The Carver Amp Challenge and the 21st Century and it's Failure

-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

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.
So, arguably, and with some induction (i.e. guessing) Carver proved distortion didn’t matter.

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

Good thread, Erik, and good points in your OP!

To clarify some historical facts about the "Carver Challenge":

Aczel’s preprint of the amp matching effort Carver performed under his auspices was written sometime prior to January, 1983, while the Stereophile article appeared in October, 1985. So Carver’s effort with Aczel was performed well before the one with Stereophile, although the people at Stereophile were apparently unaware of it when their article was written.

Also, "The Audio Critic" didn’t quite go defunct; it resumed publication in late 1987, after a lapse of about six years during which time Aczel served as president of the Fourier Loudspeaker company (which did go defunct during that time, for a variety of reasons Aczel later described in TAC as including inadequate capitalization, too few dealerships, and a warehouse fire).

The target amp that was the subject of the Stereophile article, as stated in footnote 3 on the first page of the article Erik linked to, was a Conrad-Johnson Premier Four tube amp. The target amp that was the subject of Aczel’s article was a Mark Levinson ML-2, a very highly regarded and very expensive solid state pure Class A monoblock amplifier of the time, rated at about 25 watts into 8 ohms but capable of providing huge amounts of current and much greater power into low impedances.

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.

Although Carver was able to tweak his amps to obtain a remarkable null of 70 db or so in those efforts, as I recall he pretty much admitted in an interview with "The Absolute Sound" about 7 or 8 years ago that he couldn’t come close to maintaining that deep a null in production. And as Erik pointed out, the null was achieved with one specific speaker load.

I owned both the original M400, the cube-shaped amplifier in which Carver introduced his magnetic field power supply technology but which was not "transfer function modified" to emulate a different amp, and the subsequent M400t version which was designed to emulate the ML-2.

IME the original M400 sounded very poor, with a strange glare often being present. The M400t sounded surprisingly good, and was certainly an excellent value in relation to its price and its 200 wpc power capability. The most significant weakness in my experience with the M400t, using it with fairly easy-to-drive 7 ohm 90 db speakers, was that the size of the image it projected seemed somewhat constricted. But based on my experience with the two amps it would certainly seem that Carver’s efforts to emulate the sonics of the ML-2, and the technique he used in that process, provided considerable benefit.

Regards,
-- Al