This project is a mixer, especially designed for electronic musical instruments.  It has 16 inputs, each with a level control and a three-way switch which selects between two subgroups and mute.  It has a very clean signal path, and sounds amazing!  =]

Input channel

The input channels are very simple, just passive components to directly control the input signal.  The input signal passes through a pot wired as a volume control, then through a resistor that controls the gain of the stage, and finally through the three-way switch.  There is no input blocking capacitor.



Subgroup summing amp

The summing amps for each (mono) subgroup are also pretty much straight out of a textbook.  Each one has a DC blocking capacitor, then an opamp wired as an inverting summing amp.  The output of this goes through another DC blocking cap, then another opamp wired as an inverting buffer, and finally to the output jack for the subgroup.

I built these on prototype board cos it seemed like a simple circuit;  I would make PCBs if I was doing this again!



For this build, I decided to use multiple single opamps, so I could swap them out and do comparison tests between different opamps.  I haven't done this yet because the Burr-Brown OPA604's that I tried initially sounded soooo clean!  I have a *very* nice mixing desk, and this is noticeably cleaner.  I guess you can expect that, as there is so little in the signal path.  If I ever try out any other opamps, I'll post the results here. 

I've actually jumpered out the DC blocking caps between the two stages at the moment, and it behaves just fine.

The electrolytics in the signal path are all Nichicon MUSE types.  I used 33uF where less would have done the trick, because Mouser had those in stock when I ordered  :P  The 0.1uF caps are box film types, and the 120pF caps are mica.

Power supply

This power supply should be pretty familiar, it's straight out of the datasheets for the 7815 and 7915 type regulator chips:

It's mounted as far away from the audio circuitry as possible, and uses a toroidal power supply that's way over-rated.  This is because I expect to add a lot more to this box over the next while, eg. balanced outputs and VU metering.

The opamps actually have local capacitors too, mounted right by the power supply pins.  I've used 10uF tanatalums, fed by 33 ohm resistors from the power supply rails.  You could bypass these with ceramic caps too if you wanted.

Putting it all together

To keep tings tidy, I used a strip of prototype board with thick wires soldered to the bottom as 'bus-bars' - there is a wire from each bus bar to each input channel.  These act as the summing nodes for the subgroup summing amps - they are at 'virtual ground'.  Each channel voltage after the volume pot is converted into a current by the series resistor, and these currents add together at the summing node as an input to the summing amp.  There are three bus bars wired up, but only two subgroups - this is so I can add a third subgroup later if I want to.  The third one is currently just wired straight to ground.



I've always been a firm believer in grounding things properly when you're building audio circuits.  The ground for each part of the circuit joins to a central star ground point connected to the chassis, along with the mains ground.  I've no idea if it would affect this circuit at all, but I've found that this grounding scheme fixes hum and oscillations and crosstalk in other circuits.

After finishing it, I found that most of my instruments have a very different idea of 'line level'!  So to account for the variations in normal signal level, I've adjusted the input mixing resistor to suit each channel's instrument.

All up, this was a cool project and is very useful!