[sdiy] Stereo MOOG filter / sharing expo convertors

[harrybissell at copper.net]

Using one current source with multiple 
transistors should work wery well. I'd use
a monolythic transistor array, like a CA3046
(for low cost) or a MAT-04 for better matching.

They should track excellently, imho.

H^) harry
>
>
>---- Original Message ----
>From: rutgervlek at hotmail.com
>To: Synth-diy at dropmix.xs4all.nl
>Subject: RE: [sdiy] Stereo MOOG filter / sharing expo convertors
>Date: Wed, 09 May 2007 18:09:30 +0200
>
>>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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