Roland System 700 703E 24db low pass VCF clone

Roland System 700 703e low pass filter

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:

703e filter pole

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.

3 comments

  1. 4th February 2016Daniel says:

    Great to see all these System 700 clones – the sounds are really good.
    Do you sell PCBยดs?

    Cheers
    Daniel

  2. 4th February 2016ua726 says:

    thanks!

    I fret about selling PCBs ‘cos I’m redrawing the track layout, trying to make it as close as I practically can – as I understand it, the layout is covered under copyright laws so I expect I’d get letters from Roland’s legal department if I started distributing the files or selling the PCBs.

    Other projects like the various 100m clones and the x0xb0x have used different track layouts from the originals. Not sure how where that leaves the RE-303 project which purports to be a replica, presumably down to track level.

  3. 27th February 2016brett says:

    I would’t worry to much for a couple of reasons about roland coming after you. Paul from dinsync has just set the beta pcbs out for the RE303. Its a direct copy of th 303 to the point where you could most likely use them as drop in replacements for a original 303 and he’s had no issues.
    He’s making it a diy thing and not making a fortune from it so they dont care. thinks like his and your 700 roject are for hardcore builders so they dont get huge in the public sphere.
    The ohter thing is they have there new modular range in euro to keep them busy ๐Ÿ™‚
    I’m actually rearing a 700 for a local uni and I’d LOVE to build one of your sets of 700 pcbs!

    PS if you need any help ill be working on it for at least a bit more so if you need some info especially with the trouble there is sometimes figuring out those scribbled service docs. I can confim part values for you and photograph every module too ๐Ÿ™‚

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