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Old 2nd December 2008, 07:04 PM   #1
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Default PFB line amp

Was fiddling about with a possible future line amp when I noticed I had connected the feedback backwards, as PFB.

Did some sims and this one looks nice!

At a certain value of R3 the distortion dips. Must be some interaction between the unbypassed R1 and the feedback loop.

Zout is about double ref to w/o NFB at 500ohm. And gain is about 5dB higher than w/o NFB.

Still when R3 is trimmed, THD is well below 0,1% THD. By trimming R3 the overtone-spectra can also be manipulated to diminish the odd overtones at the expense over a slightly higher 2nd order.

On the negative side is that phase shift is worsened.

This goes for sim. Think I must try it IRL.
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Old 2nd December 2008, 08:48 PM   #2
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Location: Eureka, CA
Interesting!

Does your circuit improve the AC balance of the self-splitting push-pull
output stage?

It reminds me of the paraphase splitter getting feedback from the OPT
in the later Fender Twin and AB-763 circuits.

Positive feedback is discussed in RDH4, pp314-315, applied locally
to the first stage in a 2 stage amplifier to allow greater loop NFB
and lower overall distortion and output impedance.

Also discussed on p354, for eliminating the cathode bypass capacitor
in the first stage (I played with this in a micpre circuit).

Cheers,

Michael
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Old 2nd December 2008, 09:00 PM   #3
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Think that is the secret of it. When adjusting R3 for best AC-balance you get max gain and lowest THD!
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Old 3rd December 2008, 07:08 AM   #4
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Just realized that the circuit could be looked upoan as a variation of an oldstyle paraphase splitter.
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Old 4th December 2008, 03:08 PM   #5
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This will be our test configuration, R2 will be adjustable:
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