Power Amplifier Postsurvey: Mop-Up and Afterthoughts (Vol.1, No.6: Spring-Fall 1978)

You didn't really think our "final installment" in the last issue was final, did you? New things are happening all the time and old things appear different as a result. We pick up a few loose ends and look at seven more power amps, two of them SOTA contenders.

We still don't have a program of amplifier measurements we can fully correlate with what we hear in our listening room. Our announcement in the last issue of the imminence of such a program was premature; we had gone a bit overboard after reading the engineering paper by Leinonen and Otala on the five basic distortion mechanisms in amplifiers and the most sensitive measurement techniques to identify them. Our subsequent investigations convinced us that, even though the two Finns are absolutely right as far as they pursue the subject, their methods will not pinpoint on the test bench the difference between an amplifier that sounds excellent and one that sounds even better-which is, of course, what The Audio Critic is looking for. We don't believe that anyone in the world today has a "black box" test program capable of predicting the sound of an amplifier simply by feeding test signals into the input and measuring them at the output, without looking inside the box and analyzing the topology.

There are just too many subtle non-steady-state effects that don't show up on a meter or a CRT.

What black box tests do accomplish is to separate obviously flawed products from the basically decent ones. For that reason we continue to track through all our routine measurements on new amplifiers that come in, including THD, SMPTE-IM, CCIF-IM, square waves into resistive load, square waves into reactive load, slew rate, propagation delay, recovery time, output minus input on pulse-type information, and a few other little tricks that we make up as we go along. But we must confess that our heart isn't in it; the difference between satisfactory and superb results on the lab bench isn't reflected in the sound. Poor results are always audible, on the other hand, so that the bench tests aren't a total waste of time.

We're still convinced that feedback plays the decisive role in creating the differences we hear. Most amplifier designers don't use feed back correctly because they don't fully under stand its consequences. (See also Robert Cordell's letter to the Editor in this issue and Andy Rappaport's letter in the last issue.) We get especially nervous when we measure vanishingly low THD at high power levels; it generally indicates that someone poured on the feedback indiscriminately to be able to advertise those double-oh figures. That's not where it's at, fellas; forget about the goose eggs after the decimal point and worry instead about the way the amplifier behaves with complex reactive loads.

A note on the listening tests.

The amplifiers reviewed below were compared two or three at a time by successive insertion into our 'Reference A" system (described elsewhere in this issue) and listening to each at some length, without the confusion and superficialities of rapid A-B-ing. Reference A has gone through a number of evolutionary changes but was of course kept constant in the course of any given side-by-side comparison.

In some cases it was not possible to evaluate the amplifier under test through the three-way hybrid speaker system of Reference A, and conventional dynamic speakers had to be substituted. Among the latter were the DCM Time Window, the Tangent RS2 and the Symdex Sigma. These, too, were kept constant for each particular comparison, but nearly all the amplifiers tested were eventually heard through all speakers. In addition to phonograph records played on the Reference A turntable, our source material included 30-1PS master tapes played on a modified Studer A80 with Mark Levinson electronics.

Audionics PZ3-11 Audionics, Inc., Suite 160, 10950 SW 5th, Beaverton, OR 97005. PZ3 Series 11 stereo power amplifier, $499 (8589 with VU meters). Three-year warranty. Tested

#05203, on loan from dealer.

This 100/100-watt unit isn't quite as successful in circuit concept as the later, smaller and somewhat less expensive CC-2. Not that the PZ3-1I is a bad amplifier, but Audionics has meanwhile come a long way in amplifier design philosophy, and we suspect that the days of this older model are numbered. Its sound is definitely not as transparent and unstrained as that of the CC-2, being somewhat edgy and hollow by comparison. The bass is nice, though.

On the test bench we found no obvious anomalies to account for what we were hearing.

FM Acoustics FM-600A

FM Acoustics Ltd., CH-8702 Zollikon, Switzerland.

