Wednesday, March 18, 2020

Electro Harmonix Rhythm 12 Schematic

I was able to get another obscure drum machine, an Electro Harmonix Rhythm 12.
It has a somewhat unique sound, and I wanted to understand the voices better. There is a schematic available out there, but it's not the clearest. As usual, I decided to redraw it in Eagle CAD.

There are basically 4 voices, but they can be manipulated a bit. Parts of the sounds can be muted or even cut short.

Here is the portion that actually creates the sound. The sequencing and amplification parts are not shown. 

There are four "Twin-T" oscillators present. Three are damped, so that they decay naturally. One is self oscillating, and generates a tone used in the cymbal.

The cymbal and snare noise go through single transistor VCAs. These are pretty crude, and seem to work by offsetting the input into (and out of) the active region of the transistor. This heavily distorts the signal, but it does work.

Most of the other transistor serve to mute drums for pattern variations.

Here is the beginning of a layout. It's very easy to route, and could be done single-sided with some jumpers.


  1. Hey Zack, do you know much about the Novation bass station 1? I managed to get hold of one but the line out jack terminal was missing. I've sourced a makeshift replacement but I'm unsure about the wiring. It's a two port jack. I have pictures I can send. If you can give me any advice I'd be extremely grateful, if not no worries and peace!

    1. Yes; I did draw the schematic for one. I didn't include the jacks, and I've since sold it though.
      It should be easy to tell what pins are ground and signal. Just look for the large ground plane.
      You can likely ohm it out back to the battery too.
      Let me know if you get it.

      - Zack

    2. Hi Zack, thanks for your quick reply!
      I'm not particularly great with electronics etc (I'm a film set painter!) Basically the replacement jack terminal has a long and a short pin and the circuit board has two posts (if that's what they're called) one close to the input jack and the other about 2cm away. I'm not sure which should be wired to which.
      Hope my novice talk doesn't annoy you too much! Many thanks, Jimmy.

    3. There should just be holes in the board where the previous jack was. If the new one is a proper replacement, it should fit in the same holes.
      Can you upload pictures of the board and jack somewhere? I'll see if I can figure it out from that.

    4. Nice one, I can take some pictures, where should I send them to? I don't think I can upload them to this site.

    5. Any image hosting site: Imgur, TinyPic, PhotoBucket


    7. Looks like the top hole is ground, and the bottom is your signal.
      With your jack, the long lead is probably ground, while the other is signal.
      Solder it and see if it works. Your amp/mixer should be tolerant of it being connected backwards.

  2. Thank you very much! If you are ever in London I owe you a pint or two! I'll try it and let you know how i get on.