Amplifiers: Bridging Amplifiers - sound
niedersteiner writes "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.