FM-600A Laboratory Power Amplifier, $1650. Tested #00133, on loan from dealer.

Even though this 150/150-watt Swiss amplifier sounds very good, we can't see a niche for it in a world where there exist even better sounding amplifiers for considerably less money. There appears to be a definite hardness and glare in the sound of the FM-600A (one of our keenest auditioners described it as a phosphorescent quality), which has been "bred out" of today's most successful designs. Don't misunderstand us; we aren't talking about a major flaw. But the Audionics CC-2, at less than one third the price and half the power, sounds distinctly cleaner and sweeter.

In our square wave tests, a 2-microfarad capacitor across the 8-ohm load resistor induced an overshoot of 125% followed by gobs of undamped ringing. Feedback problems, no doubt, which may have had something to do with what we heard.

Futterman H-3aa (Improved)

Futterman Electronics Lab, 200 West 72nd Street, New York, NY 10023. H-3aa vacuum-tube power amplifier, $895 (stereo pair on two chassis). Tested samples on loan from manufacturer.

This latest version of the Futterman tube amplifier still isn't the final version that will most probably be made in larger quantities and (cross your fingers) sold in stores. That one is still in the experimental stage; what we have here is the version Julius Futterman is currently making for his private clientele and delivering at the rate of approximately one pair per aeon.

This one is biased more nearly as a class A amplifier than its immediate predecessor and also incorporates a few other circuit changes, none of them particularly radical. The sound, though, is radically improved.

This is absolutely the clearest, most open, most focused sound we have ever heard out of a power amplifier. The experience is of a wholly new order. No other amplifier is quite in the same class, not even the Mark Levinson ML-2 or the Rappaport AMP-1. On the other hand, the latter are considerably more useful for typical applications, since the Futterman is quite unhappy with loads below 8 ohms or so and really comes into its own only at 16 ohms and beyond. (It's ideal for driving the superb Pyramid Model T-1 ribbon tweeter, for ex ample). Into 8 ohms, the Futterman goes into very soft clipping between about 55 and 70 watts; when pushed further it clips quite decisively. It shouldn't really be used with speakers that drop below, say, 6 ohms anywhere in the audio range-and that excludes a lot of speakers. It's a pity, especially since the Futterman isn't upset by capacitive loads at all and would be really nice with electrostatic speakers if it weren't for the extremely low impedance upper range of the latter. But that's the nature of an output-transformerless tube amplifier.

Incidentally, the Futterman uses lots of feedback, proving that if you design an open loop with as little delay from front to back as this one, you can get away with it. Tubes make it easy. The resulting steady-state distortion figures are decently low without being a threat in the DB Systems kind of numbers game.

We must come back to that utterly transparent sound, though; veteran audio cynics listen to it with their jaws several notches lower than the code permits. It's ironic that this should be happening in a world from which the power tubes Julius Futterman is using will sure ly disappear in a few years, since the only commercial demand for them is as replacements in ancient TV sets.

Kenwood L-07M Kenwood, PO Box 6213, Carson, CA 90749. L-07M mono power amplifier, $450 (stereo pair, $900). Tested #620130 and #650042, on loan from dealer.

This is one of the new family of Kenwood components making a bid for the sophisticated audiophile's dollar, but not very persuasively if you ask us. To our ears it sounds horrible.

Mushy, edgy, fuzzy, totally degraded-words fail us.

On the test bench the L-07M comfortably puts out 150 watts into 8 ohms, but when it clips it does so with hair-trigger abruptness and extraordinary violence. There's no transition between ultralow distortion and 100% garbage.

With 2 microfarads across an 8-ohm load resistor, square waves show 140% overshoot and totally undamped ringing. Obviously the amplifier has been designed for super specs with huge amounts of feedback. That makes for good reading, Kenwood, but bad listening.

Some people have kind words to say about the L-07M as a bass amplifier in biamped systems (it's DC coupled and all that jazz).

After hearing it full-range, we too are quite certain that it sounds better topless.

