Combining 2 Mono Microphones to 1 mono output with a 4 channel recorder in the middle

Question

I have an 4 Channel recorder/mixer.

As well as recording, it gives me 1/8" stereo out on a head phone output. One mike is on the left and one mike is on the right.

I use a 1/8” to 1/8” stereo cable that attenuates it to be good for line in on the cameras. I split this to 2 camera's. 1/8" plug input to the camera but the other has an iPhone input. I thought I had a 1/8” stereo to iPhone audio plug cable.

When all is said and done the iPhone is only getting one side of the audio.

I assume the stereo 1/8" female to iPhone male is actually a mono female to iPhone. So I think I need an adapter to go from a 1/8" Stereo male to a 1/8" mono female.

Looking online, I see plugs that match that spec but they are made to take a mono signal and convert it to stereo. Some seem to even enhance the signal to make it “better” stereo, which really worries me.

I am trying to go the other way and combine the left and right sides to get a mono into the iPhone.

Answer 1

Hmm, that's a rather complicated setup that could have problems. Normally combining a left and right channel is simple, simply wire the signal and ground lines together and the signals will combine to form a combined mono signal from the stereo signals.

The problem is that since you are splitting the signal, you need one output to maintain the stereo image while you need the other to be mono. I'm pretty sure that when you wire the left and right channel together for going in to the iPhone input, it is going to create a circuit that allows the combined feed to go back on the left and right channels being split off to the camera's stereo input.

You might be able to custom make something with diodes in the path to ensure that signal isn't able to backflow and pollute the stereo image, but you are talking about a pretty specialized setup at that point.

Also, as an aside, there is no such thing as a mono to stereo adapter. It's just a splitter and any marketing fluff to the contrary is just that, marketing fluff. A splitter in reverse will combine signals, but the connectors are probably all the wrong gender.

Update: See discussion below, diodes won't work for the purpose I described. Seems my electronics was a little rusty, or more accurately I forgot that the signal oscillates in both directions. The part about the issues from simply merging the signal paths should still be accurate though. A distribution amplifier (like a headphone amp) might provide what you need to provide isolated splits of the signal.

Answer 2

Analogue Mixer Desks use resistors to mix two (or more) signals.

If your individual signals (left and right) have a recognized output impedance (50 ohm, 75 ohm, 100 ohm, 150 ohm, 300 ohm, 600 ohm, 100 ohm etc) then, the two signals will "mix" to form a composite signal.

If the two signals are from low impedance sources such as amplifiers then you need to use extra resistors (1000 ohm typical) to combine the two signals and avoid the amplifiers becoming damaged or distorting.

You wouldn't put a 12V battery in parallel with a 6V battery but you can if you use both outputs connected through their individual 1000 ohm resistors. And what do you get at the mix-point... 9V i.e. the two battery voltages are mixed and this happens with AC voltages as well.

Try mixing the two outputs via 1000 ohm resistors. Take a look at this - it describes what I mean. It uses 10kohm resistors (10,000 ohms) but the principle is the same; if you think the signals won't appreciate being loaded with 1000 ohms then use 10,000 ohms.