes.
All of the recordings listed in DTR's catalog are also available on PCM digital cassettes in both the VHS and Beta formats, plus open-reel and regular audio cassettes. All are duplicated in real time, and the open-reel and regular audio cassettes can be ordered with Dolby B, Dolby C or TypeX noise reduction.
You may obtain a copy of the company's current catalog and quarterly newsletter, Direct-to-Tape News, by sending $1 to Direct-to-Tape Recording Company, 14 Station Ave., Haddon Heights, NJ 08035.
The Nakamichi OMS-1000 is the first commercially available system capable of recording, reproducing and erasing a variety of optical recording media. The system is designed as a research tool that can be used to evaluate various magneto-optical recording media, including DRAW (direct read after write) and erasable disks.
Although the OMS-1000 is designed as a general-purpose tool, it contains a built-in compact disk formatter, an EFM (eight-to-fourteen) modulator/de modulator, and precision 16-bit A/D (analog-to-digital) and D/A (digital-to-analog) converters. Built-in analog circuitry and recording level meters allow you to use the unit as an audio recording instrument. The OMS-1000's modular design allows it to be modified for special applications and ensures easy serviceability. Its electronics are controlled by six 8-bit microprocessors for versatility in many different experimental conditions. A computer interface will be available as an option.
For additional information, contact Nakamichi USA, 19701 S. Vermont Ave., Torrance, CA 90502.
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The Audio Engineering Society (AES) has come through once again. The Society's new Directory of Educational Programs is a valuable resource for people interested in pursuing a career in audio.
Compiled from the responses to an AES Education Committee questionnaire, the directory includes short descriptions of each program and its cost, certification, facilities, accreditation and principal contact.
In addition to the main part of the directory, an institutional listing by geographic area is provided. Special sections on planning a career in audio and what an audio engineer does (circa 1977) are also included […]
St., New York, NY 10165.
Monster Cable has introduced the Alpha 2, a new moving-coil cartridge. Its “micro-ridge ” stylus is said to maintain a constant tip radius that allows for better inner and outer groove tracing with low distortion. The stylus is mounted on a hollow-tube sapphire cantilever, providing a high degree of detail and increased dynamic range.
The Alpha 2 also includes a magnetic feedback control circuit that reportedly delivers extremely accurate reproduction of transients and precise imaging of instruments.
Details are available from Monster Cable, 101 Townsend St., San Francisco, CA 94107.
Gold-ens are new audio/visual cables from Discwasher They feature heavy gold plating at critical contact surfaces and hand-soldered connections. Pure copper braided shielding results in reduced : ' hum and less radio-frequency interference, while copper conductors produce improved high ends and imaging. A low-distortion foam dielectric maintains ultra-low capacitance for purer fidelity. Gold-ens rate a 75-ohm impedance for video applications without visual degradation.
For more information, write to Disc washer, 1407 N. Providence Rd., PO Box 6021, Columbia, MO 65205.
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Editorial
CRITIC CRITIQUE
Audio journalism as practiced in the three large circulation publications in the US apparently continues to follow an equipment review policy that has not only undermined any confidence knowledgeable audiophiles might have had in their usefulness, but has also spawned a score of what are quaintly referred to as “underground ” audio publications. I wrote an editorial about this American peculiarity (TAA 3/74) that elicited cries of outrage from some of my friends on the staffs of the larger magazines. The policy has been stated as follows: 'No one wants to read bad reviews. ” I presume what the publishers mean is that no one wants to read reviews of bad equipment, which is, of course, not quite the same thing.
Recently, what could be considered an exception to this rule appeared in Stereo Review. While not strictly a review, “Speaker Cables: Can You Hear the Difference? ” (August 1983) was billed as a double-blind listening test conducted by eleven participants and reported by Laurence Greenhill. The “audiophile speaker cables ” referred to on the cover of the issue were three in number: 24-gauge speaker wire, 16-gauge zip-cord and a sample of Monster Cable. It is now clear that Stereo Review's editors made major changes in what Greenhill submitted. While it was not a “bad review ” in terms of the publisher's interesting definition of that term, it was nearly a blanket denunciation of “audiophile ” speaker cables.
My original editorial reported briefly on a special session at the 1974 meeting of the Audio Engineering Society featuring five panelists representing the three large-circulation consumer magazines and Stereophile.
During the question period, one of the panelists who defended the 'no bad reviews' policy attempted to rebut one critic's unfavorable comparison of US audio equipment reviews with those published in Great Britain by saying that advertising in the UK publications came mostly from retail dealers rather than manufacturers. The dealers, he said, were much less likely to cancel advertising because of bad equipment reviews. I passed along this information to John Crabbe, then editor of Hi-Fi News & Record Review. He responded that the mix of ads in the UK publications was not much different from that in the US, but that editorial policy certainly was. It remains so today.
I bring all this up ten years later not to flog a dead horse. In my view, the issue is still germane. The policies have not changed, although I do see more critical reviews from time to time in Audio.
I do have fresh evidence that moral and professional discernment are not the exclusive preserve of our British cousins, however. The November 1984 issue of Popular Computing carries a cover story on the Dimension 68000 microcomputer manufactured by MicroCraft of Dallas, Texas. The 32-bit machine is remarkable in several details, primarily that in emulation mode, it can behave like one of several popular micros, including IBM's PC, the Apple II and some Z80 machines. But the review is decidedly unfavorable, being frankly critical of the Dimension on both its supposed ability to run other software and on its cost effectiveness.
Popular Computing has taken other tough stands editorially as well. And not merely with new, relatively small companies with unique products and small advertising budgets. Popular Computing's editor, Pamela Clark, took on IBM in an April 1984 editorial for its PC equipment design policies, particularly concerning the PCjr. Could it have been a total coincidence that IBM did a major overhaul of the PCjr in the months following the interchange? I doubt whether any words of mine will cause tremors of any sort in the boardrooms or the publisher's offices at Stereo Review, High Fidelity or Audio. But the comparison is apt. The advertising dollars potentially at risk in the computer publishing field cannot be all that different from those in the audio field. Perhaps I am all wrong in supposing we audiophiles are just as anxious to hear any bad news about audio equipment as are computer buyers about micros. Perhaps.
I would be interested to hear what you think about the issue. I would be even more interested to hear whether establishment publishers have any thoughts about lifting the iron curtain that continues to block genuine critical comment from their equipment re view columns. If they believe that audiophiles are radically different from computer buffs, I would be interested to hear the rationale. If the silence continues, I will assume that the arguments are merely self serving excuses for the most crass of motives.
In the strict sense of the English words used, I agree with those who say "no one wants to read bad reviews.” And I would say that what are being published as “Test Reports' are just that-bad reviews. They are not, in the strict sense, evaluations but merely descriptions. Although they do serve a useful purpose, they lack the vital ingredient that the US audio industry badly needs.
Critical interchange on the merits of equipment could help begin the re vitalization that audio needs.
In this issue, you will see a construction piece for a device that corrects an 11 sec interchannel phase error in many of the early compact disk players (Arnold Krueger's "CD Player Phase Corrector," p. 11). How many reviews of CD players carried this interesting information to readers? Could the reviewers have missed the fact that only one digital to-analog converter was serving both channels? Perhaps there is reason to hope.
Ziff-Davis has put Stereo Review up for sale. Just possibly some brave buyer will follow a new, independent policy. Since ABC owns High Fidelity and CBS owns Audio, we are probably not likely to see any great shift of policy there. But we could see a change if the right purchaser takes the reins at Stereo Review. We shall see.
-E.T.D.
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Audio Aids
A FAIL-SAFE MUTING CIRCUIT FOR PREAMPS
Most PREAMPLIFIERS, particularly tube types, produce horrendous pulses of speaker-destroying voltages at their outputs when the power switch is turned on or off. Several schemes mute these “thumps and bumps, ” but I do not know of any that pro vide protection on power failure with subsequent recovery. My fail safe circuit (Fig. 1) mutes the preamp out puts with normally closed form-B relay contacts so that power failure causes these contacts to close and puts.
The circuit derives its power from the pre amp's 115V AC power input through a small circuit-board-mounted transformer. A full wave rectifier comprised of D1-D4 provides a DC control voltage that is partially filtered by C1 to keep instantaneous voltage above zero.
