Balancing An iPod Output
It's tempting to want to use an iPod as a program source for driving powered loudspeakers for room testing.
It's tempting to want to use an iPod as a program source for driving powered loudspeakers for room testing. But since the output level of 1Vrms (0 dBV) may be well below what's usually required, here's a little trick for creating an interconnect cable and getting the extra level you need.
1. Make a cable using this diagram.
2. Create a stereo file from mono, and invert polarity of channel 2.
The source file is most easily generated in a wave editor. It consists of two identical channels that are reversed in polarity. This provides an identical signal to pin 2 and pin 3 of the balanced output (one of which is inverted). Balanced inputs like this type of signal, because they accept the voltage difference between pins 2 and 3 as the signal, and reject the common mode voltage between pins 2 and 3. What we are doing is giving up stereo in favor of a balanced source and 2x the voltage (+6dB).
3. Connect iPod to powered loudspeaker.
The level control can be advanced until clip/limit is indicated.A few caveats
The balancing method described is not a general-case solution for balancing an iPod. What it does is allow a balanced output at +6dBV that may be useful for a few demo tracks (i.e. a sine sweep, polarity pulse, speech track, and maybe a music track or two).
The result is mono, not stereo.
It only works with a specially modified source file.
Be sure to switch phantom power “off” before using this cable, since no DC isolation is provided.
Since the two channels are reverse polarity, typical stereo program material will play back with center channel information (e.g. vocals) canceled.
Have available a direct box with proper summing network for summing left and right outputs into a single input channel for listening to your favorite stereo music tracks (in mono).
Source: Pat Brown, Syn-Aud-Con