4 Channels Mixer Module for Modular Synthesizers

Having a nice sounding voltage controlled oscillator (being it digital or analog) is a fundamental point in a modular synthesizer. What is better than one VCO? A multitude of them obviously! :)

Whether you want to merge the audio signals coming from multiple oscillators or to blend different waveforms from the same multi-wave oscillator (who said CEM3340?), you are in the need for a mixer.

In this instructable I will show you how to realize a simple but effective four channels, mono mixer module. I will also share with you gerber files to have both the circuit board and panel board manufactured at the lowest prices out there.

Let's go!

EDIT: a new, revised and improved version of this mixer can be found HERE (Instructables).

Other Instructables in this serie:

Arduino Wavetable Voltage Controlled Oscillator

Discrete Voltage Controlled Amplifier

Arduino ADSR Digital Envelope Generator

Voltage Controlled Multimode Resonant Filter

Variable Waveshape Low Frequency Oscillator

DIY Linear Regulated Eurorack Power Supply (and Power Bus Bar)

Fiberglass Panels for DIY Modular Systems

The four channel mixer I layed down is based on a simple design you can easily find on the net. Doepfer DIY page is always a good resource to start from!

The four input level potentiometers form a passive mixer. The output of the first amplifier is the sum of the four inputs divided by four.

Both amps are configured as inverting. The first one has a variable gain, for an overall amplification that is a function of the potentiometer value connected to "gain" headers (circa 5x with 500K ohm pot). This makes up the losses in the first stage passive mixer and gives room for some further amplification.

There are two outputs: the first one is inverting and is good for AC signals (i.e. audio signals); the second one inverts the first, so it's output is non-inverting with respect to the source and is good if you want your signal voltage to stay unchanged.

Even if the PCB silk-screen calls a TL072 op-amp, we are not limited to it: LM1458, TL082, NE5532... all these are pin-to-pin equivalent and could be used in place of the TL072.

All components values are silkscreened on the PCB to make assembly easier (a reference sheet is not for everyone :))

The silkscreen shows 50K ohm potentiometers for input level, but you can also use 100K ohm pots in case you have those laying around. Different is the case of the gain potentiometer: I made some testing with 1M ohm, but the level dropped suddently to zero over a certain threshold.

The module is made up two boards: the circuit board (described in the previous step) and the panel/face plate board.

Front Panel

I layed down two different panels for this projet: a 3U, 16HP blank panel and a 3U, 16HP pre-drilled and silkscreened panel. This is because I use 1/4" jack sockets in my synthesizer (Korg MS20 had a strong influence on my first decisions), but eurorack standard calls for 1/8" sockets and you probably should select the blank one. I have written a dedicated Instructables on fiberglass panels, so you may want to take a look at it ;)

All the blank 3U panels I have layed down can be downloaded from HERE (Github). You will find 4HP, 6HP, 8HP, 12HP and 16HP panels. In case you need a blank panel standard measure I have not already done, feel free to ask: I will add it when possible.

Mixer Main PCB

Gerber files for the mixer main PCB can be found HERE (github). You can have them manufactured at THIS link (JLCPCB). All circuit board components values are silkscreened on the PCB to make assembly easier.

The circuit board is intended to be mounted perpendicular to the front panel. If your case is not deep enought, you can lock the board to the case bottom and use wires to gain electrical connection with elements (potentiometers, buttons, jacks and so on) on the front panel.

For all the boards you want manufactured, download zip files from links above, press the "quote now" button on JLCPCB site, upload the zip package containing gerber files of the board of your choice, select a nice PCB color and save to your chart. Repeat the process for all the boards you want to have manufactured, complete the order and have them shipped to you.

Follows the list of components (BOM):

Capacitors
2x 100nF non polarized
2x 47pF non polarized (optional)
2x 10uF electrolitic

Resistors and Potentiometers
1x 50K ohm
2x 330 ohm
6x 100K ohm
4x 50K ohm potentiometer
1x 500K ohm potentiometer

Integrated Circuits
TL072 Op-Amp

Diodes
1x 1N4004

Others
1x IDC connector, 5x2 pins

A special thanks goes to JLCPCB who sponsored this work by manufacturing both the main PCB proptotype and fiberglass front plate you can see in the pics of this instructable. JLCPCB is a high-tech manufacturer specializing in the production of high-reliable and cost-effective PCBs. Their customer service is very good and PCBs are the best value for your money.

Please, let me stress out that this type of sponsorship is essential to share good quality and full working circuits and layouts. It allows testing a first prototype and then share a second, improved and corrected revision of the board. This mechanism has a very positive impact in the quality and reliability of shared projects.

Thank you!