IN THIS DIGITAL AGE, it might surprise many hard-core audiophiles to see an article on 78 rpm disk reproduction. Yet many audiophiles are also music lovers for whom the performance is every bit as important as the sound. Performances such as those by Leopold Stokowski and the Philadelphia Orchestra, Serge Koussevitzky and the Boston Symphony, and various opera greats are available only on the original 78 rpm disks.
The greatest difficulty facing 78 rpm collectors, however, is the dearth of turntables and preamplifiers suited to this application. With proper equipment, it is possible to extract far more information from those record grooves than their original owners ever thought possible.
Preamp Requirements
The primary requirement of any 78 rpm preamp is equalization flexibility. The RIAA curve was not standardized until 1956. Before that time, many different equalization curves were used. The McIntosh C-8, famous as having the most flexible preamp equalization system ever offered, is the perfect solution.
Thirty-two different bass turnover frequencies, from 280Hz through 1,200Hz, are available. Thirty-two different treble roll-off rates are also available, ranging from “flat ” to 25dB down at 10kHz (see Table 1).
The bass turnover options are useful only for electrically recorded disks (1925 and later). For the reproduction of acoustically recorded disks (before 1925), a flat (unequalized) response is required, with fairly sharp low and high-frequency filtering on either side of the acoustic method's limited frequency range (approximately 150Hz to 3 or 4kHz).
Since the C-8 contains an unequalized microphone input, this flat equalization can easily be applied to the phono 1 input. The five-position rumble filter and 32-position high frequency roll-off selector provide the necessary filtering. I am assuming the use of magnetic stereo phono cartridges (such as Stanton’s 500 series) with the correct playback stylii. You must never use ceramic cartridges with “flipover ” stylus assemblies for 78 rpm (or any other speed) reproduction.
Most 78 rpm disks are cut laterally, as are monaural LP records. Many collectors might have vertically cut disks as well, including the 80 rpm Edison Diamond Discs and the acoustic Pathes. For playback of lateral disks, the left and right channel out puts of the cartridge are paralleled in phase. This sums the lateral information and cancels the vertical information (which is only noise). For play back of vertical disks, you must reverse the signal and ground on one channel when the two channels are paralleled. (In other words, you connect them out of phase.) This must be the channel that is not grounded to the cartridge case (e.g., the right channel for the Stanton cartridges).
Left ground is paralleled with right signal and right ground with left signal.
This modification adds a switch on the rear panel to change from lateral to vertical reproduction. In the vertical position, the two channels are connected as described above, summing the vertical information and canceling the lateral (again, only noise). This switching arrangement will not work with a stock tonearm cable, so I will also discuss cable modification later in the article.
You should provide two outputs from the C-8 so that you can feed both channels of your stereo preamp or integrated amp without external Y adapters. Since this modification uses only the tape output and discards the high-level section, you can disconnect the main output jack from the high-level section and parallel it with the tape out, thus providing two outputs from the monaural C-8.
Modification Preparation
To facilitate these modifications, I have included schematic diagrams for the original C-8 (Fig. 1) and the modified version (Fig. 2). The first step in the C-8 modification is acquiring or building a suitable power supply. If you have acquired a used C-8, it might not have a power sup ply, since power was originally obtained from a McIntosh MC-30 or MC-60 power amplifier via an inter connect cable with octal plugs on either end. McIntosh also made a separate outboard supply, Model D-8, which allowed the C-8 to be used with other power amps. If you have a D-8 supply, you're in business. If not, you will have to build one.
The supply I built is shown in Fig. 3. This is actually a better supply than the original D-8 because it contains a full-wave rectifier, more than twice as much filtering and a hum adjust for the filament supply. (My C-8 manual implies that later versions of the D-8 supply also had a hum adjustment.)
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Fig. 1
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I took the power transformer from an old Knight-Kit KM-15 mono amplifier, but almost any 700V, center tapped transformer will suffice. If it does not have a 6.3V filament winding, use a separate filament former i 1114). The remainder of the should not be difficult to locate. The hum adjust is not a critical value.
