My ASM1 is working! - but still some problems!

[Tony Clark]

> Finally I have finished the assembling of my ASM1 and am glad to join
> the ASM1 Comunity :-)

   Congrats!

> 1. The Oscillators seem to modulate each other even when not connected
> more than
> the onboard lines. I can hear that when I just listen to one OSC and
> change the tune
> of the other. At low frequencies this is very clear.
> 
> 2. The OSCs drift when I change the value of a voltage control on
> another module e.
> G. when I turn down the Cutoff-frequency of the Filter the OSC tunes
> down more than
> a semitone. I think there should be something wrong with the current
> supply of these
> controls but I can´t imagine what. I just connected them all parallel.
> Or is the fault in
> the OSC-Module itself?

   Both of these problems are caused by the way the oscillators draw 
power from your power supply.  The way the circuit is designed , it draws 
some very critical currents/voltages directly from the supply rails (like 
all of the frequency trimmers and pots).  Even extremely minor changes in 
the regulation of the power supply can cause the oscillators to change 
pitch.  Also, every time the sawtooth is reset, it causes some load 
changes on the power supply, also creating some supply noise that can 
possibly be heard on the other oscillator.
   Solution?  Give critical components a stable voltage/current source.  
This can be done by using a reference voltage source, like a LM336Z-5.0 
reference buffered or amplified through a GOOD opamp.  References like 
this are stable when your power supply isn't and can drive a slew of 
VCO's/VCF's with absolute precision.
   Let's face it, NO power supply on the face of this planet is perfect.  
Fortunately there are some inexpensive add-on's that can be done to make 
life much more bearable.  Utilizing precise voltage sources and stable 
opamp buffers is a good way to do this.
   Note, even doing this isn't quite perfect.  I've noticed that my VCO 
circuits still have a very very small change in pitch when plugging in 
additional circuits.  BUT, once everything is plugged in, it's all rock 
solid.  My VCO's can function from 15V to 12V without any audible changes 
(but aren't quite fully temperature compensated yet...)
   So as to your problem, I doubt that there is much you can do to negate 
those problems other than buy a REALLY good power supply.  I use a Condor 
3Amp +/-15V linear supply (took out the overvoltage regulator because 
it's a piece of crap) and it has regulation to 0.01% or something on that 
order.  There are other good supplies from Power One and etc. that'll do 
just as well.

   Hope this helps,

   Tony

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I can't drive (my Moog) 55!        |     
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Tony Clark -- clark at andrews.edu    |          COMING SOON!
http://aupe.phys.andrews.edu/~clark| 
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