How does bridging work?

Early B. · July 2004

Ok, so I understand how a 2-channel 100 wpc amp becomes 200 wpc in bridged mode. But how does a 120 wpc amp (such as the Rotel RB980BX) become 360 wpc bridged?

Will there be a noticeable difference between a 200 wpc amp vs. a 360 wpc amp in bridged mode if both are powering a Csi40?

HBombToo · July 2004

Originally posted by Early B.

Will there be a noticeable difference between a 200 wpc amp vs. a 360 wpc amp in bridged mode if both are powering a Csi40?

both what? smoke another joint... will ya:p

twin

John K. · July 2004

EB, if two 100 watt amps gave only 200 watts when bridged there'd have to be some unusually big inefficiency involved. When amps are bridged their voltages add together. One form of Ohm's law is that power(in watts)is equal to voltage(in volts)squared, divided by resistance(in ohms), i.e. P=V(squared)/R. Therefore when two equal voltages are added, power is theoretically increased four times, not just doubled, if the process was 100% efficient. In practice inefficiencies lead to about a tripling, as in the Rotel example that you mention. In order to allow the voltages to add together when the positive output terminal of each amp(rather than the positive and negative terminals as usually connected)are both fed into the terminals of a speaker the phase of one amp has to be inverted 180 degrees in polarity. If they had the same phase the two voltages would in effect run head on into each other in the speaker and cancel rather than add; but when their phases differ by 180 degrees, it can be said that one is pulling in the same direction that the other is pushing, so working together power is increased. Amps that are designed to be bridged have built-in provision to have inverted polarities. There are also separate inverting devices that are available or can be built to allow bridging amps which don't have the provision built-in.

As far as any difference between 200 watts and 360 watts, bear in mind that the Csi40 will be using about 1 watt at a comfortably loud listening level. On peaks much more is used for a tiny fraction of a second, possibly 100 watts or more, but it's rather unlikely that a 200 watt amp would ever be inadequate and that a 360 watt amp (which is only about 2.4dB higher in peak output than the 200, e.g. 117.4dB vs 115dB)would have a significant advantage.