Four attenuverters combined with a handy mixer
An attenuverter is a utility that allows to scale and invert a signal. With the control at 12 o'clock, the signal is attenuated down to zero. Turning the control clockwise functions like a normal attenuator, while turning counter-clockwise the signal is inverted and attenuated.
This module has 4 attenuverters in a 6 HP module. The attenuverters use curve shaping to have a log-like response for both positive and negative signals, to have greater sensitivity near the center of the control. All outputs are connected to a mixer via their normal connection - connecting a jack into an Out socket removes that channel from the mixer. The mixer further has a unity gain input for adding an extra signal or bussing together multiple mixers.
The unit has a 5V reference, which is normalled to the input of the first attenuverter, allowing an easy source for +/- 5V static voltages. The 5V reference can be optionally normalled to the other jacks by connecting solder bridges on the PCB.
This PCB uses surface mount technology, with 0805 passives and SOIC opamps.