I wonder if the $400 amp was the same amp that sounded like the $6000 amp? There is no question he is a brilliant engineer but I personally feel uncomfortable buying his stuff.
The Carver Amp Challenge and the 21st Century and it's Failure
Some of you may be old enough to remember this article from Stereophile. Bob Carver claimed he could make an amplifier audibly indistinguishable from some of the best from Conrad Johnson. A high efficiency (not class D), solid state linear amp vs. a linear tube amplifier.
https://www.stereophile.com/content/carver-challenge
Carver's approach was to feed a speaker via both amps at the same time using opposite terminals. The speaker itself was the measure of accuracy. Any difference in output between the two amplifiers would cause audible output.
What's super important here is Carver invented a new way to measure the relative difference of amplifiers with a real load.
That's kind of revolutionary from the standpoint of commonly published measurements of amplifiers before. Steady state, frequency sweeps, THD, IM and S/N all failed (to my ears) to express human experience and preference. I remember a reviewer for Audio, I think Julian Hiirsch, who claimed that these primitive measures were enough to tell you what an amplifier sounds like. The man had no ear at all, in my mind. More here:
https://www.soundandvision.com/content/reconsidering-julian-hirsch
And here was Carver in 1985 cleverly showing that two amplifiers which measured reasonably well, sounded differently. We should also be in awe of Carver's ability to shape the transfer function on the fly. That's pretty remarkable too but not the scope of this post.
My point is, really, Carver showed us a revolutionary way to examine differences between gear in 1985 and yet ... it did not become widespread. << insert endless screaming here >>
As far as I know (and that is very little) no manufacturer of any bit of kit or cable took this technique up. We are still stuck in 1985 for specifications, measurements and lack of understanding of what measures cause what effects and end up cycling through cables and amps based on a great deal of uncertainty.
My points, in summary:
Best,
E
https://www.stereophile.com/content/carver-challenge
Carver's approach was to feed a speaker via both amps at the same time using opposite terminals. The speaker itself was the measure of accuracy. Any difference in output between the two amplifiers would cause audible output.
What's super important here is Carver invented a new way to measure the relative difference of amplifiers with a real load.
That's kind of revolutionary from the standpoint of commonly published measurements of amplifiers before. Steady state, frequency sweeps, THD, IM and S/N all failed (to my ears) to express human experience and preference. I remember a reviewer for Audio, I think Julian Hiirsch, who claimed that these primitive measures were enough to tell you what an amplifier sounds like. The man had no ear at all, in my mind. More here:
https://www.soundandvision.com/content/reconsidering-julian-hirsch
And here was Carver in 1985 cleverly showing that two amplifiers which measured reasonably well, sounded differently. We should also be in awe of Carver's ability to shape the transfer function on the fly. That's pretty remarkable too but not the scope of this post.
My point is, really, Carver showed us a revolutionary way to examine differences between gear in 1985 and yet ... it did not become widespread. << insert endless screaming here >>
As far as I know (and that is very little) no manufacturer of any bit of kit or cable took this technique up. We are still stuck in 1985 for specifications, measurements and lack of understanding of what measures cause what effects and end up cycling through cables and amps based on a great deal of uncertainty.
My points, in summary:
- Most of what we consider state-of-the-art measurements are stuck in the 1970s.
- There are a number of ways to improve upon them
- No one has.
- We should be a little more humble when asserting if it can't be measured it isn't audible because our measurements are not nearly comprehensive
- I look forward to manufacturers or hobbyists taking modern equipment to pursue new measurement and new insights into our hobby.
