Mixing Two Audio Sources For Line Out and Recording

The need

When switching between Ableton sets, or other laptop dead air times, I want to be able to bring up ambient noise to fill the gap. I have pre-recorded sounds on a SanDisk mp3 player, which I want to blend with the output from a Mac laptop using a foot pedal. Usually I combine the two signals through a mixer, which works well and provides more flexibility, but to travel light I want to trim this down to a very small passive mixer. In short, I want two 3.5mm stereo audio sources as inputs going to one 3.5mm stereo out for the amp. I'd also like another 3.5mm stereo out for recording, which needs to be attenuated to mic level.

Initial Solution

My first pass at solving this was to build a passive summing cable, then a stereo Y plug with one branch going to a pad cable for recording. That worked, but it was a bit clunky, and was plagued by occasional cutouts and other issues related to treating the computer output as TRS when it is actually TRRS. I've given up doing it as a cable, but it does basically work, so if you want to try it that way you can check out that writeup.

Things To Be Aware Of

  1. The 3.5mm headphone jack on the computer is not TRS, it's TRRS. For a Mac, the TRRS is Left, Right, Ground, Mic. You don't want the mic connection involved in the mixing at all, since amplitude fluctuations between mic and ground are interpreted as control signals, which can cause mix cutouts as the computer audio driver reacts. The patch from the Mac needs to be TRRS, going into a TRRS socket with the mic disconnected.
  2. The 3.5mm headphone out on the SanDisk or whatever other audio source may also be a TRRS connector. If it is, it's most likely the standard TRRS configuration which is Left, Right, Mic, Ground. Again, safest to use a TRRS jack with the mic disconnected.
  3. Doing this as a box, rather than a cable, means you can see and trust all the soldered connections, and the patch connections can make better use of standard prefab cables.

Schematic

Here's the wiring diagram for the mixer. The summing portion is on the left, and the Y-plug/PAD part is on the right.