This prevents the SCR (silicon-con trolled rectifier) from turning off.
The contacts of relay K2 are closed prior to closing the preamp power switch. These contacts ground the pre amp outputs to prevent those horren dous thumps in the speakers. With power turn-on, relay K1 operates al most immediately and through its form-A contact, applies power to the primary of the preamp's power trans former. Relays K2 and K3 remain de energized with the contacts closed to keep the outputs grounded.
A voltage divider (R2 and R3) applies 63 percent voltage (about 15V) to the in verting input (pin 12) of the IC (an op amp). This results in no output at pin 14 of the IC. An RC circuit (R4 and C3) be gins to charge C3 at an exponential rate so that the voltage at pin 13 (the noninverting input) is 63 percent of the supply voltage at a time (T) of RC seconds (R in ohms, C in farads). With R4 at 1 M-OHM and C3 at 20xF, T is 20 seconds.
When the voltage at pin 13 exceeds the voltage at pin 12, the op amp (IC) turns on the full supply voltage at the output (pin 14). This voltage supplies adequate current through R6 to turn on the SCR and energize relays K2 and K3.
You may vary the delay timing for K2 ...
PHOTO 1: Mr. Ballard's muting circuit can save your speakers from life-threatening pulses.
... and K3 relays by changing R2, R3, R4 or C3. Increasing R3 increases time, while decreasing R3 decreases time. The total delay time should slightly exceed that of the last speaker thump on turn-on.
Twenty seconds is usually more than enough. Contacts K2-1 and K2-2 then open to remove the ground from the preamp's outputs. At the same time, relay K3 operates, closing contact K3-1, which discharges C3 through R5. The RC timing circuit is, therefore, reset to accept another “on” cycle immediately if required.
At turn-off, C2, which has been charged to relay K1's coil voltage, now discharges through K1's relay coil and holds it for about one-half second. This allows enough time for relay K2 to de-energize and causes the preamp's outputs to be grounded before K1-1 opens and removes power from the preamp's transformer.
Note that C3 is specified in the Parts List as a tantalum. This is desirable because of the low leakage inherent in tantalum capacitors. With a long time delay of about 20 seconds for the preamp's DC power to stabilize, R4 must be of high value to keep C3 at a reasonable value. High leakage in C3 would shorten the timing period. You can use aluminum electrolytics, but you must consider the higher leakage.
-------------- PARTS LIST
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Figs 1-3
I have marked two locations on the board (Fig. 2) for C3, although only one is needed. The one with short lead spacing is for a disk-shaped tantalum. The other location, marked ''Alt. Pos.," is for a larger cylindrical-shaped capacitor.
The muting circuit is designed so that power to the preamp's transformer bypasses the preamp's fuse. I put fuse clips in the U lead on the board to provide normal protection. The fuse is 1/2 inch diameter by 1.5 inches long and should have the same rating and be the same type as the one in the preamp.
At 115V AC input, the DC voltage across C1 is about 20.5V with all relays energized. I purposely made it higher than the rated coil voltage of the relays so that dropping resistors R1 and R7 are required. The primary reason for R1 and R7 is to isolate the stored energy in C2 so that it provides 'off'' delay for relay K1 only.
I added C4 and C5 to suppress the arc noise from current interruption at the preamp power switch and relay contact K1-1. The preferred location for C5 is directly across the preamp power switch (there might be a capacitor already there, in which case C5 is not required). In any event, I provided pads and locations for C4 and C5 on the board.
With this automatic muting circuit, turn-on and turn-off of the preamp power contributes only silence to the speakers, saving them from the life threatening pulses from the preamp's output. This silence is present under all conditions of power application or removal whether intentional or accidental.
ROBERT. J. BALLARD Johnson City, TN 37601
Equip Review
WSR THE RAMSEY BS-601 OSCILLOSCOPE
Reviewed by Benjamin L. Poehland
Tee mest PECE of test equipment most serious audio builders acquire is an analog or digital volt-ohmmeter (VOM). In to day's market, it is possible to obtain a very high performance VOM for less than $100. Before long, however, most diehard audio enthusiasts begin to hanker for an oscilloscope. Unfortunately, an oscilloscope-especially a good one-might re quire a substantial investment of $700 or more. Even a minimal dual-trace unit in kit form with limited bandwidth and few features will cost nearly $500 by the time you include accessories such as a set of quality 10:1 probes and a good calibration source. After hours of tedious construction, you might still have a unit that fails to meet all the requirements of a serious audio test bench.
But now there is hope. The Ramsey Electronics BS-601 oscilloscope is a 20MHz dual-trace differential-input instrument that comes supplied with a pair of 10:1 probes. It is loaded with features and sells for $399.95. Does it sound too good to be true? Let's take a closer look at this instrument and the company that distributes it.
Cost Effective Technology The BS-601 is manufactured in Korea and distributed by Ramsey Electronics Inc. ( 2575 Baird Rd., Penfield, NY 14526).
The company's motto is “Cost Effective Technology,” and its product line includes frequency counters and multi meters (most of which are also available as kits). All Ramsey's factory-wired gear, including the oscilloscope, carries a one year parts and labor warranty.
Here's a rundown of the BS-601's features as listed in the owner's manual:
component tester
wide bandwidth & high sensitivity
very low power consumption
high-sensitivity X-Y mode
axis (intensity modulation) e TV video sync filter
high-frequency rejection filter in the trigger circuit
front-panel electrical trace rotator
regulated power-supply circuit for accuracy.
This list, impressive as it may be, does not do justice to the BS-601. In addition to the basic oscilloscope, you receive a 28-page manual, a set of quality hook-on 10:1 adjustable probes, a separate three wire grounded line cord and two extra 1A line fuses.
The first thing that struck me about the scope was its compact size, which al lowed me to put it on my already-over crowded test bench. The light grey, vinyl clad metal case blends well with the satin aluminum finish on the front panel. A convenient carrying handle is mounted on the top cover. Underneath, the unit is supported by four plastic feet. The two front feet also support a tilt bail, that folds out.
Figure 1 shows the unit's front panel.
The standard-sized 8-by-10cm screen is light blue with an internal graticule marked off in red. I was particularly impressed with two screen details: the grey plastic bezel surrounding the screen is grooved to hold a CRT (cathode ray tube) camera, and the graticule markings have 0, 10, 90 and 100 percent calibrations that are invaluable for measuring rise times.
All the front-panel controls are clearly marked and intelligently grouped according to function.
The rear panel, shown in Fig. 2, has some features not usually found in a bud get scope. The four sturdy feet protruding from the rear allow you to use the unit in a vertical position and are convenient wrapping posts for the line cord during storage. Next to the fuse holder is a handy chart showing which fuse size to use for a particular line voltage. To select the line voltage, you just insert the heavy selector plug into the jack at the lower left so that the white arrow lines up with the desired value. The BS-601 will operate from 90 to 264V AC. The Z-axis (intensity modulation) input and grounded AC line input jacks are also located on the rear panel.
Figure 3 is a close-up of the probe set.
The center conductor of the BNC connector is gold plated, and the 5-foot length of shielded wire is thinner and more flexible than the RG-58A/U commonly used in scope probes. The spring-loaded hook-on ...
FIGURE 1: BS-601 front-panel details. (All photos by the author.)
FIGURE 2: Rear panel of the BS-601. Note the Z-axis input jack and line-voltage input selector.
... device is detachable, as is the ground clip.
On the probe handle is a small switch for selection of 1x, ground or 10 x impedance modes. Just ahead of the selector is a small hole allowing access to the trimmer capacitor. Once the probe is trimmed for best square-wave response, you cannot change the setting accidentally with normal handling. The slim profile and sturdy construction of these probes makes them a pleasure to use. If sold separately, the probe set would probably cost around $50.
Internal Construction
Having been greatly impressed with the external features of the BS-601, I wondered whether the company had paid the same attention to its innards. I was not disappointed. By loosening only ten screws, I easily removed the top and bottom covers. Despite the unit's compact size, the interior has a considerable amount of empty space in which to maneuver. Also, even with the covers re moved, the chassis maintains its mechanical integrity.