Rappaport AMP-1

A.S. Rappaport Co., Inc., Box 52, 530 Main Street, Armonk, NY 10504. Model AMP-1 stereo power amplifier, $1800. Three-year warranty. Tested two prototype samples, on loan from manufacturer.

As we go to press, there are only four samples extant of this, the world's first no feedback transistor power amplifier, two of which have passed through our hands and one of which we still have on loan. We hesitate to make sweeping predictions about the production version of the Rappaport AMP-1 based on this experience, even though we're told that the circuitry of our present updated sample is final and that the production model has no place to go but up in performance as a result of even better parts and better physical construction.

We'll report on that when we have one; the unit is scheduled to be in full production before the end of 1978.

Meanwhile we can only judge what we have on hand, and that's very impressive in deed. In a carefully controlled A-B listening comparison of a single AMP-1 with two bridged Mark Levinson ML-2's per side (that's four chassis at $2000 each), the AMP-1 was unanimously preferred by a group of rather sophisticated auditioners as clearer, more detailed, more solid, more lifelike overall. That doesn't mean that we now declare the AMP-1 to be "better" than the ML-2. First of all, only dynamic speakers were used, no suitable electrostatic having been available at the time we had all amplifiers on loan. Nor had our reference ribbon tweeter, the Pyramid T-1, arrived yet. It's possible, though not probable, that through speakers with even higher resolving power the results would have been different.

Furthermore, quite aside from sonic considerations, the Mark Levinson amplifier is built like a battleship out of the best parts money can buy and at the same time is completely modularized, so that evolutionary improvements can, and will, be implemented right in the field. The Rappaport, on the other hand, is very much an ad hoc design, the "hoc" being the best achievable sound today and never mind tomorrow.

Actually, when it comes to sheer sound quality, we prefer the latest Futterman over the AMP-1 by a small but decisive margin (see the H-3aa review above); however, the Rappaport has so much more headroom with typical low impedance loads that it's a far more practical choice in most applications. You can get 27 to 28 clean volts per channel out of the AMP-1 almost regardless of load impedance; into 4 ohms, for example, that's close to 200 watts. In fact, for sheer output capability, the AMP-1 is almost the exact equivalent of bridged ML-2's at less than one fourth the price.

Our greatest misgiving about the AMP-1 prototypes we've examined has to do with the amount of heat the amplifier generates. Andy Rappaport's design philosophy requires very high junction temperatures to be maintained in the transistors at all times (see also his letter to the Editor in the last issue), and he uses every bit of metalwork on the amplifier as one big heat sink. We don't disagree with his technical arguments on this subject but take vigorous exception to the physical design of the amplifier chassis and case. The top, the front panel, the carrying handles, the on/off switch are all dangerously hot to touch, and the finned sides where the output transistors are mounted actually exceed 100° C (212° F) after prolonged operation, making droplets of water go "pst" like an iron that's ready for pressing pants. We fear that this will give pause to audio purists with small children, pets, accident-prone in laws, or tight equipment cabinets. We see no absolute necessity for burning-hot handles just because the transistor junctions must be hot. It remains to be seen how this problem will be dealt with in the production model. The life expectancy of transistors, capacitors and other components in this superheated environment is another matter altogether, presumably attend ed to in the specs for the parts list. We certainly hope so.

As for the feedback-less circuit design, it really gets around the typical time-domain related problems at one fell swoop; our bench tests bear that out. They also indicate more than the usual amount of ordinary amplitude related distortions; where other amplifiers yield single-oh figures, the AMP-1 flirts with the point-three and point-four region when you begin to push it. We're reporting this in such a cavalier fashion because we don't attach much importance to it; we really don't think that the difference between, say, 0.035% and 0.35% THD is where the integrity of an audio signal hangs in the balance. Maybe an amplifier with an open loop just as clean as the Rappaport's and a few dB of feedback around it to bring these distortions down 0.1% or so would sound even better; maybe not. Andy Rappaport says certainly not; he believes that the perpetually out-of-date error-correcting signal applied to the input of a feedback amplifier causes a regeneration of distortion that's invariably audible.

