ASM-1 Filter Tuning...

[gstopp at fibermux.com]

     Here's the trimming procedure straight out of Electronotes:
     
     "The control voltage rejection pots are adjusted so that when the 
     coarse frequency control is adjusted throught its full range, the 
     deflection at the low-pass output is a minimum. Set both at their 
     midpoints to start with, then alternately adjust them until the 
     deflection is a minimum. For non-critical work, you can just ground 
     the (+) terminal of both 3080's and leave out TP-2, TP3 and associated 
     resistors R12-R15."
     
     TP-2, TP-3, and R12-R15 are the "offset" trimpots plus the 1M and 220 
     ohm resistors that are attached to the (+) inputs of both 3080's.
     
     What the procedure means above, is to set both trimpots so that their 
     wipers (the middle terminals) are exactly at ground. The word 
     "deflection" as used in this way means that on a scope, if you touch 
     the probe to this point, the scope trace will not move away 
     ("deflect") from ground at all - it will not move. If there is any 
     voltage at all on this point, the scope trace will jump a little every 
     time the probe touches the trimpot wiper. The deflection test is used 
     at the low-pass output to determine if these pots need to be tweaked 
     at all - if there is a small DC offset at the filter output, these 
     pots can be used to get rid of it. You must use small adjustments, on 
     each pot in alternation, to get this right.
     
     My ASM-1's have these pots set to the middle (zero volts). I found 
     that the batch of 3080's that I'm using have very good offsets (i.e. 
     none at all) - probably because I got them first-line from Harris. I 
     suspect that modern-day 3080's are of much better quality that the 
     ones available at the time these filter schematics were drawn (the 
     mid-70's). However this is just a feeling based on recent experiences.
     
     On the next ASM that I build, I think I'll just leave off these parts 
     entirely and put a jumper in the 220 ohm resistor spots, and see if 
     the filter is offset-free.
     
     The "crunchy" sound you get may be due to the amplitude of the input 
     signals being too high. As designed the ASM-1 filter input resistor 
     values allow the filter to be overdriven because this can be a cool 
     effect (like the "crunch" in the Korg MS-20) sometimes. If you are 
     connecting the VCO output straight into the filter, it will crunch 
     when resonance is high. Also, state-variable VCF's like this are 
     notorious for the uncontrollable nature of the Q. It's part of their 
     charm.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: ASM-1 Filter Tuning...
Author:  Troy Sheets <tsheets at xanadu.cyborganic.com> at ccrelayout
Date:    12/17/96 10:15 AM


I thought I sent this earlier, but I never saw it...
     
Anyway, I was wondering if somebody can explain how to tune the 2 "offset" 
trimmers on the ASM-1.  Right now, with high res, certian frequencies get 
all cruchy coming through the filter.   I bet this is the "CV feedback" 
gene explained in the notes.  I have not tweaked with these trimmers
yet, but I was wondering if somebody can give me a better idea how
to correctly trim them out before I just start tweaking them at random?
     
Anyway, I got my panal rack-mounted now, and it looks pretty cool, if 
I do say so myself... pics on my website soon!