The front and rear panels of the BS-601 are held together internally by five metal L-brackets that also serve as wiring channels and circuit-board supports. The five major component groups consist of three large circuit boards, the CRT and the power transformer. The three boards hold the dual vertical amplifiers, time-base circuitry and power supplies, respectively.
Figure 4 shows the power-supply board (with its shield cover removed) just under the CRT. Figure 5 is a view of the foil side of the supply board (lower left) and the vertical-amp board (right).
I had a little trouble determining the ex act nature of the cream-colored board material. It appears to be either a high-grade phenolic resin or a composite filled resin similar to FR-4. The foil patterns are ex pertly laid out with ground plane construction. They are very clean, with no trace of solder residues, and are professionally finished with a green solder mask. All the discrete semiconductors are Japanese, but most of the integrated circuits are American.
Film resistors are used throughout.
Many are precision 1 percent types, but a few composition types are used under the shielded section of the power supply. As for capacitors, the unit has many Mylar caps and relatively few ceramics. The electrolytics are noticeably miniaturized.
Servicing this instrument has been simplified through extensive use of plug in connectors for all board inputs and out puts. This eliminates the usual rat's nest of point-to-point wiring and minimizes the possibility of damage to the boards through soldering.
The elongated neck of the CRT containing the electron gun and deflection plates is encased in what appears to be a mu-metal shield, leaving the larger half of the CRT exposed. The CRT is securely mounted, and the hefty power transformer is bolted to a thick metal bracket
Fig 3-4
Fig. 5
... mounted separately on the rear panel.
This construction would minimize mechanical damage if the instrument were ever dropped.
Performance Specifications
Quality construction is of little value if the design is unable to perform to its specifications, listed on page 48. For those of you not familiar with scope specs, I should point out that a sensitivity of 5mV and a bandwidth of 20MHz are more than adequate for any audio measurements. In fact, the specs and features of this scope are consistent with multipurpose instruments in the $600 to $1,000 range, making it suitable for video and digital servicing as well.
A block diagram from the manual is shown in Fig. 6. Conspicuous features of the design include dual FET inputs with inherent temperature stability in the vertical amplifiers and highly regulated power supplies. How well does it per form? Beautifully.
Refer to Fig. 1 for help in locating the controls I am about to describe. When you turn on the toggle power switch next to the screen, a red LED pilot light glows, and a few seconds later, the two blue green traces appear (level set to “auto ” and mode set to “dual ”). At first, the front-panel controls seemed to have a somewhat delicate touch, but I soon got used to this. Adjusting the intensity and focus controls produced a bright, crisp display free of astigmatism effects.
Just below the power switch is the race-rotation, screwdriver-adjust pot set for setting the horizontal level of the display. For anyone who has ever gone through the process of removing the scope chassis from its cabinet, loosening the CRT hold-down screws, twisting the CRT by hand, retightening the clamps and reinstalling the chassis just to achieve horizontal trace rotations, this innocuous little control is a godsend.
First I checked the calibration of the vertical input attenuators, using DC voltages from a Heath 1G-4505 precision cope calibrator monitored by a Sabronics 2010A digital multimeter (DMM). Channel A (ChA) passed this test with flying colors, but channel B (ChB) contained a slight but perceptible error, which appeared to be linear throughout its range. I estimated the error at around 5 percent, which for most practical purposes, is insignificant.
Next I checked the calibration of the horizontal time base, using the precision square-wave output of the 1G-4505 monitored by a Sabtronics 8610A counter.
Up to the 1MHz limit of the calibrator, both channels were visually accurate during a manual sweep of the time-base range using the sweep-time/division control.
Trigger-circuit function was superb.
Figure 7 shows a 100MHz sine-wave signal from a Heath 1G-5280 radio frequency (RF) oscillator. This photo is remarkable because it demonstrates that the vertical, horizontal and trigger cir cuits all have a response far in excess of their specified bandwidths, a tribute to the thoroughness of the design. Triggering was always rock-steady, even with noisy or low-level input signals.
Figures 8 and 9 show the results of vertical/horizontal (V/H) linearity tests at 100Hz and 100kHz, using a Heath SG 1272 precision sine-wave oscillator. For this test, I placed the scope in its X-Y mode. With the sweep-time/division control in the ChB position, ChB be comes the X axis and ChA the Y axis. A display of perfect V/H linearity would, of course, be a straight line at a 45-degree angle with any nonlinearity showing up as curvature. The slight curvature shown in Fig. 9 might be due to nonlinear distortion in the generator, which is operating at the extreme end of its upper frequency limit. For any signals within the audio spectrum of 20Hz to 20kHz, distortion due to V/H nonlinearity may be considered nonexistent.
In Fig. 10, I created a 1:1 Lissajous figure by feeding exactly the same frequency from two separate sources (Heath SG-1272 and 1G-5218 sine-wave generators) into the X and Y channels. Such a test places a severe demand upon the ...
FIGURE 6: Block diagram of the BS-601. Please note that the typographical
errors appear in the manufacturer's original drawing.
... stability of the scope's V/H circuitry as well as the signal sources, and it is usually quite difficult to get a Lissajous trace to stand still long enough to obtain a good photograph. Note the perfect roundness of the trace. The four quadrants are evenly joined, and no parasitics or other visual aberrations are apparent.
Next I examined performance characteristics of the BS-601 in its dual-trace (A/B) mode. Figures 11 and 12 show the A/B interchannel phase accuracy at 1kHz and 1MHz, using the IG-4505 precision calibrator. At both frequencies, the upper and lower traces were virtually super-imposable, making the BS-601 a good instrument for measuring input/output phase shifts in audio circuits.
FIGURE 7: A 100MHz sine wave from the author's Heath IG-5280 radio-frequency
oscillator. Not bad for 20MHz band width! (Vertical ~5mV/cm; horizontal
-5 x 0.2 usec/cm); FIGURE 8: Vertical /horizontal (V/H) linearity at
100Hz using the X-Y mode, both channels 20mV/cm.
FIGURE 11: A/B phase accuracy at 1kHz, vertical equals 0.2mV/cm. FIGURE 12: A/B phase accuracy at 1MHz, vertical equals 0.2mV/cm. Ringing is due to sloppy lead connections onane author's test bench.
FIGURE 13: Input/output comparison of a small bench amp driven to clipping at 1,000Hz. The upper trace is ChA at 5V/cm. The lower trace is ChB at 2V/cm. FIGURE 14: Differential waveform of Fig. 13. The waveform is inverted due to a 180-degree phase shift in the test amp. FIGURE 16: Another example of Z-axis modulation.
Another valuable scope feature is the ability to view the waveform resulting from the difference between two signals, such as the input and output of an amplifier. The BS-601 is capable of A/B differential performance, as illustrated in Figs. 13 and 14. In this test, I deliberately overdrove a small bench amp until its output became clipped, as shown in the upper trace of Fig. 13 (the lower trace is the input signal). The analog difference between the two signals is shown in Fig. 14 (ChB position switch set to “invert ” and mode switch set to “add ”). The clipped-off portion of the input signal is visible in the resultant waveform, along with some slight vertical distortion that was not obvious in the dual-trace mode.
1 did discover some caveats about using this feature. The ChB inversion switch introduces some baseline anomalies, which were especially noticeable at the highest sensitivity setting of the vertical attenuator. In general, you should avoid a sensitivity greater than about 200mV/cm.
Also, it is a good idea to adjust the signal source level to a setting that will permit you to use the input attenuators on both channels in their calibrated ranges if you wish to quantify the magnitude of the differential waveform.
One of the facts of life with oscillo scopes is that most of them, even fancy ones costing several thousand dollars, tend to be rather poorly shielded with respect to stray magnetic and RF fields. In this respect, the BS-601 is superior to my plastic-cased budget scope, but would probably fall somewhat short of Pentagon standards.
I conducted a rough test of magnetic shielding by feeding a 1kHz sine wave in to both channels to make a suitable display and then brought a car speaker with a large magnet structure into the vicinity of the instrument. At a distance of about 12 inches, the display began to show the warping and bulging characteristic of magnetic interference. I observed similar effects with my plastic-cased budget scope at a distance of around 22 inches, indicating that the shielding of the BS-601 is at least partially effective.
