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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



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