Best,
E
57 responses Add your response
"His "motel-room" modified amplifier sound was so similar, Stereophile Magazine editors could not tell the difference between his amplifier and one costing more than $6,000. This amplifier was marketed as the M1.0t for about $400.00. Bob Carver may have single-handedly debunked any number of theories about sound quality by using physics, blind and double-blind testing and unbiased measurements (such as "gold-plated speaker wires sound better than copper wires", etc.). Carver successfully copied the sound of the target amplifier and won the challenge. The Stereophile employees failed to pass a single blind test with their own equipment in their own listening room.". (Preceeding in part from Wikipedia). Enough said. Carver came through in spades. Nonetheless, keep knocking what he pulled off but first figure out which terminal is Positive and which is Negative. |
Also kind of interesting to point out that the other way, making the Conrad Johnson amp sound like the Carver, in an afternoon with a bucket of parts, would have been nearly impossible. Maybe he could have taken feedback off the output? I'm not sure but lowering the output impedance of a tube amp sounds a lot harder than raising the impedance of a SS amp. |
Not explicitly, but the notion of a transfer function is a complicated subject. @almarg Yep, and noise and distortion can be a part of it. I’m just trying to parse out what Carver did during this testing. We think of him as a god, faithfully recreating the output of one amp with a complex load in a way no other EE could possibly do. I still think the test design was still ingenious, but if we limit our thinking to Carver changing just the frequency response and output impedance, then this seems like something any competent amp engineer could do in a day, which then explains what happened, which then informs us of what was important. Of course, it’s 3 stages of supposition, but it’s fun to think about. Best, E |
I guess I don't understand this comment. Distortion is why tube amps sound warm and smooth (some lower harmonics and not so much higher orders) while transistor amps sound brighter and harder (not much lower orders but what higher orders exist are easily heard by the ear which is very sensitive to higher orders). I don't see how cables can affect that. Because distortion wasn't taken into account in Bob's test- just the way the amp interacted with a speaker was, it was mostly about output impedance. But even if a tube amp and a solid state amp have the same output impedance they will have a different distortion signature (variable depending on topology as well) so they wouldn't sound the same. |
-70 dB puts it in the range of distortion.Agreed. Although in terms of voltage -70 db is only around 0.03%, which may or may not be audible depending in part on the particular distortion components it comprises. ...have you ever seen a transfer function equation that included distortion? Not explicitly, but the notion of a transfer function is a complicated subject. For example, click on each of the four jpg’s I’ve provided here, which are excerpted from one of the EE textbooks I used way back when. And I believe I was correct in saying that the transfer function of an amp, in the traditional EE sense and in the sense of what Carver was **attempting** to match with his null test, encompasses anything and everything about the relation between output and input. Best regards, -- Al |
And since distortion is one of the main things that we audiophiles hear as differences between amps, it would seem that the Carver approach is moot. Not moot, contradictory, and kind of goes with some of the subtle things I've heard with cables, that the sound of an amp is much more affected by the impedance changes in a cable than we have thought up until now. |
Using the complex impedance of the speaker in between two amps is how it happened. This still seems really problematic, as Al pointed out- the speaker used as a load and how that interacts with the amplifier is a pretty big deal! Now if this is indeed all about output impedance, way back in the 1950s Fisher, EV and a few others made amplifiers that employed current feedback as well as voltage feedback. Voltage feedback in an amplifier reduces its output impedance; current feedback increases it. The feedback was controlled by a knob marked 'damping factor', 'damping control' or the like. With an amp set up like that, theoretically there's your 'transfer function'... |
No, I don’t think that is correct, Erik. The transfer function of an amp is the relation between output and input Right, but have you ever seen a transfer function equation that included distortion? If the transfer functions of two amps are precisely matched, or in the case of Bob’s prototypes matched to a null of something like 70 db with a musical test signal, Right, and -70 dB puts it in the range of distortion. What I’m getting at is that if the two amps matched exclusively on gain vs. frequency and output impedance, then you’d get to this point. What’s left? Distortion and noise. |
Ralph ( @atmasphere ), I believe Erik is correct on this, in the first part of his post just above (but see below re the second part of his post). Note the description of the null test I provided in an earlier post (dated 3-3-2019): The null test, as described by Aczel, was performed by connecting Carver’s amp and the target amp in the normal manner to speakers that were at inaudible locations, and connecting a monitor speaker and/or a meter at an audible location between the + terminals of the two amps, both amps being provided with identical inputs. Also, note that the two amps were not connected directly together, but rather through a monitor speaker or a meter, presumably some sort of voltmeter having a wide bandwidth. As I mentioned in that earlier post, though, two major flaws in Carver’s approach were inability to maintain a deep null in production (which he pretty much admitted many years later, in the TAS interview I referred to), and the fact that the deep null he obtained with his modified prototype was obtained with just one particular speaker load. I’ve never seen a transfer function that incorporated distortion. No, I don’t think that is correct, Erik. The transfer function of an amp is the relation between output and input, not just in terms of voltage gain or any other single parameter, but in terms of everything. If the transfer functions of two amps are precisely matched, or in the case of Bob’s prototypes matched to a null of something like 70 db with a musical test signal, then everything about what the amps are doing (gain, distortion of all kinds, noise, etc.) is presumably matched very closely. But as we’ve both indicated, just under the particular test conditions, including the characteristics of the particular speaker load. Best, -- Al |
