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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?
I'm terrible at understanding circuits... I've been looking at the schematic here: 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
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
I'm looking at having someone make me a gerassic organ or two

I had an idea

so I've seen people use a switch with a bunch or capacitors connected that can essentially be swapped out via the switch

this is great!

so my idea splits all the 12 or 16 capacitor values between TWO selections switches...

and then

it uses a push button switch like the 3 keys normally have that would select between the two capacitor tuning switches

my thinking is that you could "jump octaves" with the new capacitor switching key thus adding 8 more note possibilities with just the 4 keys

am I totally off here for some reason?

and then you could get as wild as you want
instead of two capacitor multi selector switches have three with two buttons associated one could switch between bass and mid range and the other would switch between the first one and a higher voice

I would think you could also add the third switches output across the first so instead of selecting between three ranges you could have your main selector switch between bass and a higher voice then your secondary selection switch would add to the capacitor value only when it is pressed there for your range selection switches (2) could select 4 distinct ranges instead of just 3

bringing your key combinations up to 64 (or 32 if the secondary range key was only switched in and not added to the main range key) with 5 keys altogether 2 range selection and 3 note selection

ok I hope that makes sense I'm not great with electronics but the idea comes from something I had made on the shnth a while back

any thoughts>?
also for a little icing on this cake
when I see people playing the organ all I can think is wouldn't it be great to replace those knobs with joysticks MY ASS IS BLEEDING

4 joysticks 3 for tuning keys 1 for overall pitch
5 momentary keys (3 pitch, 2 range (as explained in my post above))
1 overall decay knob
and a potential for 64 notes (I'm sure with overlap)
hey there!
I hope this thread is not dead...
I'm planning to build a Gerassic organ (first with some friends and if we like it we wanna make a group builds of different paper circuits if there will be more people interested, so if you're in Berlin (DE) and want to join - pm me)
and I'm wondering about the hairy caps possible values and a possibility to add a switch to be able to choose the caps while playing (i believe this mod has been done already)
miƂego dnia!
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