Mcintosh used 2502, but 500 to 1,000 is fine. Lowest hum will result with the control at one extreme of rotation. Six wires will come out of the supply. They should be stranded, about 2 feet long, twisted together and terminated with a male octal socket, which plugs into the back of the C-8. Pins 5 and 6 of the male socket should be connected to the power transformer primary winding. This eliminates the hazard of a shock from the plug's male pins should it be unplugged while the preamp is on. You may shield the filament and high-voltage wires from the 117V loop in the cable. Ground the shield at one end only. Twisting the filament pair tightly will help minimize any residual hum.
Test the supply with an unmodified C-8 first. You can change the value of the 47k resistor to accommodate power transformers with somewhat different secondary volt ages. The important thing is that you have about 335V DC at the junction of the 12k resistor and the 20uF capacitor in the C-8 (as shown in Figs. 1 and 2). In normal operation, the supply should be away from the preamp to prevent the phono preamp from picking up hum from the power transformer. The 2-foot power-sup ply cable facilitates this.
With the power supply operating correctly, you can begin the modifications. You must make three principal changes to adapt the C-8 for 78 rpm reproduction.
1. You must remove the aural compensator switch and reconnect it as a flat/equalized switch.
2. You do not use the high-level section (V3 and associated components). V3 remains intact, however, since supply voltages will change if it is removed. Output will be taken on ly from the tape output. The preamp will invariably feed a solid-state preamp with a 50k or less input impedance, so you must use a larger coupling capacitor. The original volume control becomes a level con trol at the tape output.
FIGURE 3: Suggested power supply for the C-8.
375v DC 4mA POWER PLUG; THIS FUSE MUST BE 1A. A 2A FUSE WILL BLOW AS SOON AS THE SUP PLY IS TURNED ON BECAUSE OF THE ADDITIONAL CURRENT REQUIRED TO CHARGE THE FILTER CAPS. (MCINTOSH USED 8 uF CAPS IN THE D-8 SUPPLY, SO 1/2 A WAS SUFFICIENT.)
3. Because this will be a phono only preamp, you disconnect the phono 2, microphone, tuner and tape inputs and add a switch to select lateral or vertical reproduction.
Since this design is nearly 30 years old, the front panel of your preamp is probably corroded and otherwise dull in appearance. You can remove the front panel by removing the six knobs, the hex nuts that hold it in place and the pilot lamp. The original pilot amp is awkward to remove and reinstall. I recommend discarding it and replacing it with the Radio Shack 272-331 (red). You may lightly brush the removed panel with fine to medium sandpaper. (This will not affect the engraved lettering.) Then wipe off the dust with a soft cloth and spray the panel with two coats of clear acrylic spray coating (Krylon
#1301). This leaves a very attractive brushed gold finish.
Switching the Switches Before reinstalling the front panel, remove the bass control and the aural compensator switch. Clip the wires connecting these controls at their circuit-board connection points and at the control terminals. Clean pins 1 and 5 of the aural compensator switch so that you can make new connections at these terminals.
Replace the front panel.
Install the aural compensator switch in the hole previously occupied by the bass control. Its terminals can face the bottom of the preamp. Install the bass control in the hole previously occupied by the switch. Moving the compensator switch minimizes the wiring length required for the switch's new function. The bass control is inoperative in its new position, but the original appearance of the C-8 is preserved.
The treble control is also inoperative, since the high-level section is not used.
Before reinstalling the preamp's knobs, wash them with soap and water and a soft brush. Install a Radio Shack 272-331 red lamp in place of the old pilot lamp and connect it to the existing wiring.
Connect pins 1 and 5 of the old aural compensator switch as shown in Fig. 2. It is easiest to make these connections to the two terminals on S1B-Rear. The wires on the S1B terminals are yellow and yellow /white in my C-8. With this switch closed (turned to the left), the five bass compensator switches are bypassed, and the response is flat. The five treble compensator switches still operate.
