A ‘Lossless Combining Amplifier’ was the first project I undertook to get rid of all the DIY audio bits I had lying around.

The plan was to build a 16 channel summing amplifier to sum the individual outputs from the PC DAW – Digital Audio Workstation – using two DOA – discrete operational amplifiers – as the output stage.

Why sum the outputs when I can mix internally and bounce the 2 track output?

To make good use of a rack of DIY Class A Line Amps that were built sometime ago. They were roughly based on the Neve 1272 line amp that used a stripped down version of a Neve 1073 Mic Preamp circuit.

One less gain stage and fixed at 55dB. As mic amps great on drums and guitars, but only really effective in close micing applications. The absence of the later gain stage meant they saturate the sound at higher gains. Not a nasty saturation at all, but certainly coloured. This can be good or it can be bad. Depends on the application and your taste in colouration. Recording ’sensitive material may need cleaner pres with more gain. As line amps though they’re perfect. Eight of them should give me a decent mix.

The Amplifier

16 inputs are fed to two banks of resistors, odd numbers left, even numbers right for eight stereo pairs.

As output pans are fixed either left or right 16 switches are incoperated to throw the L/R signal into the centre.

This makes sure we don’t waste a stereo pair if we only need a mono output. Just pan the signal hard L/R on the DAW output and throw the corresponding switch to centre it.

Inputs are terminated with two 25 way D-SUB connectors. As the unit housed in a 1u casing this squeezes the most amount of space from the crowded backplate.

The idea of hand drilling a hole for an IDE socket, plus room for the XLR outputs and 16 jack inputs would be impossible. The D-SUBs tidied things up a lot and cut expense.

There is no volume control. The was no room for volume control. The unit could probably do with a trimmer to drop the output a touch as once you send some tracks through it for mixing it can get quite hot on the output. (I often like to drive the line amps quite hard to get a bit of saturation.) Not a big issue if we use the input trim on the 2 track mix input, yet I may still rewire the transformer for a 2:1 step down and see if this helps.

Anyway

Here’s The Partslist

1u 19″ Case
1 IDE type socket
1 PSU 3 rail with adjustable voltage
2 D-SUB connectors female
16 SPST single pole single throw switches
32 100k input resistors
2 125uf capacitors
2 Op Amp circuits
2 feedback resistors
2 feedback capacitors
2 output transformers 1:1 600ohm
2 XLR outputs