Here's what I'm thinking about building next. There is one cheap audio transformer in the sidechain but it would not be necessary to use one there. Control voltage is brought to tube cathodes so no need for input transformer. The stage after the tubes should be able to handle approximately 25V of common mode voltage and using 0.1% resistors should guarantee good common-mode rejection. If each triode has quiescent current of 5 mA, plate resistors up to 4.7k can be used and 25V common-mode voltage won't be exceeded when going to deep gain reduction. Since the audio signal will be much smaller than the common-mode voltage, the combination of differential and common-mode voltage won't be much larger than the common-mode voltage alone.
This is probably the cheapest way to build high quality tube compressor since there are no expensive audio transformers needed.
Edit:
Attachment removed
Edit: Here is the original schematic https://tube.ghr.fi/varimu-t.pdf
Schematics to what the thing evolved to can be found here Variable Mutual Contuctance Compressor
This is probably the cheapest way to build high quality tube compressor since there are no expensive audio transformers needed.
Edit:
Attachment removed
Edit: Here is the original schematic https://tube.ghr.fi/varimu-t.pdf
Schematics to what the thing evolved to can be found here Variable Mutual Contuctance Compressor
Last edited: