Connecting a balanced output to an unbalanced input is a little trickier.
You need to know something about the circuit that feeds the balanced output in order to do it properly.
Although output connectors can come in various forms, the 3-pin male XLR and ¼-inch TRS jack are the most common.
Conventional wiring, essentially standardized around 11080 when the Europeans and Americans settled on which pin was hot is:
Pin 1 Shield/ground
Pin 2 Signal High (non-inverted)
Pin 3 Signal Low (inverted, or not driven)
TRS jack equivalents are nearly always:
Tip Signal High (non-inverted)
Ring Signal Low (inverted, or not driven)
With that said, beware of vintage equipment, which may be wired with the signal leads reversed!
Balanced Output Circuit Topologies
Throughout the tube era and carrying through the early years of semiconductor design, nearly all balanced outputs employed a transformer. Not only does a good transformer provide a perfectly balanced output, it also isolates the signal leads from ground and, with tube equipment, converts the high voltage and high output impedance of a tube amplifier circuit to a lower impedance and voltage, making it compatible with most inputs.
A transformer is the most expensive, and some still consider it the optimum, approach to balancing an output, but even the best transformers add a bit of distortion. High-quality transformers, however, are still found in some of today’s boutique audio devices, primarily for their (desirable) coloration, superior ground isolation, and high noise immunity. A good transformer is the result of sophisticated design engineering and manufacturing, though today, lower quality transformers find their way into audio products primarily as a marketing feature. (“Ours is better than theirs because it has a transformer.”) But with respect to balancing, even poor transformers are pretty good.
To unbalance a transformer output, it’s necessary to connect pins 1 and 3 together. Plugging a TS plug into a TRS jack usually will do this automatically. This does no harm to the hardware, the signal quality, or the output level, and the sonic characteristics of the transformer are maintained. The only compromise with this connection is that there’s no common mode noise rejection because by grounding one side of the transformer, the output becomes unbalanced.
Note that if you don’t ground the low (pin 3) side of an output transformer, but instead leave it floating, your receiving device will see a very low-level signal that is lacking in low frequencies. This is because there’s nothing but stray capacitance between the transformer windings and ground to complete the circuit. Capacitors are poor electrical conductors at low frequencies, a valuable characteristic when you’re building a filter, but not when what you really want is a piece of wire.
Electronically-balanced (or “active-balanced”) outputs come in a several flavors. Each of the two output legs carries an audio signal that’s referenced to ground, identical in voltage but opposite in polarity. When the voltage on pin 2 is +1 volt with respect to pin 1 (ground), the voltage on pin 3 will be -1 volt. If you measure between pins 2 and 3 with a voltmeter, you’ll see 2 volts, and that’s the voltage that a differential input sees.
There are two common (read: “cheapest”) circuits used for active-balanced outputs. One adds a unity-gain inverting operational amplifier (op-amp) after the non-inverted output to get the inverted output. In another configuration illustrated here, two op-amps—one configured as a non-inverting amplifier, the other wired in the inverting configuration—are fed the same signal. This is the type of output configuration used for the main outputs of PreSonus StudioLive mixers.
Op-amps sometimes can become “unhappy” when their outputs are shorted, so simply tying pin 3 to ground, while electrically valid, isn’t a good way to connect this type of balanced output to an unbalanced input. It might work fine, but in the worst cases, the shorted-out op-amp could be damaged; the short circuit could be reflected back to the non-inverted output, causing the desired signal to distort; or the grounded output stage could start oscillating, usually at a frequency well above the audible range.