[sdiy] ASM VCF Problem

[Ken Stone]

if the "horrible noise" is a clear sine-like tone that's frequency varies
with the CV, it is self oscillation of the filter - quite common, and
usually desirable. A lot of people want this ability. It does sound like it
is a bit aggressive though - perhaps you have too much gain or too much
feedback in the circuit for whatever reason.

(The above does not apply to all filters though - I get a heck of a lot more
than just sine waves out of my Sallen Key filters - period doubling and all
sorts of wonderful things.)

Ken

>Hi list,
>
>(Previous message didn't make it thru to the list so here it is 
>again...)
>
>Got a little problem with the VCF section of my ASM-1. Everything is 
>working (filter sweeps, etc) but when I twiddle the cutoff pot I've got 
>a big loud noise comming in. In fact it's only when the pot is in the 
>positive domain. So from -15V to 0 the filter sweeps normally and as 
>soon as it is from 0 to +15V I hear a terrible noise that replaces the 
>sound of the vco.
>
>Have tried changing the ic's and the trannies from the expo section 
>with no luck. Re-checked my work but still bogus.
>I'm lost here...
>
>Does anyone have experienced the same problem or has a clue on what's 
>going on??
>
>Any help appreciated,
>
>Fred.
>
>
_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au or ken at cgs.synth.net
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
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