ORGANS! Cause polyphonic synths are expensive

so go easy on my folks, this is my first project…

so anyways, screwing with an old Yamaha organ… Still learning on how it works, what I’ve learned so far, is it only has two oscillators feeding a bank of frequency divider chips, the chips feed switches on the key beds, so every key has the sound behind it, and the key makes the connection, and lets that particular note through… so I have almost 8 octaves with more notes than I have fingers… My first thought was to replace the original base frequency oscillators with 1v/oct oscillators which can generate a few different waveforms. was also thinking of adding a divide in also, to basically to be able to plumb in any sound I want and make it playable through the organ, essentially making it like a sampler(ish) which leads me to my first question, and you feed varying frequencies into a divider circuit? does that work?


My guess would be that the top octave generator chips and the dividers can only deal with square waves. You could probably modulate the driving oscillators to some extent, certainly enough to create a vibrato, but potentially enough to create an FM synth like effect. That could be interesting to try.
Do you have a model number for that organ?
Do you have access to its schematics?

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Usually you can feed in any waveform you like but you always get a square out. You can vary the frequency all you want though. All a divider does is output a pulse each time it counts up to the set number of denominator pulses, so if you feed in a chord presumably what you’ll get out is the low note(s) PWMing the highest note in the chord. I’d be interested to see that in practice though.

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I think I’m going to order a cheap oscilloscope to see on the waveforms, its a yamaha bk-20s which I’ve found no hits on, the boards all say bk-20c, and there is a 20cs listed, so guessing its that. I had to buy a paper schematic (service manual) for 9 dollars on ebay, I believe the way it works after the frequency dividers is it shapes the waves, as ive seen a few oscillators after the chips with drawn waveforms, how they work no FN clue… kinda figuring this out as I go… I’d like to find a way to have say the lower keybed modulate the top keybed, or something like that… and this was a free organ, with about $100 invested and can be recycled if I fubar it so pretty much approaching this with a no holds barred mentality…

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Personally ive heard a lot of people say cheapo oscilloscopes are garbage, but i cant agree with that. I got me one of them DSO based dealies for a reasonable price. Its good for audible range which is what i use it for. Its also all assembled in a case so thats a plus.

This is the one i got:

$38.69 / £30.02

Good luck with your project! Throwing yourself in the deep end is a fun way to learn I reckon. Even if I fail, i usually learn heaps, so thats worth a lot personally.

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This one is mine:
I think I paid $200-300 for it new, but you probably don’t need 100Mhz for audio work. Originally I needed that resolution for my… time machine. :alien: :robot: :robot: :robot: :alien:


okay here we go… so since I was lazy and didn’t feel like finding the top octave oscillator, I decided to feed my in behringer D to c7 for testing, because that sucker is easy to get to… it then feeds all the divider chips and lower C notes from there. so all notes are regular except for C. found that the divider chips don’t like anything but square waves. at least from the D. I noticed that the output from the D wasn’t as high as the organs oscillator, so that might be why… not sure. also found that when the synth had sounds close in frequency fed in from the D, there were some weird resonance things happening, and the low bass from the foot pedals was awesome sounding… so going to maybe get a small preamp to see what that will get me. not really sure what the hell im doing, but seems like im heading in the right direction…

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Oh, you definitely are. This kind of experimenting is hard to come by :heart_eyes:


Ok so I think I have my plan of attack. I’ve decided I like the way most of the presets are, but I want it to have a lower base note, and better filters. also it seems like the divider chips really only like square waves, But introducing a triangle wave at LFO frequencies makes some neat sounds by confusing the divider chips I’m assuming. I think I am going to turn this into a keyboard, so that means removing the 12 pedals, so, what I think I want to do, is hopefully come up with a sequencer that will trigger bass notes, and also be able to trigger drum sounds. I think I’m going to take a peek into my volca beats for that.

now onto some questions… for starters I was wrong on the top octave oscillators, the schematic calls for two main oscillators feeding a pair of dividers, then onto more dividers what I found is the top octave is generated by 12 transistor oscillators then fed to all the dividers. what I want to do is be able to pitch shift, as well as modulate, while keeping the scale. should I redo the top octave oscillators with chips? or should I just find a schematic for the thing, and connect in some LDR’s for modulation?

also I would like to introduce a minimoog or ms20 filter, can I get away with feeding chords through a filter? or should every note have a filter… or maybe every octave? whats a good blend of cost vs performance?

