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Firepowerforfreedom 22nd December 2010 11:32 PM

Oscillator organ to synthesizerómission impossible?
I've got an old spinet organ I'm trying to decide what to do with, and my latest idea is to turn it into an analog synthesizer (polyphonic or otherwise).

The first thing I need to know is what makes an analog synth different from an oscillator organ, on an electrical level.


indianajo 24th December 2010 05:55 PM

uh, no, not really. You don't list a brand. Most brands of organ before 1980 were chock full of electrolytic capacitors that are all dried up now, so they all need replacement just to make it an organ again. They generated triangle waves typically, then filtered them down to organ sounds. After 1980 cheap organs were full of LSI organ chips that might work now but are irreplaceable if they don't. The exception is Hammond, but you know what Hammonds are worth.I believe (I never owned one) Moog synths had triangle wave, sine wave and envelope generators (VCA), and a patch board. Hammonds were basically additive analog sine wave synths with a tonebar mixer control instead of a patchboard, and Hammonds mixed with transformers, not resistive dividers & amps ("mixers"). The H100 Hammond model, often selling for $100 these days, has the patchboard in the back in addition to the tonebars in front. It also has a rudimentary envelope generator (percussion). I'm having a ball planning modifications to mine- getting rid of the unreliable tube sockets (bad connections) and putting in solid state stuff with additional percussion curves (envelopes) etc. There is tons of space in this old thing for SS tricks, including plenty of contacts for MIDI.

stoc005 24th December 2010 06:08 PM

Most, if the they have a top octave oscillator per note... a total of twelve with digital dividers for the each lower octave, use diode gating. The keyboard turns on the correct note using the diode gate and it is routed to filters to form the voice. The newer ones are microprocessor controlled, digital again but the voice is usually digital samples. A diode gated organ can't easily be converted to analog, I think.

Firepowerforfreedom 24th December 2010 06:13 PM

It's a Conn 427M caprice. Not sure if it uses one oscillator per top octave note. I'll check later and let you know. Conversion to solid state is also on my list of ideas.

I'm not so much interested in converting the whole organ to analog, only a single manual. My plan would be to take what I have on hand (plus a little extra) and reduce it into a small analog synth.

Firepowerforfreedom 24th December 2010 06:47 PM

According to my oscilloscope app, the flutes are pure sines. The strings seem like filtered sawtooths, and the diapason seems like a filtered triangle.

Firepowerforfreedom 24th December 2010 07:37 PM

Yep, single oscillator per top octave note.

Firepowerforfreedom 24th December 2010 07:38 PM

Wait, no. There are only six top octave oscillators—one per every two notes.

There's an oscillator per note for everything else, though.

Firepowerforfreedom 24th December 2010 07:41 PM

Here's a pic of the back: Austin Leeds's Photos | Facebook

indianajo 24th December 2010 10:49 PM

Looked at your picture. Tube city. The good news is that this thing is so old, there is probably only the one electrolytic cap, the can on the power supply. Those are available from FP at or Maybe one on the upper right, look to see if that yellow thing has a plus on one end or minus on the other.
The bad news, is that these were not very stable in pitch, and needed tuning frequently.
Sines and triangles gives you basic opportunities in the synth area, but there is not a lot of room for S.S. toys. I would either replace the electrolytic and keep it with the back plexi-glassed over for comedy relief- or convert it to Midi by connecting the keyboard to Midi scanners and hooking them to a PC. I had a computer that had more tubes than this, about 1966. It was dumber than a $.99 calculator. But it was an awesome collection of fifties hardware, with a bad magnetic drum drive not compatible with floppies. I'm confining my tube mania to a ST70 amp, a PAS2 preamp (nothing visible, ****) and maybe a bogen 10W amp to convert to higher-fi.
Have you seen Arcade Fire's (band) round projection B&W "TV" with artificial snow? Retro is in, if you are trying to be really different. They also have a pipe organ on stage. You could intercept the keys on the way to the oscillators for midi relays, dress up in a white lab coat & Einstein hair, and play Adam Ant covers in bars with this thing.

Firepowerforfreedom 24th December 2010 11:06 PM

Here's a pic of under the swell manual. Are those yellow tubes also electrolytic capacitors? Austin Leeds's Photos | Facebook

I had thought about the midi route, but I cant find anything cheaper than $500. If you know of a place that could give me a whole kit for less than half a grand, I'd really appreciate it.

I like the look and feel of this organ, I just hate that it requires multiple people to move. My goal with this thing would be to get it small enough to take places for performances. What would be my best option for this?

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