By the Staff of The Audio Critic

In this second installment of our search for the perfect power amplifier, we come a lot closer. We even nominate a new reference standard (at least until the next go-around) and also have other good news to impart.

As we explained in the introduction to Part I, we're convinced that a correlation can be established between the audible and measurable characteristics of a power amplifier, al though the road that leads there is a rocky and winding one. Since this survey will have a concluding Part III in the next issue, we'll wait until then (as we said we might) to report just how far down that road we have, or haven't, been able to travel. We're still very much in the process of sorting out the tentative correspondences we've found so far between our listening room and laboratory observations, and we certainly don't want to be guilty of premature or superficial generalizations, despite the blandishments of some fairly juicy data. One thing is certain: the Hirsch-Houck type of testing is insufficient and leads only to the discovery of one SOTA amplifier after another. We're looking for just the opposite: a meaningful test or series of tests that no amplifier in the world can pass without some difficulty. That way progress will be measurable; right now, if you read the commercial magazine reviews, perfection is being routinely attained.

Just so we won't leave you in total suspense, we're willing to confess to continued partiality to the CCIF intermodulation distortion test, with 14 kHz and 15 kHz mixed 1:1.

We aren't quite sure, however, just how fine tuned a tool it is; gross differences in the resulting sidebands from amplifier to amplifier invariably reveal audible differences; differences of a few dB may be imperceptible. We've also noticed interesting correlations between phase shift and sound quality, even though the resulting time delay at the frequencies in question may not, on the face of it, appear to be significant. And so forth and so on: we're knee deep in clues, without firm conclusions. Please be patient; give us another couple of months to find what no one else has in thirty years-or to report with some certitude that we're stuck.

The continuing framework.

Everything we've said about our listening test conditions in Part I still applies to the re views below, but this time there's more. To wit; In addition to the Mark Levinson ML-1, we also used the Hegeman HPR/CU (proto type) as a reference preamplifier. The latter is an outstanding unit, but please, don't jump to any conclusions. We haven't tested the two against each other, nor the Hegeman against the Rappaport, nor any of these against other preamps with SOTA claims that have recently appeared on the scene. We have no definitive opinion on the subject. As you can see on our back page, the question will be resolved in a new round of preamp tests to be reported in the next issue. Needless to say, we never changed two links in our reference chain at the same time. Whenever we compared two different power amplifiers, the preamp remained the same, whether it was the Mark Levinson or the Hegeman.

We also used other speaker systems this time in addition to the Dahlquist DQ-10. These included the Koss Model One/A electrostatic (a difficult test load for some amplifiers), the 15-ohm Rogers LS3/5A (used full-range as well as with the Fundamental Research subwoofer), and the DCM Time Window. Again, any two amplifiers under comparison were always listened to through the same speaker, although the more critical comparisons were repeated with several speakers.

We keep mentioning comparisons of two amplifiers, one against the other, since this new crop of power amps did require some careful A-B-ing to shake down the finalists. No won der; quite a few of them were superior to our previous top recommendation.

Otherwise, our reference system remained the same, except that the Denon DL-103S (played through the Verion transformer) permanently replaced the EMT; also, the modified Stellavox 'Stellamaster' tape deck introduced in our speaker survey in this issue was used extensively with the master tapes mentioned there.

On to the reviews, then.

Bryston 4B Bryston Manufacturing Limited, 17 Canso Road, Unit 1, Rexdale, Ont., Canada M9W 4M1. Model 4B basic stereo power amplifier, 31250. Three-year warranty; manufacturer pays return freight. Tested #4139 and #4140, owned by The Audio Critic.

This is it, music lovers and audio freaks --the best power amplifier we've been able to find so far. Of course, we haven't tested them all. (Two that we should definitely look into, and haven't yet, are the Electro Research and the Mark Levinson ML-2. And there are others.) But this will be the one to beat, from now on.