Seems a bit drastic, though I appreciate a fully fledged organ with pedals isn’t exactly useful to people who have no experience with the footwork.

For inspiration on use of the pedalboard and MIDI integration to get more interesting sounds from the organ format, see Barbara Dennerlein.


That’s up to you. There’s a lot of industry discussion about polyphony vs. paraphony and their definitions. If you don’t have any enveloping to the filter (or any VCAs for that matter) then it couldn’t make any difference if you had one or multiple vcas. It’s hard to imagine how you’d trigger them. If you have a Model D you probably know about the Poly D, which only has the one filter and volume shape for all four oscillators at once.

working on that… there are a few ways I think I could trigger the envelope… there’s a “sync start” for the drum machine, that starts the beat once any key is pressed. as far as vca’s and envelope generators etc ill add those in as needed… but even If I don’t automate the filters, is there a rule of thumb for how many notes one should run through a filter? whats nice about this, is I can add more filters… so really I should just try it and see what happens…

by the way - i recently found out that the yamaha electone e75 organ is basically a cs80 in organ-format. there is an article about this (just in german though):
maybe it would be a nice project for @lookmumnocomputer to get one of these and make an actual cs80 out of it?


Add an MS20 style filter (just the one) : it’s a quick win that will dramatically expand your sound. If you don’t want to “play” the pedals then keep them and use them as controllers for other additions and changes.
Add a delay circuit, a soft pot strip even an arduino could add another polyphonic voice for your bass notes. I ramble but only because I’d love to have an old organ to mess with. Best of luck!


Ok, so I think I may have an answer on my oscillator question… I was discussing this over facebook and the idea of a microcontroler came up… Hey I have this arduino over here collecting dust. its got 12 digital outputs. whats stopping me from writing a code to have it pulse square waves on one octave on all channels and then have it change them pitch bend etc as dependent on whats coming through the analog inputs, basically making an arduino dco… is that a thing? I was told the atmega 328 can’t handle pulsing that many outputs, why not? its got 12 outputs…

It can kind of do it, but I’m guessing the thing you’ll run into is that for 12 voices you’ll probably start having some hiccups over timing. Having to calculate the duty cycles will start to tax the processor to the point that it will start to miss high/low flips at the times they’ll need to occur to sound like constant tones. Add to that the demultiplexing required to poll 61 key values and compare them to last reads and it’s going to get dicey, but as Sam says, “don’t be afraid to try it!”

well… how much “deformity” is the question… it could be a benefit… I’ve been reading the reason they stopped using the divider setup was the division was “too perfect” so some abnormal glitches might be cool… who knows… I think I have enough things kicking around to make it work and see what happens… if it doesn’t work because lack of crunch upgrade to a teensy i think is the next logical step and I had some time noodling around through the single filter in my behringer it sounds pretty cool… however I think for neat effects it should have 3 filter stages, as well as digging into the different filters it already has… got a lot of work ahead of me i think :smiley:

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One thing I’d recommend if you go that route is to get a couple of prototype headers (or the IC sockets like Sam uses if you’re using mini/micro form factor duinos.) if something goes wrong you can pull out the microprocessor and start over easier that way.

While I think it would be unrealistic to expect twelve-note polyphony from a single Arduino, this video with its associated links indicates how versatile it can be.

Also remember you can buy an Arduino nano for around £3 so if you want multi-voice polyphony is just a matter of buying more and programming each one to produce however many voices it’s capable of. With N nanos you could effectively code high quality N-voice wave table synthesis, for whatever value of N you want.

See this audio library for more ideas:


Ive been working on a sampler like project with the samples stored on eeprom, this code looks harvest-able for that. Noice.