When designing a stereo power amplifier, the issue of interchannel crosstalk is always a concern. Now that amplifiers with up to seven channels for home theatre are becoming more common, the crosstalk issue is that much more important, if only because the channels are likely to be more closely packed. Here I deal with one aspect of it. Almost all power amplifiers have output coils to stabilise them against capacitative reactances, and a question often raised is whether inductive coupling between the two is likely to degrade crosstalk. It is sometimes suggested that the coils which are usually in solenoid form, with length and diameter of the same order should be mounted with their axes at right angles rather than parallel, to minimise coupling. But does this really work?

I think I am pretty safe in saying there is no published work on this, so it was time to make some. The coil coupling could no doubt be calculated (though not by me) but as often in the glorious pursuit of electronics, it was quicker to measure it.

The coils I used were both of 14 turn of 1 mm diameter copper wire, overall length 22 mm and diameter 20 mm. This has an inductance of about 2 H, and is pretty much an 'average' output coil, suitable for stabilising amplifiers up to about 150W/8 ?. Different coils will give somewhat different results, but extrapolation to whatever component you are using should...