So, I kind of let this thread go in preference to newer threads talking about the same idea, that we’re stuck, but I wanted to take this up: One vast, gaping hole in Bob’s idea way back then is the output impedance of the two amps involved in the test. If not identical, the one with the lower output impedance will dominate the results. @atmasphere Au contraire, mon ami. The clever EE trickery Bob does forced him to alter the output impedance of his amp to match the other. Using the complex impedance of the speaker in between two amps is how it happened. Amp1 (+) =====> (+) Speaker (-) =====> (+) Amp2 The goal being to null the voltage at the speaker terminals with music, which, as you allude to, could only happen if the impedance of the amps matched. Now, thinking of this, Carver claimed he manipulated quite a bit, but it may have been all he did was change his amp’s impedance curve. Indeed in future amps, that was the only difference, whether you wanted low or hi Z outputs. I think the story if not the legend is he was able to change the distortion profiles. What if the latter part was bunk. His claim was to match the "transfer function" of the amps. That means voltage into a given load. I've never seen a transfer function that incorporated distortion. So, arguably, and with some induction (i.e. guessing) Carver proved distortion didn't matter. If your distortion is low enough what matters is the output impedance. Well, I've taught myself something, thank you guys. :) |
One vast, gaping hole in Bob's idea way back then is the output impedance of the two amps involved in the test. If not identical, the one with the lower output impedance will dominate the results. And if its not obvious, another serious issue is simply that neither amp will be operating correctly if connected to another amp in a null test! This is the sort of thing you don't do at home, lest ye 'let the smoke out'. |
Of course it is understood that Bob Carver used a measurement device to tune his amplifier to match the two amplifiers, right? He didn’t sit down with an oscilloscope, he used the output of the loudspeaker to identify the differences. He did use his ears, not his eyes, and he did use a loudspeaker to do this. Like for example a waveform that has low distortion however isn’t a match to the original despite not having more or less distortion isn’t going to sound the same. And yes a computer can analyze this difference. This difference can be shown graphically or like the speaker did, be represented audibly. I am agreeing that despite the sound reproduced by a device having less noise, than another with more noise where the wave form is closer to the original can and often will sound better than the former in resolving as an approximate to the original. This is measurable. Is it possible that science hasn’t discovered all of the parameters that we can perceive with our hearing? Maybe not, maybe we've been sold on the wrong metrics. Perhaps there’s bigger fish to fry. |
Excellent article and thread. Thank you. The best thread I have found in a long time. In regards to the "glare" reference of Al's, I have found that with speaker placement ( I have Klipsch Klipschorns, KLH 30's, and CF 2's (you can tell I am a fan...lol)) and a little tweaking of the signal path the glare disappears. I agree with that Bob is a genius. The simple fact that his 400 (in all of its iterations) is an original masterpiece of engineering that changed an entire paradigm, and is still running with the big guys almost 40 years later, is testament to that. That applies to just about all his products including his M500t, of which I am blessed to have four. As to any criticism of the "sound" of carver amps, music reproduction, like art, is in the "ear" of the beholder. The simple fact that Bob Carver opened up "upper class" sound to us middle class and poor folk, did indeed build a better "mousetrap", that is still sought out and used in the 21st century, is testament to his creative genius. |
Bruno Putzeys claims that he can make his n-core amplifiers sound in any desirable way. The OEM line is bufferless, manufacturers can bring their own sound with the buffer construction. I tested some of them (including those based on the expensive SonicImagery Labs opamps ) and for me the winner is the basic Hypex “tester” buffer costing a fraction. I understand that neutrality is not everybody’s cup of tea though... |
tomic6012 , glad to see another Audionics fan hear. I've had a BT2 preamp and CC2 power amp off and on at three different times starting in 1980. I still have my last BT2 that I use as a backup to my Audio Research SP6. They have amazingly similar voicing. I've used a pair of CC2's in mono mode with the power-hungry Thiel 3.5's ..... a match nobody would think to recommend but one which worked beautifully. I don't think I've ever had another amp that reached the "back of the stage" as effectivly as that pair. |
Seriously the speaker null part of the test is essential IF you really want to move the ball, just don’t pick a lame easy load... back EMF, phase angle, reactive and reflective impedance.... none of that shows up in the “ Julian Hirsch “ testing..... give that null test some thought... start with an Apogee... pair of em.... |
