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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.

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Be aware, that combining left and right into mono runs the risk of constructive or destructive interference. Opposite phases will cancel each other out, while the same phases will add to each other... at least to some degree. Good mixing engineers will check their mix in mono to account for this, but it's certainly something to be aware of. –  JoshP May 17 '13 at 20:43
    
The setup as you describe it seems horribly hacky. What exactly are you trying to achieve? –  leftaroundabout May 18 '13 at 8:57
    
Yes it is hacky. I have two cammeras one shooting HD that I will edit and an iphone that streams live to the internet. I want to do a little post sound work as possible. But I am recording the sound so worst case I have to sync the sound with the HD video. Combining the sound is not that big a deal. Because the each is from a different lapel microphone. and only one is in the shot. –  BigAl May 18 '13 at 15:47

2 Answers 2

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.

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I was only an EE for one semester, but won't adding diodes just clip the top or bottom of the signal? I think you'd actually want a buffering opamp to prevent backflow. –  ObscureRobot May 17 '13 at 23:21
    
@ObscureRobot - could be, I think it depends on the diode, but I thought there was a diode that restricted the direction of current. It's a bit fuzzy for me though as the last actual electronics class I took was my high school electronics class in 8th grade and that was, oh, about 1998. Haven't used it particularly much since (I'm a software developer and A/V guy, not an EE. :) ) –  AJ Henderson May 18 '13 at 3:07
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All diodes restrict the direction of the current, but an audio signal goes both above and below zero. In other words, the current is flowing in both directions. So a diode will just clip half of the wave. That's why audio design is so full of Operational Amplifiers (OpAmps). Anyway, this is pretty far afield from the OPs question. –  ObscureRobot May 18 '13 at 3:28
    
@ObscureRobot - ah, right. Well, it's still relevant since it is going to pollute the stereo signal unless he uses a very specialized way to merge the signals together. –  AJ Henderson May 18 '13 at 5:18
    
Yeah, diodes clip in a very harsh way, which is why they are often used in simple fuzz circuits. You couldn't do it like this unfortunately. –  Rory Alsop May 18 '13 at 12:02

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.

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That looks right is there anyone selling this. Also is needs to be compact because I dolly all this around on my tripod. (Shooting Individual booths at trade shows) –  BigAl May 18 '13 at 15:55
    
I'd imagine there are but i'd make it from two resistors but if you haven't got a soldering iron it's gonna be tricky –  Andy aka May 18 '13 at 18:10

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