The Bryston 4B is actually two separate mono amplifiers on one chassis. The only thing the left and right amplifiers have in common is the line cord. Each channel will deliver up to approximately 40 volts across just about any load that doesn't approach a short circuit- meaning 200 watts into 8 ohms, 400 watts into 4 ohms, 500 watts into 3.2 ohms, you name it.

Since it's a truly unflappable voltage source with a brute-force power supply that never seems to run out of juice, the amplifier simply doesn't care whether it sees an obscenely complex load like some of the full-range electro statics or a virtually pure resistance like a Cizek box speaker. It just keeps rollin' along. That's probably the main secret of its listening quality, but since its designer has expressed the engineering philosophy behind it in his own words, which we're reproducing herewith, we'll refrain from second-guessing him. We can't resist wondering aloud, though, why there's all the fuss about pure class A output stages both in this country and in Japan, when in Canada class AB can be made to sound this good with this kind of power.

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Comes The Resolution, All Power Amps Will Sound Great

Editor's Note: Whenever we're sufficiently impressed by the performance of an audio component, we try to brain-pick its designer for some kind of correlation between engineering philosophy and sound quality. Having been thus impressed by the Bryston 4B power amp, we managed to elicit the following observations from Chris Russell, a very serious and very long-haired young man who not only designed the circuit but also, with his brother and father, runs the company.

The Audio Critic:

In an era when power amplifiers are attaining distortion measurements approaching a thousandth of one per cent over a wide range of frequencies and output levels, essentially falling to background noise levels which are theoretically inaudible, why is there the continuing search for an amplifier which is entirely accurate to the ear musically? Whence springs the departure from theory which seemingly separates objective and subjective realities? Is it possible we are hearing things we cannot (or do not) measure? Let us investigate.

The latest thinking on the problem of dynamic distortion mechanisms in amplifiers centers largely on the popularization of Transient Intermodulation Distortion. As it is known, this is a feedback-defeating phenomenon related to the ultimate speed or slew rate capability of the amplifier. Feed an amplifier circuit a transient signal faster than it can handle and it gene rates a momentary burst of distortion trying to catch up.

It should therefore be possible to "perfect" an amplifier by either low-pass filtering the input or pushing the slew-rate limit to stratospheric values. That this engenders only some improvement leads us to the inescapable conclusion that it is a gross oversimplification of a very real problem. It is even possible that exclusive concentration on this one particular aspect of an amplifier's behavior can actually have detrimental consequences on overall musical reproduction.

In point of fact, there are dozens of phenomena which can encroach upon an amplifier's ability to handle a musical signal with negligible distortion. Most are transient-related.

Many are low-frequency problems.

For instance, when an amplifier de livers a pulse of low-frequency energy to a loudspeaker, the current flow tends to pull down the power supply voltage. This change in voltage supply throws all the sensitive standing cur rents and bias voltages within the amplifier into a state of flux, muddying the signal. Tying both channels to the same power supply exacerbates the problem greatly, as then one channel can affect the other. Split up the power supplies and regulate the voltage to all the sensitive stages, and there is an immediate and obvious improvement in clarity, transparency and resolution.

An interesting word, that-resolution-for it implies much more than appears at first glance. Resolution denotes focus, the ability to delineate subtle and delicate details in the musical picture, and its absence can have as much to do with the removal of something as with an addition such as harmonic distortion. It is entirely possible for a circuit to "lose" subtle information, or at least to hide it be hind the coarser fabric of the overall musical framework.

It appears to be this loss of resolution which offends the ear rather grievously in many circuits which display a certain amount of non linearity in the signal processing, and which hope to straighten out the problem with feedback or with "complementary nonlinearities" further down the line.

Case in point: Another low-frequency vagary which often creeps into amplifier circuits is related to high standing currents in class A stages.

When a transistor conducts a good deal of current, at a rather high voltage, it heats up-since voltage times cur rent equals power. When the signal causes the voltage to swing up and down, the power dissipation varies from top to bottom of the cycle. At mid to high frequencies this tends to integrate over the cycle, but at lower frequencies the transistor chip will have time to vary as much as tens of degrees Celsius from one end of the signal swing to the other. Since most of the transistor's characteristics, including gain, base-emitter bias voltage and frequency response, vary with temperature, this causes nonlinear distortion. Further, much of it is high order. Common sense dictates, there fore, that we should run low-level class A stages at no more current than necessary, and at reasonably low voltages if possible. (Especially in view of the fact that, other things being equal, transistors distort less in every way when they handle less current.) This way the junction will always be near room temperature and not subject to change with signal swing.