The issue behind us hard nosed business types is that designing a reliable product, the real engineering work.. which is viewed as boring to many IS key to long term survival.... Bob and his now rather extensive lineage of products and companies in the wake, do not have a reputation for longevity or reliability. as an aside, we were a very early PS Audio dealer - they offered quirky products, oversized power supplies, frequently great sound for $$$ and I remember only one amp and zero preamps in the service department. i could set my watch for Tuesday morning Frat boys dropping off the Phase 400 for a weekly fix.... about a 2 week break even payback to switch to a Hafler DH-200 and 4 weeks for a DH-500 we sold a bunch of those.... no hate at all... as I said, hope the 75 is a sweetheart ! |
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lowrider57" I’ve read the stories where Bob was considered a showman or a barker. I can see how those in the industry resented that. Also, his history of company ownership raises concern about the future of his current highly regarded tube amps. Will there be any authorised service when he hangs it up?" It is not likely that is why all of his companys have gone "belly up" as Americans say. |
@spatialking , interesting story. I knew Bob’s ego was beyond the pale back in the day, but that was very ballsy behavior. Clearthink, I’ve read the stories where Bob was considered a showman or a barker. I can see how those in the industry resented that. Also, his history of company ownership raises concern about the future of his current highly regarded tube amps. Will there be any authorised service when he hangs it up? I’ve owned two of his amps. A high powered Carver, which at that time, power was king for a young headbanger. Next was a Sunfire 300, which I consider a first step into a serious home audio system. |
spatialking"The Defense rests our case" There was no case to defend the facts speak for themselves you're "client" should have taken an American "Plea Deal!" |
lowrider57"why are there so many Bob haters?" There is no reason to hate Bob Carver and no reason to be infatuated with him either. Every company he has ever owned has gone out of business except for his current company and that is only because he has not had time yet to let it run out of business many of his products, designs, and technologies were only for show such as his silly "magnetic" amplifier and auto-correlator circuits and then of course the famous "Flame Linear" designs that weren't properly conceived, specified, or manufactured he is a promoter, a showman, a "carnival barker" as Americans say! |
@Elizabeth, @Lowrider - I think I can shed some light on Bob Carver "haters", although I think the term "hater" is really too harsh, at least for me. I met Bob at a CES show many years ago. We were both a lot younger then and back then he had a personality that just didn't work with me. I am not sure what it was. Perhaps, I didn't like his bragging about the sound in his booth as a number of his competitors I visited had surprisingly better sound. That was one of my jobs at CES, as a design engineer I had to check out the competition, check out the sound, and meet the designers, and get a feel for their philosophy. (Analog circuit design is about 1/3 science, 1/3 philosophy, and 1/3 art form.) Yet there he was Bob telling me his booth had the best sound at the show, and how the sound did this, and did that; yet my ears told me it didn't do any if the things he was claiming. I guess he expected me to believe what he was saying rather than what I was hearing. The reason I walked over to his booth is one of our dealers came by our booth and mentioned how bad that sound was. So bad he decided right off not to pick up his line. In any event, I can see how Bob's personality back then could rub people the wrong way. I still wouldn't use the term hater though. Not too surprisingly, I never purchased any of his equipment since meeting him that first time. BTW, wasn't he the lead designer at Phase Linear? I seem to think he was. |
I own a M400T and still use it in my pool table room. I also owned and sold dual M500T’s. I consider Bob to be a true maverick with his innovative creations. Alas, Bob is getting up in years and is mostly on the retired genius path. So, future products may be limited. But, he did tell me that he was working on a new tube preamp. After many years of using various amplifiers, I purchased the Bob Carver Raven 350’s and enjoying them as I write this. There was a problem with the XLR output in my unit that was easily fixed, so anyone buying one intending to use the balanced out should contact Bob Carver Corporation or myself for the correction. ozzy |
I don't think that manufacturers want a revealing measurement that could be used by consumers to understand real world performance, might hurt sales for some high dollar equipment. I have had two Carver TFM-42 amps driving a pair of Caver Amazing Silvers for around 30 years working flawlessly. I just upgraded speakers to the Legacy Whisper XDS (delivered Saturday), maybe after some time I will take a look at the Carver 350s. |
i will add your list ignored the great contribution of Dr. Matti Otala...TIM and I have and cherish one of the first amplifiers to exploit his findings..the lowly Audionics of Oregon CC2 capable of driving the 3 ohm Apogee stage...and so sweetly.... running some 3 ohm Thiel 2.3 with it this eve....very nice... |
To give an example of what I'm talking about as really revolutionary, consider the system Meyer's sound labs uses to tune pro audio systems in real time as the audience arrives. THAT is, relatively, revolutionary, and takes huge advantage of DSP and computer processing power to coordinate hundreds of speakers at a time. Being able to push the noise floor from -70 dB to -100 dB is not. It's the same measurement, but with better gear. And as @elizabeth has pointed out, you can buy for a couple of hundred dollars what would have cost tens of thousands in 1985. |
What good would it do to recreate this test? I'm not saying we should do so verbatim. I'm saying that this was a ground breaking test that brought together several aspects of amplifier performance in ways the Stereophile suite does not. I'm saying it illustrates how limited we actually are in understanding how amps differ. I don't agree that most of our measurements are stuck in 1970 technology. Can you name a new measurement since 1985 which has become part of the vernacular? I wish I had today's stuff back in my audio design days! Same. But adding precision is not a new measurement. It's just more precise.
