Building a 2 Voice "Poly" Synth. Need to pick some brainz

So, I decided to build a 2nd MFOS Soundlab Mini-Synth MKII. The idea is to make it into a 2-Voice of sorts. Right now, my first SLMS is controlled by a midimplant. I will be wiring a 2nd one for the new SLMS. I’m building a MIDIpal out of through hole parts on perf board for the poly allocation.

At first, I was just going to slap the 2 together in a wood case and be done with it. Then I decided I was going to get a hand brake to bend some sheet metal myself and make a base and throw some wood sides on it. Well, if I’m gonna go that far, I should incorporate the additional ins & outs, which I will mount level along the bottom. Also add a panel to the left for the MIDIpal screen and knob. This will then give me some room to add dedicated pots that control cutoff, res, etc.

Now, the idea here is to keep each SLMS independent, so I can use them as 2 seperate monosynths, but have the dedicated pots either independently selectable with a switch or an overall MONO/POLY switch that will move the controls from each synth to 1 knob.

I’d like to have the following single pots to control both at uniformly:

  • Cutoff
  • Resonance
  • LFO Rate
  • S&H Rate

I imagine it’s going to be a bit more involved than just rerouting the pots wiring to another pot with a switch in the middle. Of course, I cannot mix CV’s without a mixer, so, I’m picturing a small perf board with some 074’s and some switches and that should work.

Any thoughts and suggestions would be greatly appreciated!

@ v’cent, I’ve got .1% resistors on the filter & VCO’s where it matter. Thanks again. I’ve never used dual gang pots do that never crossed my mind. I ordered 1 of each for now to test. If it works and I want to get em closer to matching, ill order more. They’re not cheap, that’s fo sho.

Seems about right, I would recommend ordering ~10 of the pots, so that you can get the most closely matched ones - Yes, some drifting will still happen (remember to match other resistors in the filter if possible), but that’s difficult to fix… +It adds a bit of charm, doesn’t it?

Happy to have been able to (sort of) help :slight_smile:

Now that I think about it, I should build a dedicated LFO & S&H with selectable destinations.

In theory, something like a dual-gang pot could be used with a multipole switch to mix and match what controls what.

Unfortunately, the shitty tolerance of potentiometers will then gang up on you, and make the whole thing not work as expected, unless some form of compensation was used.

I guess some form of digital pot could be a solution as well, but I have zero experience with those.

@ v’cent, hmm, that’s a pretty good idea. May look into that. I could order a bunch and test each and try to find the one that has the closest match. Maybe add some trimmers to dial them in. I’m trying to stay away from digital (other than the midipal and midimplant). This will definitely be a very involved project. Pretty stoked! Should have all the parts to build the midipal tomorrow. That will be my first perf board build, other than simple PSU’s.

EDIT: Found some Alpha’s with a 5% tolerance. Much better than the usual 20%!

Embrace the differences. The poly/duo modes on the perfourmer are really great because you can’t ever get any of the voices to sound exactly the same. I don’t know what the tolerances are on the pots inside it but I can set the voices close enough for cookie by ear and I’ve learned to think of the little differences as pluses rather than minuses.

@ soup, agreed. I’m leaving every parameter to chance. I really wanted to be able to grab 1 knob for the cutoff & filter. Only reason why I’m doing that. 20% tolerance may be a bit to extreme to get anything useful, but 5% should be the bees knees. Can’t wait to start testing this! This thing will be a beast.

Think I got it figured out. 2 Dual Gang 5% pots and 2 CD4053’s with 1 DPDT to switch between mono & poly mode. Word.

Check the schematics. If you only need a voltage for your LFO you might be better of with a single pot and some OpAmp Voltage Followers routed to each LFOs VCin. This way both LFOs will get xactly the same CV.

@ frank, I’ll look info that. I was thinking of adding a shared LFO with a rotatary switch to with different destinations (filter, VCO or both), but that could be another feature. This thing is slowly getting bigger & bigger.

So, thought I’d give an update on this.

Keeping a polysynth (even a 2 voice) analog is a lot more complex than I had originally thought. I wanted it to stay somewhat simple, but that just wasn’t possible. I believe I ironed out all of the details though. The “Global” panel will have the following:

  • 1 LFO
  • 1 S&H
  • 2 Modulation Source/Destinations. Source: LFO or S&H. Destinations: Osc1, Osc2, Filter. Both destinations go through a CV Mixer then out to both voices. I may add a couple more destinations.
  • 2 Portamento’s. Each portamento will go to each voices V/oct. They are asymmetrical, so there will be a choice of slide in, slide out or both. There will only be 1 amount pot, but this may change. I may also add a circuit that will route it to 1 - 2 or both.
  • Global Cutoff & Resonance Pots. This was the tricky one. The amount of circuitry needed for this is pretty crazy. I’m using some 4053’s to do the switching from local to global control. I read that the 4053’s work best with current, so I have to convert the voltage to current, then back. I based if off of this schematic.

Both LFO and S&H are separate modules from the 2 voices. They are specifically for the global control panel. Each voice will still have it’s own LFO and S&H.

The attached image is the Global Control PCB.

Other than that I just need to figure out how I am going to tackle the actual voice allocation. The first thought was using a MIDIpal. Another thought was to strip all of the apps out of the code except dispatch. The only issue I have with this is if I went this route, I’d like to do away with the Display and move it to either a MOM toggle to turn it on and off, but that brings up another question of how to set the first midi channel. Probably the best thing to do is learn how to write code. Ideally I’d like it to be 1 clickable encoder and 2 of these. So, if anyone is bored… :slight_smile:

EDIT: to reiterate, the 2 voices are the MFOS Soundlab Minisynth MKII’s. Also, the LFO & S&H will both have patch points, so nothing is limited to the mod matrix. It will be a semi-modular poly-synth.