In electronics terms this ‘Lossless Combining Amplifier’ is one of the most basic things to build. There is practically nothing in there in the way of components. The amplifier section is simply a pre-assembled op amp module, or even an integrated circuit such as the NE5534, with three surrounding components and a transformer on the output.
Infact the most difficult thing is the metalwork for the casing.
Drilliing 16 holes for the switches and lining them up exactly takes some precision measuring. Luckily there is a program called Front Panel Designer that …

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. …