I made a rough test of RF shielding by feeding a 1MHz unmodulated sine wave into ChA via a shielded BNC cable with sweep-time/division set at 0.2usec. I then brought a small portable AM receiver into the vicinity of the scope. With the receiver's antenna a few inches from the front panel, I tuned the dial to 1,000kHz.
At this point, I heard a strong heterodyne squeal from the receiver's speaker, indicating RF leakage from the scope case.
As I rotated the tuner dial further, I heard progressively lower-volume squeals at various places on both sides of the dial, indicating the presence of harmonics. When I moved the receiver and its antenna 6 inches away from the scope, I no longer observed the heterodyne effects. Thus, the RF radiation, while present, is rather weak.
I should point out that magnetic and RF shielding effects cannot really be classified as a fault, but you should take them into account when using the instrument on a test bench. Avoid placing the unit too close to heavy power transformers, speaker magnets or other field-generating components, and beware of inducing un wanted signals in nearby equipment having unshielded high-impedance inputs when working with high frequencies.
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CRT HV Calibration Voltage Power Requirements Weight Dimensions 48 The Audio Amateur 1/85 Vertical Deflection 5mV to 20V/division on 12 ranges in 1-2-5 step with fine control DC-DC to 20MHz ( - 3dB) AC-10Hz to 20MHz ( - 3dB) Less than 17.5nsec Less than 3% 1 M-OHM shunted by 20pF, + 3pF (max. input-600V,, or 300V DC plus AC peak) ChA, ChB, dual and add Approx. 200kHz Better than 60dB at 1kHz ChB can be inverted Time Base Automatic and triggered. In automatic mode, sweep is obtained without input signal.
0.2usec to 0.5sec/division on 20 ranges in 1-2-5 step with fine control and X-Y x5 at all ranges Less than 3% Trigeeri Internal-1 division or more External-1V,, or more Internal, ChB, line or external Positive and negative, continuously variable level control, pull for auto 20Hz to 20MHz or more AC, HF Rej, TV (each + or -) At TV sync, TV-H (line) and TV-V (frame) sync are switched automatically by the sweep time/division switch.
TV-V-0.5sec/division to 0.1msec/division TV-H-50pusec/division to 0.2usec/division Horizontal Deflection 5mV to 20V/division on 12 ranges in 1-2-5 step with fine control DC to 1MHz ( - 3dB) 1 M-OHM shunted by 20pF, + 3pF 300V DC plus AC peak or 600V,, X-Y mode is selected by the sweep time/division switch.
ChA--Y axis ChB-X axis Z axis-TTL level (3V, approx. 50V); plus bright, minus dark Other Specifications 2kV 0.5V,, +-5%, 1kHz square wave AC-100V, 120V, 220V, 240V, 50Hz, 60Hz, 19W Approx. 7kg 162 by 294 by 352mm (HWD)
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More Tests
I conducted some miscellaneous tests to check two special features of the BS 601-intensity modulation and the component tester. I was curious about the Z-axis input jack on the rear panel and spent some time learning about the effects of intensity modulation. Figs. 15 and 16 show the type of display resulting from intensity modulation of the input wave form. Best results were achieved when the Z-axis input was a square wave having an amplitude of about 3.5V,,. Although this feature is probably of negligible value for most audio work, it does facilitate measuring instantaneous voltage within a waveform and also permits the use of an external time base.
Another BS-601 feature that aroused my curiosity was the built-in dynamic component tester. After fiddling with it for awhile, I decided that the component test feature was a bit of a gimmick. I have two complaints: the manual lacks important information concerning proper setup of the component-test feature, and the tester's dynamic range has significant limitations.
The manual instructs you to use the tester by setting the input attenuators of both channels to 5V/division, setting the time-base knob to X-Y mode and pushing the component-tester button. When I did all this, I found that the component tester would not function at all. To make the tester functional, you must also set the input coupling selector switches of both channels to their ground positions. This is not stated in the manual. You can obtain a more even and slightly more sensitive display by setting the Y input to 2V/division instead of the 5V suggested by the manual.
In actual component measurements, I found that resistance values below about 1,000 ohm gave the same display as a short circuit, while values above about 220 k-Ohm appeared as an open circuit. A similar limitation surfaced with capacitors: I could not distinguish values below 0.01uF from a short-circuit, while those above 10 uF were indistinguishable from an open circuit.
The measurement range of inductors and zener diodes was most severely restricted. I could not distinguish inductance values below 1H from a short-circuit, which precludes the use of the tester for quick-checking crossover coils and other inductances in the millihenry and microhenry range typically found in audio or RF circuits. I was unable to establish an upper limit of inductance response because I could not find any values higher than about 3H among my collection of coils and transformers.
Presumably, the lack of dynamic range for inductance testing is due to the relatively low test frequency of 60Hz. In testing zener diodes, I could not distinguish units rated above about 15V from rectifier-type diodes. In fact, quick-checking rectifier, switching and LED-type diodes was the only function the component tester could perform without any limitations.
In all fairness, the component-test feature does add value to the instrument, and at worst, it in no way detracts from the BS-601's other excellent qualities.
Finally, I conducted a few brief tests to check power-supply regulation and calibration voltage. For the supply-regulation test, I plugged the BS-601 into a bench Variac and fed both channels with a 1kHz sine wave to obtain a stable display. I set the line-voltage selector on the rear panel to 117V and slowly lowered the line voltage to see how the display would react. At 93V AC, triggering became unstable, although the magnitude and brilliance of the display remained unaffected. I observed no visible effects when I increased the line voltage to 142V AC.
When I varied the line voltage rapidly back and forth from 85 to 140V AC, the trace stability and brilliance were unaffected, although the display amplitude did rise and fall slightly. In sum, the high degree of regulation in the power supplies of this instrument make it relatively immune to all but the greatest variations in the AC line.
I also gave the square-wave calibration jack on the front panel a once-over. The signal available at this jack was specified as 0.5V,, at 1kHz. My measurements indicated that this output was at its specified level and that the frequency was 988Hz with a 50 percent duty cycle. All these figures are well within the 5 percent specification. This calibration signal is produced internally via a dedicated logic gating circuit, a type of design more commonly found in higher-priced scopes.
Budget-priced units typically derive their front-panel test signals from a trans former winding using the 60Hz line frequency.
Documentation
The BS-601's 28-page manual is both satisfying and frustrating. It includes the vital technical data for calibrating and servicing the instrument, along with the operating instructions, specifications and description of the controls. Detailed schematics of the three main boards appear in fold-out pages opposite the X-ray views of the boards, which facilitates trouble shooting. Although no troubleshooting hints or charts are provided, there is a brief description of each major circuit. I tried a few of the adjustments described in the calibration section, but I would have liked more detailed descriptions.
My most serious complaint about the manual is its overall presentation. The author is obviously not very proficient in the English language. The manual contains many misspelled words and some clumsy grammar. This is not the first poorly written manual that I have seen supplied with a professionally designed and manufactured product. People who market such products should realize that poorly presented documentation can seriously tarnish the image of even the finest product. On the other hand, someone less fastidious than I could argue that the manual does contain most of the vital in formation required to operate or service this instrument and that we are infinitely better off to have the manual with its faults than to have no manual at all.
Overall Assessment
Given the BS-601's low price and excel lent performance, I feel a little guilty at picking any more nits, but here goes.
Although Ramsey does back up this unit with a one-year parts and labor warranty, this policy is not stated in the manual or any of the paperwork that accompanies the scope. (I found out about it from Ramsey's ad in Radio Electronics and later re quested written confirmation from the company.) Somehow I always feel better when there is a warranty card to fill out and a fancy piece of paper listing the warranty provisions. I would also like to see G-10 glass-epoxy used for the circuit boards, sockets for the ICs and descriptive information on the probes, although I must admit that the instrument performs beautifully without these frills. And it would be nice if the ground clips on the probes were 15 inches long instead of 5 inches.
These minor complaints cannot diminish my assessment of the Ramsey BS-601 oscilloscope as a professional instrument offering superb features and outstanding performance at a bargain-basement price.
This instrument belongs on every serious audiophile's test bench.
