Eurorack electric guitar preamplifier

BACKGROUND

I am designing and building a series of Eurorack modules for interfacing modular analog synthesizers. As I am not a musician, I am more interested in learning than in actual sound quality. Which does not mean that the designs are of poor audio quality, but you might find 12-bit analog processing and, definitively, you will not find tube valve amplifiers. You may check my other Eurorack module projects.

As these projects are really quite basic, I believe they might be a good starting point in the electronic music and audio electronics. I have found a good reference for designing some circuits for the projects in the following books:

• Ray Wilson (2013) "Make: Analog Synthesizers: Make Electronic Sounds the Synth-DIY Way", Make Media, Sebastopol CA
• Nicolas Collins (2020) "Handmade Electronic Music: The Art of Hardware Hacking", 3rd Edition, Routledge, New York

ELECTRONICS

The goal of this module is to have a simple preamplifier for boosting the output of custom made Eurorack modules with low output gain. In particular I use this preamp circuit for boosting the output of an Arduino based sound effects processor, with an output range of [-1.65V... +1.65V].

I have made three different versions of the electronics:

• A simple protoboard based circuit (schematics v0.0), to test the basic preamp
• A PCB prototype (schematics v1.0), to test the overall circuit, which showed some flaws
• A final PCB board (schematics v2.0), which is the one discussed in this section, and whose schematics and bill-of-materials can be found in the attachments section.

WARNING: although the version discussed here is v2.0, in the making-off section there are pictures of the other versions, which use a different component numbering.

The circuit is based on three opamp stages. The first one is a generic non inverting buffer with a gain of 2. The second stage is an inverting opamp for the tone adjustment, which includes two 10k ohm potentiometers for adjusting the bass and treble gain levels, and a defeat switch which bypasses the tone control settings. The third stage is an inverting opamp for overall output gain control, with a maximum gain of 4.4, which is adjusted with a 100k ohm potentiometer. With the two inverting opamps in cascade, the signal is returned to its original phase. I use the NE5532 low noise opamp for the three stages. They can be substituted by the LM4562, which may work a little bit better.

The output of the preamplifier is routed to both audio output jacks and to a voltage clipping indicator. This clipping indicator uses an opamp in a window comparator configuration, and uses two voltage references -Vref and +Vref. When the output of the amplifier says inside the voltage window [-Vref... +Vref], the green LED is lit. Otherwise, the red LED is lit. I use the LM358 opamp for the window comparator. It does not pay off to use a more expensive one, but anyone similar will do.

This indicator is handy in order to avoid output signals that are too "high" for other modules. Depending on your Eurorack setup you may want to change the default value of 20k ohm of resistor R20, which sets the Vref value. In my setup I use a more common 22k resistor, which stays on the safe side with Vref=6.3V for a total output of 12.6V peak-to-peak. As a guide you can use the following resistor values:

R20 |  Vref
-----+-------
 47k |  8.4V
 30k |  7.2V
 20k |  6.0V
 10k |  4.0V

WARNING: it is recommended to use 1% tolerance resistors, because the values of some of them are required to be very close, for instance all the resistors in the opamps.

CONSTRUCTION

I first started making a prototype with a protoboard v0.0, with a minimum of functionalities: just the two stages of the amplifier (I added a third socket, which I did not finally use). You can find the image below.

It worked reasonably well, and after some hours of use with my synthesizer setup, I decided to climb up another step of the ladder and go for a professional-like PCB prototype v1.0. With not that many components, I used KiCad and manual routing. I decided to use JST-XH (5 pins, which I only use 3 of them) and JST-PH (2 pins) connectors for wiring the panel-mounted elements (jacks, pots, etc), just because I use them for my day work projects and had them at hand. I produced a functional board, as shown below.

1/4W resistors are fine. I used 1W resistors for R17, R4 and R5 because I had not 1/4W at hand. I made a BIG mistake with the v1.0 schematics (I swapped the + and - inputs of the U2 and U3 op amps, the perils of cut&paste), so I had to scratch a few traces and re-route with thin wires. You can find a picture below, with one scratch easily visible halfway between D1 and R20, and other between U2 and R4. The annexed v2.0 schematics have been corrected, and I have also included some additional capacitors at the power input side.

I cut a 3mm acrylic plastic sheet to fit into a 3U/14HP shape (133.4 x 70.8 mm), and then drilled holes for the 3.5mm female jack connectors (one for signal input the other for signal output), a 6mm female jack connector (a duplicated output), the potentiometers, LEDs and switch, and the 3mm fitting screws. I made the cuts and drills by hand. I should CNC them, and put it in the TODO list. You can find the results below, with a 3.5mm to 6mm jack adapter:

I finally produced a definitive PCB board v2.0, the one attached to the project). with all the mistakes corrected and with only JST-PH (2 and 3 pins) connectors to save some precious space in the board. You can find below a 3D mock-up of the final board.

TESTING

I have made a short video of the preamp. The base sound is "Popcorn" (Gershon Kingsley) played by an Arturia MiniBrute 2S. The preamp is located in an Arturia RackBrute 3U, together with different modules. Preamp input is fed with the output of a digital reverb from a custom Arduino. Enjoy the result.