Imbalanced outputs

Although I earn most of my living writing software, I also work on audio whenever I get the chance. A few days ago one of my clients, a bass player, came to me with a piece of equipment that was causing a buzz.

The equipment in question was powered by an AC wall adapter, and it had a balanced output for sending the signal to the PA system. Balanced signal transmission is used in professional audio in order to reduce induced noise problems: the idea is that the signal is sent along two wires simultaneously, but with opposite polarity. At the receiving end, one voltage is subtracted from the other, eliminating any common noise that might have crept into the cables and leaving only the intended signal. The problem my client had was that whenever he used this output, it caused a terrible buzz in the PA system - rather counterproductive.

It turns out that a lot of what the industry calls "balanced outputs" really aren't. There's a popular belief amongst equipment designers that to make a balanced output, what you need is two signals in opposition - that is, you split the intended signal, send it unaltered onto one wire, and then flip its polarity (that is, multiply the voltage by -1) and send that to the other wire.

This is malarkey, as has been pointed out by luminaries like Douglas Self and Bill Whitlock. Having two voltages in opposition is irrelevant; if that mattered, then when the signal was zero (dead quiet), noise would no longer be eliminated. What actually matters is that the impedance on the two legs is balanced, so that any induced common-mode noise is the same on the two legs.

This is pretty old news, but has been largely ignored in the industry. So I was not surprised to discover, on tracing the circuit in my client's equipment, the following "balanced" output stage (I've eliminated a few unimportant details, like DC blocking capacitors):



It does just what it was meant to - the voltage on pin 3 will always be -1 times the voltage on pin 2. But look at what happens when the AC adapter is plugged into the wall:



There's always a little bit of leakage between the windings in a power transformer, perhaps a few picofarads. 120V from the wall leaks through that capacitance, into the power supply, into the signal ground. From there, I've traced the two main current paths to the output. Notice that one path goes through about 21.8k of resistance before getting to the output, while the other sees only 1k.

This 22:1 imbalance, with the tiny leakage through the transformer, was enough to generate a couple mV of differential signal - a lot, in the terms of audio signals, which rarely exceed one volt. By replacing this output stage with one that was truly impedance-balanced (based on an SSM2142 chip), I was able to reduce the noise signal by a factor of 30 - enough to get it below the noise floor of the unit.

The cost of the extra components was about $8 retail. But frankly, if cost were an issue, it might have been just as good to have gotten rid of the entire inverting amp stage and simply connected pin 3 with a 1k resistor to ground. The impedances would be balanced. The differential voltage would only be half as big as before, but the common-mode noise voltage would be reduced by much more than half, so signal to noise ratio would be better than with the "balanced" output the designer came up with.

Many software errors come from using a common design pattern without understanding the problem it's aimed at. This was the same mistake in hardware.