[sdiy] LFO's showing up on VCO

Sean Ellis tensiontype at hotmail.com
Tue Nov 11 22:41:42 CET 2008

I built an ASM-1 and the rather poorly designed SBM from EFM (I doubt many 
people actually finished one these to 100% operation) and in both problems 
like this were traced to poor grounding. Make sure you run a generous amount 
of the panel grounds back to the power supply. At first I treated all ground 
connections the same with front panel wiring but with the SBM I had to 
separate every module's ground connector and give each one a dedicated wire 
to a central ground location just near the power supply. It fixed this type 
of cross modulation.

There were many other issues with the SBM that were related to power supply 
alone. There was just too little trace space allocated to power and in 
particular ground signals. The ASM-1 wasn't nearly as bad and was a pretty 
painless build.

----- Original Message ----- 
From: "Dan Snazelle" <subjectivity at hotmail.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, November 11, 2008 9:58 PM
Subject: [sdiy] LFO's showing up on VCO

> I am not sure of the technical name for the problem of circuits (lfo's 
> especially) influencing your Main VCO'/'s, but it's a problem i have had 
> before. The difference this time is I am working with a pcb (asm2) that
> has all of the circuits on ONE board, utilizing a single PSU but with a 
> +/-10 v reference for the VCO. ( i had hoped the reference would get rid 
> of this sort of problem)
> Now that all the circuits are working, i am finding myself pulling out my 
> hair due to HEARING the other periodic circuits making my VCO output 
> unstable. (even with the CV's disconnected, i can still hear them messing 
> with the waveform)
> at this point i am not willing to add a second power supply to this board 
> and i dont think it would fit anyway.
> SO...i am looking for ideas.
> I tried removing the LEDS attached to the LFO  but that didnt seem to 
> help.
> HOWEVER i am wondering if giving the LEDS their own ground (straight to 
> the PSU) would help.
> I am using a power one +/- 15 volt supply
> here is the link to the circuits:
> http://www.elby-designs.com/asm-2/asm2.htm
> thanks so much
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