AM981 CV Mixer

Op Amp based precision CV mixer

Moog developed the 981 four input mixer in November 1967 and two prototypes made it into the first shipment of the Modular IIIc. It was a transistor based design that appears to be intended for audio only but clips the negative portion of the signal, which probably explains why it didn't make it into production. It was replaced by a 994 Multiples module in production IIIc's.

I have updated the design and taken inspiration from the Moog CP3A Mixer in the System 55. It is an Op Amp based circuit, which is intended for precision mixing of CV signals, for example when driving VCO's. The design has a very low offset (error) on the mixed outputs and does not clip the negative CV's like the original 981.

The module is designed to complement the Behringer System 55 Modular System, with a black anodised aluminium 8HP panel with 4 potentiometers. The AM981 uses precision resistors and Op Amp, to deliver an accurate CV mixer with a variable gain from zero to x2. It is DC coupled and not intended for audio signal use.

There are three signal inputs with attenuating potentiometers, one input with no attenuation, and a final master gain potentiometer.

https://www.amsynthstore.co.uk/AM981_CV_Mixer/p1461448_21466430.aspx