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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
11,784 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
16,041 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
erik_squires OP
16,041 posts
 
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.
atmasphere's avatar
atmasphere
11,784 posts
 
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.
Sorry, this should have been included with my prior post.

lgldsr73
7 posts
 
"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.
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