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Old 20th September 2013, 02:45 PM   #21
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bingo! no NFB no oscillation! and now???
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Old 20th September 2013, 02:53 PM   #22
popilin is offline popilin  Argentina
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Distributed capacitance in OPT, see post #19

Start with (3.-)
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Old 20th September 2013, 03:22 PM   #23
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I'm tryng but no way for now.... oscillation appear as I connect FB....

Why the other amp, the same identical amp, is rockstady stable???
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Old 20th September 2013, 03:29 PM   #24
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Try reversing the secondary wiring, perhaps positive feedback in place of negative.
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Old 20th September 2013, 03:31 PM   #25
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alredy done, result in a squealing amp...

I need to find the old book of sistem theory or automatic controll....
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Old 20th September 2013, 03:47 PM   #26
popilin is offline popilin  Argentina
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The amount of GNFB that an amp will tolerate is governed primarily by its OPT.
Interaction between input stage and OPT determines the stability of an amp when GNFB is applied.
So I insist to adjust the value of C in the feedback loop.
As I said before, an RC between cascode anodes, may also help.

Edit: Put C=2uF in the output, probably both channels will oscillate.
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Last edited by popilin; 20th September 2013 at 03:58 PM.
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Old 20th September 2013, 03:56 PM   #27
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As my friend popilin said, first remove the cap in // with the R in the FB loop. The amplifier may be not unity gain stable at higher frequencies. Doing it (Remove C) you are warranting the same amount of feedback to any frequency. Then, when the amp is stable, try increasing this cap in small amounts, example, 100, 120, 150 pf or place a variable trimmer. Then, tune the amp using square wave as it has been said.
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Old 20th September 2013, 04:14 PM   #28
pergo is offline pergo  Italy
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Add 10ohm+100nF Zobel at secondary winding
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Old 20th September 2013, 05:21 PM   #29
kward is offline kward  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
As my friend popilin said, first remove the cap in // with the R in the FB loop. The amplifier may be not unity gain stable at higher frequencies. Doing it (Remove C) you are warranting the same amount of feedback to any frequency. Then, when the amp is stable, try increasing this cap in small amounts, example, 100, 120, 150 pf or place a variable trimmer. Then, tune the amp using square wave as it has been said.
I've been through almost this exact same experience recently. Removing the cap in parallel with the FB resistor cleared up the oscillation. Adding small amounts of capacitance back in let me know where it became unstable. In my case, 330 pF made my amp oscillate (at about 1.5 MHz), but 220 pF was solid as a rock.
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Old 20th September 2013, 06:58 PM   #30
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with or without feedback capacitor the amp is not stable.

I've added RC between the two anode of the 12AX7, 33k + 100pF and no oscillation occour...

from the music I'm listening there is no kind of problem, excelent resolution as the twin amp...

the question is: how much is "legal" this solution on an hi-fi amp?

is the first time I build an hi-fi amp so I don't have any comparison with other project
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