We suspect that he is at least partly right; how ever, there must exist a threshold of perception in this process as in all auditory phenomena.

We expect to have more to say on the subject in the future; meanwhile we can report that Andy, who recently had his 21st birthday, is entering adulthood with a very good first amplifier.

Series 20 Model M-22 Series 20 (a division of Pioneer Electronic Corp.), 75 Ox ford Drive, Moonachie, NJ 07074. Model M-22 class A stereo power amplifier, $790. Two-year warranty. Tested #X13200510M, on loan from manufacturer.

Pioneer is doing a bit of high-end colonizing here, just like Kenwood, but at least they had the decency to assume an alias for the purpose. Besides, this is a much more respectable effort than the Kenwood amplifier reviewed above.

The sound of the M-22 is pleasant and non-fatiguing but not as transparent and detail ed as that of other class A amplifiers we've tested. We hear a dull, veiled, almost opaque quality that in our judgment keeps this beautifully made product out of the top category, even on a value-per-dollar basis. On the other hand, it's rugged, reliable and sonically inoffensive, which is more than we can say about most amplifiers. The 30/30-watt power rating at 8 ohms reflects the penalties of class A operation but is quite conservative; at 40 watts it's good-bye Charlie.

Our routine series of measurements revealed no satisfactory reason for the dull sound of the M-22.

Stax DA-80

American Audioport, Inc., 1407 North Providence Road, Columbia, MO 65201. Stax DA-80 class A DC stereo power amplifier, 31700. Tested #0187, on loan from dealer.

This class A stereo amplifier from Stax Industries of Tokyo gives you 45/45 watts into 8 ohms, exceptionally clear, sweet and well balanced sound, and a royal pain in the pocket book. We wish we could say it was worth the money, but we can't. The Futterman easily beats it at its own game (clarity and unstrained ease) at about half the price and is somewhat more powerful to boot; the Rappaport AMP-1 is also far superior sonically and provides twice the power for only $100 more. In Japan, if your brother-in-law owns a hi-fi store, the DA-80 is probably still a terrific buy. We certainly don't fault it, except by comparison. The best is the enemy of the good.

In our lab tests the DA-80 behaved very well in every respect; we were amazed by its bandwidth, evidenced by a small-signal rise time of 500 nanoseconds. If you've read our review of the Cotter NFB-2 subsonic/ultra sonic filter, you know how little store we set by that sort of thing, but it's always interesting to observe the numbers games audio designers play.

Recommendations

Our top choices entail certain unavoidable trade-offs this time, so we must emphasize the necessity of referring back to the actual reviews to avoid simplistic conclusions based on this summary alone.

Best-sounding power amplifier tested so far, regardless of price, but with serious practical limitations: Futterman H-3aa (latest mod only, more into class A).

Best-sounding general-purpose power amplifier, with serious reservations about physical design: Rappaport AMP-1.

Best-sounding power amplifier requiring no caveats: Mark Levinson ML-2 (or bridged ML 2's for more power).

Close to the best at a much lower price: Audionics CC-2.

+++++++++++++++

Power Amplifier Summaries and Updates

All of the following were reviewed in Volume I, Numbers 2, 4 or 5. Some are of little or no current interest either as SOTA contenders or even as good sound per dollar.

Audionics CC-2

Audionics, Inc., Suite 160, 10950 SW 5th, Beaverton, OR 97005. CC-2 stereo power amplifier, $489.

One of the all-time "best buys' in high fidelity, even at its new price. Within its power limitations (70/70 watts into 8 ohms, considerably more into lower impedances) it comes amazingly close in clarity, resolution of detail and unstrained ease to the best amplifiers, regardless of price. Current samples we've checked are better than ever; watch out, though, for some earlier ones delivered with the bias set too low, an easily correct able condition. In its bridged mono mode, rated at 225 watts into 8 ohms, we aren't quite as sold on it; we'd like to see a heavier power supply, with greater current capability, for the kind of service high-wattage amplifiers generally end up in.

