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Old 16th March 2009, 11:29 PM   #1
DamienH is offline DamienH  Ireland
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Default Using 12AX7 as input with feedback

Hey,
I'm designing an amplifier stage ( closed loop gain of 12 ) and I'd like the input valve to utilise some feedback. The coupling capactior's fequency response is a low-roll off @ 1Hz, is this something I should be worried about? I.e. What kind of an impact ( if any ) would it have on the amplifier's sound?
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Old 17th March 2009, 12:44 AM   #2
SY is offline SY  United States
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What you have to worry about is not just the rolloff from the coupling cap, but all the rolloffs between the point from which the feedback is taken to the point at which it's returned. Now, how is that managed? You could write a book on the subject and a few hundred people have...

Basically, though, you have to look at the relative phase between input and feedback and make sure it never goes positive because of the phase shifts from the intervening rolloffs. If you can scrounge up a copy of Crowhurst's "Understanding HiFi Circuits," there's a very lucid discussion with some incredibly useful charts.
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