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Knowing how bridging amplifiers affects your system can prevent costly equipment failures.
Stereo amplifiers are actually two amps in one chassis, both usually powered from a common power supply. Bridging connects these two amplifiers together to function as one amplifier into one load. (A load is one or any number of loudspeakers connected together.)
There are two ways to connect these amplifiers to each other, parallel or series. Parallel bridging doubles the current available to the load, series bridging doubles the voltage on the load. It is the series connection that is most commonly used when "mono bridging" your amp.
When voltage is doubled the power goes up four times on the same load. Let's illustrate this point by using one of my old SAE P50's as an example. This amp is conservatively rated at 50W per channel into 8 ohms, producing a total of 100W into two 8 ohm loads (50W from each channel). When this amp is bridged (it is now mono) it will produce 200W into one 8 ohm load.
Connecting the two 8 ohm loudspeakers together in parallel gives you a net load of 4 ohms. The bridged amp will now try to dump 400W into this load (200W to each loudspeaker). Each amplifier section "sees" this as a load of only 2 ohms (half of the 4 ohm net load), which is very nearly a dead short. The current flow becomes 4 times what it would normally be into a single 8 ohm loudspeaker (one per channel in stereo mode) and the amp tends to heat up trying to deliver this much current.
Also consider what your speakers must now contend with. This same amp that was once producing a comfortable 50W (with 3dB headroom) is now giving it 200W. Apply these ratios to amplifiers of more power (a 100W per channel stereo amp becomes a 400W series-bridged amp) and loudspeakers soon become overwhelmed.
Unless you are comfortable applying these numbers in your system, approach series bridging with caution.
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Copyright © 2001 Anton Niedersteiner. All rights reserved.
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