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Can anyone explain how the gerassic organ works please?
MUFF WIGGLER Forum Index -> Ciat-Lonbarde  
Author Can anyone explain how the gerassic organ works please?
cygmu
I'm terrible at understanding circuits... I've been looking at the schematic here:
http://ciat-lonbarde.net/paper/gerassicorgan.jpg and trying to figure out what happens.

So far I have this:
the three switches select the switch channel on the 4067. Input pin 13 switches between the top and bottom row of pins, connecting the hairy cap either to supply voltage or to ground. This makes the first op amp (bottom left) into a schmitt trigger oscillator because the capacitor is charging from the supply voltage when the top row is selected and discharging to ground when the bottom row is selected. The second op amp is a follower so it buffers the triangle wave from the capacitor, sending the output up to the transistor stuff at top left to drive the speaker.

That's as much as I can understand. I don't grasp at all what the top part of the circuit is doing. Is it a sort of envelope generator? Are the capacitors on the momentary switches there to generate a pulse of current when the switches are pressed so that you get a sharp attack, like a plucking sound?

If anyone has any understanding of this circuit to share, I'd be delighted!
mike page
I think you've got it. Thats how i understood it worked from looking at it
cygmu
Haha, I didn't really expect a reply after more than a year, so thanks!

I am a bit better at this stuff now and have at least half an idea how the top left section works. There is a differential amplifier for the output, with bias current generated by the third PNP transistor. The base of this is driven by the voltage at the 10uF cap which is brought low when a switch is pressed, turning on the output amplifier; when the switch is released the cap recharges through the 22k resistor and the x resistor, so x gives you control over the decay, I believe.

But it's a bit of a funny circuit anyway smile
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