I have received a letter from M.j. O'Connell, sales and marketing manager of Ramsey Electronics, stating that the errors and omissions in the operator's manual are being corrected. Mr. O'Connell also mentioned that Ramsey will soon include warranty registration cards with each unit. Such responsiveness is refreshing and adds to my positive review.
Troubleshooting
PROFESSIONAL vs. AMATEUR AUDIOPHILES
I BET MANY AMATEURS have thought about turning their hobby into a profession.
After all, that's what I did. Some of you probably fantasize about how great it would be to get paid for doing what you enjoy. Well, that's one way of looking at it. But let's talk about what being a professional audio technician really means.
I decided to learn audio repair because I was too cheap and suspicious to pay anyone else to mess with my equipment.
Past experience had taught me that most technicians were incapable of properly handling audio equipment, especially mine. Following the old adage “If you want something done right, do it yourself,” I sold all my camera equipment and bought a sine-wave generator and VTVM (vacuum tube voltmeter) with the proceeds. Then I started to read and practice. Brave friends would drop off their ailing equipment, half-hoping to get a free repair, half-dreading the return of their prized stereo in a box full of unrecognizable parts.
Failure was as common as success in those days, but persistence paid off.
Their equipment actually got fixed most of the time! I even ran a profitable repair service right out of my apartment, satisfying the service needs of complete strangers! 1 was on the way. Thirteen years, 18,000 units and three employers later, I am still at it. Would I do it all again? Well... maybe.
The trouble with turning a hobby into a profession is that you no longer have a hobby. After fiddling with amps, receivers and turntables all day, it is difficult to get interested in your own equipment. Besides, the attitudes I have toward audio as a hobby and servicing as a profession are quite different.
Servicing is a business in which volume of units repaired is the main concern. Its motto might be “Perfectionism is just another excuse for not getting the job done. ” The tender loving care, the hours of tweaking and refining so relished by amateurs have no place in the competitive milieu of a high-volume ser vice operation. I have known many a perfectionist technician who would spend hours making every unit work at spec or better. Unfortunately, most of those people are now doing something else, either because their low production got them fired or because they tired of the grind.
That is not to say that competence is missing from this profession, but an amateur works for love and a professional for money. A professional in any field has to do the best job within the limits of what is practical, rather than what is possible. Few customers are willing to pay what it costs to fine-tune a piece of gear. After all, stereo is just “audio wallpaper ” for most. Amateurs are a minority.
Technical competence is only one of the many talents necessary to succeed in the retail audio service business. If you work in a typical small shop, you will probably be handling customers. A degree in psychology with a minor in salesman ship will really come in handy. Most people are not in the best of spirits when they come into the shop with an ailing unit. They often ask why the unit broke down, since the salesman told them it was very reliable. That always amuses me. In what areas of life don't failures occur? The only things that cannot fail are things that do not work to begin with.
Everything breaks sooner or later-you, me and the stereo.
You will also learn how to question nonobservant customers so that you get solid symptomatic evidence without making them feel stupid for not paying more attention to the failure pattern.
This can be quite a trick.
On the plus side, some customers not only pay the rates without complaining, but are also grateful when you do a good job for them. Sometimes you even get a tip. The best ones are those who write letters to your boss, telling what a great job you are doing. In general, you will probably find most customers pretty tractable and quiet-as long as you treat them fairly.
Nevertheless, salesmanship is an increasingly important skill for technicians. Most shops are small operations, where the person who services often has to handle the counter as well. When a customer brings in a cassette deck that cost $100 and the labor rate is posted at $62.50, you better be prepared with a good rap.
Technical Work The actual technical work you perform is much different from the work you do as an amateur. Amateurs build; service technicians clean and replace parts.
Twenty percent of the units I service re quire a maximum of cleaning and a minimum of creative thinking. When you do not have the spray cleaner out, you will probably be stuffing output transistors or ICs into receivers and amps. Unlike the thrill of building your own equipment, fixing that of others becomes quite tedious. The occasional 'challenge' is more an annoyance because it cuts into your production and usually turns out to be the result of a “technician error.” On the plus side, working with a bench full of sophisticated test gear gives you the chance to train your ears to hear what 0.3 percent wow and flutter sounds like. After a time, you find that your ears and senses are adequate for most repairs, and you rarely use the test equipment unless you require hard, numerical specifications.
As a service technician, you can fix your own equipment and usually have access to test gear you could never afford as an amateur. You also learn where to get parts, how to twist service representatives' arms to get those special, scarce components, and how to teach customers patience in waiting for back orders.
You will probably be the first one on your block to see the new circuitry as it hits the market. Since you will have an inside track on what units break a lot, you might even become a hero to your friends by steering them clear of likely lemons. Best of all, parties need never be boring again. You will be busy answering questions such as 'Can I hook an auto motive power booster to my Marantz 2270 and get more power?' and 'Why does my Lloyds compact sound distorted when I turn the volume past 3 o'clock? ” Technical competence, while important, is secondary to customer relations skills in this increasingly competitive retail environment. The prime goal is to make money and keep the customers happy. Squeezing the last half-percent of performance out of a piece of gear is not part of the job. If that's what you are looking for, I'd say stick with the joys of amateur status or search for the rare work environment where dollars-billed per-hour take a back seat to uncompromising attention to performance.
Good luck! Demagnetizing: Myth or Fallacy?
Do you remember the story of the emperor's new clothes? His tailor and subjects conspired to make a fool of him by pretending to laud his beautiful raiment, even though he was actually naked.
Everybody just kept telling him how great his clothes looked, even though he was wearing none, and soon he believed what they were saying. I have a suspicion that the benefits of demagnetizing tape heads might be similarly exaggerated. Let's narrow the range to home cassette and reel-to-reel recorders.
I seldom degauss the tape heads on the Pioneer RT1020L reel-to-reel deck that sits in my living room. It probably logged 100 hours of play before I decided to go through the ritual a few weeks ago. First I measured the signal-to-noise (S/N) ratio and frequency response of the deck, then I demagnetized the heads. There was no measurable change in performance. I then decided to devise a more scientific test using cassette machines, since I see them by the score every month, while reel-to-reel machines are scarce.
Starting with four virgin TDK AD cassettes, I set aside two of them as S/N test tapes. One was to be the working tape, the other an untouched reference de signed to gauge degradation of the working cassette. After demagnetizing and cleaning the heads on the recording deck, I recorded two pink-noise response tests on the remaining two cassettes and played each once to be sure they matched. I stashed one away as a reference and used the other to measure the S/N ratio, unweighted, and the frequency response at 1kHz, 4kHz, 8kHz and 16kHz, both before and after demagnetization, on ten cassette decks. Not one showed any significant difference in before and after performance. I was using a typical hand-held AC-powered demagnetizer. All decks were two or three-head machines with combined record and play head assemblies. None had been de magnetized recently.
You have probably all read conflicting reports on the question of demagnetizing tape heads and would like to find the definitive answer. Can any of you help? Send your questions or comments, along with a stamped, self-addressed envelope, to me in care of TAA or leave a message in my CompuServe EMAIL “box,” #72356,1355.
Book Reviews
NEW PENGUIN GUIDE
Reviewed by Patrick. Amer The New Penguin Stereo Record and Cassette Guide (Penguin, $12.95) is an indispensable resource for audio amateurs who are serious about expanding their collections of classical music recordings. (I avoid the term “record collector ” whenever possible. To me, it connotes someone who accumulates, but does not use. Music on records is to be played, not simply stored.) The new guide concentrates on records issued between 1977 and 1982, but refers to earlier records whenever they remain the prime recommendation for a given work. It is limited to records avail able in England, but this generally broadens the selection rather than narrowing it. This is not a “complete ” guide to classical records in print, but it is large (978 double-column pages) and must contain reviews of 8,000 to 10,000 records. I have found no other single source reference like it, and I know of no American equivalent. There is a real need for a cumulative index to Fanfare, which would fill the gap to some extent.
How about it, Fanfare editors? The most striking and valuable characteristic of The New Penguin Guide is its overall soundness and the judicious ness of its appraisals. I have developed confidence in the recommendations of Messrs. Greenfield, Layton and March, who are senior reviewers for The Gramophone. Their comments on many of the records I own or have purchased on their recommendations have been right on the mark.
