I finally got around to getting the parts I was missing to complete several modules. Here is a list of what I put together and what I’ve learned, over the last six months.
- Ken Stone’s Peak/Trough, essentially a slight modification of a 1973 Serge module. I built this on stripboard using S8050/S8550 transistors which I had a few around and could not find many uses for them (layout contributed here). A very easy built which can be useful in creating complex modulation sources by combining different signals, among other use cases (which I have yet to explore).
- @moritzklein Diode Ladder VCF, as modified by @benjiao (whether this is actually a ladder topology is debatable, I think, but it should not matter). I implemented the suggested resonance mod by adding a 10k trimmer to the feedback resistor (I think there is a sweet spot around 12k) along with sockets for the feedback diodes. A managed to kludge the Eurorack PCB set behind a Kosmo size panel that features a transparent window to show the flashing clipping LEDs for effect. Overall, a nice aggressive sounding filter, with a good range in the CV control, although I found the range of the cutoff potentiometer somewhat limited. Perhaps, one could try a lower value potentiometer with resistors on either side for finer control? I also did not find much use for the passive LPF potentiometer. Maybe, one can repurpose it as a clipping control to regulate the amount of signal that goes through the LEDs, by moving it before the output buffer op amp. I will probably build another one with these mods, but I should have a couple of PCB sets (no panel) up for trade.
- Midcentury Modular’s Slew Limiter, with precision rectifiers and independent controls for rise/fall times. I built this point-to-point (save for a tiny bit of stripboard for the TL074) using A500k potentiometers and played around with the selection of the capacitors to find useful spots for both linear and log outputs. This can be very useful for how simple it is, for portamento effects, as a simple A/R envelope, LFO waveshaper, or even a very crude lowpass filter.
- @analogoutput Precision ADSR. See my post above in this thread. Arguably the best (with most features?) analog DIY ADSR, so I got the parts to build another two of those. This leaves me with two spare main boards for trade (no control boards or panels, I found it easy to kludge everything behind a handmade panel where I used potentiometers with solder lugs).
- Korg’s MS-20 LFO featuring two waveforms of variable shape. I used the stripboard layout from this blog which included Marjan Urekar’s useful mods and it worked well. There is some uncertainty about the FETs used in this circuit. The layout linked above suggests BS170 MOSFETS but the MS-20 service manual indicates 2SK30GR and 2SK30O in the parts list (unclear what goes where). Similar parts sometimes show up as MOSFETS in the datasheets but the schematic in the service manual clearly uses a JFET symbol. Urekar used BF245A which is a JFET. I used sockets and tried 2SK30A (from AE, supposedly JFETs) and they worked fine (note the reverse pinout of 2SK30 compared to the BS170 though). I noticed that sometimes at the extremes of the skew/symmetry control I get a drift in the frequency. It’s kinda finicky as you need to twist the knob a little to get it back right, but it doesn’t bother me. Also, I found that the triangular output may need to be boosted to become more useful, so one might try to configure the added buffer at the triangular output as an amplifier.
- EFM’s Dual Voltage Processor by Chris MacDonald, loosely based on the Buchla 257e, with subsequent modifications by Peter Grenader and Matthias Hermann. I also implemented the mods suggested by @EddyBergman on the PCB contributed by Daniel Karlson (as listed the project page linked above). I noticed some discrepancies between the schematic and the PCB silkscreen after I started populating the PCB so I had to add a bodge wire among other things to correct my mistakes. Daniel quickly updated the documentation in the PCB files, so no other person will go through my careless mistakes. I haven’t used this much, but I am sure I will as it has several useful features. A couple of extra boards are up for trade.
- Doepfer’s A190-2 MIDI interface. I originally intended to build a MIDI to gate/CV interface using an Arduino, and got the parts for it, before I found this module for sale locally. The price was right because of a broken potentiometer, hence the change of plans. I replaced the potentiometer and “Kosmofied” it behind a bigger panel, mirroring the outputs to 6.35mm jacks. For the CV and Gate outputs, however, I added an @analogoutput buffered multiples stripboard to take advantage of the panel real estate and get two additional outputs for each. By the way, does anyone know how to switch retrigger on? I am not a big fan of playing legato on a synth. I tried through program change 6 via my MIDI keyboard but no luck. Obviously, I am doing something wrong!
- Michael Barton’s BMC96 Rectifier/Recombiner. A niche but very interesting module that allows you to combine AC and DC signals, rectify and split the positive and negative parts, send them for external processing and recombine them. I built this on my compact stripboard layout style but I made a few careless mistakes which necessitated the addition of a couple of bodge wires. If anyone’s interested in the corrected (bodge wires added) layout contact me privately. Michael Barton has a PCB for sale which works nicely with the Kosmo format.
Naturally, there were also a few (half?) failures along the way:
- Kassutronics’ ASR Envelope on stripboard. My layout was so compact I was almost certain it had mistakes, so once I saw it was not working, I abandoned it without hesitation, especially after having completed the Precision ADSR.
- Skulls & Circuit’s VCF-3 using this verified layout. See the dedicated thread here for troubleshooting this module since at least one other person had issues with this.
- Rene Schmitz’s Sample and Hold, replacing the JFET with two 2N7000 MOSFETS as suggested by Trevor van Ginkel. The circuit worked fine on breadboard and in the modular, but at some point, for some reason, it stopped working. I was already planning to replace it with a proper JFET Sample and Hold featuring the YASH clock by Schmitz since the clock module that I used was giving me problems (see below), so I did not make any effort to troubleshoot it. I might come back to it later, try to fix it and repurpose it in some other project.
- Circuit Abbey’s Tick/Tock Clock (one of the first modules that I’ve built on stripboard) was putting out an annoying audible ticking. I tried to solve this by replacing the NE555 with a CMOS one, and by adding filtering capacitors in the module that was the most affected, but when I realized that the ticking extended to other analog synths outside the modular, I had to pull it out. Not that I expected much from a simple 555 timer circuit… This one will be scrapped for parts.
Upcoming projects: The tube/valve VCA that I have on the breadboard and a VCO.