Feedback

Modjeski got his hands on a Futterman OTL amp, looked into it's design, and discovered that Julius employed an extremely large amount of negative feedback. A search will bring up more details on Roger's thoughts on the Futterman, negative feedback in general, and their relationship in different amp designs, including of course OTL's (he designed and built a number of them).

@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.

Thanks @clio09. That partial-quote is but one example of Roger’s depth of knowledge and wisdom in amplifier design, and why his passing is such a loss to hi-fi.

Ralph (@atmasphere), what do you think of Mr. Putzeys’ statement that ...

... the idea that feedback causes TIM is probably most noteworthy for being not just wrong, but also the exact opposite of the truth. TIM happens in the input stage. An increase in global feedback makes the input stage work less hard. That causes a disproportional reduction in TIM.

@almarg Bruno is spot on here. The problem is that certain elements of the amplifier design can lie outside the feedback loop, but nonetheless be affected by the operation of the feedback. When you're dealing with something like this you get distortion.


Feedback has mostly been used in the last 40 years to decrease output impedance so that the amp can behave as a voltage source. That is why in most cases there is brightness/harshness that isn't part of the original signal since the feedback needed to get the amp to behave this way is always going to be less than what is needed for the feedback to have the ability to correct for distortion it adds.


One issue that's not seen a lot of attention is the simple fact that a speaker does not need more than about 20:1 for damping and many need considerably less. If overdamped, you get a phenomena often called 'tight bass' which is pleasurable but a coloration. So right here you can see that adding the feedback that is really needed for the amp to do its job right is overkill for the needs of the loudspeakers. 


I've seen nothing to address this; audio is still a developing field.

Thanks, Ralph ( @atmasphere ). So would it be fair to say that feedback can cause TIM if the amount of feedback that is applied is insufficient to prevent it, but in many designs it is not practical to apply that much feedback "due to all the frequency poles which limit phase margin and decrease Gain Bandwidth Product." (Quoting from one of your earlier posts).

If so, I believe that would reconcile Mr. Putzeys' statement with the seminal work of Dr. Matti Otala, mainly in the 1970s, that was referred to by @tomic601 in an earlier post. 

Thanks! Best regards,
-- Al