Here's something I'm working on right now.
http://ghr.fi/varimu/varimu2.pdf (couple resistor values are wrong on the schematic. I'll upload corrected schematic later.)
No input transformer and the control voltage is brought to cathodes. Impedance for the control voltage is very low but it's not a problem for solid state control amp. Only transformer used is on the vari-mu stage plates. Input stage is cross-coupled NE5532 and it drives the vari-mu stage differentially. If one input is grounded the gain goes down 6dB. Other than the input stage and the CV to cathode there's nothing unusual going on.
I got the circuit complete enough today to do some testing and it seems to work very well. For these initial tests I used Edcor XSM 10k/10k as the plate transformer and I can't understand how anyone can use them in their builds, they pick up hum and buzz like crazy. I would need a huge chassis to get them far away enough from the power transformer. I'll replace the transformer with Lundahl 1540 later on.
I don't know if anyone has done a vari-mu with CV to the cathode. To me it seemed like the easiest way to get rid of the input transformers, keep fast attack times and make a cheaper vari-mu with better performance. It works great, even though some purists might object to the use of op-amps and transistors. I'll do some more testing and measurements and post some specs later.
http://ghr.fi/varimu/varimu2.pdf (couple resistor values are wrong on the schematic. I'll upload corrected schematic later.)
No input transformer and the control voltage is brought to cathodes. Impedance for the control voltage is very low but it's not a problem for solid state control amp. Only transformer used is on the vari-mu stage plates. Input stage is cross-coupled NE5532 and it drives the vari-mu stage differentially. If one input is grounded the gain goes down 6dB. Other than the input stage and the CV to cathode there's nothing unusual going on.
I got the circuit complete enough today to do some testing and it seems to work very well. For these initial tests I used Edcor XSM 10k/10k as the plate transformer and I can't understand how anyone can use them in their builds, they pick up hum and buzz like crazy. I would need a huge chassis to get them far away enough from the power transformer. I'll replace the transformer with Lundahl 1540 later on.
I don't know if anyone has done a vari-mu with CV to the cathode. To me it seemed like the easiest way to get rid of the input transformers, keep fast attack times and make a cheaper vari-mu with better performance. It works great, even though some purists might object to the use of op-amps and transistors. I'll do some more testing and measurements and post some specs later.