News from the lab: Eurorack Synth!


Back in the 1970’s I was thinking of a career in electronics for music, e.g. amplifiers, tape decks, speakers and most of all synthesizers. My career took a very different path, but I have kept up with what’s going on in the world of synths ever since. When the trend got into digital technology (starting already then with the Synclavier and the Fairlight) I sort of lost interest, because I could see that “everything” soon would be possible, and that the creative work in synth development would mainly be the programming. It is not that there is anything wrong with programming, it is very creative in its own way, and I still write code for other purposes. Anyway, developing analog circuits is still something I really like to do, and ten years ago I learned that modular analog synths were coming back. I started designing a rather complicated bus structure, and made a rack in the 19″ Eurocard format, using the 64 pin standard connector. I made some VCO designs using the XR2206, made a prototype, but never finished any PCB or any other modules. Again I lost interest, and now it is clear to me that I started out with a too complicated design.

Three years ago Jonathan Reus told me about Eurorack and especially the Doepfer synths that gradually had become more and more popular. He pointed out that I could make modules using this format, and maybe even sell some small series. Another advantage would be that I could buy a few modules and make a faster start, test my own designs, an all in all show some quick results. I decided to give it a try, and put a small dual row rack together from some odd parts I had already. To me everything starts with the VCO, and preferably several! I have over the years made a lot of different designs, and for some time I was really trying to make “the ultimate VCO”. A fresh start on the VCO seemed to be the right thing for me to do now.

I think it was around 1990 that I came across the XR2206 and XR2209 VCO-IC’s. Especially the XR2206 has got several nice builtin features like FM, AM and sine shaping. I had a few ’06 in my drawers, and even though they are not available any more, I decided to make my first module with one of these, and here it is (very small picture since it is not completely finished yet 🙂 )   I packed a lot of features into it: Sine-, square-, triangle- or ramp-wave VCO/LFO with FM, AM, VCA, FSK, PSK, sum-inputs, three six-stage programmable envelopes (Arduino ProMini inside)…. maybe a bit too much, at least the front panel seems a bit crammed. The square window is a small I2C display showing the parameters of the envelopes that you set with the dials below. I 3D-printed the orange knobs on my Prusa. I will try printing complete module mechanics for future modules, and probably put some adhesive aluminum foil on the backside of front panels as shielding. I will put up the schematics here on my blog as soon as it is finished and working well.

As it has turned out, I am not too satisfied with the frequency stability and especially not the output gain linearity of the XR2206. Also, the different waveforms are not all present at the same time. The power supply setup that is necessary to make a good split-supply all-DC-design, with no capacitors in the signal path, is rather complex and takes too many trimpots to my taste. For those same reasons I decided to make a brand new Super VCO design, with an all-analog/no-trimmers design, eliminating almost all parameters that can create instability and nonlinearity. The “heart” of the module is now finished, and it works perfectly. I had prototype PCB’s made by Eurocircuits, and soon a couple of extra functions will be added as a separate board, together with module frontpanel, wiring, etc. Details will follow here on the blog, when everything is finished.