In Beethoven's works alone, the guide steered me away from the recent recordings of the Piano Trios by the Beaux Arts Trio in favor of their mid-1960s set, which is marvelous; confirmed my affection for the Vienna Octet's recording of the Septet; called to my attention the very fine recording by the Alban Berg Quartet of the Opus 18 Quartets; caused me to search out the Vegh Quartet's superb account of the late quartets on the Valois reissue; and led me to Bernard Roberts' direct-to-disk recordings of the piano sonatas on the Nimbus label.
The reviews in the guide comment on sound quality, but the authors give much more weight to the musicality of the performance than to the fidelity of the re production. The guide is mildly pro-digital, but not excessively so. If your own view is that digital recording is a major retrogression in recording technique, as is mine (see my review of the Telarc Rite of Spring in TAA 3/81, p. 56), this new guide is invaluable. It will go out of date very slowly because so few of the new records, being digital, are of truly acceptable sonic quality, and it summarizes the golden age of stereo recorded sound.
The fact that the guide includes many labels that are hard or impossible to find in American record stores presents somewhat of a problem, although serious buyers of classical records already know about many European mail-order record services. Unfortunately, the guide does not list records from some of the superb small European labels (e.g., Accent, Pierre Verany and Claves) with which a record collector should be familiar.
A more dangerous trap is the guide's favorable comment on the English pressings and releases of some American records. You are apt to forget that the domestic product is frequently inferior to the European release. On the very high recommendation of the guide, I purchased three of Murray Perahia's records of Mozart piano concertos with the English Chamber Orchestra on CBS Master works. Two of them had substantial areas of non-fill, and one of those had a stuck groove. I returned both recordings and replaced them with Philips recordings of the same works by Brendel, Marriner and the ASMF, which are excellent performances and much better pressings.
The New Penguin Stereo Record and Cassette Guide is a pleasure to read. I am sure music lovers will find much to de light and enlighten them.
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Newsletters. These publications are often of high technical quality and are full of worth while information even if you don't attend many meetings.
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Group Buying. This can be an effective way to obtain obscure items from abroad, including audiophile disks.
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For more information, see the club listings in the Classified Ads of this issue.
Letters
CORRECTIONS:
DIDDEN POWER AMP I AM BUILDING JAN DIDDEN'S POWER AMP (TAA 4/83, p. 7; 5/83, p. 30), but have run into several problems, some of which I have been able to solve myself.
These include the following:
1. The 2N3920 FET Mr. Didden specifies in the power monitor parts list (Table 4, 5/83, p. 36) is not a FET but a bipolar power transistor. Using some logic and a transistor book, I discovered that the 2N3820 is a P-channel FET, so I exchanged the 2N3920s for 2N3820s.
2. The values for P1-P4 in Table 4 appear to be reversed. I think P1 and P2 should be 2.5k trimpots and P3 and P4 2500 pots.
3. BC and BF transistors (Table 4 and Table 1, 4/83, p. 9) are almost impossible to find in the US, so I substituted the MPS 8599 for the BC 556 and the MPS A92 for the BF 423.
4. I could not find the TIC44 thyristor listed in the auxiliary amp parts list (Table 2, 4/83, p. 14), so I substituted the TIC47.
5. I used fuses in the auxiliary amp on the +V,. and -V,_ rails (Fig. 5, 4/83, p. 12) as a safeguard. I think 10A is a good value.
6. D3 is reversed on the power monitor schematic (Fig. 7, 5/83, p. 31) and stuffing guide (Fig. 19, 5/83, p. 38).
This error cost me a dead LM13600 and a dead Q1.
Now for the problem I could not solve. In the test, Mr. Didden states that the power monitor should shut down with 0.5V at the input (TAA 5/83, p. 33), but mine shuts down at 1V. Something is wrong. Any suggestions will be appreciated.
GLEN CLARK Fort Lauderdale, FL 33311
Mr. Didden replies:
I would like to thank Mr. Clark for in forming me of the errors in my article. I thought I had caught all the mistakes, but.... I will deal with each question in turn.
1. The FET in the power monitor circuit should indeed be a 2N3820 and not a 2N3920.
2. Mr. Clark is also correct in assuming that P1 and P2 in Table 4 should be 2.5 k-Ohm) and that P3 and P4 should be 2,00.
3. As for the replacement transistors, I have no access to data on the types Mr. Clark lists, but I have used different (European) types and have never had any problems. Just make sure the breakdown voltages are high enough and the devices have a reasonable gain and f,.
4. The important parameters for the thyristor are the holding current and the trigger current (less than 5mA and 200pA for both the TIC44 and the TIC47). Do not replace the MJE 200 and the MJE 210, though.
5. Fuses in the auxiliary amp are probably a good idea, although I did not include them in the V, lines. (I think I subconsciously wanted to prove some thing to myself!) You will probably use slow-blow types (unless you want to fuse at 30A or so, which defeats the purpose), so you can get away with 5 or 6A, depending on your music and speakers.
6. Zener diode D3 is shown on the power monitor schematic and stuffing guide with the wrong polarity and must have appeared wrong on the board. (I have not seen the board myself.) I am puzzled by Mr. Clark's problem with the power monitor. Although he does not mention whether it is a problem with just one of the circuits or all of them, I can offer some advice. (I assume he is talking about the top half of the circuit and that he could calibrate.) With no input on the I, terminal, the voltage across R19 should be about 15V. Assuming you have +65V supplies, the V,, will be about 58V. To balance the current through R19, you will need 32pA through R17, which means 0.55V (32 divided by 58) at the input. Check that you have this current through R17. If you then increase the voltage at I, a trifle, you should see the potential at junction D5-Dé start to in crease above + V,.. The circuit should shut down when terminal 9's voltage reaches about 5V above V,.. You might have to be patient. With a small excess current, it can take several seconds before shutdown occurs, but it should certainly happen right away with a 0.6V input.
Finally, I have one hint for testing the main power amp. In the last paragraph of the article (5/83, p. 39), I recommend testing with + V_ not connected first. Doing so might, however, indicate a problem where none exists.
This is because there is no supply for the LF411 that sets the quiescent cur rent, which will produce a large offset at the output. It is better to test with + V, connected through 10 ohm resistors before mounting the remaining output transistors on the main amp.
CORRECTION:
DUFRESNE/MARTEL DRAWINGS Danie. DurresNE and Pierre Martel have informed us that Figs. 15a and 15b of their article (TAA 3/84, p. 21) were labeled incorrectly. The captions are correct, but the figures should be switched.
AMPZILLA II TRANSISTORS STALLED
Jim BONGIORNO REPORTS that the Japanese manufacturer of the output transistors specified for his Ampzilla
Im, 200W/channel power amp (TAA 4/84, p. 7) has given no definite date for delivery of the devices. Those who wish to hear progress reports on the project may write DG852 into one of the boxes on a Fast Reply card from a 1984 issue of TAA. You may also send a self-addressed, stamped envelope to A-Train, PO Box 4835, Santa Barbara, CA 93103, or call (805) 963-1122, evenings only.
-Ed.
BORBELY MOSFET MOD
A TAA reaper recently discovered that the DC 100 MOSFET (TAA 2/84, p. 13) oscillates with the 2SK135 and 2SJ50 transistors installed. I had a similar case in Norway with the old 2SKj34/ 2SJ49 devices, and I observed the same problem when working on my 200W amp. Since I was using another type of device, I also thought it was a function of the transistors. So I went back to the data sheets and reexamined the MOSFETs. It turns out that the feed back capacitance of the N-channel device is much smaller than that of the P-channel one. I believe this is the main reason for the N-channels being much faster than the P-channels.
Equalizing the feedback caps seemed to be the most efficient way to approach the problem.
After testing this on my breadboards, I modified all my amplifiers, regardless of the type of transistors used. I also proposed this solution to my Norwegian customer, and it cleared up his problem. Therefore, I suggest using the mods indicated in Fig. 1 on both the DC 100 and the Servo 100 circuits.
