Due to it´s exponential characteristic the A-138a is well suited to mix audio signals. Control voltages can be mixed as well but you have accept an imperfect behaviour of the pots.
If the first input is not connected you can use the first potentiometer as an offset control for creating a constant offset voltage. By a jumper you can determine if the offset is active and if it goes from -5V to 0V or from 0V to +5V.
four inputs, one output
3U Eurorack module, 8HP wide, 35mm deep
10mA current draw