The one unfortunate fly in the ointment is that transistors tend to increase in frequency response with increased current flow. Thus the Great Slew-Rate Race dictates that many designers opt for speed rather than linearity and ignore this common sense ideal. (Recently I have seen some amplifiers with low level stages dissipating so much current that they require small power devices on their own heat sinks, running blazing hot to the touch!) Since amplifiers tend to have gobs of feedback available at low frequencies, this problem is often hidden from steady-state distortion measurements, but it is still audible as a loss of resolution. Unfortunately, once the finesse in separating tiny nuances is sacrificed, it can never be regained later.

This is why I say that, to the ear, distortion is distortion. There is no point in taking a step backwards in one area in order to gain in another.

Speed and slew rate are very important parameters, but only insofar as they relate to obtaining the maximum benefit from a circuit which is maximally linear at all frequencies, as well as from the amount of feedback which is employed. (After all, feedback can and does further reduce distortion.) The important watchword is overall musical accuracy.

Thank you very much for your interest.

Sincerely, Christopher W. Russell Bryston Manufacturing Ltd.

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Because the Bryston 4B sounds just about unexceptionable to our ears. On the top end it rivals the Electrocompaniet amplifier, which is our standard of excellence on that count-open, sharply etched, un-smeared, fast and sweet at the same time. And of course the 4B eats the Electro for breakfast in overall drive capability and load handling. The Bryston also has the best bass we've heard so far, free from the least trace of looseness, hangover or artificial "warmth." It just goes down, down, down, tight and unshakable. The midrange may not have the uncanny clarity of the Futterman, but it's close (damn close), and the overall quality of the Bryston is more open than that of the Futterman. Actually, as a high-power amplifier for all seasons, the Threshold 800A was the one the Bryston had to beat, which it did quite handily. By comparison, the Threshold, a very good-sounding amplifier in its own right, seemed a bit heavier, thicker and more closed down in its sonic presentation, with just a least bit of the aforementioned "warmth" factor.

Incidentally, the above observations apply only to a Bryston 4B that has been turned on for half an hour to an hour. Listened to cold, the amplifier sounds a little hard and glary.

The warm-up seems to stabilize the circuits.

In the laboratory, the Bryston 4B appeared to be irreproachable, with point-double-oh or low single-oh distortion on every possible THD and IM test. No problem at all on the 14-plus 15-kHz killer test; gorgeous square waves right down to 20 Hz and below; in fact the phase shift at 20 Hz is an incredible 1° (don't forget, this is not a DC amplifier) and totally unmeasurable by the time you get up to 100 Hz. The slew rate appeared to be a little over 40 V/uS, which is somewhat short of the *'greater than 60 V /uS" specified, but then who is to say that Bryston's technique for this rather difficult measurement isn't superior to ours. In any event, you can double the slew rate by throwing the two channels into a bridged mono configuration at the flick of a switch (what a nice feature!), in which mode the available power is 800 watts into 8 ohms. That's more than one horsepower.

As a footnote to all this, we must refer you back to The Admonitor column in our second issue, where we took Bryston to task for claiming to have the world's best power amplifier, period. We still stand firm on that admonishment, even though the claim seems less outrageous after our tests. When an ad makes a superlative product appear like a hype, it's time to switch to another ad writer.

Oh yes. What about the Quatre DG-250, our previous top choice? Not quite in the same class with any of the amplifiers mentioned above.

CM 914 Audio International, Inc., 3 Cole Place, Danbury, CT 06810.

CM 914 Stereo Power Amplifier, 3449. Tested #3129, on loan from manufacturer.

