nw2s::m 4x4 active audio mult

The nw2s treatment for the humble mult.

We've built one circuit that can be configured as either buffered or passive. The buffered version contains a high-performance Analog Devices OPA4134 audio op-amp. There's plenty of space for your fingers to work the cables. The four groups of 1x3 mults are normalized together, turning it into a 1x12 mult. It can be run from +/-12V to +/-18V rails for enhanced audio performance. It uses both a keyed power header and and our near zero voltage dropout MOSFET reverse voltage protection circuit.