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Tuesday, April 17, 2012

Duophonic Module Assembled

Module before racking
Module in the rack and working
Doepfer MTC 64 MIDI to TTL voltage converter
The High Octaves reed relay PCB which sits under the Dopfer MTC 64 board

Top view of module. Top to bottom: MTC 64, HIgh Octaves relay PCB, Low Octaves PCB, Duophonic PCB
Side view of module. From left to right: MTC 64, High Octaves relay PCB, Low Octaves relay PCB, Duophonic PCB.
Side View of the module. From left to right: Duophonic PCB, Low Octaves relay PCB, High Octaves relay PCB, MTC 64

The Duophonic Keyboard PCB is fully working and it is sweet. This was the first "module" I built when I originally started this project in the mid 1970s, so it is fitting it is effectively the last module in this resurrection project. It is big and deep. Using reed relays instead of solid state relays meant I had to split the Pratt-Read keybed emulator into two PCBs instead of one. But the very low resistance and zero leakage of the reed relays made it worth the trade off.

If you have an old Pratt-Read keybed, or another switch-per-key on a common buss device, you don't need the relays or MTC 64. You can make a resistor chain on the switches, like in the original and hook it up. You can also make/configure your own custom controller to drive the keyboard electronics.

Essentially, here is what is going on. The MTC 64 takes a MIDI note on/off and turns on an individual TTL level (or turns it off, you can configure it) for each note number from 0 to 63. I take 0 through 60 (for a standard 5 octave controller with 61 keys) and connect each one to its own reed relay. This acts like the buss/key system on the old keybeds. On the other side of the relays is a resistor chain so that when a key is pressed, it taps into this chain and sends this signal to the "buss" input into the electronics. So, the Duophonic electronics sees exactly what it expects, a voltage divider resistor chain, just like the old Pratt-Read keybeds.

This particular module acts like a MIDI to Duophonic Keyboard converter, but there is no reason you can't just build the Duophonic electronics into a dedicated keyboard, or even an external module.

The Duophonic Keyboard was pushing the technology at the time when most others were still using monophonic keyboards. The Duophonic was  both a low note priority monophonic keyboard and a two note keyboard.

There are some interesting "features" of this particular keyboard. The upper voice tracks the lower voice when an upper key is not pressed; there is no sample/hold for the upper voice due to the way it's generated.

Next, you can either have a new trigger/gate on each low key press event, or only a new trigger/gate when the previous key is released before the next key is pressed. Nyle calls this legato or lift playing respectively. The upper voice does not generate a gate/trigger.

There is a pitch bend knob. I chose to make this a big knob with a finger spinner to make it easier to turn. Also, because the pot is not spring loaded, I chose one with a center detente to make is easier to feel when I was back to center. Nyle added an electrical dead spot in the Pitch Bend, too so if you miss the detente, you will still be OK.

There is a Portamento knob, and I added a jack for a normally closed foot switch so I could dial in a Portamento and then hit a foot switch to activate it.

There is also an Auxiliary Scale which can be tuned for other scales and it can be switched in and out.

There is a reverse switch reverses the keyboard, and a Range switch lets you shift the keyboard output up or down by octaves.

Now I have all the pieces to my Synthasystem, I'll try to get some additional video/audio up of all the pieces working together.

Next, since the keyboard is done, I'll start ramping up on the Microcon!