On Fri, 06 Jun 2008 00:25:53 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
>On Tue, 20 May 2008 21:57:48 -0500, flipper <flipper@[EMAIL PROTECTED]
> 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.
Are the two stages DC or cap coupled? AC coupling should break the LF
oscillation since it wouldn't propagate to the next stage to then be
fed back. (That's why I cap coupled mine in the PC Speaker amp)
Dynaco used a similar circuit (DC coupled) in their ST35 but AC
coupled the (positive) feedback resistor, which should also kill the
motor boating since the PFB would 'go away' at the low frequencies.
That is, unless it's coming through B+.
In both cases you need to watch out for the negative and positive
feedback time constants or else you may end up with net (excessive)
PFB at the low frequencies (or you've made a 'bass boost' tone control
;) ).
> 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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