If true (not doubting it) that's at the core of our problem as end users. Best, Erik |
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I also don’t want to get hung up on the null speaker test, because I think there are a number of ways to get more data and be more accurate. I wanted to point out this test came out in 1985, and nothing in popular reading has come out since its equal or better. That’s what I call stagnation. What the Carver amp challenge shows is the space available for innovation and research. To stop now is like stopping astronomy when we see the moons of Saturn for the first time. "Hey, that's all there is!" Best, E |
the null test works well in an anechoic chamber to match pairs of speakers.... but so few do it....the reason why ? Well first ya need a chamber, and when they don’t null out, ya got to know what to do, and want to do something about it vs just passing it along.... We could just as easily today measure the difference via the speaker terminals, and listen to it with headphones, or save it for spectral analysis. Carver was surely using a scope to figure out exactly what was going on and zeroing in with help. |
There is a lot going on... the how with reliability ( never a Carver strong suite imo and experience running a service department in a college town ) may not be shared much because it is valuable IP and patents are expensive and not always worth it... the amp I am listening to has five parts in the signal path, no emitter resistors, floating ground outputs, tube front end, liquid cooling so bias is a set point, ten regulated power supplies, built in HRS isolation.... etc, etc..... the null test works well in an anechoic chamber to match pairs of speakers.... but so few do it....the reason why ? Well first ya need a chamber, and when they don’t null out, ya got to know what to do, and want to do something about it vs just passing it along.... color me fanboy :-) |
@maplegrovemusic Erik - Douglas Schroeder just reviewed a amplifier that does what you are describing . Think it was a Goldnote amp . Something about changing the dampening factor by a switch . Yep, similar idea about Carver's challenge with the CJ. Nelson Pass has also written a little about using current source amps with certain types of woofers, with some interesting conclusions. Best, E |
My points, in summary: Who knows how many more audiophiles there might be if they had not had their minds poisoned by Julian Hirsch? For years I was under his spell, so bad it took many trips and listening sessions to finally realize what a crock of bull it is thinking we can measure music in any meaningful way. Thanks to him and a whole bunch of others misled into his camp we had to go through a whole generation of high powered amps that measured great but sounded bad. We had to go through the CD! Gad! Suppose it is too late to ask Stereo Review for my money back? Seriously though, the damage is incalculable. Cable construction. Frequency response. Timing. Double-blind testing. On and on. All kinds of seemingly unrelated things that when you dig into them all are based on the false assumption that measurements matter. And granted its a little complicated because sometimes measurements do matter. Where the damage is done is at the margins where instead of asking if this is an area where measurements matter people simply assume that they do. Which is wrong. Forced to choose, no measurements at all build everything entirely by ear, and the opposite, I'll take by ear every time. I'm just thankful there are so many talented designers like Ted Denney and Keith Herron and Duke Lejeune who understand when and how to use measurements, and when to trust their ears. |
Erik - Douglas Schroeder just reviewed a amplifier that does what you are describing . Think it was a Goldnote amp . Something about changing the dampening factor by a switch . https://www.dagogo.com/gold-note-pa-1175-mkii-solid-state-stereo-amplifier-review/ |
