• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ECC88 mu-follower

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Interesting circuit! The pot varies both open-loop and closed-loop gain - I assume that helps maintain stability?

Spot on!!! It is inspired by the Telefunken V76 which does much the same thing with pentodes. My improvement is to ensure stability also at the LF end of the spectrum by dc coupling the NFB. The NFB lies between 20 and 30dB at all values of closed loop gain from 6 to 40dB.

This technique of altering open loop gain at the same time as closed loop gain seems to be endlessly rediscovered. It was used in a discrete op-amp design a couple or three decades ago:

http://www.technicalaudio.com/pdf/Valley_People/Valley_People_Trans-Amp_LZ.pdf

and is very useful in virtual earth mixers as an aid to stability.

Cheers

Ian
 
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Yes, DIY is supposed to be the basis of this site. We help, but we don't do it for you.

Hi!



Yes that is putting in nicer words what I wanted to say. Didn't want to appear harsh. Remember that the answers you get will probably not be better than the questions you ask. You need to get over that first step to be able to ask good questions, only then you can make progress

Thomas

Hey guys, I do not want that you make nothing for me, I only ask to do it myself following the people that wants to help.
 
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