[sdiy] Stereo MOOG filter / sharing expo convertors

[Rutger Vlek]

Hi everyone,

I'm currently working on a new project and I'm trying to find a cheap way to 
create multiple Moog Ladder filters for several voices. The voices are 
actually SID-chip stereo-pairs as presented on www.ucapps.de. Each pair 
would be working as single voice to create a 4-voice synth with 8 SID's. The 
PIC controller for the SID pair can also control a CV-module to generate a 
control voltage for an external filter.

I was wondering about the pro's and con's of creating a stereo Moog Ladder 
filter while sharing a single expo converter. The transistor at the end of 
the expo converter functioning as a current-mirror (if i understood 
correctly) could be doubled to create the path for controlling the second 
transistor ladder. This way a single CV could control both filters...in 
theory I think this could work, but how about practice. Is anyone 
experienced with this kind of setup?

I'm particularly concerned about possible differences in cuttoff frequency 
for both filters. Little differences are okay (I don't have temperature 
compensation in the expo), but I don't want extremly different responses for 
the left and right filter.

Greetings,

Rutger

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