Turning the switch one notch to the right restores bass equalization. I recommend labeling the front panel for these two positions (flat/equalized) and lining up the knob indicator accordingly. If the knob indicator markings have worn off, you can touch them up with gold modeling enamel (Testors #1144).
---------- LEFT CHANNEL EXISTING CABLE 2 INCHES OF EXISTING RIGHT CHANNEL CABLE COVERED WITH SHIELD EXTENSION
Next, reconnect the tape output.
Refer to V2B in Figs. 1 and 2. Remove the wiring between the volume control and the circuit board. Remove the 0.47uF coupling capacitor at the cathode of V2B from the circuit board. The terminal on the front side of the board where one end of the 0.47uF capacitor was connected is the junction of the V2B cathode and the 1.8k resistor. Connect the new 4.7 uF capacitor from this terminal to the volume control terminal closest to the bottom of the front panel, as shown in Fig. 2. This capacitor should be polypropylene or polyester.
Remove the yellow wires connecting the tape and main outputs to the circuit board. Connect a twisted pair from the center pin and chassis connections of the tape output to the remaining terminals of the volume control. The wire from the center pin connects to the control's wiper. Parallel the tape and main outputs, as shown. The modified circuit should look like Fig. 2.
Finally, connect a 1M, 0.5W resistor from the V3A grid to ground. Al though V3 is not used in the modi fied C-8, it must remain installed to ensure correct supply voltages. The 1M resistor replaces the disconnected bass control and provides nor mal biasing.
Phono Input Changes
Disconnect and remove the phono 1 and phono 2 input level controls (located on the rear panel), the phono 2 load control and the associated wiring. You need not remove the micro phone, tuner and tape level controls, NEW TWO-CONDUCTOR SHIELDED CABLE SPLICED TO EXISTING RIGHT CHANNEL CABLE but you should disconnect them from the selector switch. (The modified C-8 will work only in the phono 1 position.) The phono 1 jack will be the left input from the stereo tonearm cable.
The right channel input must be insulated from the chassis. The 0.25-inch nylon insulators provided for the Old Colony Phono Jack B fit perfectly in the hole where the phono 2 level con trol was. The lateral/vertical switch should be a mini toggle (Radio Shack 275-663). Mount this in the hole where the phono 2 load control was.
Wire the phono 1 (left) and new insulated phono 2 (right) jacks to the lateral /vertical switch, as shown in Fig. 2. You can connect the 150pF capacitor and 47k load resistor across the phono 1 jack. The 1k resistor can go directly from the phono 1 jack center pin to the selector switch.
This simplifies the wiring.
If the selector switch is accidentally turned to the phono 2 or microphone positions, the phono preamp will oscillate. To prevent this, connect terminals 3 and 5 of S1A-Front to ground, as shown. The modified input circuit should electrically duplicate Fig. 2. Finally, clean all the switches and contacts with Cramolin R-5 spray (a 5 percent solution diluted in Freon TF). This completes the C-8 modifications.
You might wonder whether using higher-quality capacitors and resistors will improve performance. I do not know whether polypropylene capacitors and metal film resistors will benefit 78 rpm recordings, but considering the differences I have heard with different turntables and tone arms, it would not surprise me. If necessary, you should also replace the V1 and V2 tubes. The Telefunken 12AX7s from Old Colony are excel lent replacements.
The Tonearm Cable
Before you can use the preamp for vertically cut disks, you must modify the tonearm cable. I chose an Audio-Technica AT-1005 II tonearm.
Being a medium-mass arm, it works well with the lower-compliance 78 rpm stylii manufactured for the Stanton 500 series of cartridges. Tracking force for the D5127 stylus is 3 to 7 grams. I have found 5 grams to be a good setting for most disks. I should point out that high tracking forces are necessary to keep the stylus in the groove if records are slightly warped or off center. In my opinion, Shure Brothers made a serious error in designing high-compliance stylii for 78 rpm reproduction. Even on perfectly flat disks, the run-out grooves are usually un-trackable, causing the stylus to play the label.
