MIDI Polymoog « Thread Started on Oct 27, 2009, 10:55am »
Hi people,
I've had my polymoog since 2001, but it has been a long, long term project to bring it back to life... One of the main problems was that the keyboard contacts were in a really poor state, so I ripped out the original contacts and modified the keybed slightly to take the contact boards from a velocity sensitive MIDI master keyboard, intending to MIDIfy the voice boards soon after. The poly has been in use for a few years now as my master keyboard, but it's just in the last few weeks I finally finished the MIDI to voice board driver circuitry (photo attached). So now I have a Polymoog that sends and receives MIDI. It's quite a different beast when you can sequence the notes and just tweak the filter and resonators. Not sure if anyone else is going to be interested in it, but it should be easy enough to modify the original contact boards so they could be digitally scanned, which would make installation a lot simpler. If there's sufficient interest, I could do a run of complete polymoog MIDI retrofits, or possibly just bare PCB sets for self-assembly. These would consist of a set of 3 PCBs that sit under the original keyboard, each board driving one voice board. The wires that run from each of the key contacts to the voice boards would be cut, and re-routed through the MIDI boards. Also the original keyboard connections are routed to these boards, to supply the control signals needed, and to drive the CV/gate outputs and filter tracking. If you'd be interested, drop me a mail.
Re: MIDI Polymoog « Reply #3 on Oct 27, 2009, 1:45pm »
Thats brilliant Colin!
Any thoughts on doing a similar conversion for the Minimoog to bin the flakey Pratt-Read mech? Might be a decent potential market there (and I'd be at the front of the queue).
Wow, a MIDIed Poly. Is that a custom board you had made up - looks very neat.
There are three boards, all of the same design, but with some parts only populated on the top and bottom boards. I had them made by PCBCart.com Each board drives 24 notes, with one note unused on the top voice board. They mimic the original polymoog keyboard circuit - a cap/resistor pair for each key is charged with a DC voltage proportional to the key velocity, then the normal AC trigger signal is switched through by an analogue switch. It's a brute force solution, not really any other way to do it...
Any thoughts on doing a similar conversion for the Minimoog to bin the flakey Pratt-Read mech? Might be a decent potential market there (and I'd be at the front of the queue).
I've got a flaky mini keyboard here too, but the mini is MIDI'd up, and always played from the poly. I'd keep the Pratt-Read mechanism, they are much better than a modern plastic action, but those contacts are way too expensive and tedious to replace. I'm pretty sure you could replace the mini contact board the same way I did the polymoog. I have the contact board mounted on top of the keybed, positioned so that the bottom of each key presses a contact directly. I've attached a photo taken during the conversion. I had to infill the bottom of each key, to create a flat area to hit the contacts. The key spacing worked out fine. The contact PCB came from an M-Audio 88 key controller, with a bit chopped off the end. There are plenty of cheap 49 key controllers around. If you used one of those, hooked up internally to a MIDI to CV, then you'd have reliable control of the mini, with the original keyboard feel, plus the option to use the mini as a polyphonic MIDI master keyboard.
Re: MIDI Polymoog « Reply #6 on Oct 27, 2009, 4:37pm »
Yep that's the sort of thing I meant Colin - just replacing the bussbars and spring contacts with the contact bed from summat modern (e.g. I have a D10 I could scrap). I need to do it as my bussbars are fubar and we all know how flakey the mini's P-R gets over time anyway.
Pity I don't have the skills to DIY it as per your suggestion. Fancy a project...?
They mimic the original polymoog keyboard circuit - a cap/resistor pair for each key is charged with a DC voltage proportional to the key velocity, then the normal AC trigger signal is switched through by an analogue switch.
Wow! Does this mean that it will respond to incoming key velocity? If so, you must have spent some time figuring out the velocity vs time relationship.
Wow! Does this mean that it will respond to incoming key velocity? If so, you must have spent some time figuring out the velocity vs time relationship.
My circuit extracts the DC difference between the two key busses, scales it according to velocity, then 'writes' it to the resistor/cap pair just before turning on the analogue switch to the mod card, so the amount of DC kick is precisely controllable in software. The RC pairs in the polymoog have a time constant of around 5ms, so there's an exponential decay of the 'DC kick' over that time. In theory, incoming MIDI velocity should be mapped to that curve. But I found a linear response is more playable. The curve is adjustable in software, so I guess I should make it switchable, and maybe try some other curves.
Joined: Jun 2005 Gender: Male Posts: 675 Location: Brighton UK
Re: MIDI Polymoog « Reply #9 on Oct 30, 2009, 2:55pm »
Hello Colin,
I have a Polymoog 280 Keyboard and have always dreamed of using it as my master midi keybd. The physical keyboard and contacts on mine are ok but would I still need to replace parts from (say) an M-Audio keyboard? I'd be happy to join in and DIY or indeed pay you (!) to make my baby midified...
I have a Polymoog 280 Keyboard and have always dreamed of using it as my master midi keybd. The physical keyboard and contacts on mine are ok but would I still need to replace parts from (say) an M-Audio keyboard? I'd be happy to join in and DIY or indeed pay you (!) to make my baby midified...
An original keyboard with good contacts can easily be converted for digital scanning, and thereby MIDI output. I actually have a spare polymoog keyboard mechanism I was going to test the modification out on, but a friend of mine who lives a 2 hour drive away has it at the moment. Next time I see him, I'll get it back, and work out the required circuitry.
Indeed, final cost for a DIY kit (a set of boards plus pre-programmed CPUs) would depend greatly on the number of orders I got. I use PCBCart.com for small runs of PCBs, and their prices reduce significantly as the quantity goes up. For a ready-built retrofit, I'll need to sort out the design for keyboard scanning logic, work out a total bill of materials, and guess how long it would take to stuff a set of boards for each one. I'm pretty sure it'll be the cheapest polymoog MIDI kit on the market though ;-)
Indeed, final cost for a DIY kit (a set of boards plus pre-programmed CPUs) would depend greatly on the number of orders I got. I use PCBCart.com for small runs of PCBs, and their prices reduce significantly as the quantity goes up. For a ready-built retrofit, I'll need to sort out the design for keyboard scanning logic, work out a total bill of materials, and guess how long it would take to stuff a set of boards for each one. I'm pretty sure it'll be the cheapest polymoog MIDI kit on the market though ;-)
Cheers, Colin f
I still think the Minimoog-owning world will beat a path to your door for a similar mod. Or this particular corner of it will anyway
I'd be quite keen on the Polymoog version if it wasn't ruinously expensive, problem is my PSU has packed up and now that Tony Allgood has pulled out of repair work I'm struggling for a tech to get it into shape.
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