A simple open hardware audio mixer (active with with op-amps)

In order to record the cockpit intercom and radio communication with ATC during flights in an ultralight plane, I have developed a simple audio mixer. Namely, the pilot's headset I am using has a 4-pole jack output with separate signals for earphones and microphone, while for a recorder I need a single audio signal mixing both sources. I have thus developed a simple active mixer with two op-amp ICs, which allows to adjust separately levels of both signals, has high enough input impedance and low output impedance. It's a very simple circuit, but fulfills the needs. The PCB fits in a small plastic box with a compartment for a 9V battery.

Here is the schematics in PDF and full design files in Kicad.

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