This makes Stantons the only choice for this application.
A normal tonearm cable will cause severe hum when you switch the pre amp to vertical reproduction because the right channel cable's shield is now the signal carrier. The cable modification is shown in Fig. 4. You can apply this to most tonearm cables, but I have shown an AT-1005 II.
First, remove the cloth cover from the cable with a single-edge razor blade or small scissors. Be sure you do not cut any wires. The left channel (white) cable remains in stock form, as does the black ground wire. Cut the right-channel (red) cable, leaving 2 inches extending from the five-pin plug. Remove 1 inch of the outer insulation, being extremely careful not to cut into the shield. Connect the center conductors of a two-conductor shielded cable (Radio Shack #278-1276) to the shield and center conductor of the 2-inch cable. Insulate both with electrical tape.
Using a 2-inch length of shield removed from a piece of RG-8 cable, you can extend the new shield over the 2-inch length of original wire.
This will ensure that no portion of the cable is unshielded. Insulate this end of the cable with electrical tape.
(Heat-shrink tubing will leave the cable too stiff.) At the other end of the new two-conductor cable, connect the center conductors to a new RCA plug and the shield, via a short length of wire, to the black ground wire. Be sure to observe the polarity of the two center conductors, as shown in Fig. 4.
The left channel of the modified cable is connected to the phono 1 input on the modified C-8, and the new right-channel cable is connected to the new insulated phono 2 input.
You might not wish to modify your existing cable if it is the only one you own. In that case, you can order a spare cable from Audio-Technica (or whoever manufactures your tone arm) for modification purposes.
Using the Modified C-8
With the C-8 modifications completed and the new tonearm cable connected, the preamp is ready for use. The paralleled tape and main outputs should connect, via a normal stereo cable, to the spare, auxiliary or other high-level input on your stereo preamp or integrated amplifier.
The volume control has very little effect below 8. In actual use, it will normally be left at maximum level (10) anyway. When turning on the unit, allow it to warm up for one minute before turning up the volume. Turn back the control when switching from flat to equalized, as switching produces a pop. Backing the control down to 7 eliminates the pop. The selector switch must be in the phono 1 position.
Play acoustical recordings in the flat position, with the rumble filter set at 4 or 5. Rumble-filter curves are shown in Fig. 5. Set the treble compensator switches for lowest surface noise, without removing the high frequency information in the music.
You will normally need at least 15dB of attenuation at 10kHz. The ear is the best judge. Treble compensator settings are shown in Table 1.
Play electrical recordings in the equalized position, with the bass turnover switches operating as shown in Table 1. Table 2 shows approximate bass turnover frequencies and treble roll-off rates for some ...
FIGURE 5: C-8 rumble-filter curves, microphone and phono channels only.
... 200 rpm labels. These settings do not hold true for every record on these labels. If playback is bass heavy, you must switch to a lower turnover fre quency. If the treble is excessively noisy or bright, you need more treble attenuation. Once again, the ear must be the final judge. Set the rumble filter at 3 for most electric disks.
Serious collectors will also find an octave equalizer useful but not necessary. If you opt for an equalizer, the Dynaco SE-10 and Hafler DH-160 are ideal.
Most 78 rpm disks were cut later ally. It is easy to determine whether the lateral/vertical switch is in the correct position, since the wrong set ting results in loud surface noise and no music. Also remember that more than half of all 78s were recorded at some speed other than 78. The 78 speed (78.26 rpm to be precise) was not standardized until the late 1920s, but many recordings made well into the 1930s deviate from the standard.
For instance, most Victor acoustics and early electrics play at 76.5 rpm, although many are at other speeds, including 78.26. Columbia acoustics seem to vary between 79 and 80 rpm.
Edison Diamond Discs and most Pathes (both cut vertically) run at 80 rpm. The Edisons seem quite uniform. Very early Victors and English G&Ts (pre-1903) are often between 70 and 72 rpm.
