4 Channels Audio/CV Mixers for Eurorack Synthesizers

This follows a previous mixer project I rolled out some time ago for my DIY eurorack synthesizer. After years of use and some time spent on the theory, I ended up seeing more clearly the limits of that project and matured some awareness on mixers.

Here I wanna share with you what I have learnt.

Someone once said: "No single mixer module can do everything well".

That Someone continues: "You could go nuts trying to make one design do it all".

Absolutely true words, in my experience: If you try to realize a "one-fit-all" mixer, you will end up with something with flaws.

Mixer design cannot lie out of it's final use. Will the mixer sum audio signal or control voltage signal? Is that audio signal "hot", like the eurorack standard calls for, or "line" level?

In this instructable I will show you my new set of DIY mixers for eurorack synthesizers.

I realized a simpler one, for audio signal processing, and one for CV signals processing.

I will describe the approach used and technical solutions adopted.

I will share with you Gerbers to have modules realized by pro manufacturers and help you have two fundamental pieces of a synthesizer at the smallest price possible.

Yes, you will have a lot of fun in the process, so give DIY a try! ;)

We are dealing with two mixer modules here, designed with very different applications in mind: one for audio, the other for control voltages.

The audio mixer circuit is simpler, use a small amount of components and I was able to lay the whole circuit down onto a single PCB (main board).

The full audio mixer module calls for the aforementioned main board and a front panel aluminum PCB with no components at all.

The CV mixer module is instead made of three PCBs: main board (the one with main circuitry), front board (the one hosting potentiometers and jacks) and an alumium front panel.

Again, the CV mixer module front panel has no components.

AUDIO MIXER BOM

Main board

Integrated Circuits, diodes, transistors

1x Dual Op-Amp TL072

1x Atmel ATtiny85 microcontroller

Resistors, potentiometers, trimmers

1x 270 Ohm resistor

1x 1 KOhm resistor

1x 27 KOhm resistor

7x 100 KOhm resistor

1x 10 KOhm trimmer (B25P)

1x 50 KOhm trimmer (B25P)

4x 50 KOhm potentiometer, logaritmic

Capacitors

2x 47 pF ceramic capacitor

4x 100 nF ceramic capacitor

4x 4.7 uF electrolitic capacitor

3x 10 uF electrolitic capacitor

Others

1x IDE connector header 8x2

5X mono jack connector, 3.5 mm female (PJ301M)

1x SPDT ON-ON switch (MTS302)

CV MIXER BOM

Main board

Integrated Circuits, diodes, transistors

1x Dual Op-Amp TL072

1x Quad Op-Amp TL074

Resistors, potentiometers, trimmers

2x 1 KOhm resistor

15x 100 KOhm resistor

4x 1 MOhm resistor

Capacitors

6x 47 pF ceramic capacitor

2x 100 nF ceramic capacitor

2x 10 uF electrolitic capacitor

Others

1x IDE connector header 5x2

2x female header connector, 8 pin

1x female header connector, 6 pin

Front board

Resistors, potentiometers, trimmers

1x 1 KOhm trimmer (B25P)

4x 50 KOhm potentiometer, linear (WH148)

Others

2x male header connector, 8 pin

1x male header connector, 6 pin

6X mono jack connector, 3.5 mm female (PJ301M)

You are also in the need for a soldering station, some solder wire and some spare time to assemble the module of your interest.

Please notice that in the audio mixer it is best to use logaritmic potentiometers because of the pitch logaritmic tracking of human ears. The CV mixer, intead, works better with linear pots.

Eurorack synthesizers are modular systems renowned for their flexibility and deep sound design capabilities. Mixers are among the essential modules, and play a critical role in combining and routing signals.

Broadly speaking, mixers can be divided into two categories: audio mixers and control voltage mixers. Though their names suggest similar functions, they serve very distinct purposes in modular synthesis.

Audio mixers

Audio mixers are designed specifically for combining audio signals. These signals are typically in the audible frequency range (20 Hz to 20 kHz) and are generated by oscillators, samplers, and other audio-producing sources.

Audio mixers are commonly used to combine multiple oscillators to create thick, harmonically rich sounds, sum signals processed through different effects chains, modify the audio level and balance for the final output, and so on.

In eurorack world audio signals are HOT (up to 10Vpp) and need some attenuation before hitting external hardware. Just to cite a very applicable example: "line" level, the most common input level for main mixers, is 10 times lower than eurorack.

