I finished this a few months ago but was without a digital camera until recently (I don’t have a smart phone) so couldn’t document it.

This mixer is the spiritual counterpart to the Pathos mixer, both names derived from the Greek concepts of persuasion. Pathos referring to an emotional appeal and Logos the logical. Keen students of Greek philosophy (Not myself, I just googled it) will also note there is a third ‘pillar’, Ethos, which is the name for a future mixer I’m slowly pulling together.

In the spirit of cold logic, this is a completely opamp based design with low distortion and noise in mind. These weren’t issues at all in the Pathos mixer but it is more on the colourful side of things.

The jist of the design is a two channel mixer with a built in isolator.

Each input channel has a line and phono input, two band EQ based on the Studer 169, an EFX send and output volume. They also contain a summing amp and meter driver to run the single VU meter.

The master audio bus is summed on the Isolator pcb and split up into the Low, Mid and High bands. This is again summed and then sent over to the Cue PCB for overall volume control. This signal is buffered and sent to the two centre meters for individual monitoring of the left and right channels. Seperate potentiometers are provided for the Room and Booth outputs. Both outputs are balanced using ThatCorp 1646 ICs on the IO card.

The Cue card contains everything else. The EFX send is summed here, and driven through a textbook line amplifier taken from Douglas Self’s Small Signal Audio Design. Good Book. This signal is sent to the IO card where it meets a mono switch, passively summing the left and right sends for use with mono efx. Once back the signal meets another line amplifier and then is injected into the main summing bus via a return level potentiometer.

The cue signal is mixable between each of the two channels and controlled by a potentiometer.

The design was all done as a large singe PCB intended to be snapped apart for the separate boards. It is essentially 5 separate PCBs, from left to right: Channel A, Isolator, Cue, Channel B and the input and output board.

To reduce clutter on the main boards, the output balancing stages, EFX mono switching and some PSU decoupling is on the output board, but it more or less serves to convert all I/O to ribbon cables which are used for the majority of connections in the unit.

All panels are just laser cut 3mm acrylic with a paper printout glued on with a gluestick. I usually acrylic for prototyping, I’m a cheapskate.

Design wise, I managed to contort all the wiring (other than the Meter signal, illumination and headphone output) onto IDC ribbon cables, so it was quite a quick build. The grey 10way ribbon cable carries +/-17V to all cards. Below it on the front of the unit is the 16way main audio trunking lines containing master summing, headphone outputs, EFX sends etc. The top of the unit has another 16way ribbon cable to shuffle the Isolator signals around to each card and back again. Next to it is a small 10way ribbon cable used to run the post isolator master signal for volume control on the Cue PCB.

See More:

Part 2

Part 3