I guess he removed a BUNCH of his posts then.
Not that I remember, anyway.
Its a simple fact that we could have sold a lot more amplifiers if we only used negative feedback. The reason that we don't is that its use causes the amp to violate a fundamental rule of human hearing: how we perceive the volume of a sound. Negative feedback can increase trace amounts of odd-ordered harmonic distortion (while overall otherwise decreasing THD). Its the trace amounts (specifically the 5th, 7th and 9th harmonics) that are used by the ear/brain system to figure out how loud a sound is.
So when these harmonics are altered even slightly, the amp will sound brighter and louder than it really is. This is one reason why two amps on a bench can measure flat but one is bright and the other is not.
Many speakers (not all, by any means) are designed to expect the amp to have some sort of voltage response at its output that is or approaches a constant voltage characteristic. With tubes this pretty well means that you will have to run feedback. The result will be unnatural brightness and unnatural loudness cues. IOW, with such speakers **it will never sound real**.
If the result will never sound real, why bother?
more on this topic: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
I note that the Atmasphere S-30 has an odd number of output tubes per channel. I have not seen a schematic of this amp and would be interested in looking at one. I suspect the odd tube may be the driver for the other parallel output tubes and Ralph may be going into positive grid voltages to get more power. Any comments?
We built the driver circuit in our amps to be able to push the power tubes into the class A2 (grid current) region. The 6AS7G remains quite linear in this portion of the curve and so allows us to do so. Conversely, the tube does not go into cutoff very easily either, so (as long as the amp is driving the right load) they are class A2 designs. There are those that claim that this is not 'true' or 'pure' class A, and such statements would be true if we were claiming that the amp was A1. But its not, and does trade off some linearity for more power. However, we also gain instantaneous overload recovery, with no issues of bias stability, both of which are problems in most OTL designs.
The S-30 has an odd number of tubes only because it worked out that way- as many of you know, OTLs have an economy of scale, the smaller you make them the less efficient they become. So the additional tube sections offered by the 5th 6AS7G were helpful.