Doepfer A-138b Mixer exponential.
Logarithmic mixer for mixing up to 4 signals to one output. Works best with audio, but CVs are possible, too. If there´s no jack in input 1, you can use the level-control 1 as an offset control.
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.