The Carver Challenge Page 3

This output nulling technique is not a Carver innovation. It has been known for years to be a possible way of comparing amplifiers, at least in thoery. But it could never be made to work with amplifiers having slightly different group delay and phase-shift characteristics, because any loss of signal synchronisation impairs the effectiveness of the cancellation. In other words, it wasn't used because all amplifiers are very different—the test was too sensitive! But phase shift happened to be only one of the many parameters for which Bob planned to compensate. Hearing of this level of sophistication made LA and I begin to suspect that Bob just might be able to pull this off after all (footnote 5).

We were still pretty confident that he couldn't, though. After all, 66 years of amplifier design have still not resulted in any way of pinning down the subjective effects of every measured imperfection—even if we had measurements for them all, which we don't. The beauty of Bob's approach, however, was that he didn't need to know what all those objective imperfections were doing; all he had to do was eliminate them.

Neither LA nor I had any idea what "adjustments" would be involved, but I, for one, was convinced that the area that would ultimately stymie Bob was that of harmonic distortion content. I have long believed that some of the major sonic differences between amplifiers were related to the relative and absolute amplitudes of their harmonic distortion components. (It is known, for example, that the amplitude of the high-order harmonics—the third, fourth, fifth, and sixth harmonics—become progressively weakened in the signal from a tubed component, and remain relatively constant from a solid-state device.) I was a little shaken when I learned that a half-dozen small potentiometers that Bob had wired into his amplifier were "distortion pots," which enabled him to change the amplitude of any "spurious" harmonic as desired, independently of the other harmonics!

That first listening test in Bob's room was an ear opener. He had already achieved a surprisingly effective null—a 50dB reduction below the level measured at each amplifier's output. But there was still a substantial amount of sound coming from the Rogers speaker, and that sound was some of the filthiest, dirtiest crud I have ever heard!! Bob explained that he had nulled out most of the things that both amplifiers were doing right, leaving only such things as distortion and frequency-response deviations. Yes, I thought, and those are going to be the hardest things of all to null out.

Bob explained that a 50dB null meant that the difference between the two amplifiers amounted to about 0.3% of the total output of each. The dramatic audibility of that 0.3% came about because he was driving the amplifiers to rather high output levels, and because of the ugly nature of what was left in the sound.

At this point we ran into a problem. The AC line voltage at La Posada was quite low, meandering around 106 volts much of the time. This would quite obviously throw off both amplifiers, enough so that they would probably not perform the same way with a more normal line voltage. I loaned Bob my Variac.

The next day he had managed to boost that 50dB figure to 70dB, and felt ready to try some listening. By this time the difference signal between the amplifiers was audible only with an ear glued to the Rogers LS3/5A, even with the output of the amps cranked up. There was no doubt that Bob had achieved something impressive, but we questioned whether it would translate into true duplication when driving real-world (and difficult-load) loudspeakers. We moved the project to my listening room.

The Listening Comparisons
The signal sources for our listening tests were to be both CDs and LPs. The CD player used was a Sony 520-ES, the analog player a SOTA Sapphire turntable with Well-Tempered Arm and Ortofon MC-2000 cartridge, with Ortofon's T-2000 step-up transformer. The preamp was a Conrad-Johnson Premier Three.

Program sources were as follows, for the following specific sonic attributes: "The Portrait" and "Peter the Hermit," from Growing Up in Hollywood Town (Sheffield CD-13 and Lab 13) for depth and perspective, HF maturalness, bass heft and tightness; Respighi's Church Windows (Reference Recordings RR-15) for breadth, depth, bass range and control, and massed string tone; Beethoven & Enesco Violin & Piano Sonatas (Wilson Audio Specialties W-8315) for tonal accuracy, depth , and imaging specificity and stability; "Improvisations" by Jim Keltner, from The Drum Record (Sheffield CD-14/20) for high-end openness and timbre and low-end attack, control and range; and McBride's "Mexican Rhapsody," from a badly worn copy of Fiesta In Hi-Fi (Mercury Living Presence SR90134) for treatment of HF stridency and mistracking.

We made no effort to do A/B testing, since we feel it does not replicate normal listening conditions, and there is still insubstantial evidence that A/B testing reveals small differences as well as does prolonged listening to each unit under test. In our tests, one amplifier would be wired into the system and auditioned as long as we wanted, using a wide variety of program material that always, however, included the material listed above. Notes were made of anything we heard that we thought different from the other amplifier, and those specific points were checked again when we went back to the other amplifier.

A Good Beginning
We were not too surprised to find that there was no longer a dramatic difference between the Carver 1.0 and the reference amp. In fact, what surprised us was just how similar they sounded. They were almost a perfect match, except for a slight difference in perceived depth and perspective, a marked difference in low-frequency range and control, and a noticable difference in high end smoothness. We were pretty taken aback by the similarity, but, because the differences were reliably audible, we were still confident of our abilities to hear differences between the two amps. And, because the differences were important in type, though small in degree, the expensive reference amp was unthreatened.

In spite of the really amazing feat he had pulled off so far, Bob was disappointed. With 70dB of null, he assured us, they should sound identical. They didn't; it was back to the test bench and soldering iron for Mr. Carver.

It took another day to find the source of the trouble and work on correcting it. The trouble, it seemed, came from my Variac, which could not deliver enough current to meet the brief, but very high demands of the reference amplifier when playing music into demanding loudspeakers rather than mockups. Back at the hotel room, Bob had been trying to match his amp to one that was working with one hand tied behind its back. The matching that had produced a 70dB null in the hotel collapsed to 35dB in my home, so it was necessary to produce a new model of the reference amplifier as it performed with adequate current availability. Fortunately, my line voltage was normal (115V), so the Variac could be dispensed with. Bob was discouraged at having to do his entire analysis and modeling over again, but glad of a problem concrete enough to be addressed.

A Second Stab
After another day, Bob seemed convinced that he had done it. We gathered for another listening session, and, indeed, it sounded as if he had. The high end stridency we had noticed in the 1.0 was gone (or, as it turned out, was just as present in the reference amp). Depth presentation, midrange solidity and 3-dimensionality, imaging, high-end sweetness—in short, all the characteristics one normally finds important in amplifier evaluation—were identical.

Footnote 5: Actually, I was impressed—but I still doubted the relevance of the null test to the actually driving of loudspeakers. Bob's imitation loudspeaker might not stress an amp or store energy and feed it back to an amp to nearly the same degree that our real reference loudspeakers would do. Plus, I had once upon a time picked up the differences between ½" of steel lead from a capacitor to a crossover as opposed to ½" of copper lead—and these two amplifiers had much bigger differ- ences than ½" of wire.—Larry Archibald