Several amateurs had problems finding noninductive wire-wound resistors for R39-R42 (0.2292). An inductive resistor can do a lot of harm in the amp: it can cause instability, and it can affect the sound adversely, especially in the upper audio range. I inserted the resistors to enable adjustment of the bias current in the output stage and to check the matching of the output devices, but they are not essential for the DC operation of the amplifiers. I suggest, therefore, that they be removed from the circuit after the bias adjustment. This allows you to use inexpensive resistors for the adjustment. An alternative way of adjusting the bias is to monitor the drain current of the out put devices. In this case, no resistors are needed for the adjustment.
With the output coil L1, shown in Fig. 7 (TAA 1/84, p. 14), we have more than two full sine waves of ringing when the amp is driven with a 10kHz ...
FIGURE 1: Modifying your DC 100 or Servo 100 circuit in this way can eliminate oscillation.
Fig. 2
... square wave and is loaded with 82 in parallel with 1uF. This ringing can be reduced by changing L1 to the one shown here in Fig. 2. The coil is made of 1.4 to 1.5mm wire and is self-sup porting. The circuit board has to be drilled to accommodate the increased wire diameter. The new coil has an in side diameter of 16mm, with 13 turns.
These modifications also apply to both circuits and have been tested extensively. They improve the upper midrange and the high frequencies.
ERNO BORBELY 8031 Neu-Gilching, W. Germany
AUDIO & COMPUTERS
TAA READERS WHO OWN personal computers would no doubt like to use them in building and testing audio equipment. Although the list of commercial programs for audio amateurs is very small, Programs for Electronics and
Hobbyists by David Leithauser is an excellent resource.
This book includes 13 BASIC pro grams for the Apple II, which you can easily convert for use on other computers according to instructions given in the first chapter. I have converted four of the programs already, and although I am a novice, doing so was a minor task. The second chapter lists a main menu from which you boot the specific programs. The master menu lists options for capacitors, resistors, inductors, RC circuits, RI circuits, CI circuits, RCI circuits, op amps, time circuits, filters, transistor circuits and general math for electronics. Published by Wayne Green Books (Rte. 101, Peterborough, NH 03458), the book sells for $14.95 plus $1.50 for shipping.
Another source of information is Micro magazine (issues 72 and 73), which published a program for using many scientific formulas and several conversions audio amateurs might find helpful. Here again, you can make minor modifications to apply the information to your particular computer.
EUGENE C. WALLS
Scottsdale, AZ 85257
PSRR ARTICLE REFERENCES LIMITED I was GLAD TO see the article “Make the Right Match ” (TAA 3/84, p. 14), which dealt with power-supply/amplifier interaction. Especially interesting was the table on page 20 showing the power-supply rejection ratios (PSRRs) for different circuit configurations.
When the authors say that “no one has investigated the relationship between the power-supply characteristics and the quality of the output signal, ” however, they ignore much of what has been published. In particular, I ad dressed PSRRs in my article “Power Up-An Overview of Power Supply Considerations” (TAA 3/83, p. 16).
Specifically note the section on noise (p. 18). Although mine was a compact article, it covered many of the areas that influence audio performance.
Have Mr. Dufresne and Mr. Martel read it? It was not listed in their references.
In addition, various articles on preamps, pre-preamps and power amps written by Walt Jung, myself and others address the importance of the power supply/amplifier relationship. Our comments and designs were based on discovering and sorting out what is significant for audio use, including the effects and interactions of the designs.
The list of references in the Dufresne/Martel article was too narrow, as the concern for power-supply considerations emanates from amplifier designs and listening.
Dick MARSH Livermore, CA 94550
Mr. Dufresne replies:
We acknowledge that the volume of the published matter dealing with power supplies is considerable and have in the past found great inspiration from various sources, including Mr. Marsh's articles. We were, however, not trying to repeat this information, but rather to build from it.
The article we put together was not so general as to be an overview of power-supply considerations, but was meant to focus on a supply's purpose, on an amplifier's PSRR and on how to meet that amplifier's power-supply needs. We all realize, we are sure, that the fulfillment of the promise of truly high-fidelity analog audio is going to take a concerted research effort. Our article tried to shed light on an important interaction that we felt had been insuf […]
UNCONVENTIONAL DEVICES, PROBLEMS
WHILE ERNO BORBELY'S INTENT is laudable ('New Devices for Audio from National Semiconductor, ” TAA 5/83, p. 7), I think he should emphasize more clearly that when using a device in an application unintended by its manufacturer, you might be in for some unpleasant surprises. In addition, it is essential that you study the manufacturer's data sheet carefully, as much for what is omitted as for what is written. Many parameters that are important in audio use are unspecified for “non-audio ” devices because the performance is inadequate with respect to those parameters or the manufacturer cannot guarantee consistency over time and throughout large production runs.
A case in point is the AHS50xx P-channel JFET analog switches. These are actually “second-source ” devices for several members of a much larger group developed by GE/Intersil-the IH5009-38 series. While monolithic fabrication promises close matching of transistors on the same chip, this is not guaranteed as it is for the 2N5564-6 N-channel J-FETSs, and users should expect worst-case mismatches of perhaps 15 to 20 percent. The switches are also unspecified for noise, so I would not think of using one in, for example, a phono preamp without measuring it.
My experience with other semiconductors has been that if a manufacturer does not quote a noise spec, the device is likely to be very noisy.
A more treacherous pitfall Mr. Borbely fails to note is that these transistors are made with intrinsic clamp diodes between the chip substrate and their sources (IH5009-24) or gates (IH5025 38). To avoid short-circuits or “latch up ” in the presence of large signals when using the devices as general-purpose amplifiers, you should bias the substrate more positively than any other electrode-carefully observing the chip's specified 30V breakdown limit. Even after taking this precaution, the diodes still contribute a small amount of extra input capacitance that might be critical in certain applications.
In short, using a device in an unintended application is nearly always a 'good news, bad news ” story, and I think Mr. Borbely and other authors should try to tell the whole story or at least post a conspicuous warning to less-experienced readers.
Scott MAROVICH E. Palo Alto, CA 94303
Mr. Borbely replies:
I agree with Mr. Marovich about using a device in applications unintended by its manufacturer. Many parameters that are important in that application are unspecified, either because performance is inadequate or because the manufacturer cannot guarantee consistency. There is also the possibility, however, that the manufacturer simply did not test the device for those parameters because he was not interested in them. Since I had reason to believe that this was the case with the AH5011/12/20 analog switches, I reasoned that it was worth investigating the matter in detail.
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Ambisonic
As my three-part series on Ambisonics (TAA 3/84, p. 7; 4/84, p. 38; 5/84, p. 36) neared completion, I learned that Integrex had discontinued its Ambisonic decoder kit-which I had already reviewed to run with Part III.
In addition, about a year earlier, IMF had ceased production of its decoder, which had essentially the same circuitry as the Integrex unit but a better power supply and higher-quality ICs.
Although I did not have time to survey the market before the last issue of TAA went to press, I have since learned that the only decoders now available in the US are two models The AD?7 is a basic Ambisonic de coder with all the features required for ordinary playback of UHJ and B-for mat Ambisonic material and enhancement of stereo recordings. It is a good value at $175. (Minim tells me they are willing to take a loss on the AD7 to encourage the sale of Ambisonic decoders.) The AD10 is more advanced and is priced accordingly at about $500. Both units are available from Quad Inc. ( Box 19, Capron, VA 23829-0019) and Audio + Design, Calrec Inc. ( Box 786, Bremerton, WA 98310). In this review, I will concentrate on the AD10.
Despite its high price (partly due to import duties and partly to the use of hand-selected components), the AD10 is diminutive, measuring 8.5 inches wide by 8.25 inches deep by 2 inches high. Since Ambisonics is an optimized surround-sound system, there is no need for a mess of logic circuitry to enhance separation.
Besides the usual switches to select the speaker layout/distance settings and UH] (two-channel) or B-format (three-channel) decoding, the unit has two controls marked “position ” and “focus. ” The former permits you to select your apparent distance from the performers. In the “back ” position, hall ambience is emphasized at the expense of direct sounds. In the “front ” setting, ambience is diminished, and frontal sounds appear closer. A middle, unmarked position presents the record “as recorded. ” This control is not equivalent to the front/back balance control in quadraphonic systems. Rather, it changes the apparent perspective by varying the level of the X (front/back) signal sent to the speaker-feed matrix.
