Photocoupler Limiter in BBD-Delay

Haible_Juergen#Tel2743 HJ2743 at
Mon Oct 21 19:40:00 CEST 1996

I did some hacking on my new Amdek Delay this weekend.
As with all BBD-based delays, it's fun to have "eternal" repetition,
until the whole system clips and overloads. Sometimes the
distorted sound is nice, but sometimes you'd like to have things
smoother, and most of the time you don't want the feedback
noises to go that far beyond the input signal level.
In short: This thing needed a limiter in the feedback loop.
I wanted to keep things as simple as possible, so I tried
a compressor that Gene Stopp had published some time
ago, with a single photocoupler (LED/LDR) in the feedback
path of an opamp.
I looked for a proper location in my Amdek Delay, and I found
that there was an inverting amplifier between the BBD output
and the feedback potentiometer, a gain of 10 set by 470k
and 47k resistors. Couldn't find a better place for the LDR!
I soldered the LDR directly across the 470k, so that any current
thru the LED would reduce gain. Then I used the output of this very
same opamp stage, and built this simple half-wave rectifier:
opamp-output -> 1k-resistor -> 47uF-elko -> LED of photocoupler
 -> gnd. I also soldered a diode antiparallel to th LED to prevent
damage during the negative half wave.
That was all. No external time constant - it's all built-in into the
LDR's inertia. The whole thing works great. You don't hear a
difference in normal operation (below the LED's threshold), but
if the feedback builds up, it's nicely limited. There is still a light
distortion in this case, but a very pleasant one.
Thanks to Gene who brought up this circuit, and thanks to the
people who still build LDR's !


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