[sdiy] MG-1 first mods, envelope-extractors

[Metrophage]

It appears that last week, I made only one wee-morning mistake when
putting together that envelope follower with gate and trigger
extractors. Still, it doesn't yet work quite as I had hoped, in context
of the MG-1.

The circuit is from Electronotes, as reprinted and explained by Barry
Klein in chapter 3 of his book. When I had started it initially, the
gate and trigger LEDs I added to it were pulsing in accordance with the
input, but the envelope was not doing anything to the MG-1 VCA. Since
the 33k resistor on the second amp is supposed to set the gain,  tried
some changes to other values. Lesser values resulted in the extractors
getting sloppy, so I assumed this to mean less gain. Increasing the
value to between 100k and 220k results in an envelope that the MG-1 can
use. Sounds quite neat!

As the gain resistor approaches 100k, the gate comparator outputs a
constant signal, which means that there are no further triggers either.
 Barry said: "Op amp A3 is a comparator with hysteresis and generates a
gate signal if the input rises above 500 mV peak to peak." It appears
then that with the envelope follower gain set with a resistor of about
75k or more, that there is always more than 500 mV! I figure I can
either replace the 33k resistor, and add scaler amps; or I can change
the sensitivity of the gate extractor. Or...?

Changing the gate sensitivity is probably quite simple, but I do not
know how to set it. I'd guess that since the envelope goes to the third
amps non-inverting input through a 1k resistor, with a 300k feedback
resistor; that the threshold is set at the same amps non-inverting
input. There is a 39k resistor which is connected to +V, and a 470 ohm
resistor connected to ground. I tried increasing either value by
clipping additional resistors to these, but with no noticable effect.
Next obvious step would be to try lesser values, but I'll need to
remove the ones that are on there. I suppose I will try these things
tomorrow! I recall that in Tom Henry's "Electronic Drum Cookbook", in a
rather different trigger extractor he used a 1k pot between two similar
values to manually set the sensitivity during play. This would be very
cool! Can hardly wait to play with this more tomorrow.

CJ 


		
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