Fixed-Architecture Mono Synth Builds

thanks, so if you use a selector switch then you are limited to one waveform at anytime from the oscillator, but if you take each waveform to a pot and mix the outputs together either passively or active mixing you can vary the level of each waveform to suit, a saw at 100% volume with a square at 33% etc. Turning the pots all the way down is off for each wave like a switch, turning them all the way up is like turning the switch on for each wave, but anything in between gives you variations. Synths do it in various ways, some have a selector switch followed by a level control, some have switches for each wave followed by a level control, some have level controls for each wave and no switch.

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I completely get your drift now, thank you. Was there a particular diagram or schematic you found helpful or have just come across that utilizes this? I searched “switchable wave-form oscillators with mixing pots schematic diagram” and realized again, asking here might be the only or best choice.

There’s lots on the web, but just look at synth panels like the pro1 etc and you can see what waveform mixing it has, a slide switch for each wave and then a level control, this is standard for most early sequential synths. I just went with levels for each wave as it seemed more practical, of course if my oscillator generated 7 different waves like the minimoog then I might haev a hard time with 7 level controls, but really the minimoog is unique as it has no PW or PWM control so it has many variations of pulse to choose from, whereas other synths let you vary the pulse width from a control so only one pulse output is required. So for my synth design I have 3 waveforms per oscillator, saw, PW and triangle, easy enough to mix with pots.

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I understand. It is a lot simpler than I was making it out to be in my head, haha. Many many thanks for taking what limited time you must have from your next-level projects to help out the noobs.

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The wiring of it all was what I was hung up on. But it is as simple as hooking a pot up where the jack would be. Madness.

Well I would make sure you put some sort of mixer after the pots, a summing amplifier is what you need connected to the wiper of each pot with a resistor. Look up summing amplifiers, you will find them very useful and I’m sure there is at least one in that norm synth design where he mixes the the oscillators together before the filter.

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Ahh, ok. I think I need to take a deep dive into some of the more descriptive and thorough full synth block diagrams to make more sense of this. I will also start now with a dive into summing amplifiers. That is something I’ve only seen a few times, and this where I see an example of how things can be so tricky.

There not difficult and I use them all the time to sum waves or voltages, basically an opamp and n+1 resistors for as many inputs as you need. There are inverting summing amps and non inverting, so you have to be careful with voltages so they don’t swing negative if you want a positive output.

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Here’s a helpful op amp guide I made a while back that might help.

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I have spent a lot of time looking at various verified VCO diagrams and trying to make sense of how I would convert them to be internally wired without any of the 3.5mm jacks. I’m essentially trying to copy schematics this early on to keep the margin for error low. That being said, I was looking at Eddy Bergman’s site, at his 555 and 3340 VCOs, and I could not for the life of me figure out I would practically wire that up. Having read a great deal on summing amplifiers and the difference between inverting and non-inverting, I am more confused than before. Assuming I’m doing everything linear 15v dual power supply (not sure how relevant that is at this juncture) is there a standardized method to converting a diagram when it has a lot of the waveforms as patches instead of switches/pots etc? If I am clearly out of my league here, where should I begin to tackle this? I am struggling because everything seems to be euro or kosmo, so there must be some standard change made I am not understanding. I get it’s a summing amplifier. But I don’t see that in action in anyone’s diagrams ever.

I wish there was a way to see how people hook up there modules together in a fixed-architecture format. A bunch of stripboard diagrams all connected together. Does such a thing exist?

this is what I’ve been puzzling over as an example…how to make all that switchable instead of patchable. summing amplifiers, I’m guessing, but where and how

Suppose you’re going into a VCF from here.

You could:

  • Use a rotary switch, send the three waveforms from the VCO into the rotary switch and the common terminal of the rotary switch into the VCF. Then you can select one, but only one, waveform to go into the VCF. Advantage: Simplicity.

  • Use three potentiometers, send the three waveforms from the VCO into the potentiometers and the wiper terminals of the pots to a mixer whose output goes into the VCF. Then you can use whatever mix of the three waveforms you want into the VCF. Advantage: Flexibility.

Mixer schematic from

You can use however many inputs you want; of course you would use direct connections instead of jacks. Here you’d use three inputs for the three waveforms. You could omit the mute switches if you want, you can always just turn the pot down to zero.

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Ok, I think this makes sense, thank you. And if I’m reading this correctly the summing amplifier is a part of the mixer’s circuit, after the waveform inputs are mixed down (or rather, that’s what is mixing them down? I tried to learn more about that particular mixer circuit but the link is dead

Here’s an example of switches used to select or mute the waveform which are then summed into the volume control for osc A and the same for osc B, Osc A & B, plus a noise source are probably summed into the filter input as well which in fact is done by the CEM3320 filter input stage acting as a summer.



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I’ve laid something out here that shows a bunch of stripboards connected together: Verified Stripboard Layouts! - #509 by savt22

Hopefully it’s clear, the gate out on the envelope generators should read just ‘gate’, they’re the gate connections from the Pico. Hopefully everything else makes sense how it’s connected - I’m going Pico to oscillators, oscillator outputs to VCA, VCA to mixer (the mixer is the TL072 on the VCA board). I’m also going Pico to Envelope generators, envelope generators to VCA. It assumes you’d build multiples of some of the boards for the full 6 voices.

As it’s based on the PolyKit DCO too you can swap between mono, stacked, and poly.

I ran the mix out into a single ms20 filter. I had an extra envelope generator running off an OR gate, which is just diodes and a resistor to ground, so the envelope was triggered by any of the notes, which ensures it’s always open when any note is played if using the envelope control. Then I added an lfo too.

One of my earlier posts also has a power supply layout which works too.

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If you want to do it on a PCB rather than stripboard I have a layout you can download Gerbers for

And I definitely recommend the 1N4004 to 1N5817 swap (on the +12 V output)


Been doing some reading but I’m about to dive in deep on these comments. Thanks folks! Savt22 this helps me immensely, this is kind of exactly what I was looking for.

That schematic just did WONDERS for me thank you.

I’m a bit surprised at the non inverting configuration. I guess in a fixed architecture there isn’t a problem with potential overvoltage. And if you have positive-only signals, phase reversal won’t be a problem either… here there’s a negative bias on the pulse wave, but the ramp is positive so the average at the + input will apparently be no lower than -4 V. So I guess it should work in this case. But not necessarily in a different case.

I’m definitely gonna be using your PCB just to minimize margin for error on that. Thank you. I made a note of the swap on my print version. Sadly I’m in the middle of several client pedal builds so I have a minute before this gets out of the R&D phase and I see what I need to order. Torment!

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