The focus control is more subtle. It alters the phasiness of frontal sounds, sharpness of imaging. I have yet to receive an instruction manual, so I do not know the 'proper' setting for this control. Apparently, it replaces the “forward preference ” switch on simpler decoders, allowing you to ad For most listeners, the most useful decoder function is “stereo enhance. ” (The AD7 also has this feature.) Enhancement can vary from a subtle ex traction of ambience to a complete wrap-around effect, with any in-be tween gradation. With the enhance control turned fully counterclock wise, the sound is regular stereo. As you advance the control, the image slowly widens and the ambience in creases until it suddenly 'pops loose' and fills the room. Generally, the ambience sounds coherent and not “tacked on. ” In fact, the ambience enhanced sound is usually more “natural ” than playback without the decoder. This suggests that the in creased spatial accuracy of the reproduction more than compensates for any minor sonic degradation of the decoder's circuitry.
Even more remarkable is the ADI10's full surround synthesis. For the first time, it is possible to hear sounds coming from the sides clearly.
In my system, they were a bit phasey, but there was no question about their position. Full surround synthesis is heavily dependent on the mixdown the master tape received, with some stereo recordings sounding as though they are “true ” surround and others turning into a confused mess no matter where the enhance control is set.
The AD10 cannot work miracles, but its superior side imaging, combined with the ability to vary the enhancement continuously, increases the chances that a record will sound more like a live performance.
An internal examination of my AD10 (an early production unit) revealed high-quality parts and careful assembly. The case is heavy and rigid and should survive some fairly severe abuse. Controls are clearly marked (albeit in rather small letters) and logically arranged.
William Sommerwerck
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The tests I conducted are described in detail in my article. Based on this data and the actual tests in different audio applications, I found that the switches performed well. As Mr. Marovich points out, the AH50xx devices are not specified for noise. Consequently, I measured the noise performance, and as I reported in the article (p. 15), they are not good enough for phono preamp use. They performed very well, however, in the line amp I presented in Fig. 11. I am aware of the substrate diodes (they are clearly shown in the data sheet), but in the applications I presented, they did not cause any problems, so I did not mention them in the article.
Naturally, in spite of all the tests I made, there is still no guarantee that all devices will perform as expected. Reading my article carefully, I cannot see that, I ever implied such a guarantee.
But to comply with Mr. Marovich's wishes, I post a warning in this respect to less-experienced readers.
After having said all this, I still believe that properly used, the AH5011/12/20 analog switches per form well in audio circuits, and I am sure some audio amateurs will agree with me.
TAKING CUSTOMERS TO THE CLEANERS
I HAVE JUST FINISHED reading the editorial “Should Schematics Be Trade Secrets? ” in TAA 4/84. I agree that schematics and parts lists cannot be protected successfully as secret documents.
I have recently lost a battle along these lines with Whirlpool. My five year-old washing machine stopped working, and upon examination, I found two diodes on a circuit board open. Although the diodes were identified only by the company part number, by looking at lead and body size, I guessed their ratings and replaced them with two diodes I had on hand. The machine worked fine.
I then wrote to the company explaining what had happened and the 'fix' I had made. I also requested information on the manuals containing the ma chine's schematics. I received four nice pages of mechanical information and a suggestion that I purchase the manual.
When I sent the money for the manual, I made sure Whirlpool understood I was expecting a schematic of everything, including the circuit board. The schematics in the manual I received showed everything but the circuit board. The board was shown as a rectangle with the connections to it.
In the meantime, the strip of LEDs that indicate which cycle the machine is in failed to light, so I replaced a scorched AW resistor on the board to remedy that problem. I sent off another letter explaining that all the information I had received was nice, but I still had the same problem I started with-no schematic of the circuit board. A letter requesting that I call the company's customer relations people on their WATS line soon arrived. The gentleman on the phone told me he had requested schematics from the engineering department for other customers in the past, and the engineers would not release them. He was very sorry, but there was nothing he could do.
Well, there is something I can do, and that is never buy another Whirlpool product.
The Consumer Protection Agency for Maryland was also of no help. And a local parts dealer quoted a board price of $100 plus the old board, which is about $99.60 too much and at least four weeks without a washing machine.
In the early days of audio, I bought a few Heathkits after looking over schematics and descriptions supplied by the company. I am sure their policy of providing that information helped rather than curtailed sales. I will be anxious to see what other response your editorial brings.
GEORGE A. ROBINSON JR.
TO PUBLISH OR NOT TO PUBLISH?
As PRESIDENT OF a company (Phoenix Systems Inc.) that routinely publishes schematics and past president of a company (Phoenix Audio Laboratory) that does not, I would like to offer a perspective that differs somewhat from that of the editorial in TAA 4/84.
There should be no doubt that schematics are the manufacturer's property.
Good circuit designs do not come cheap. The value of keeping a schematic confidential varies with the nature of the product, the nature of the competition, the life cycle of the product and the age of the design. I would categorize the “borrowers ” of this confidential property into four groups-competent competitors (comp-comp), incompetent competitors (incomp-comp), personal users and the intellectually curious.
The comp-comp are capable of de may have already done so, but they will often “reverse-engineer” a competitor's product to estimate manufacturing cost and, by extrapolation, profit margins. They will also analyze the design to check for any (unprotected) new techniques worth borrowing.
Withholding schematics for the first six months to a year after the introduction of a product will not stop reverse engineering, but will raise the stakes. It is much more difficult-thus more ex pensive-to trace out a complex circuit board layout. In defense of reverse engineering, it is about the only way to tell whether a competitor is infringing on one of your design patents.
More problematic to manufacturers is the incomp-comp. These people can range from the small guy knocking off ten units a year to large Taiwan manufacturing facilities that have not had an original idea in 20 years. The following anecdote reveals how creative some manufacturers are.
In the fall of 1981, I was called upon by an electronics magazine to design and write a construction article about the then-hot CX (remember CX?). I had all of six weeks to deliver a finished product. I worked closely from the CBS "typical circuit'' that was offered to all licensees, replacing the gain element with a higher-performance unit and generally bringing the design in line with my other products. I noted a discrepancy between the system time constants and the circuit, so I changed the values to agree with the system specifications and notified CBS. I later learned that a drafting error on the typical circuit had caused the error. I also learned that two other licensees had copied the schematic, error and all, and had shipped 20,000 to 30,000 units with incorrect system time constants.
(it was a 10 percent error in attach time, and CBS later changed the en coder to agree with the mistake!) So much for originality among manufacturers. It is also widely known that most vacuum tube power amp designs were lifted from a tube manual or one of the published ''classic'' (read ''wide y copied'') designs.
The third group of borrowers are the personal users. They have more of an impact on their employers, from whom they borrow those mil-spec resistors and capacitors, than they do on the design's owner. That can change, how ever, if they decide to knock off a few units for their less-handy friends, some of whom might have actually bought the unit.
The last group of borrowers, the intellectually curious, are the most benign. Circuit designers hone their skills by analyzing other people's work, thereby building a library of design techniques. To ensure that the level of circuit design will advance, protected designs (patented) are published by the patent office, followed by articles in journals such as those from the AES and IEEE. Circuits that are not protected are usually kept confidential until the commercial advantage has diminished, and then they are published.
I appreciate what TAA is doing to provide another vehicle for distributing such knowledge.
In conclusion, I would not discourage hobbyists from borrowing a circuit if their goal is to increase their knowledge. The world will always need good analog designers, and a small loss of revenue is a fair price to pay for increasing the pool of knowledgeable designers. If a hobbyists only intention is to get something for nothing, however, he or she is enriching him/herself at the expense of the system. The more people steal, the less information that gets published.
I have one final comment on the publication of schematics without values.
A student of circuit design will learn a great deal more about a design that way. I will grant that it makes analysis more difficult but not impossible.
What it does stop is the blatant copying of a design by someone who has no intention of trying to understand the circuit.
John ROBERTS Manchester, CT 06040
TECHNICAL QUERY Write it please: Our phone doesn't do theory.
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