Hi Peter,
I believe RoadRanger nailed your primary issue.
When combining two channels to one, at minimum you want resistors in the circuit so the signal from one channel doesn't damage the circuitry on the other channel.
The best, most flexible interfaces have each channel transformer isolated.
Interfaces that combine two channels to one are often useful when channels are running short.
Keep it stereo if playback is critical.
There are a number of physically small interfaces that have tiny transformers in them.
Physics say these are nearly worthless at lower frequencies.
I like the Radial Stagebug5.
It can be used as two balanced TRS channels, or one combined mono XLR channel.
http://www.radialeng.com/r2011/stagebugsb5.php Modest price, very good quality.
The one thing that could be improved is the 1/8" plug.
With some phones, you have to remove the case to plug in the 1/8" connector.
Some of these units come with skinny/extended connectors that allow the phone case to be left on.
I ordered some 1/8" skinny/extended plugs to put on these units.
Amphenol KS3PC-AU
The 1/8"TRS to dual 1/4" cable is the cheapest way to go.
Keyboard Magic - most portable devices are pretty modest output and you will get best signal-to-noise ratio running them wide open.
The transformers in those direct boxes are definitely a good idea.
SteveGarris - see first note about using a circuit with resistors.
Your equipment may not die immediately, or may be robust enough to withstand the drive voltage, but I prefer not to gamble...
The DJMate doesn't have a detailed enough description for me to tell how good it is.
Transformer has to be tiny.
You definitely DON'T want to be running it at the stereo setting in to a mono XLR input.
If you are going in to a +4 balanced input the Whirlwind IMCOML line level combiner has very low signal loss.
Thanks and good health, Weogo
