70's electric analog organ synth DIY thingy

so, i’ve found an electric organ (GEM Rodeo 61, it’s an Italian organ) from the late 70s with accompainment, drums and so, I also have the schematics
I’ve decided to turn it into a polyphonic synthetizer, since i wanted to get into synthetizers and learn more about electronics. I’ve figured out how to do most of the things like triggering the drums and making a sequencer, using the internal LFO, adding pitch bend, messing with the octaves and even cv input for the oscilator
the only thing i can’t quite figure out is changing or shaping the waveform from square to sawtooth
at first i thought all i had to do was slap a sawtooth wave oscillator instead of the square wave one, but after searching online for datasheets of the ICs under the keyboard, i’ve found that they are clock dividers that divide a very high frequency into lower, audible frequencies. But they only work with square waves, and I’ve read that whatever you send in those chips, they always output square waves

Master frequency generator aka oscillator, LFO, and divider chips

i’ve searched online for circuits that change waveforms, i found very few and just one or two worked nicely,
i tried this one, and modified it a bit, it works with all frequencies but needs adjustment for almost every frequency
WhatsApp Image 2021-11-04 at 13.32.36

slightly modified circuit

what happens if I increase the frequency

i’m taking this project of mine as a way to improve my electronics skills
so, does anyone knows if there’s a way to fix this problem? Thanks


Yes - what you’re looking at is a Top Octave Generator that then gets divided down to make all the notes of the octaves below and so forth.

It’s a classic organ/string machine way to get polyphony.
All the dividing is done digitally - which is why everything is a square wave.

I don’t want to discourage you, but doing the same thing with other wave shapes isn’t easy. You’d most likely have to convert each of the notes separately, so you’d be looking at 61 copies of which ever circuit you decide on.

You can turn square waves into triangle waves with an op-amp integrator circuit - but that too will need tuning for each particular frequency - you’d need to carefully select the integrator cap and input resistance for each note if you wanted the waves from all the notes to have the same amplitude. Once you have equal amplitude triangle waves you could use a rectifier similar to what you’re already doing to turn the triangles into a saw tooth.

I’d be keen to see what you get out of this so please do keep us posted!


thanks, i might consider doing it in a future now that i know that I have to make the circuit 49 times… (the first 2 octaves have the same tones). Maybe I could start with an octave and change the affected one with a rotary switch

one more question, in the future I’ll certainly add an envelopee for the notes’, but i’ve noticed something that I don’t like

this is the internal “envelope”, there are 3 modes that affect the attack and/or the decay, at first you might think that there’s nothing wrong, right? the “envelope” is monophonic.

what i mean

how can i make it “polyphonic”? do i need to add an envelope for every note😬? and what about filtering? i saw that Sam, in the gameboy mega machine, built a polyphonic filter and envelope, should i do something like that?

It’s paraphonic which is not an uncommon economic design choice.

The options to make it paraphonic would be to either make a filter and envelope for each note (not economical IMO) or build say 6x filters and envelope and develop a voice allocator to get 6 voices of polyphony (this is relatively hard)

My suggestion would be to embrace the limitations of the instrument and look at what you can do from there.
Some “easy” suggestions:
Better paraphonic filter with cutoff and resonance controls (ms-20 & the Moritz klein diode ladder filters spring to mind)
Chorus (research the solina strings organ)


I’m with @JaggedNZ here. There are so many possibilities without redesigning the wheel.
The amp and keybeds are brilliant so I’d tap in there and add fx or even another discrete synth such as the nano version of the solina string synth.
Adding a simple bypass between the organ and the amp would let you add guitar pedals or other DIY filters on an add hoc basis.
Tapping the keybed with an Arduino could give you midi or even midi to Cv also.


I’ve built the nano Solina and looked into the code, and my recommendation is, particularly as he has a working top octave generator, to stay well away from it. The nano just does not have the chops to compete with it.


True, it’s not a Powerful build, and a bit shonky up top, but tapping the keybed for midi and mixing the output with the amp’s input is good fun. I did this, building Jan’s solina into a cheap electronic piano that had midi and good speakers.
There are plenty other projects that could patch in without damaging the original organ.

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I used to have 2 organs from the early 70s. One Farfisa (pretty good brand) and a CRB Elettronica (can’t find anything about it)

The CRB had 12 LC oscillators, one for each note. On the oscillator pcb’s there was also a divider IC. I’m guessing the triangle wave eas generated in the IC as well (turns out there were a lot of organ/instrument specific IC’s back then)
The 4’ ‘pipes’ were sine wave or triangle or square. The 8’ and 16’ were square wave only. The note percussion envelope never worked properly.
This was a compact organ.

The Farfisa (80 kilogram home organ) had a 13 note top octave generator ic. 13 note means it generates both the high c and the c one octave below. The lowest 12 notes were divided down for the other octaves. There was an insane amount of switching going on. Sp12t switches and also logic gates.
Every voice (clarinet, flute, oboe, violin, cello, bass guitar and another few) had their own filter board.
It was often just transistor driven RC filters, but some were quite complex.

I’m guessing if you want full polyphony with individual envelopes for each note you would have to find a way to trigger the envelope when you press the corresponding key. Most organs just used a foot pedal for volume control (manual envelope)

In short most organs work like this:
Top octave generator>divider>formant filter>summer>volume control (manual envelope)
Good luck :slight_smile: