ruffrecords
Well-known member
One problem with tube mic pres is varying the gain over a reasonably large range. This is usually a problem caused by the difficulties in closing the NFB loop at dc in tube circuits. That is why you see lots of tube mic pres with gains variable only over a small range (REDD47, V72 etc). I solved this problem in the EZ Tube Mixer project but it took one and a half tubes to do it. So I wondered if I could come up with a simpler variation on the same theme that would allow you to build a one tube mic pre with single REV log pot to vary the gain over a wide range. Here is the result:
#
It is only a sim at the moment but it seems to work. It is basically a modded version of the EZ Tube Mixer mic pre stage with a single ended output instead of the SRPP and the gain of the amp can be taken right down to 0dB. With a 1:10 input transformer and 4:1 output transformer it will have up to 48dB of gain and drive +20dBu into a 600 ohm load with less than 1% distortion. Gain of the amp is set by R8 and can be varied from 0dB to 40dB so the pre can have a gain of 8dB to 48dB.
Cheers
Ian
It is only a sim at the moment but it seems to work. It is basically a modded version of the EZ Tube Mixer mic pre stage with a single ended output instead of the SRPP and the gain of the amp can be taken right down to 0dB. With a 1:10 input transformer and 4:1 output transformer it will have up to 48dB of gain and drive +20dBu into a 600 ohm load with less than 1% distortion. Gain of the amp is set by R8 and can be varied from 0dB to 40dB so the pre can have a gain of 8dB to 48dB.
Cheers
Ian