Feedback

Now some designers have taken a different approach, sculpting the distortion signature to include the 2nd and 3rd harmonics. Its been shown that if these lower orders are present (both treated by the ear as 'richness', 'bloom', etc.) in great enough degree, they can somewhat mask the presence of the higher orders. Nelson Pass takes this approach in his designs, which are some of the best sounding solid state amps out there.

https://www.passlabs.com/press/audio-distortion-and-feedback

This is an article Nelson Pass originally wrote and was published by 6Moons a while back which goes into more detail regarding Ralph's comments on Nelson's use of feedback. It also talks about implementation of feedback, both global and local.

There is another article elsewhere which I cannot find at the moment in which Nelson mentions that small amounts of feedback, the amount of which I cannot remember are fine, as are large amounts in excess of 30dB. So again, there seems to be some consistency in this line of thought that excessive amounts of feedback could be beneficial. I know that the last amplifier designed by Roger Modjeski used greater than 35dB of feedback, and as Nelson himself has mentioned regarding his First Watt designs the application of global negative is determined on whether or not the amplifier sounds better with it or not.

Properly implemented feedback is not a bad thing. I have some amplifiers that use it and some that don't and enjoy then all.

@tomic601 the idea of the feedback switch on the RM-9 was not about adjustable feedback, it was about adjustable gain (Nelson Pass talks about the feedback/gain relationship in the article as well), which is why it is referenced as a gain switch. Roger wanted users to have a means to use both passive and active preamps with the RM-9, so he put that feature in to allow the gain to be adjusted accordingly. However, after more questions from users about which position sounded better than he wanted to deal with (Roger's stock answer was, "whatever position sounds best to you") he removed the switch and the feedback/gain was factory set. The RM-10, while not having a switch, has instructions and a chart in the manual for adjusting the gain by swapping out some resistors and capacitors.

Here is another little snippet from Roger. This time on IMD which he felt was very important to limit in circuit designs:

"I intend to write a paper on the more complete story of 2nd harmonic distortion theory. I will say here that it must be minimized (but not at the expense of adding other distortions). What listeners don't seem to realize is that large amounts of 2nd is fine for a single note but not for a full orchestra. As I like to say "Please tell me what is the second harmonic of Beethoven's 9th? The amplifier is having a little trouble figuring it out". Once music becomes a complex signal, the second harmonic argument goes right out the window. The amplifier can only act upon the instantaneous voltage it is given and has no idea what notes are being played. At that point, it's Intermodulation Distortion we had better be taking care of. How many of us have heard our single-ended amps make a mess of a symphony yet play a solo voice beautifully?"

@bdp24, the Futterman H3 which Roger referenced for his last OTL project used an incredible amount of feedback, but even more interesting were the large number of 1" ferrite beads inside the amp that Julius omitted from the schematic.

I wish Roger had written more about the use of feedback in amplifier design. His knowledge of the subject was in my opinion better than that of transformer design and tubes. Roger technically only manufactured one OTL amplifier, the OTL-1 which saw very limited production. The RM-6 design was purchased by Counterpoint and became the SA-4. Contrary to popular belief Roger did not design the Beveridge OTLs, although he did QA and test them. Here is a link to the story of the OTL-1: https://www.audiocircle.com/index.php?topic=126907.0

If you scroll down to the Circuit Description you can read some of Roger’s thoughts on feedback. A portion of which I will quote here:

In the OTL-1 the input tube literally rides on the output terminal. So as the input rises the output follows it perfectly in phase. Rather than being an injection point for feedback this point exists in the fundamental circuit. This allows the amplifier to react immediately. In conventional amplifiers the feedback comes through the output transformer (with considerable phase shift that is load dependent). The feedback then needs an injection point which is often the cathode of the input tube or grid of a differential amplifier. Internal delays in the loop (phase shifts at low or high frequencies) can cause the negative feedback to become positive and make the amp oscillate at low or high frequencies or both. Many amps are unstable without a load and many have low frequency instability that causes the woofer cone to wander about its rest position.