Since the Quatre DG-250 seems to be leaving a very poor track record as regards reliability, this neat little 100-watt-per-channel amplifier from the successor of C/M Laboratories is the best we've been able to find in the same price range as an all-around work horse alternative. The CM 914 sounds more open than the Quatre on top, although we still prefer the sound of the latter on an overall basis. The CM 914 is a little less focused and transparent in the midrange and not as firm in the bass. The very fact, however, that the CM can give the Quatre a hard time in an A-B comparison shows that it's a better amplifier than most of those reviewed in Part I, including the bigger and more expensive CM 912a.

(It's interesting to note that the 912a incorporates current limiting whereas the 914 does not.) Our laboratory tests revealed some peculiar ringing on square waves, which was some times intermittent, sometimes stubborn and ineradicable. Since it existed in both channels, we doubt if it was due to some kind of minor defect in a single component. The ringing, excited by the leading edge of any square wave, appeared to have a frequency of approximately 1MHz and a decay time of about 3 uS. Whether it had anything to do with what we heard . . . who knows? Otherwise the amplifier exhibited single oh distortion within its power range on our THD and IM tests, except on the 14-plus-15 kHz CCIF, where it did a little worse though still acceptably (better than the Threshold 800A for example). Its strongest spec seems to be phase shift; there's hardly any to speak of between 20 Hz and 20 kHz.

As audio purists, we can't get too excited over this amplifier; on the other hand, we must admit that it represents decent performance and value when measured against what else is available.

DB Systems DB-6

DB Systems, PO Box 187, Jaffrey Center, NH 03454.

DB-6 Precision Power Amplifier, $650. Five-year warranty, manufacturer pays return freight. Tested #200730, on loan from manufacturer.

When a 40-watt-per-channel power amplifier of super-austere appearance costs $650 (i.e., considerably more than the much more powerful Quatre DG-250, Quad 405, CM 914 or GAS Son), one would expect it to produce extraordinarily accurate sound at moderate levels. The DB-6 sounds clean, sweet and non fatiguing, but not accurate. Hence we must call it a disappointment.

Example: We were listening to an unaccompanied quartet of voices through the Bryston 4B. Each voice stood out in perfect relief, without the slightest tendency to cover up the other three. You could actually 'hear' the spaces between the four singers. We switched to the DB-6. The sound blended into a pleasant, euphonious glop. It wasn't unmusical; if we hadn't known what was really on the record, we would have been satisfied. Strange amplifier.

It's interesting and instructive to compare the DB-6 with the Electrocompaniet amplifier, which has a somewhat lower power rating (it also costs less) but is otherwise similar in its techno-image: a cute little black box for the purist. The Electro has a slew rate of 100 V /uS;

the DB-6 is much slower at 15 V/uS. The Electro does a little better on the 14-plus-15-kHz CCIF test, but not much. (Neither is in a class with the Bryston 4B, even at low power levels.) Both are uncomfortable with low-impedance loads that tax the current capability of the power supply. Does the Electro sound considerably more accurate (a la Bryston) because it's faster? Or is that only the by-product of the actual causative factor? Or is it purely incidental? These are some of the nagging questions we're trying to find answers to. (Lots of luck, fella, say some pretty smart engineers.) One thing that isn't responsible for the shortcomings of the DB-6, for sure, is harmonic distortion. The specs say less than 0.005% from 20 Hz to 20 kHz, and it's the truth-we've verified it. Double-oh and triple-oh distortion figures seem to be the big thing at D B Systems, and we're very much beginning to wonder whether that isn't the root of the problem.

Ultralow distortion can only be achieved with lots of feedback; lots of feedback will cause . . .

there we go again.

Another thing we aren't sure about is the net effect of the subsonic filter in the DB-6 (12-dB-per-octave Butterworth with 15 Hz corner frequency). There has been some talk of audible group delay introduced by certain types of subsonic filters, but not such simple net works. At any rate, the resulting phase shift at 20 Hz is 64°, which means a delay of just under 9 msec at that frequency.

We wish we could either like the DB-6 a little more or else be able to explain a little better just where the design went wrong. Neither of which wishes can change our conclusion: too little for too much.

