Zero feedback and class A,which is better?

I hadn't thought about matching OTLs with auto-formers as I would have thought the autoformers negate the OTL-ness but the answer is probably that I was not thinking creatively enough. Sounds like a great idea.

Ralph,
Is the reason why SS amps sound "bright" on ESLs (because of the constant power demand) because ESLs see low impedance at high frequencies, and the SS amps double down in power at the low impedances, making SS amps, on an ESL-specific frequency response curve, relatively "bass-shy"?

T_bone, you got it exactly right. Most SS amps make constant voltage with respect to load; on ESLs that is not the right response.

With respect to the ZERO/OTL thing, we made a device similar to the ZERO about 10-15 years ago. It got to be a bit of a chore explaining why it did not negate the 'OTL character', so eventually we stopped. Paul Speltz picked up the slack.

Here is the explanation: OTLs already have low output impedance and do not make DC, so some of the bigger requirements transformers face have been reduced. To start with, the device does not have to block DC, so its an autoformer with no secondary. This is a lot simpler. 2nd, we're only going from 16 ohms down to 2,3 or 4 ohms, so the turns ratio and distributed capacitance is such that the device has bandwidth out to 1MHz, something that is impossible for a regular transformer. In fact the technology only works if you have a low output impedance and no DC- so its fine for an OTL- particularly in the light of a speaker load that the amp would not work with otherwise! Finally, in the case of our amps, you do not negate the triode-class-A-single-stage-of-gain-fully-differential-zero-feedback aspects. So it is a nice solution. Paul sells a lot to the single-ended crowd as well.

Which is better?

Neither. Comparing apples and oranges. Some may go as far to say that a zero-feedback Class A would be the ideal amp.

(Actually, the further a "zero-feedback" amp is biased into Class A, the better it will sound. To work well in a wide range of applications, amps of this nature are best when they have lots of output devices, drawing a ton of current. I live in Texas, so we changed over to making switching amps. Got tired of turning off the system 4 months of the year.)

I have to chime in here having listened to MA-1 MK II.3 through ML Summits and the ZEROs would be required. The MLs sucked the life out of an otherwise very fine Amp....Steve

Hmmmm, im running the Martin Logan Aerius I's with bi-wire with the SimAudio Moon P-3 preamp,the Moon W-5 Amp, and the Moon Equinox SE CD Player, and the Martins just love this setup, more so then my $10,000 Theils....Go to the moon man. u wont regret it with the Martins....Rick