Mixing Two Audio Sources For Line Out and Recording

The need

Being able to smoothly bring up pre-recorded sounds from a second source allows for sound layering, and transitions between times when there is no laptop output. I'm using a SanDisk mp3 player into a Behringer FCV100 pedal. Running that and my laptop into the mixer works great, but to travel light I wanted to trim that down into a small passive box. What I need is two 3.5mm stereo audio input sources going to one 3.5mm stereo out to 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, even though it fundamentally works. If you want to try it that way see 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.

refs