Roland System 700 704 VCA clone
After all that faffing around peering at photos of circuit boards and the dodgily reproduced schematics, here’s a stuffed and working Roland System 700 VCA clone board.
It turns out that learning Kicad/Osmond was worth the candle, it was so much less tedious to arrive at a layout that mirrors the schematic. And it looks a lot nicer too. The board says “V1” on the silkscreen because there was a later version of the VCA that used a (cleaner sounding) BA662 for the OTA rather than a CA3080.
Speaking of the CA3080, the original schematic (see Florian Anwander’s site) calls for it to be selected, but doesn’t say what for. I’m presuming it should be chosen for low CV feedthrough, but as with the System 100 VCA all I can think to do is to try a few different ones to see which sounds the best.
There are a couple of problems with the schematic. Q1 is written as a 2SC100GR but it’s more likely a 2SC1000GR, and the sketchily annotated Q2 should be a 2SA493GR. R22 should go to -15v rather than ground, and R17 on Yves Usson’s version should be 68k (as on the original schematic) rather than 100k.
I’ve seen a photo of an original 704 VCA which has the DC offset trimmer at VR7 at 100k rather than 50k, so I went with that. Also the board layout shows a 0.1uF mylar cap at C4 rather than 0.01uF.
It kind-of worked straightaway in that it did the VCA thing, responding to a CV input from my clone System 100 envelope, but did so with a set-your-teeth-on-edge distortion. Not the nice kind of fuzz.
I suspected the 2SC1000BL I’d used in the push-pull amplifer/buffer after the 3080 – the schematic specifies a -GR variant – but narrowed it down to something in the LED level detection circuit. Stringing a 15nF capacitor (…as in the service manual) across Vdd and ground of the CD4001/IC3 abated the noise.
The original schematic is confusing when it comes to showing how the CD4001 (which is the brains behind the level indication LEDs) should be powered, being apparently run off both the +15v line and the 14.4v line via a crossed-out diode. When laying out the board I took the crossing out to mean “don’t do this” but it’s probably more like “this is an addition”, or in this case “this is a bodge”.
Here’s how it goes, not that a VCA demo is very exciting, bass going boom on the scope – square wave from System 100 VCO into 703b VCF with the cutoff turned right down so I could check for distortion into the 704 VCA set to linear response, controlled by a System 100 envelope – all clones.
The exponential response is much faster than the linear input for the same envelope settings. I’ve not played with it much yet, but it seems like it’ll make for a really snappy-sounding voice.
My board design was a bit, um, idiosyncratic, let’s say, with wonky haphazardly curved thick traces, no ground pour, and some dodgy spacing like this, although it might helped if I’d used a smaller voltage cap:
But it was my first(ish) go, and I was trying to balance getting it right/making it pretty with finishing it. For future boards, where there’s room I’ll try and put the reference on the outside of the body of the component so it’s visible after soldering when I’m trying to debug.
Here’s the original schematic updated with corrections:
Update: Sep 2016
Via Matrixsynth, a picture of the new style VCA.
Note the BA662 chip replacing the CA3080. The owner of this reckoned he preferred the old style 3080 version.