Electrocompaniet (Part II)

Electrocompaniet, Toyengt. 14, Oslo I, Norway. ''The Two-Channel Audio Power Amplifier," $505 to private users, $344 to dealers, FOB Oslo. Tested unnumbered sample, on loan from owner.

Since our original review, we've found out a great deal more about this lovely little amplifier. First of all, it does really "exist" from the consumer's point of view; you can write for it to the above address and, sooner or later, they'll send you one. (For all we know, by the time these words are in print, there'll be an American distributor; don't count on it, though.) Secondly, it's every bit as good after greater familiarity with it as we first reported. Thirdly, the 100-watt-per-channel version is still only in the laboratory phase, not in production.

Our second sample of the amplifier was a little different from the first physically, but not in sound. The highs are still SOTA; completely unsmeared, etched but edgeless, totally trans parent. The rest of the range is equally good, as long as you don't ask the amplifier to drive loads it doesn't like at levels it can't handle.

Specifically, complex loads such as the Koss Model One/A, which drops down to rather low impedance at some points in the audio range, distress the Electro to a surprising degree. Even at moderately loud listening levels, the Koss/ Electro combination sounded hard and irritating. On the other hand, the 15-ohm Rogers LS3/5A makes the Electro very happy; at that impedance the current demands on the power supply are reduced and the sound is of the utmost purity.

Interestingly enough, the amplifier will deliver 14 volts or so into any load without actually clipping (i.e., 12.5 watts into 16 ohms, 25 watts into 8 ohms, 50 watts into 4 ohms);

however, at 4 ohms the power supply has trouble providing the necessary current and high amplitude signals begin to be envelope-modulated by the 60-Hz line frequency. That's what we obviously heard on the Koss. Otherwise the Electro behaved very nicely on the test bench; even the 14-plus-15-kHz CCIF test went well, though not brilliantly. So, we must ask again, is it the 100 V/uS slew rate with 1MHz bandwidth that makes this circuit sound so good, or is that the symptom rather than the cause? Tune in the next time around and see if we can answer the question.

Incidentally, the Electro has a rather peculiar input with a 1000-ohm impedance and mustn't be driven from a high-impedance source such as a tube preamp. Input sensitivity for full output is only 0.35 volts. Electrocompaniet makes a matching preamp, which we haven't seen, that has a 10-ohm output impedance, just the right gain for the power amp, plus a 70-kHz low-pass filter, presumably to protect the power amp against transient over load. Interesting.

Our recommendation: think about your speaker load, your loudness requirements, and consider this power amplifier very carefully if you want close-to-SOTA sound with the afore mentioned limitations at a reasonable price- reasonable even with freight and import duty added.

Futterman H-3aa (Part IT) Futterman Electronics Lab, 200 West 72nd Street, New York, NY 10023. H-3aa vacuum-tube power amplifier (mono), 3260; stereo pair, $520 (no longer made to order at this price; see below about availability). Tested #105 and #106, on loan from manufacturer.

Our test samples arrived as promised (see Part I) and confirmed just about all our initial impressions. This is unquestionably one of the world's finest power amplifiers, but not an amplifier for all seasons-all applications, that is.

First of all, the Futterman doesn't like to see low impedances, such as are presented by the Koss Model One/A electrostatic, for example. It just isn't on its best behavior at 4 ohms; it can't deliver enough power, for one thing, and clips badly when pushed. Go just a wee bit higher and the impedance match improves; for example the DCM Time Window, which is still a fairly low-impedance speaker, never sounded better than when we connected it to the Futterman. At 8 ohms the situation is pretty well stabilized, and at 16 ohms the amplifier comes into its own; but how many 16-ohm speakers are there? The Rogers LS3/5A is rated at 15 ohms but shouldn't really be used with the Futterman, which can deliver about 150 watts into that impedance and blow out the rather fragile KEF drivers. (We tried the combination, anyway, and it sounded gorgeous-but the Rogers/Electrocompaniet duo was, if anything, even better.) As regards specific sound quality, the Futterman seemed to have the most amazingly transparent and un-blurred upper midrange (female voices, etc.) of any amplifier that has crossed our path; superb highs (but not quite as fast, etched and open as the Electrocompaniet or the Bryston 4B); excellent bass if used with extra output capacitors (more about that be low), and a slightly compressed overall sound in comparison with the Bryston (our reference standard). In other words, better than any other amplifier in some respects, better than all but two or three others in every respect. It must be that at least one unidentified kind of distortion, the kind that dumps spurious products into the all-important upper midrange, has been more thoroughly eliminated in the Futterman than in any other amplifier.

