ASM1 LFO influance?

Magnus Danielson cfmd at swipnet.se
Thu Aug 10 00:26:26 CEST 2000


From: e <edgarfov at zahav.net.il>
Subject: ASM1 LFO influance?
Date: Thu, 10 Aug 2000 00:47:32 +0300

> Hi all

Hi Edgar,

> small/big problem
> The LFo of my asm1 seems to influance the hole synth somehow
> (offcourse it is unpached!!)
> example
> i probe the vco and when i play with the lfo rate
> some VERY minor fluctuations seems to appear to the amplitude of the vco
> the fluctuation are apparently an lfo function on the pitch.
> but it is barely noticed , only by scope or very concentrate hearing.
> it appears in both vco's and both shapes.
> 
> when i changed the rate i can hear the rate faintly changes on both vco
> thats how i related the lfo for the cause.

To me your description matches a flaw that I know of in the ASM-1 VCO design
(and really in most of the others using this form of expo/sawtooth core) which
makes the amplitude of the sawtooth dependent on the +Vcc -> GND voltage.
Naturally, if you have leakage from your LFO to the powerline, then your
VCO will suffer. What really happends is that the +Vcc is used as a + 5V
reference using a 20k/10k resistor dividor chain. This + 5V is being used to
compare the sawtooth voltage so that when it has reached it's peak value
(that is, at the reference voltage) the reset will kick in. Now, if +Vcc is
being modulated, naturally the amplitude will modulate along with it. However,
by automagic the frequency of the sawtooth remains the same (well,
approximatly, the canceling is not perfect).

I have proposed the usage of a voltage reference in order to get rid of
(rather reduce) these kinds of problems.

What happends if you happend to drive your LFO (which is simplest to cut out)
from a separate +15V source?

Cheers,
Magnus



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