Balanced impedence?


Can anyone explain why, when running balanced connections between a preamp and power amp, the output and input impedences should be equal? With single ended connections it makes sense that a low impedence ought to mate well with a vastly higher impedence. Why are balanced connections different, impedence-wise?
rockvirgo

Showing 2 responses by rockvirgo

Thanks for your response Gs. I now see that many power amps do typically offer high balanced input impedances.

What led me astray are these lines from the Threshold S/350e owner's manual: "The balanced setting provides a standard 600 Ohm professional connection, and the unbalanced setting provides 50,000 Ohms load through an RCA connector." and later, "Moving the switch toward the XLR connector will select a 600 Ohm input characteristic. Moving the switch toward the RCA connector will select a 50,000 Ohm unbalanced input characteristic. Make the appropriate selection as required by the equipment sourcing your Threshold amplifier." and finally in the spec sheet, "Input impedance: 50,000 Ohms unbalanced, 600 Ohms balanced." Maybe my question should be: what is the standard 600 Ohm professional connection?
Thanks Herman, the link is helpful. Given the link's explanation that a matched impedance connection will attenuate the signal by 6dB and thus require 6 dB more gain, can I assume, that because balanced signals typically double the output voltage, that using matched impedances is a way to gain-match balanced connections with signal ended connections, or maybe compensate for the voltage doubling rather than ignore it?

In the past, I've noticed that running a low impedance balanced source into a high impedance preamp connection raises the overall gain in the system and reduces the useful range of my volume control. I've not used the balanced input on my power amp but I'd like to one day. I'm still searching for its contemporary preamp or a suitable substitute that I can run balanced.

Electrical theory aside, I can see how a standard could be useful to an engineer who might wish to preserve favorite or sweet-spot control board settings. Thanks for your comments.