ASM-1 oscillators

gstopp at gstopp at
Wed May 8 17:53:31 CEST 1996

     I've hooked up the prototype to a keyboard and played with it - it's a 
     big mess of wires but it works pretty good.
     The VCO's are next to each other on the circuit board, but as far as I 
     can tell there is *no* cross-influence between them at all. I can tune 
     them to an identical (zero-beat) frequency and they exhibit the very 
     slow evolving timbre sound that is characteristic of two very close 
     oscillators. You can hear the second harmonic drift in and "take over" 
     and fade out again which is a good indication of un-synchronized 
     sawtooth mixing. I have adjusted the volts/octave trimmers so that the 
     VCO's are beat-free across 3 octaves (the size of my test keyboard) so 
     I think that they will have excellent tracking.
     There is what some may consider an excessive number of bypass caps on 
     the circuit board. One of my design techniques is, upon completion of 
     the circuit layout, to go back and stick power decoupling caps 
     everywhere they will fit because they're cheap. Maybe this helped.
     If an interlocking effect is desired then I think that it would be 
     possible to use an attenuator on the sync input to adjust this.
     BTW the VCF has the same Q feedback limiter circuit as the Oberheim 
     SEM (back-to-back diodes plus resistor divider) which prevents runaway 
     oscillation of the filter at high Q settings. This seems to work well 
     and the component values could be adjusted by the builder for a 
     different max Q value if desired. If the sawtooth waveforms are run at 
     full volume into the 100K VCF inputs (unity VCF gain) at high Q 
     settings you will hear a kind of screeching distortion which reminded 
     me strongly of the Korg MS-20 filters. If you pad down the filter 
     inputs to about 60% the sound becomes very clean.
     The VCA's work well, as do the envelope generators. The noise 
     generator volume is quite loud but there is a trimpot for this and 
     mine is turned up all the way.
     I'm working on a panel to hold all the pots and jacks so that I can do 
     full testing of all envelope times etc. The batch of circuit boards 
     will probably be ready in a week or two.
     - Gene
     gstopp at

______________________________ Reply Separator _________________________________
Subject: ASM-1 oscillators
Author:  Joachim Verghese <jocke at> at ccrelayout
Date:    5/8/96 1:30 AM
I was wondering, with these fast-reset sawtooth VCOs, are there any 
hints of VCO interference (interlocking) at all? How does local supply 
decoupling affect the performance in this context?

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