Custom Eurorack Synth modules

This is a collection of custom synthesizer modules designed around the Eurorack standard.

The goal is to design these from scratch; I can use textbook and datasheet example circuits, and I can look at other circuits for ideas and inspiration, but I’m not cloning an existing circuit but instead coming up with my own designs.

Some of these modules are emulated in Max MSP to preview how the circuits will sound.

Modules

This is a work in progress; different modules are at different stages of development.

Utilities

In the process of building these modules, I’ve also built some tools to assist.

• Headphone breakout board
• Breadboard supply adapter
• Transistor matcher

Source

This project is hosted on Github. The repository goes back quite a long way, evolving out of coursework from GT ECE 4893 Analog Circuits for Music Synthesis with Dr. Lanterman.

In the spirit of the open-design movement, all source code is licensed under the permissive MIT license, and circuit designs are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike license.

Eurorack

Eurorack is a loose standard for small modular synthesizers. It uses:

• 10 or 16-pin ribbon power: +/-12V, 5V digital. Pinout
  • 
• 3.5” mono patch cables
• 3U tall rack mount, split into 5.08mm horizontal units (HP)
• Audio signals are typically a maximum of 10V peak-to-peak (i.e. between -5V and +5V)
• Control voltages can either be unipolar or bipolar. Bipolar control voltages are typically 5V peak-to-peak (i.e. from -2.5V to +2.5V), unipolar voltages between 0V and 8V. The V/Octave scale is used for pitch information
• Trigger, Gate or Clock signals are digital 0V-5V pulses typically used for timing and event signaling

In addition, I’m targeting:

• minimum 100K input impedance
• maximum 1K output impedance
• connecting rail voltages to signal inputs should not fry any components (although it doesn’t have to perform correctly)
• “semi-semi-modular”
  • modules are fully modular and independent, but have additional connections on the back for connecting other modules to default inputs/outputs if a jack is unplugged. This provides the benefits of both semi- and fully-modular
• Try to keep jack placement consistent, i.e. jacks on top with knobs on bottom

Common Design Components

• TL07x
  • Basic audio-quality op-amp
  • single, dual, or quad
  • note: not rail-to-rail, and asymmetrical clipping
  • slew rate minimum 5 V/us, typical 13 V/us
  • could be trivially substituted for any other general-purpose op-amp
• LM13700
  • dual operational transconductance amplifier
    • effectively an op-amp with control of the output current
    • could also think of it as a current-controlled resistor
    • makes voltage control easy
  • sensitive inputs:
    • I_abc is limited to 2mA
    • differential input voltage is only linear at +/- 20mV
      • using linearizing diodes, linear region can be extended to +/- 60mV
    • differential input voltage is limited to 5V
  • treat amp bias input as V- + 1.2V for the purpose of calculating I_abc
  • 50 V/us slew rate
• DG403
  • dual SPDT analog switch
  • very generous on control ranges and input limits
  • switching time in 100ns range
  • on resistance in the 45 ohm range
  • Unfortunately no DIP package - SOIC is largest size (10mm long)
  • preferred over CD4066
    • CD4066 has max 20V supply differential, which makes it frustrating to use with +/-12V supply
    • when off, CD4066 connects the pins to the minus supply voltage, which makes it impractical in many situations
• Tempco resistors
  • resistance varies with temperature. Useful for canceling out temperature dependance
  • not strictly required - classic synths would take some time to “heat up”
• 2N3904 / 2N3906
  • General-purpose BJTs
• DMMT5401 (NPN) and DMMT5551 (PNP) matched pair transistors
  • similar characteristics to 2N3904, 2N3906
  • useful for current mirrors and exponential converters
  • easier than trying to match transistors by hand (but here’s a guide for matching)
  • Unfortunately only come in SOT26 (3mm long)
• 2N2222
  • General-purpose Diodes
• Alpha 9mm pots
• Thonkiconn 3.5mm jack