Audio Research D-100 Audio Research Corporation, 2843 26th Avenue South, Minneapolis, MN 55406. Model D-100 stereo power amplifier: no longer available in the version reviewed.

The 100/100-watt solid-state D-100 we tested has been superseded by the similarly rated D-100A, at $1195. We didn't particularly like the D-100, even for $200 less; it sounded rather blunted and lacking in high-frequency transient detail as well as in bass definition.

Bryston 4B

Bryston Manufacturing Ltd, 574 Westmore Drive, Rexdale, Ont., Canada M9W 4M1. Model 4B stereo power amplifier, $1295.

Quite possibly still the best of the very high-powered amplifiers (200/200 watts into 8 ohms, 400/400 into 4 ohms), even though it sounds just a wee bit hard and zippy compared to our current favorites. We've heard from fairly reliable sources that the latest production units are substantially better than what we tested; if that's true we may want to take another look soon.

CM 912a and CM 914

Audio International, Inc., 3 Cole Place, Danbury, CT 06810. CM 912a and CM 914 stereo power amplifiers, 3899 and $449, respectively, when reviewed.

We're not quite sure whether these units are still available; this company seems to be taking the disco sound systems route.

It matters very little, in any event, since both the CM 912a and the CM 914 have been left far behind by newer developments in the same price range. We were never fully convinced of the: engineering savvy of these people and we've also had a very negative experience with them as regards their business ethics.

DB Systems DB-6

DB Systems, PO Box 187, Jaffrey Center, NH 03454. DB-6 stereo power amplifier, $595.

This is a 40/40-watt amplifier designed with lots of negative feedback to achieve the ultralow THD figures dear to the heart of this company. It sounds strangely homogenized and lacking in spatial detail, a time-domain distortion of far greater consequence than some low-order harmonics. Not our cup of tea.

Electrocompaniet

Electrocompaniet, Toyen Gt. 14, Oslo 1, Norway. The Two-Channel Audio Power Amplifier (reviewed at an early stage of its development): current price NA.

We've been promised a greatly improved new version of this almost legendary little amplifier (25/25 watts into 8 ohms, 40/40 into 4 ohms), which was already very close to SOTA when we had our first look at it. The circuit was very up-to-date in concept, originating from Matti Otala, and the sound was exceedingly clear and detailed, surpassed only by our latest generation of top choices. We're looking forward to reporting on the improvements, probably in the next issue.

GAS 'Son of Ampzilla' and 'Grandson'

The Great American Sound Co., Inc., 20940 Lassen Street, Chatsworth, CA 91311. 'Son of Ampzilla' stereo power amplifier, 8519; 'Grandson' stereo power amplifier, $379 (with meters).

We're lumping these two together because they represent the same design philosophy, originating from the same source, with incorrect use of feedback in our judgment. The sonic results are quite disappointing; the Audionics CC-2, which is competitive in price and power output with the Son, sounds astonishingly more transparent, detailed and non-fatiguing. As for Grandson, we find it un-listenably hard and sizzly. Both Son and Grandson ring like crazy with capacitive loads.

Luxman M-4000

Lux Audio of America, Ltd., 160 Dupont Street, Plainview, NY 11803.

Model M-4000 stereo power amplifier, $1595.

A beautifully made, rugged, obviously reliable 180/180 watt unit, inexcusably hard and opaque in sound for this price category. We suspect typical feedback problems, probably traceable to mandatory Japanese specmanship.

Mark Levinson ML-2

Mark Levinson Audio Systems, 55 Circular Avenue, Hamden, CT 06514. ML-2 class A power amplifier, $2000 (per mono chassis).

