I've been experimenting with Monotrons since around the time of the original's release. I took an interest in the Duo model more recently though. The extra oscillator opens up some modulation options, and paraphonic possibilities. I thought it would be fun to digitally control as many parameters as possible.
After experimenting with using OTAs to control things, I determined that digital potentiometers would be easier. Unfortunately, there has been an ongoing shortage with the digipots that the project calls for. Because of this, the project has been shelved for some time. While working on this, I developed some modifications that don't require digipots though:
- Oscillator sync
- XOR ringmod
- Squarewave anding
- Sub oscillator
- PWM
- filter HP input
- VCF crossmod
I'm posting now to at least keep the schematics from sitting on my hard drive. I hope to follow this up with a breakdown of each mod.
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| Full schematic |
Hey your blog is really cool! I was wondering if there was ever a follow up on this?
ReplyDeleteI was wondering where you connected the high pass filter to seeing as c31 is under the epoxy?
ReplyDeleteHi Mauli, the epoxy comes off very easily. Try some rubbing alcohol and maybe a little heat.
DeleteI would remove C31 from the board, and connect a wire to the pad that joined with Q13 and F1. Then you can re-connect one side of C31 via the wire, and put your signal into the other side (that was connected to ground previously).
Hey thanks for the answer I’ll try that and thanks so much for labelling the monotron it’s a very useful resource. I wanted to add independent CVs for each of the voices so I could have a duophonic synth I’m still a newbie with electronics though, I’m assuming to send a voltage to the second oscillator, I would have to send a voltage to Q3 Im also assuming I can remove the frequency pot of the first oscillator and replace it with a fixed voltage divider so I can choose the octave range. I’m just not to sure what IC3B or IC3D are doing I assume they just filter the cv from the TI microcontroller.
ReplyDeleteYou're welcome. I'm glad you find it useful!
DeleteYes, IC3B is a filter/buffer for the PWM from the micro. IC3D is actually part of an exponential current source that scales the input voltage exponentially, and converts it to a current for the oscillators.
Putting a voltage into Q3 will change the pitch of the second VCO, but it will only *offset* it from the first VCO. You'll want to build a second exponential current source for the second VCO if you want to really control it independently.
I see, thanks for the answer again, so I assume I would have to use a different transistor wired up like q2 is, I heard that in exponential converters matching the transistors is very important, I guess I’ll have to replace q3 or do you think I could use a bc548?
DeleteThe type of transistor isn't critical, but them matching is. For good performance, you want two transistors with hand-matched Vbe. They should ideally be kept at the same temperature too, so it's good to thermally couple the pair together.
DeleteYou can experiment with whatever parts you have laying around, but it will probably be very difficult to tune/keep in tune.