Patch schematic questions

Hardware Development
1

Hi, I’m planning on designing my own pcb for eurorack modules, a bit like the Patch but with a few modifications. e.g. Having the daisy on the back not the front, no extra audio codec, no oled screen, more CVs.

Looking at the Patch schematics, I had a question regarding the op amps used for the audio input and output.

Each input and output has uses 2 op-amps, one is obviously used for giving a 1/10x gain to convert eurorack levels to something the audio codec can handle and vice versa. Does anyone know what the purpose of the second op amp is? It doesn’t seem to do anything but invert the signal. I’m wondering if I can get away without it, or does it offer some level of necessary protection? Comparing this to mutable instrument schematics where most of them seem to do with single op-amp.

Same question regarding the CV outputs as well, could I lose the second op-amp and handle the inversion in software? This seems to be the case for the CV inputs which use a single one.

If these could be removed it would save a lot of space on a through hole pcb.

2

For the audio output, using a single op-amp channel per side would be totally okay.

The decision to use two channels each was made so that the phase of the audio in the dsp would be the same as what comes out of the hardware. Primarily to avoid any confusion when writing waveforms to the OLED (“why is my ramp wave a saw wave?”, etc.).

The CV outputs are a bit different since they’re unipolar positive.

A single op-amp channel can be used under two conditions:

  • Supplying a negative reference to the inverting input node (similar to the CV input circuits)
  • Using a non-inverting configuration.

There’ll be examples of single-supply circuits for the CV Inputs and outputs on the Daisy Field schematic when that becomes available.

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