Eurorack - Input Output

If you're looking for a way to send and receive audio signals, then the In-Out Eurorack module might be just what you need. With its ability to convert audio signals from line standard to Eurorack standard, this module can be used to add external effects and devices to your modular setup, or to simulate a waveform generator by plugging in a Walkman broadcasting the desired waveform. This module is inspired by the one proposed by Haraldswerk.de.

Specifications

• Attenuates one Eurorack signal to line level
• Amplifies the input signal to Eurorack level
• Amplitude control for input and output
• Runs on +/-15V and +/-12V without changes
• Power consumption around 10mA each rail

Prototyping

To ensure correct dimensions and design properties, the front panel was designed in Kicad. The white circles in the second image show the position of the knobs and nuts for the various jacks used in this design. After validating the position of the interfaces, the panel graphics were developed using InkScape, with the borders of the draft fixed using a DXF file from freeCAD.

To speed up the process of developing the panel ergonomics, a 3D printed panel model was created with one layer of height only to save money. Note that Alpha type potentiometers are typically used to stiffen the structure by adding mechanical support, but these are not shown in the images.

The front panel PCB with the graphics applied on the silkscreen can be seen in the following picture.

The Circuit

The outgoing signal is attenuated with U1B, with RV1 setting the output signal level. The incoming signal is amplified with U1D, with RV2 setting the input signal level.

I also used LTSpice to simulate the analog circuit in order to choose component values and requirements while saving money and time.

The simulation shows the attenuation between the +/-10V entry and the line output, as well as the attenuation due to the R3 resistor/potentiometer.

As this module is for audio use, the minimum requirement is to have a flat curve between 20 Hz and 20k Hz, and the range of use is larger. Note that the TL071cp OpAmp is not in the LTSpice library, so the ideal OpAmp model was used instead, which may cause deformation of the frequency response of the module, but not on the audio range.

I used KiCad to design both the circuit and the PCB layout. After entering the circuit footprint and other necessary details in KiCad, I arranged the components to fit the desired design as shown below. Finally, I produced the gerber files to proceed with manufacturing the PCB.

Conclusion

In conclusion, designing and building a Eurorack module requires careful planning, attention to detail, and expertise in circuit design and PCB layout. Through the use of software tools such as KiCad and LTSpice, it is possible to simulate and validate the design before manufacturing the actual module. Additionally, 3D printing can be used to validate the panel ergonomics and ensure proper fitting of the components.