Roland System 700 703E 24db low pass VCF clone
Time for another System 700 module recreation: this time I’m having a go at the later 703e 4-pole/24db OTA-based low pass filter.
It went together pretty easily with the schematic being near-enough correct, although as with the 704 VCA I had to add a 15nF capacitor on the 4001 +V pin to ground to banish some noise.
I made some mistakes this time as well. Initially I put the +VLED power protection diode in the wrong way because I got the silkscreen wrong, due to a mismatch between Kicad and the footprint in Osmond. Also it seems I can’t count, I put 13 pins on the connector rather than 15 – although it doesn’t really matter, they weren’t all used anyway.
Once I’d chopped it out we get this kind of thing, recorded using an MC-4 controlling two System 100 oscillators into the 703e in, no VCA, straight out into the mixer and Audiofire 4.
I’m pleased with how it sounds, although it does sound fairly similar to other Roland OTA filters. The closest thing I’ve got to it is an SH-09, which uses BA662s and mylar capacitors against the CA3080s and polystyrene caps in the 703e.
Another thing that’s different to the other Rolands is that each OTA has a trim pot attached. Here’s a redrawing of one of the poles showing one of the presets (RV9) at top left:
The System 700 user manual from Roland Japan includes calibration instructions for the rest of the modules, but not the 703e. Maybe there is some extra documentation around somewhere, but I’ve not seen any.
My guess was that they were for nulling out voltage offsets on each OTA, so once I’d worked out to pull the plug on the filter input and attach the oscilloscope to the output of each OTA, I could see a decent offset from ground. Twiddling the appropriate trimmer dragged the output back to 0V, and once I’d done each in turn, the waveform on R33 (before the output cap) was centred around ground.
Post-calibration I gave it a bit of a test, although it didn’t really sound any different. Here’s my second jam with the System 100 oscillators and envelope, the MC-4 providing the CV and gate and the 703e clone. Soz about the length, I was having fun:
Slide pots from Futurlec have arrived now so the original format 700 clone dream lives on. I’ve got more boards coming shortly so there should be plenty to do over the Christmas break.
Here’s my redrawing of the schematic. In the LED driver circuit, I suspect there should be a diode between the +14.4v power input and Vdd on the 4001 (as on the 702) but I’ve not tested that yet.