Re: 'PC Speaker' Tube Amp

On Tue, 20 May 2008 21:57:48 -0500, flipper <flipper@xxxxxxxx> wrote:

.....snip!.....

I'm beginning to get the feeling I could talk till I'm blue in the
face so a better idea might be for you to pull out RDH4 and read
Chapter 7, Section 1 (v) "Combined positive and negative feedback at
the mid-frequency" and Section 2 "Practical feedback Circuits" (xi)
"Combined positive and negative feedback."

Figure 7.51a is similar (principle is the same), but not identical, to
the one I employed . Mine has two less resistors but takes a bit more
work balancing idle currents and gain.

OK, I pulled that old book out and checked it out, and I see the
light.
I haven't opened that book for a number of years, but I was
surprised to see how well they covered feedback issues, that dozens of
other engineering texts that I have don't even suggest.


Here I am replying to an old thread.
I tried using a positive internal feedback loop in a SET amplifier
that has a global negative feedback loop. By coupling two unbybassed
cathode resistors to each other through another resistor, I was able
to reduce the 2nd and 3rd harmonics by about 10db, with a small change
in overall gain. I'm impressed with this "trick"! Really, it is a very
small modification, and distortion is reduced by a factor of 3!
Too much positive feedback, and the circuit would oscillate at
about 1-2 Hz. High frequency stability wasn't affected, since there is
not so much feeback that there is any overshoot or high frequency
instability.
I'm not sure if the "purists" would be annoyed at putting feedback
on a SET amp, but, in listening tests it sounds much better WITH
negative feedback. I suspect that's largely because my speakers do
not maintain a constant impedance across their frequency range, and
they are designed for a constant voltage (low Thevenin resistance)
amplifier.

-Paul

.

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