The need for lowering the Eurorack audio level, sooner or later, makes not that important if the modular mixer "accidentally" modifies the incoming signal amplitude, in example adopting the cheap strategy of series current limiting resistors for protection pourpouses.

To our ears, a change in amplitude makes it quieter, but doesn't change the timbre or other quality aspects of the audio signal.

Another aspect strictly related to audio, underlies in the human auditory system. This responds logarithmically to changes in sound intensity, this means that potentiometers to be used in audio mixers should be logaritmic (often indicated as "A") for a "correct" behaviour.

CV Mixers

Control voltage mixers are intended to mix modulation sources signals. These are not audible sounds, but rather signals to control module's parameters such as pitch, filter cutoff, amplitude, etc.

One of the most common use case is the mixing of expression voltages like aftertouch, velocity and keyboard tracking, but also LFOs, envelopes, and other modulation sources to create complex CV patterns.

These mixers need precision and stability over output voltage, and are more complex to design than audio processing mixers.

Precision and stability requirements are especially necessary for VCOs pitch control, being the most demanding application in synthesizers.

With such a target application in mind, common simple solutions used in audio mixers are no longer applicable.

Just to cite an example, using series current limiting resistors for output protection could be ok if the output is used for modulation of a filter opening or an oscillator PWM duty cycle, but definitely not to control a VCO pitch over a wide octaves range. This is because of the voltage drop that resistor could cause.

With all these info in your bag, you should now be able to select the right mixer for your task with a little more awarness than before :)

This project follows a previous 4 channels mixer module. The time had come to focus it more to its final application (audio mix) and remove some unecessary feature.

Improvements

First: no variable gain. Eurorack oscillators signal is HOT and needs attenuation before hitting external hardware. Going higher than unity gain in the final stage makes no sense, so we are allowed to save some good amount of panel space by omitting a dedicated gain potentiometer.

Second: no inverted output. To human hears, direct and inverted audio signals are perceived the same. The most basic audio mixer could use a single op-amp in inverting configuration and you won't notice.

Built-In Reference Tone

A feature I always use with my analog oscillators, the built-in digital reference tone for tuning is something I could not live without.

This feature was already present in the previous mixer version, but it is here upgraded by the addiction of a direct control over the pitch and an RC filter in it's output stage with a roll-off frequency of just under 6kHz.

The sketch I have written for the reference tone is also brand new, with a direct square wave generation instead of the adoption of the basic "tune" Arduino function. This made the wave's tuning control way more effective, without the audio glitches the function was generating when moving between semitones.

Reference tone's pitch and volume can be set directly from the front panel through two dedicated potentiometers. These are two trimmers firmly soldered on the PCB, but made acessible through two holes on the front panel.

Circuit

This new four (monaural) channel audio mixer module's circuit is based on a very simple but effective design by Doepfer.

The circuit is built around a single dual op-amp in single package. I used common TL072, but one could use any other, pin-compatile op-amp of choice.

A human ear characteristic is it not being sensible to audio DC bias. Anyway, when two (or more) waves are mixed, the resultant wave is actually affected by relative bias. This is why every input of this mixer has an AC coupling capacitor following its voltage divider.

If the audio source is well known to be already AC coupled, one could omit these and replace them with jumpers.

>>HERE<< is a Falstad's circuit simulation of this mixer.

Module Design

The mixer circuit is made of only few components, and I dedicated due efforts to lay the circuit down onto a single PCB (plus the front panel) instead of two stacked PCBs.

All components values are silkscreened on the PCB to make assembly easier (I hate reference sheets).

There are no exhotic components, nor special care is in the need in components selection with the exception of potentiometers.

In the first iteration of the mixer I used linear pots because cheaper and easier to find than logaritmic. Human perception of volume follows a logaritmic law, instead, so in this more "focused to audio" mixer module I had to adopt logaritmic potentiometers by default.

Potentiometers are wired in voltage divider configuration, so even if the silkscreen shows 50K ohm potentiometers for input level, but you could use 100K ohm pots in case you have those laying around.

The module calls for +12V, -12V and +5V. The 5V line is in the need for the digital tone generator, so you could omit it in case the reference tone is of no use for you.

It's 3U tall, 8HP wide and made of only two boards instead of three (front panel and main board).

Not bad, if you ask me!

CV Mixer Challenges

When dealing with control voltages, you often don't actually need a perfect tracking of the incoming voltage. Think i.e. to the common CVs from your keyboard (velocity, tracking, aftertouch): they could be lowened to some extent at mixer out and you won't even notice.

Different is the case when you want to mix voltages for pitch control. A classical example is the application of some vibrato, where you add a certan amount of "oscillation" to your pitch voltage.

