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In Reply to: RE: Carver Challenge PDF file online posted by andy_c on January 10, 2008 at 11:02:41
What I find interesting, having now read the article, is that Carver was adjusting the harmonic distortion content (among other things) to get his null. This harmonic pattern seems to be rather material to the overall sound of an amplifier.
Now if the distortion of one or the other amp or both is already quite low (I have seen THD measurements for amps well below the 0.03% of the null he achieved) then what was the crap coming out of the speaker that they could hear at -50db?
The other interesting thing is how exactly did Bob do it? It suggests a very intimate knowledge of what the actual devices are doing and how to coax or reduce various noises and distortions from them. It also doesn't seem like something just any designer can do and would thus put a premium on talent of building an amp.
Would the identical sound response hold up over a variety of loudspeakers, whose loads can be radically different? Some dynamic parameters of the amps could be relatively different with wildly varying loads. Electrostatic speakers come to mind...
Also, in the absence of a "reference" amplifier (a very good and likely very expensive amplifier where the sound is achieved "the old-fashioned way") Bob has to rely on his own taste to get a sound he likes. If his ear is well tuned to live unamplified music then maybe he generates a very good sounding amp by tweaking to sound like this but he has no null reference anymore. That is the tricky part, reliance on what someone else did the hard way first. As there seems to be no agreement on this it doesn't make sense to rely on one or two marketed expensive amps. Still, it would be very interesting to make a Carver SS amp sound just like a Lamm ML2 (arguably one of the best) but with a lot more power reserves! I would buy it if it could.
Another point is that it is clear from this article that it would not be trivial to make cheap SS amps sound like top class tube, hybrid or SS amps. It seems that Bob has a rather special understanding (genius if you will) of how his amps work and maybe there are only a few guys in the world so able to make such tweaks. Making the distortion/phase/frequency response (ie. transfer function) to match another amps closely I think would not be a trivial exercise and would end up costing a lot of man hours thus making the product substantially more expensive (still less than the "reference" though I imagine). It also suggests that the distortion pattern of preference is not necessarily the one that is the lowest (as has been suggested by many people over the years).
Finally, the null test seems to be MORE effective at matching two amps then simply taking static THD measurements ever could. In the end Bob used his ears with a null test through a real loudspeaker. Sure he analyzed the distortion and such to know what adjustments might be necessary to get in the ballpark so to speak, but it seems he listened to eliminate the rest.
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