Here's a bizarre problem ....

[Ric Miller]

Howdy do, all fine chaps?  I've run into a bizarre new problem:

Over the last 4+ years, I've never had trouble with Ernie (my modular synth) as
far as power surges goes.  I had my 7 LFOs whirring away nicely, and no
problems with my VCOs, and their pitch.
Recently, I fired off my Hammond/GFC power supply to Darwin, while I worked on
finishing his Connor power supply.  I've been using the Connor now for a couple
of weeks, and I noticed an odd problem this past weekend:
It seems that when the Positive Voltage LEDs in the LFOs are lit up, the LED
itself sucks enough Positive Voltage to cause any oscillator being heard, to
drop in pitch.  As soon as the Positive Voltage LED goes out, then the pitch
comes back to where it was.  This pitch drop is by a very small percentage, but
IS audible.
I first tried putting 0.1uF caps onto the opamp's power supply inputs, of the
opamp that was driving the LED.  This made no difference.
On the LFO that I was testing (I've found this problem happening from ALL of
the LFOs), I changed the 470 ohm resistor for the blue LED (indicating the
voltage coming out of the Squarewave output) to a 1K resistor.  (Dropping the
milliamp drive to the LED from ~31ma to ~15ma.)  There was STILL no change in
the pitch interaction with the VCO.  (I tried 3 different VCOs, and they're ALL
reacting this way.)

The only other thoughts that I've had so far, are changing the 0.1uF caps on
the opamp (1 opamp driving the LED and Squarewave output, from an LF347 quad
opamp) to a 1uF cap to see if this helps.

All of the modules that I build, have 100uF caps, on their power supply input
lines.  I've been doing this for years, as it cleared up this type of problem,
when I was first building circuits.

And I never saw this problem happen when I was using the Hammond/GFC power
supply.  But now I see it with the Connor power supply.  Does anyone have any
other clues as to what's happening here?  This is a problem that I HAVE to stop
from happening!  :o

Thanks muchly for any help!  :)
Take care all,
Ric

Aural Research Facilities - Samples of some modules are now available at:
http://www.pangea.ca/rmiller or http://www.pangea.ca/~rmiller
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