System 700 711 output module clone-ish
I’ve had this board hanging around for…about three years? That’s how excited I am by it. This is an old photo, ‘cos it currently has about an inch of dust on it.
It’s all a bit kind of just OK – the phase shifter is alright, the spring reverb is nothing special (nowhere near as nice as the Tombola spring board) and the test oscillator is my own version and that sounds… acceptable, as far as a test oscillator goes.
For the spring, I replaced the TA7136AP with a stripboard adaptor using a TL071 and kept everything else the same, as far as possible.
There’s no specification for the reverb tank as far as I can see, so I can only guess Roland used the Z-3F tank they used in the Space Echo and System 100 103 mixer. Somewhere I’d seen suggested that the Accutronics 8AB2D1A would do the trick, so that’s what I’m using.
I did some underwhelming demos at the time – here’s a familiar-sounding bassline going through the spring:
And a blippy-ish loop, starting from dry:
For the phaser I roughly matched the 2SK30A FETs as far as I was able to using the method outlined by RG Keen in JFET Matching for Effects. They’re not super-cheap, so it’s expensive to buy loads just to match eight.
These demos might be an even greater test of your patience – hold on: here’s some twiddling with white noise:
And here’s an annoying loopy bassline, first with the resonance turned down, then turned up:
For the 1KHz and the A octave test oscillators I couldn’t find any of the large variable inductors, so I went with a couple of phase shift oscillators with a bit of extra simple RC filtering afterwards.
The octave divider chip is an LM3216 – at the time I designed the board, this was fairly rare (£12 from Little Diode), so to avoid using a sought-after IC for such an unexciting purpose I designed the octave divider around the CD4024.
I say “designed”, I probably nicked it from somewhere.
Here’s a sample of the 1KHz test tone – three seconds is as much as I can take
Looking at the spectrum analysis the odd harmonics are accentuated, so I guess it’s a bit squarer than it should be, but it’s OK.
The board still needs a front-panel, and I’ve spent way too long trying to work out whether it’s worth doing a PCB or just a stripboard for the mixer section.
Either way, it’s the summer and while it’s not piddling down or throwing a gale I’ll be out riding my bike.