DIY Eurorack Modules

Eurorack is an excellent platform to experiment with electronics, especially if you happen to be heavily into synthesizers. You can make something cool and useful with only a drill, a soldering iron, and a few dollars worth of parts. Here are two of my most successful experiments:

Harmonic Percolator Clone

In researching some audio electronics stuff, I came across a youtube video about this bizarre distortion pedal. The circuit, designed by Intefax (not exactly a household name) in the 1970s, is rather unconventional for a two transistor fuzz. It embues any signal you feed it with a horrific trebly distortion, and as a result it was not especially popular in its day. Apparently this pedal was partially responsible for the aggressive, almost metallic guitar tone that is Big Black’s signature sound. I am a pretty big fan of Steve Albini’s music, and I figured this sort of fuzz would be great for the aggressive synth music I like to make. The schematic is really simple too.

Curiously, this circuit calls for both a silicon and a germanium transistor. Germanium transistors are one of those artifacts in analog electronics that supposedly have magical properties. They are also much more expensive than the now-ubiquitous silicon ones. Since I have the luxury, I decided to build this in such a way that I could swap out this transistor and hear if the difference is really worth it.

Listen for yourself:

Clean
Synth with Silicon Transistor
Synth with Germanium Transistor
Drums with Silicon Transistor
Drums with Germanium Transistor

Please excuse my obnoxious examples.

I think I can tell that there is a difference, but I'd be hard pressed to say which is 'better'. When I was a kid I had an old alarm clock with a red segmented display and fake wood panelling. It had the most horrible alarm tone you could imagine and I truly hated that clock. This experiment makes me think it might have had some germanium components.

Spring Reverb

Real electromechanical spring reverb has always been fascinating to me. It’s a pretty simple and common DIY electronics project, but I couldn’t find an easy eurorack version. I took this as an opportunity to design my first module completely from scratch.

The basic principle is this. An audio signal is fed to a transducer (a speaker for instance). Instead of vibrating the air, the transducer vibrates a spring. The spring is attached to a contact microphone, which turns the signal back into voltages. Because the spring is, well, springy, the signal bounces up and down the spring and decays slowly, giving it that familiar metallic reverb sound.

I wanted this thing to have a dry/wet knob, and it is pretty common for eurorack reverb modules to have a feedback input normalled to the output.

This is the exciter I used, which is basically just a speaker without the cone. Speakers require power amps, and I wasn’t interested in building yet another lm386 amp so I found these inexpensive ready-made ones on amazon.

I used a good old piezo disc for the contact mic. Piezo discs have really high output impedance, so my circuit would require a special preamp for handling that.

Here is the circuit I came up with:

U1A is the preamp for the piezo disk.

D1 and D2 are overvoltage protection.

R1 is for input impedance to handle the large output impedance of the piezo element.

U1C is a mixer to combine the input signal and the feedback signal before going to the exciter power amp.

C4 is a large filter capacitor for the exciter power amp.

Here's how the module sounds:

It seems to work! It's a little trashy, but when used sparingly I think that will work in my favor for the type of noisey techno I like to make. There's a lot I'd like to do to improve the module. I think enclosing the spring would probably make it sound better, plus maybe some padding to isolate the transducers. For V2 I will definitely be printing out a PCB. This circuit was beyond what is reasonable to solder onto perf board. Maybe I'll get some actual knobs, too.