In the laboratory, the Futterman was no match for the finest solid-state amplifiers, such as the Bryston 4B. All our usual tests came out a little worse, though none of them disturbingly so. It seems that a tube amplifier is a whole different species, and the same rules don't apply. Which may prove, in the end, that the rules are wrong, since it shouldn't make any difference to the electrons how we make them flow, as long as they flow just so.

We mentioned the output capacitors in the Futterman, which in the standard model add up to 2400 microfarads per channel. Julius Futterman supplied us with an extra 4200-microfarad cluster for each amplifier, so that each of our speakers could be coupled to the tubes via 6600 microfarads. And it made a difference. The bass was drier, more solid, more realistic than with 2400 microfarads. Whether this has anything to do with phase shift and time delay, we have no idea. It certainly has nothing to do with amplitude response. Even the standard model is flat at 20 Hz. It may be that electrolytic capacitors have some peculiar distortion which is minimized by multiplication in parallel.

The main problem with the Futterman is where to get it. We've been informed that Julius Futterman has stopped taking orders from private individuals at the ridiculously low price listed above. He is now selling every amplifier he makes by hand to The Sign of the Golden Ear ( Riverview, Michigan), the out fit that plans to market the amplifier in a commercially packaged version, which to the best of our knowledge doesn't exist yet. The price of this latter version was originally planned to be $989 for a stereo unit or stereo pair. That's all we know.

If it's any consolation to you, a Futterman amplifier was just as hard to get in 1956.

Quatre DG-250 (follow-up) For manufacturer, price and other particulars, see original review in Part I.

We regret to tell you that our original endorsement of this amplifier must now be heavily qualified. It still sounds as good as ever (though not quite as good as the top amplifiers in this second batch), but just too many reports of failure in use have reached us to be ascribed to normal attrition. Since for every failure reported to us there must be a good many others we'll never know about, the total number should logically be a substantial percentage of production, since not that many Quatre DG-250's have been made thus far.

And that's not all. The typical DG-250 failure seems to be particularly drastic and destructive. Here's the scenario we've been hearing over and over again: The predriver transistor fails and a huge amount of DC, representing a major portion of the rail voltage, appears across the output. Since the amplifier has no protective circuits to sense this type of failure (don't ask us why), the DC passes through the speaker load. The speaker either isn't fused or the fuse doesn't blow fast enough.

The speaker goes 'arrgh' and that's that. No more woofer-and maybe no more other things.

Not nice at all. Inexcusable, in fact.

Out of just three Quatre DG-250's that have passed through our own hands, one of them did this. A second one conked out non destructively. That's two out of three. And similar cases we personally know of add up to a solid two figures, not one. On top of it, we hear that Quatre has been dragging their feet in responding to the situation. New circuit boards to replace the burned-out ones take for ever to receive, we hear; and we've had reports of noncooperation and evasiveness by the front office. In all fairness, we must remind you that we spend most of our time and energy on equipment tests and editorial work, not on double checking the authenticity and accuracy of com plaints. But there must be a fire of some sorts where there's so much smoke. And you should have seen the smoke that came out of our DG-250 . . .

So, if you buy a Quatre, you're on your own as far as we're concerned. Enjoy the good sound, but don't say you haven't been warned.

The amplifier may easily turn out to be more expensive than you thought.

RAM 512 RAM Audio Systems, Inc., 17 Jansen Street, Danbury, CT 06810.

