Minimoog VCF

[Rene Schmitz]

Hi List,

regarding the discussion on the mini-filter I've taken the time to 
draw my changes to the schematic. A working version of the all-transistor
filter can be found at http://titan.cs.uni-bonn.de/~schmitz/rsmoog.gif
Unlike the circuit found at Sean Costellos website, this one oscillates
reliably, though the amplitude of the oscillation rises with frequency. I
don't know if this is part of the minimoog sound or not? (Any opinions?)
I think this is due to the high-pass(es) in the resonance loop (the 10uF
cap with the Q-pot in series, the .22 decoupling caps). 
If this makes a difference, then this version would sound closer to the
mini than Tom's version. (Which I nevertheless find very economical, and
elegant!) But here I'm on thin ice!
I raised the overall gain somewhat, added summing resistors for the audio
ins. (@ 2Vpp the filter starts to clip)
I also added the CV-summing resistors to the schematic, and show how I
hooked up the Q-pot. I didn't use matched transistors for the LTPs, since
all the dc-offsets are decoupled anyway. 

Hope this helps to put these filter-clones to life :)


Bye
 Rene







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