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Join Date: Mar 2007
Location: Long Beach, California
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It can swing +/- 60 volts because the power rails are just over +/- 60 volts, but the minus speaker terminal is tied to zero volts. ie: The plus terminal swings +/-60 volts while the minus terminal stays at zero volts.
In bridged mode, the minus terminal is replaced with the plus terminal of the second amp, and is driven by the compliment of the input to the first amp. So while the first amp's plus terminal swings toward +60 volts, the plus terminal of the other amp swings toward -60 volts, giving you a total swing of 120 volts across the two plus terminals.
A single amp can't swing +/-120 volts because the power rails are only +/-60 volts.
We used to call the former "unipolar drive", while the bridged mode would be called "bipolar drive" (not to be confused with Class A and Class B).