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Pin 1 Shield/ground
Pin 2 Signal High (non-inverted)
Pin 3 Signal Low (inverted, or not driven)

TRS jack equivalents are nearly always:
Sleeve Shield/ground
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.