This is almost unquestionably the world's best-made power amplifier, further enhanced in value by completely modularized construction to facilitate future updates (it comes apart like an Erector set). At this writing, we rate it behind the latest Futterman and the new Rappaport in sheer sound quality (see reviews above), but that rating could quite possibly change in view of the ML-2's evolutionary capability. Since we have various non sonic reservations about both the Futterman and the Rappaport, the ML-2 remains our top choice for those who wish to play it absolutely safe and don't care about the cost. As a result of fully class A operation into all loads down to 2 ohms, the out-

put capability of the ML-2 is limited (not quite 15 volts out into all such loads), so that bridging two of them per side becomes necessary for power-hungry applications. For $8000, that gives you approximately 210 watts per channel into 4 ohms, half as much into 8 ohms. Call your friendly neighborhood loan shark.

Quad 405

Acoustical Manufacturing Co. Ltd., Huntingdon, Combs. PES 7DB, England. Quad 405 "current dumping' stereo power amplifier, $480.

We recommend this only as a reasonably priced choice for driving the Quad electrostatic safely; as a general-purpose amplifier the Audionics CC-2 gives considerably better sonic results at the same price. The ultimate voltage-limited amplifier for the Quad electrostatic is the Mark Levinson ML-2.

Quatre DG-250

Quatre, 21356 Deering Court, Canoga Park, CA 91304. DG-250 Gain Cell stereo power amplifier: no longer available in the version reviewed.

The original version turned out to be unreliable and a frequent destroyer of speakers, forcing us to withdraw our recommendation of it despite some highly attractive sonic qualities, plus not far from 200 watts per channel, for very little money. The new DG-250C ($575) may or may not have solved the problem; we haven't tested one yet. In any event, the Audionics CC-2 is sonically superior to the original version.

RAM 512

RAM Audio Systems, Inc., 17 Jansen Street, Danbury, CT 06810.

RAM 512 stereo power amplifier: no longer available in the version reviewed.

The last version we saw listed is the 512A at $1250; if it sounds as hard, bright and smeared as the original 512 we tested, it isn't worth that kind of money, even at the increased rating of 200 watts per channel into 8 ohms.

SAE 2400L

Scientific Audio Electronics, Inc., PO Box 60271, Terminal Annex, Los Angeles, CA 90060. 2400L stereo power amplifier, $850.

Somewhat opaque, closed-down, lacking in transient detail and yet slightly aggressive on top. The feedback blues, no doubt, confirmed by circumstantial evidence on the test bench.

It's nice to have 200 watts per channel at this price, but not with this kind of sound.

Threshold 800A

Threshold Corporation, 1832 Tribute Road, Suite E, Sacramento, CA 95815. Model 800A class A stereo power amplifier: no longer available in the version reviewed.

The nearest thing to this discontinued unit on the current Threshold list is the Model 4000, also a 200/200-watt amplifier with "dynamic bias" for alleged class A operation (ahem, ahem), but at a much lower price-a mere $1825. Apparently the super deluxe packaging of the 800A didn't prove to be practical. It was never worth the original $2275, anyway; even the Bryston 4B sounded more open and better focused at about half the price, and since then there have been further advancements at several price points. We must add that this company has from the very beginning struck as being into "'head trips' about advanced circuitry and laboratory measurements rather than the realistic analysis of what we actually hear.

Yamaha B-2

Yamaha International Corp., 6600 Orangethorpe Avenue, Buena Park, CA 90620.NS Series B-2 stereo power amplifier, $850.

There was a lot of ballyhooing about this 100/100-watt vertical-FET power amp when it first came out, but there are any number of better amplifiers today for the money. The B-2 was over-bright and irritating even for its own generation; by late-1978 standards it isn't acceptable. Too bad; it was very beautifully made and well conceived in its inputs, controls, meters, and other aspects of physical design.

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[adapted from TAC]

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Also see:

Preamps Without End (They Still Keep Coming)

Various audio and high-fidelity magazines