Human perception of pitch is extremely accurate and requires closely matched and/or trimmable components, so designing such a mixer calls for some special precautions.

Circuit

First of all, we adopt a buffering stage at the input of any of the 4 channels as to not load down the output of whatever module is carrying the pitch.

The buffer is here made of a quad op-amp (TL074) in voltage follower configuration.

The buffered signals from each input are then fed to an attenuation stage before hitting the mixing stage of the circuit.

Attenuation is simply made with linear potentiometers in voltage divider configuration.

The mixing stage is similar to the audio mixer's one we have seen in the previous Step, but with "pure" unity gain in sight.

A gain trimmer is a good way to compensate for losses from the previous circuit sectors. This is placed in series with the appropriate feedback resistor dimensioned for a teoretical unity gain.

I adopted a multi-turn trimmer for fine gain control, which had the welcome side-effect of keeping the module width way smaller than a potentiometer would allow.

The use of 1K ohm current limiting resistors at the direct and inverted output placed inside the feedback loop is a further care to limit the voltage drop at the output stage.

The 1K Ohm resistors serve not only as short-circuit protection for op-amps (every time you patch, the tip of the mixer output is shorted to the grounded sleeve of a jack), but also protects it from static charge damage, for example when a cable is inserted one side only.

This module calls for +12V and -12V. It's 3U tall and 8HP wide, like the previous mixer.

I layed down a Falstad simulation of the circuit you can toy with >>HERE<<.

To turn an ATtiny microcontroller into a "perfectly steady" digital tone generator to be used in the audio mixer module, we need to first program it.

The ATtiny85 microcontroller cannot be directly programmed with the Arduino IDE like a prototype board. It requires an intermediate board (such as an Arduino UNO) to act as an ISP (In-Circuit Serial Programmer).

Don't worry: it's a straightforward process if you follow these instructions carefully!

1: Turn Your Arduino UNO into a Programmer

The first step is to configure your Arduino UNO to work as an ISP:

1. Open the Arduino IDE.
2. Connect your Arduino UNO to your PC.
3. Go to File -> Examples -> ArduinoISP and open the ArduinoISP.ino example.
4. Upload the ArduinoISP sketch to your Arduino UNO.

2: Install the ATtiny Core

There's no more need to install ATtiny Core (by Spence Konde) on Arduino IDE v2 since it is already there. Give it a check, anyway.

3: Connect the Arduino UNO and ATtiny85

Now it's time to wire your Arduino UNO to the ATtiny85. Use a breadboard and it's cables for this, or create a little permanent circuit on perfboard if you prefer.

Here follows the wiring to program your ATtiny on breadboard:

(please notice that UNO pinout refers to silk screened pin numbers, while ATtiny pinout refers to package pin numbers)

Connections:

Arduino UNO → ATtiny85

5V → Pin 8 (Vcc)

GND → Pin 4 (Gnd)

D13 → Pin 7 (SCK)

D12 → Pin 6 (MISO)

D11 → Pin 5 (MOSI)

D10 → Pin 1 (Reset)

4: Upload the Sketch

Select the appropriate microcontroller and clock source:

1. Go to Tools -> Board and select "AttinyCore -> ATtiny45/85 Optiboot".
2. Under Tools -> Clock Source, select "8MHz Internal" (this should already be set by default, but double-check).

Open the sketch you want to upload.

Now, go to Tools -> Programmer and select "Arduino as ISP".

Burn the bootloader:

1. Go to Tools -> Burn Bootloader.

Upload the sketch:

1. Select Sketch -> Upload Using Programmer.

Note: Burn the bootloader each time you upload a new sketch.

Many thanks to the nice girls and guys at JLCPCB for sponsoring PCBs manufacturing for these mixer modules.

Without their contribution this project would have never reached the current level of development, like many others projects of mine now.

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By registering at JLCPCB site via THIS LINK (affiliated link) you will receive a series of coupons for your orders. Registering costs nothing, so it could be the right opportunity to give their service a due try ;)

All Gerber files and sketches I realized for this project are stored >>HERE<< (Github). The repository hosts both my previous mixer ("VERSION_1_(OLD)") and the new versions ("VERSION_2").

My projects are free and for everybody. You are anyway welcome if you want to donate some change to help me cover components costs and push the development of new projects (I have a new one on a nice sub-oscillator module that could be interesting for some of you... :) )

>>HERE<< is my paypal donation page, just in case ;)

I don't make modules to make money: I collect pennies to make modules ;)