RAM 512 Stereo Power Amplifier, $1150.

Three-year warranty; manufacturer pays return freight.

Tested #10547, on loan from manufacturer.

In our CES report we made the statement that Richard A. Majestic, who is Mr. RAM, was originally the M of the former C/M Laboratories, which is now Audio International. That was an error. Dick was an employee at C/M, not a founding partner. We also made the statement that his RAM 512 power amplifier was an "all-out design." That was no error, except in the intended sense. The amplifier we tested turned out to be an all-out design all right-all out to make money by tweaking super specs (high power, double-oh THD, etc.) out of a basically flimsy box and selling it at an exorbitant price.

If you're getting the impression that we're a little upset and indignant about this, you're quite right. The price of the RAM 512, either retail or wholesale, is only a few dollars below that of the Bryston 4B, our new reference standard. And the comparison between the two amplifiers is a joke. Compare the power sup plies (single vs. dual), the heat sinking (hardly any in the RAM, just convection cooling), the quality of parts, the general construction (the Bryston weighs 50% more)-we could go on.

Without knowing the prices, you'd guess a cost ratio of about two to one. What the RAM gives you for your money is a pair of big, flashy dB meters plus peak-reading -20 dB LED's, and lots of protective devices to keep the tweaked circuitry from going up in smoke or blowing out your speakers (at least that's conscientious, unlike Quatre). Even so, the RAM can't give you the same maximum volts into 4 ohms as into 8 ohms, which the Bryston does without batting an eyelash; and even into 8 ohms the RAM gives you a little less power, being rated at 170/170 watts.

Of course, the most important comparison is just listening-and, again, it's no comparison. The RAM 512 sounds a little over-bright, hard and glary, though not disturbingly so; it gives a nice feeling of depth, but doesn't have anywhere near (and we mean anywhere near) the transparency and definition of the Bryston (or the Futterman or the F Electrocompaniet, for that matter, at much lower prices). If the RAM cost, say, somewhere in the seven hundreds, we'd be more charitably inclined toward it and would probably weigh its positive and negative qualities with greater deliberation. At $1150, however, it's put up or shut up.

On the test bench, the RAM 512 displayed a peculiar kink in both the leading and trailing edge of a square wave (any square wave), which is definitely not normal or acceptable. (Over biased transistors?) Phase shift at the lower frequencies was also considerably higher than we're comfortable with, although it's inevitable with the AC input coupling employed and we don't claim (not yet, anyway) that this is an established criterion of audible quality. On top of it, the amplifier went into protection on both channels toward the end of our tests and we couldn't make it work again without looking into the cause-which, quite frankly, didn't interest us at that point.

Our conclusion: give us more amplifier for the money, RAM, or else take out those meters, give up a few watts, and cut the price in half, Threshold 800A Threshold Corporation, 1832 Tribute Road, Suite E. Sacramento, CA 95815. Model 800A class A stereo power amplifier, 32275 (discontinued). Two-year warranty, manufacturer pays all freight. Tested #770408, owned by The Audio Critic.

Even though the Threshold 800A has been discontinued as a specific model, we consider it well worth reviewing, since this company is strongly committed to the patented active bias class A circuitry it incorporates. A smaller and less expensive version, the Threshold 400A (a 100/100-watt power amplifier at a mere $1147) is still on the market; and a successor to the 800A, tentatively called the 8000 and built on two separate mono chassis, is soon to be re leased. (The 810 we had mentioned in our CES report was shelved in the prototype stage.) The 800A was undoubtedly too expensive and impractical to make: a huge but beautiful black monster, with massive handles and two enormous meters calibrated in watts at 4 and 8 ohms, practically hand-wired throughout- a real status symbol. The rated power is 200 watts per channel into 8 ohms; a conventional (constant bias) class A amplifier of this power would have to be the size of a small refrigerator.

The Threshold system of active or dynamic biasing reduces the power dissipation requirement at idle by a factor of four; hence the large but still viable dimensions of the 800A. A critique of the Threshold patent is beyond the scope of this review; we have, however, heard a few impeccably qualified technologists speak of it with a certain degree of disdain.

