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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:

  • 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


erik_squires
atmasphere's avatar
atmasphere
11,575 posts
 
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.



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'...



erik_squires OP
15,657 posts
 
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.

almarg's avatar
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almarg
9,670 posts
 
-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

atmasphere's avatar
atmasphere
11,575 posts
 
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.
erik_squires OP
15,657 posts
 
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
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