SERGE BY RANDOM SOURCE
The modular synth circuits designed by the German company Random*Source, channeling the genius of Serge Tcherepnin and Juergen Haible among others, are among the finest to be found in modern synthesizers, despite being relatively old designs in many cases. It has been a pleasure to lend my iron in order to bring these great modules to the US:
Serge Resonant Equalizer
A veritable feast of nice components, the Resonant Equalizer is an absolutely mental module which resonates or attenuates fixed frequency bands ranging from ultra-low to a piercing 11k. Earlier builds used OPA2604s but more recent ones have OPA2134s. The Feedback knob on Random*Source’s version is a huge upgrade over earlier versions, and I daresay with this circuit it’s one of the harshest-sounding analog modules I’ve ever heard. (In a good way!)
Serge Variable Q VCF
Even among the plethora of other classic Serge modules, the VCFQ stands out for its incredible versatility and amazing sound, still unlike any other filter design. Random Source updated the circuit for use with the awesome THAT2180 pre-trimmed VCA instead of the rather difficult Serge gain cells (with Serge’s blessing), and the results speak for themselves.
My personal favorite use is as triggered percussion via the Trig In and LP or BP output, often with a nice sine FM input, but hey that’s just me — there’s really not much you can do wrong with a VCFQ. Warning: you might end up wanting two!!
Serge Variable Slope VCF (VCFS)
A lot of people probably think of the VCFS as an afterthought compared to the colossal VCFQ, but that’s what a fool believes — its morphing Slope feature is magical, particularly in the ultra-throaty Bandpass mode, and self-patched inversion using some sort of CV processor produces a really fat sine VCO as well. Like Random Source’s VCFQ, this VCFS uses THAT2180s and OPA2134s, a perfect fusion of old and new.
HAIBLE BY RANDOM SOURCE
Random Source Haible Krautrock Phaser 
Haible Krautrock Phaser w/ Van Daal Electronics enclosure
Record snobs everywhere know that there was strange magic afoot in the control rooms of 1970s West Germany recording studios, as the demands of popular artists such as Kraftwerk, Tangerine Dream, and Cluster with funds to employ real live engineers resulted in many sonic innovations which are now taken for granted. Anyway, one of the most distinctive sounds on these records is that crazy phaser–it’s everywhere, and it always sounds great!! Well, “that crazy phaser” is the Gerd Schulte Compact Phasing “A,” now a holy grail piece of kit.
A cursory Google of an OG Compact Phasing “A” demonstrates that it is not quite the sophisticated unit that musicians from the 90s onward have come to expect. This hot mess of a circuit uses real incandescent bulbs to control light-dependent resistors, which in turn control the filters which make up the phaser, and the original wasn’t grounded, didn’t have much in the way of heat management, and a few other issues. Thankfully, we were for a time graced with Juergen Haible’s electronic genius, and in 2007 he reverse-engineered the Schulte and made some PCBs featuring notable improvements which are documented here: http://jhaible.com/legacy/compact_clone/. Mr. Haible is of course no longer with us, but thankfully our friends at Random Source will continue to manufacture the PCBs.
Anyway, one of the main catalysts for this project was the availability of the lovely Van Daal Compact Phasing “X” case (https://vandaal-electronics.com/product/compact-phasing-x-case/). It took a while to get the various parts together, but in the end it was worth it!!
Haible Living VCOs
An incredible module indeed from the legendary Juergen Haible — very fat and stable ultra-analog VCOs packaged as a powerful triumvirate with a great master 1V/O control section with octave up/down, vibrato, portamento, along with linear detune for each oscillator and four waveform outputs per oscillator bursting with character and harmonic content.
The secret with patching these VCOs really comes down to the effectiveness of the master control section. By freely routing 1V/octave (which can have portamento and vibrato involved as well) to each of the oscillators at the flip of a switch, nice FM ratios can be roughly maintained over a full range. I’ve had one myself for a few months now and feel I’m still scratching the surface.
