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Eurorack Comparator Module

 A Eurorack Comparator module in 6HP.

Like the Analog Switch this module comes from my journey to create a noise gate with modules. Most of my attempts to make a monolithic noise gate have required some form of comparator to determine when the signal goes above (or below) the noise floor, so making it an independent module is likely to be useful.

The core of this module is a dual comparator LM393. The input signal is rectified by the op-amps that buffer the input so both inputs on the comparator are operating in the positive range but one of them is the negative half of the input and the other is the positive half. This creates a window from -LEVEL to +LEVEL where the comparator outputs GND and if the input goes above/below ±LEVEL the output is 10V (or 5V if desired).

I made several simulations of the circuit to improve idle noise issues and allow adequate adjustability of the desired level to detect. The latest simulation is here. There's a simple RC filter on the signal input with a19.4kHz cutoff frequency. This helps reject any non-audio signals some of which I found would trigger the comparator. A more robust filter could probably be created with the two op-amps used to invert/buffer the input, but I didn't feel like going down that rabbit hole.

The LEVEL the comparator checks against is voltage controllable or can be set with the knob. There are a few trim pots on the PCB to allow the output to be trimmed to the desired range(s). I like signals to be 0-10V, but 0-5V should also be possible. The logic outputs are both 0:+10V and make good inputs to the Analog Switch module or as gates to some other module.

The module started as usual on the breadboard.

Once satisfied with the design I had a PCB made and designed a front panel for the module.
Issues on v1 PCB:
  • Negative of the LM393 should be to GND like on the breadboard... cut the trace to -12V and short C8 to GND with a 0 Ohm.
Calibrating the output takes a few simple steps:
  1. Adjust the LEVEL knob to mid-range and provide an input signal that can trigger the comparator.
  2. Adjust the output voltage of the INV. OUT output to the desired 0-10V level with trim pot A (R_ADJ100).
  3. Adjust the output voltage of the OUT output to the desired 0-10V level with trim pot B (R_ADJ101).
Module Width: 6 HP
+12V Current: 12mA
-12V Current: 3mA

Design Files: vc_comparator