Balanced vs. unbalanced

Bob...RF signals will not get through either the (+) or the (-) input of a balanced interface, so talk about how such signals are cenceled is meaningless.

The additional circuitry is a unity gain stage, which inverts the signal. Unity gain stages are easy to design and build with virtually no distortion or noise, so I wouldn't worry about that. The balanced interface is much more immune to noise pickup, and almost unaffected by wire characteristics (which make some single-ended interconnects so expensive).

On the other hand, especially for short runs, single-ended interconnects are fine. Only replace them if you experience a problem, or if the equipment you bought, for other reasons, happens to have balanced interfaces. (Or if you need status points among your audiophile friends).

By the way, the noise immunity of balanced lines can be achieved using active circuitry only in the receiving end of the connection. The HI wire of the balanced connection carries the single-ended signal. The LO wire is simply connected to the single-ended return (usually ground) through a resistor of value equal to the output impedance of the active signal. The LO wire picks up the same noise as the signal wire, and the noise is cancelled by the differential input circuit of the receiver. Clever.

Davemitchell...Please explain how a differential amp input stage having frequency response appropriate for audio signals can reject signals at RF frequency.

Bob...Exactly my point. Although some op amps may be usable at RF frequency, the ones used for audio applications, especially when wired up in an audio circuit probably max out at 250 KHz or so.

Balanced interfaces do a great job on 60 and 120 Hz hum.