Haible 
Haible Dual Wasp Filter
One of my first Euro modules was the Doepfer A-124 Wasp VCF, a surprisingly versatile but ultimately very nasty and acidic filter which abuses the CD4069 hex inverter IC to attain the gnarly sound that enchanted William Bennett, among others. The original EDP Wasp is an amazingly cool synth now available in DIY form as the Jasper, and the Doepfer module still remains in production, a great starter module for sure given its character and great price.
I still have my Doepfer module, but the R*S Haible Dual Wasp honestly blows it out of the water. To start, it’s two discrete filters with cool CV sync and mode crossfading, both within each filter and over series/parallel routing between the two. Then there’s the sound, which still has that Wasp nastiness but adds a lovely overdrive/distortion circuit great for beefing things up.
As of March 2019 I’ve built roughly eight of these monsters, and am always looking to build more!
Haible Tau Phaser (4U)
I had already built a couple of the Eurorack version of the Tau. With these sadly out of print for the time being, I set my eye on building Random Source’s lovely 4U module. The original circuit was designed for 15V, but as many are adopting big 4U Random Source systems, I made it for 12V to be fully compatible.
This was not without its issues, despite Random Source’s documentation. Though there are allusions to RS1’s value being 10k5, a link is necessary regardless of supply voltage. As two stages are to be shorted for 12V use, two matched transistor pairs on my build are likely there for decoration. Still, this one works a treat, and unlike the Eurorack version, allows for the boosted resonance mod.
SERGE MODULAR IN 4U
Finally, some Serge Modular as it was always intended–nice spacious 4U panels and banana connectors for instant patching fun. Complete panels of SERGE are a challenge, but in the end it’s always worth it.
73-75 Serge Modular Homebuilt System
The master Jon Nensen of The Human Comparator/zthee fame has graced us with some lovely full panels of early Serge circuits. (All for sale here: http://www.73-75.com/) Among them is the all-through-hole Homebuilt two-panel system. By Serge’s standards it’s relatively crude, but it’s important to learn about his pioneering modules and the sounds are still tremendous, if a little rough around the edges!
Miscellaneous Eurorack
Meng Qi Voltage Memory
Nice arduino-based touch keyboard-type module from Chinese synth lord Meng Qi. Each touchplate can be used to recall six simultaneous pre-programmed voltage along with a gate output when touched, a unique feature set which can create amazing changes in a patch or really trick out a CV-hungry module like a Noise Engineering Basimilus Iteritas Alter or Mutable Instruments Rings. This was my first foray into Surface Mount soldering.
The Human Comparator Syncussion SY-1
The Pearl Syncussion is a legendary two-channel electronic drum module released during the Disco era which contains an amazing array of awesome percussion sounds within its easy-to-grasp interface. Its 6 oscillator modes, selectable by rotary knob, allow a user to emulate bells, toms, snares, claps, and more (triggerable manually or via analog trigger input) or totally space out using the onboard LFO and S&H modulation. Though original units are hard to find, they are still heavily in use to this day due to their obvious usefulness for techno, electro, etc.
In 2016, The Human Comparator released an instantly popular, near-identical clone of the original Pearl unit, thus making these legendary sounds available to the masses once again! It’s a truly old-school build, with big beefy circuit boards and BOMs typically still call for nasty old 5% tolerance carbon film resistors (I definitely used 1%s for a lot of things here). Also, THC’s version (as opposed to the more “modernized” PsyCoX SY-1m) calls for manually-matched NPN and PNP transistor pairs, a very tedious process even when using the awesome Ian Fritz transistor matching method (look it up!).
Overall, it is a very satisfying machine. The sounds are just great for cutting through a mix as an electronic kick, snare, hi-hat, crazy space thing, or bell. It has a really easy control interface, to the point where it’s not hard to dial in some approximation of a previous sound even though there’s no patch recall.
Dinsync RE-303
The blog documenting my ongoing Dinsync RE-303 project and modifications can be found here: https://jackschoonover.com/re-303-modifications/