Balanced Power Amps

If you decide to connect your Squeezebox directly to a balanced power amp, you may well encounter a grounding issue that causes a loud hum.

Let's first consider the traditional way to connect an unbalanced output to a balanced input. The signal from the output goes to one of the signal inputs (usually the "hot" input on pin 2 of an XLR, but it could just as easily be to the "cold" input on pin 3). The ground from the source is connected to the other signal input. The screen of the interconnect cable is usually only connected at the balanced input end (on pin 1) and is left floating at the source end in order to avoid ground loops.

This arrangement causes the balanced input to regard the difference between the signal and ground as the signal (and since the ground is nominally at 0V, this difference is indeed just the desired signal).

In most systems where an unbalanced source drives a balanced power amp, the source is a preamp, and preamps tend to have a ground. But if you use a source which has a floating ground, such as a Squeezebox, then any mains ripple on its ground line doesn't get shunted to earth and appears to the balanced power amp as part of the signal and gets amplified along with everything else. The result is a loud hum.

The solution would appear to be obvious: connect the screen of the interconnect to the ground at the source end. This will shunt the Squeezebox's ground to earth via the power amp's ground. And in many cases this will indeed solve the problem.

But there is an added subtlety. Some balanced power amps don't have the screen of their XLR inputs connected directly to ground. (For example, the power amps in my ATC speakers have a 47 Ohm resistor between pin 1 of their XLR inputs and ground. I don't know why this is so - I'm not an amplifier designer, but I'm sure ATC have good reasons for including it). So connecting up the screen at the Squeezebox end only establishes a partial shunt to earth. The hum is reduced, but not eliminated. The solution here is to connect the ground plane of the Squeezebox to the mains earth - preferably the same one used by the power amp. I used the screen of the Squeezebox's COAX SPDIF output, which was not in use in my system, for this purpose.