Out of all the various dodgy looking stripboards in my 100M clone, this one turned out the best (let’s just not mention the LFOs, they were a pain in the arse).
Determined to get it going first time, I made sure I’d done all the cuts, checked for shorts between tracks, powered up the board without ICs – was generally super careful, and then only later found that I’d missed out a couple of capacitors. Silly.
I made a daft mistake with the hold in the sample and hold. It worked, but the held voltage was drifting downwards rather quickly. Compared it to my System 700 board and realised my mistake – I’d used a 2.2nF (222) capacitor rather than the specified 22nF (223).
(Actually I was tempted to use System 700 boards rather than faffing with stripboard, but they’re so lazily massive that they didn’t fit in the panel space I’d designed).
I compared the white noise to the pink, and the white noise just sounded really quiet. Which is weird because it’s coming out of the same place, it’s just being filtered differently. Here’s the relevant bit of the schematic.
I compared the outputs on the oscilloscope – didn’t take a photo, but the white noise was chopped off at 0V, whilst the pink danced around both sides of the ground point as you’d expect.
Anyway, it’s pretty obvious from looking at the schematic that the R31 (attached to the transistor output buffer, Q4) should be connected to -15V, and the track layout bears that out.
The ring mod is based on the LM1496 like the rest of the Rolands, and you’ve probably got three of those already, but anyway.
Here’s a tiny demo of clanking and white noise twiddling. The ring mod was in better calibration when I put it together, it’s bleeding a bit from one side at the moment.
There would be a demo of the sample and hold, but I’m presuming the lag pot has disconnected itself behind the panel, as it’s verrrrrrry smoooothed ouuut, barely even recognisable as a sample and hold right now.
Here’s the layout, and wonky BOM.
It includes the indicator for the clock out, which has separate ground and +V power supply connections, presumably in an attempt to keep the audio path clear of any clicks from switching the LEDs. On the original (and on mine) this is powered by 22V, but it should work off 15V.
Here are the Gerber files for my version of the Open Music Labs BA662 replica.
I’ve called it the tall clown ‘cos it uses BCM847DS and BCM857DS dual transistors, rather than the tiny PMP4201/5201 in the OML version. It’s a bit easier to hand-solder than the OML original, but not much.
I’ve tested these in my System 100M clone and they seem to work, but ymmv.
The filenames follow the iTead naming convention, and the board size is 22.0 x 12.1 mm.
If you prefer, here’s a link to the (untested) project on Oshpark – three boards will cost $2.05.
Thanks to Open Music Labs for doing all the hard work on reverse engineering the circuit. This particular layout is my fault, so blame me for that.
Here’s a handy map of where things go – it’s from rev A, but the components haven’t changed position. The resistors are both 0805 sized.
If you can’t be arsed with soldering such tiny components, the OML versions are distributed through Synthcube, try them instead.