Clearly, a variable-speed turntable is essential. I use a Netronics 350F, which, unfortunately, is no longer available. Other possibilities are the
Lenco L75 and the older B62 and B61. The latter two have terrible tonearms that you should replace if you are a serious collector. Thorens still manufactures several models with 78 speed and variable pitch. You can find some hints on modifying turntables to incorporate 78 speed in TAA 4/77 (p. 46) and 1/79 (p. 36).
As I said, my speed recommendations do not hold true for all records on these labels. The best way to determine speed is by locating the musical score to the selection and adjusting the speed to match a pitch pipe, such as Linder's Master Key, available in most music stores. This is no problem with instrumental selections, but opera singers occasionally transposed arias to suit their own voices. If you are familiar with the timbre of a singer's voice and the playing speeds of other records from the same time period, it should be ...
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Table 1
BASS TURNOVER & TREBLE FILTER CHARACTERISTICS
Turnover Bass Frequency Treble (hertz) Decibels of Attenuation (at 10,000 )
TABLE 2 APPROX. CHARACTERISTICS FOR SOME 78 rpm RECORDS
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...easy to determine whether a selection has been transposed down from actual score pitch. Score pitch is, however, pretty reliable for most vocal selections. You can assume the
“A equals 440Hz ” standard for most American and European disks made after the turn of the century. Some early European piano-accompanied disks might be closer to A-435 or 438.
The 2.7-mil standard 78 rpm stylii are not ideal for all disks, although they are acceptable for most. Owl Audio Products carries many special Stanton stylii for the 500 series cartridges. Contact Owl for further in formation.
I am sure you will be amazed by the improved sound of your 78 rpm records when you use proper play back equipment. A high-quality tonearm and cartridge, combined with proper equalization and filtering, will reveal music not previously heard from these grooves. I cannot agree with the purists who insist that old records must be played on the original phonographs for proper results. The original phonographs were incapable of retrieving all the information contained in the record grooves, and most of them caused severe record wear because of excessively high tracking forces. I hope this project will encourage collectors to look more closely at the beautiful music on some of their old recordings.
3. Owen, Tom and John C. Fesler, “Electrical Reproduction of Acoustically Recorded Cylinders and Discs, ” AES Preprint #1854, presented at the 70th Convention, New York, 1981.
4. Owen, Tom and George Alexandrovich, “A Technical Look at the Stylus/Groove Interface in Recording Past and Present, ” AES Preprint #2048, presented at the 74th Convention, New York, 1983.
5. Read, Oliver and Walter L. Welch, From Tin Foil to Stereo, Howard W. Sams & Co., 2nd Edition, 1976.
6. Tremaine, Howard M., Audio Cyclopedia, Howard W. Sams & Co., 2nd Edition, 1969, pp. 699-701.
ABOUT THE AUTHOR
Mr. Galo is a recording engineer and electronics technician at the Crane School of Music, State University College of Arts and Science, Pots dam, New York, where he also teaches courses in sound recording and audio technology. For three summers, he has been on the faculty of the Saratoga-Potsdam Choral Institute. Mr. Galo has a B.M. in music education and an M.A. in music history and literature from the State University College at Potsdam. He is a member of the Audio Engineering Society, the College Music Society and the Association for Recorded Sound Collections.
PRODUCT SOURCES:
Specialty stylii for Stanton 500 cartridges. Owl Audio Products Inc. PO Box 616, Ansonia Station New York, NY 10023
AT-1005 II tonearm, extra cables Audio-Technica US. Attn. Jeff White 1221 Commerce Park Dr. Stow, OH 44224
Stanton 500 cartridges and D5127 stylii Lyle Cartridges 365 Dahill Rd. Brooklyn, NY 11218
Cramolin R-5 spray, Cat. No. CL-880-R/5F, 6 oz. can Caig Laboratories Inc. 1175-0 Industrial Ave. PO Box J Escondido, CA 92025
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