Their comments ranged from *'class A in name only, not in fact" to "unnecessarily complicated without genuine advantages" and even to "class A is pure bullshit, anyway." On the other hand, the Mark Levinson ML-2 is a constant bias class A design with some pretty good credentials behind it, so we'll hang fire on this one until we have an ironclad case one way or the other.

Since the proof of the circuit is in the sound, we must report that until the arrival of our two Bryston 4Bs, the Threshold was our reference amplifier for a number of weeks. It sounded more solid, sweet and open than the Quatre DG-250 (big deal, at about five times the price);

it wasn't quite as uncannily transparent in the upper midrange and lower treble as the Futterman, but unlike the latter it could drive low impedances without any problem; nor did it have the totally unsmeared definition of the Electrocompaniet, but with its vastly greater power capability it was a considerably more versatile and practical workhorse. We were quite pleased with it and listened to it with a great deal of satisfaction.

Then came the Bryston 4B and the pecking order changed somewhat. The 4B has a tighter, drier, more thoroughly lifelike bass; a more open, luminous quality throughout; and more snap and definition on top, without edginess.

The Threshold sounds slightly thicker and darker; with sweeter but less finely delineated highs; somehow more closed down overall.

Minor but noticeable differences, between two outstanding amplifiers. We've heard it said that the Threshold was trimmed in to make the Dayton Wright electrostatic sound as good as possible, since the two have the same channels of distribution and are generally demonstrated together. We hope that's not true; a power amplifier shouldn't be used as an equalizer or compensator; however, delivery of our Dayton Wrights was delayed (see the speaker survey in this issue), so we still don't know more than what we're told-which can be notoriously unreliable.

Our laboratory results were most interesting: the Threshold 800A did quite poorly on the 14-plus-15-kHz CCIF test, with 13 and 16 kHz sidebands averaging 12 to 15 dB above those produced by the Bryston 4B at the same power levels. This makes us wonder about the sonic correlatability of the test itself; after all, the Threshold does sound better than a lot of amplifiers that throw less obvious sidebands. It's the first significant exception so far, though. Just to follow through, we measured the standard (meaning SMPTE) IM distortion of each amplifier. Lo and behold, the Bryston was almost a whole order of magnitude better, with double-oh figures throughout as against the Threshold's single-ohs. How about that? The Threshold also showed more phase shift at the lower frequencies than the Bryston (8° vs. 1° at 20 Hz, 2° vs. 0° at 100 Hz), but we can only repeat our reservation about the significance of this as stated in the RAM review above. In most other respects, such as slew rate and THD for example, the two amplifiers were quite comparable. The Threshold, how ever, doesn't have the Bryston's mono bridging feature that doubles the slew rate and quadruples the power.

Our suggestion, then, if you can afford the Threshold, is that you buy two Bryston 4B's, which won't cost you all that much more, throw the bridging switch and enjoy the fastest two 800-watt channels in the business. Even if faster doesn't mean better (which remains to be seen), it's nice to have that kind of headroom.

Unless, of course, you feel underprivileged without class A. In which case you might want to wait for the Threshold 8000-or get in touch with a Mark Levinson dealer. He'll show you what class A, and privilege, can do for you.

Recommendations Pending the conclusion of this survey in the next issue, which may still hold surprises, dark horses and even scandals, here are our current top choices.

Best power amplifier so far, regardless of price: Bryston 4B.

Best sound per dollar: Electrocompaniet (if you can get one and are satisfied with 25/25 watt power capability).

Special situation for the connoisseur: Futterman H-3aa (somebody will get one next week or next month-why not you?).

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[adapted from TAC, Vol.1, No.4: July/August /Sept 1977]

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Also see:

The Realities of Noncommercial Audio Journalism

The Present State of CD Player Technology: Who Is Doing It Right? By David A. Rich, Ph.D. Senior VLSI Design Engineer, TLSI, Inc. Adjunct Assistant Professor, Polytechnic University