For my own, and anyone else's info, I found this article:
This article explains the difference between balanced and unbalanced audio, when it is best to use which, and how to go about connecting a balanced output to an unbalanced input and vice versa.
Most domestic audio equipment has unbalanced audio inputs and outputs. This means that the audio output (left, right, or mono) appears on a single wire, and is referenced to "0V" or "Ground". Typical connectors used are RCA "phono" connectors, DIN plugs/sockets, and 0.25" (6.3mm) or 3.5mm jack plugs/sockets.
Unbalanced audio is fine for the domestic environment, and for line-level signals in a typical broadcast studio. Problems start to occur if the signals are being carried over long distances, especially if the source and destination have separate mains supplies.
A lot of professional equipment will have balanced audio inputs and outputs, usually on 3-pin "XLR" connectors. A balanced audio signal consists of a pair of wires carrying the audio signal in anti-phase with each other. Relative to 0V, if one wire carries a positive voltage, the other carries an equal and opposite negative voltage.
The 3 wires used in a typical XLR lead are often referred to as Ground, Live and Return, or Ground, Hot and Cold. "Live" and "Return" carry the "in-phase" and "out-of-phase" versions of the audio respectively. Calling the signals "Live" and "Return" makes it nice and easy to remember which goes on which pin of the XLR plug/socket:
X = Ground
L = Live
R = Return
The advantage of balanced audio over unbalanced audio is it's ability to reject external interference added as the signal is carried over the wire. The receiving equipment takes the voltage difference between the two wires as the input signal. Interference will usually get added to both wires equally, and so gets ignored by the receiving equipment.
Often, it is necessary to connect a balanced output to an unbalanced input, or vice-versa. The 100% correct and official way to do this is via a transformer or an electronic converter made from an op-amp. There are also some reasonably-priced ICs especially designed for the task - check out the SSM2141/2/3 from Analog Devices for example.
Assuming you are within the relatively benign surroundings of somewhere such as a radio studio however, there are simpler ways to connect balanced and unbalanced inputs and outputs. All result in unbalanced signals on the wires, so don't do this if you fear interference!
To connect an unbalanced output to a balanced input, simply connect the unbalanced signal to the "Live" side of the balanced input, and connect the Ground side of the output to both the "Return" and "Ground" of the input. Some unbalanced outputs assume they will be connected to a high-impedance input, so if your input is switchable between high-impedance and 600 Ohm, make sure it is set to high-impedance. If it is low-impedance only, you may be out of luck! If you're desparate to get a signal, try using the headphone output to feed the equipment - just make sure you turn the levels down to start with, and turn them up slowly until you get a reasonable level. The sound quality may not be brilliant, but you will get half-decent sound - better than trying to cope with the very low input level caused by feeding a high-impedance output into a low impedance input!
A word of warning. XLR inputs on mixers are also used for Microphone inputs. These assume an input level of somewhere in the region of -60 to -40dB. Feeding them with a 0dB input level is not recommended. Make sure that you use only line-level inputs, or that the input has been switched into line-level mode. If you only have a mic-level input, you can make do by using a resistive divider to reduce the input level. An attenuation of 60dB is a factor of 1000, so put a 10k resistor and a 10 ohm (or 100k and 100 ohm) resistor in series between the output and ground. Now connect the live and return pins of the input across the 10 ohm resistor.
To connect a balanced output to an unbalanced input, there are a couple of choices.In both cases, the "Live" side of the output is connected to the signal input.The other connection depends on whether the output is transformer isolated (in which case the "Return" side is connected to the input ground) or electronically balanced using ICs (in which case the Ground pin of the output may be connected to the input ground instead). The latter will result in a 6dB (i.e. factor of 2) loss of amplitude, and perhaps reduced signal-to-noise ratio.
Why have we got two possibilities? If you try to get a signal between a transformer balanced output and ground you will get nothing - the signal is not referenced to ground at all. On the other hand, if you connect the "Return" output of an electronically-balanced output to the input ground, you will be shorting that output to ground, and the chip may not like it. From an audio fidelity point-of-view, it is obviously better to use the "Return" output rather than the "Ground", so how do you know when it is safe to do so? If you're being professional, you could open-up the equipment, find-out which IC is being used and then go and read the datasheet. The more practical solution is to "suck it and see" - try it out and see if the audio sounds distorted or looks distorted on an oscilloscope - you're most unlikely to cause permanent damage to the chip as it will have built-in short-circuit protection.
If you regularly need to connect balanced outputs to unbalanced inputs, you may like to make up a lead with a switch in circuit to select whether the return or ground connection on the balanced output is connected to ground on the unbalanced input.
Nigel Dallard, Chief Engineer, Winchester Hospital Radio
9 October 1999
Now, obviously, car audio does not use 3-pin connectors, so the balanced outputs from the HiFi HU must be a +ve and -ve pair, rather than a +ve and 0V that you'd get from the base system.
I understand a lot of what that article says, and it looks like it's going to be a case of trial & error to get it right. Luckily I have an oscilloscope so it will be relatively scientific.
1. Fit RCA plugs to the speaker wires, and firstly see what the output is like without any modification of the signal. I don't expect that to work, because the amp will expect a 0V signal the the RCA sleeve, not an inverted copy of the +ve signal.
2. The article differentiates between tying either the -ve signal to input ground, or connecting the input ground to output ground. In car audio, everything is grounded to the chassis anyway so I'm hoping that it won't matter but I'll try it and see.