This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
When you're told a stereo power amplifier can be bridged,
that means that it has a provision (by some internal
or external switch or jumper) to use its two channels
together to make one mono amplifier with 3 to 4 times the
power of each channel. This is also called "Monoblocking"
and "Mono Bridging".
Tube amps with multiple-tap output transformers are simple to
bridge. Just connect the secondaries in series and you get
more power. The ability to select transformer taps means that
you can always show the amplifier the impedance it expects, so
tube amp bridging has no unusual stability concerns.
The following discussion covers output transformer-less amps.
Bridging these amps is not so simple. It involves connecting
one side of the speaker to the output of one channel and the
other side of the speaker to the output of the other channel.
The channels are then configured to deliver the same output
signal, but with one output the inverse of the other. The
beauty of bridging is that it can apply twice the voltage to
the speaker. Since power is equal to voltage squared divided
by speaker impedance, combining two amplifiers into one can
give four (not two) times the power.
In practice, you don't always get 4 times as much power. This
is because driving bridging makes one 8 ohm speaker appear like
two 4 ohm speakers, one per channel. In other words, when you
bridge, you get twice the voltage on the speaker, so the
speakers draw twice the current from the amp.
The quick and dirty way to know how much power a stereo amp can
deliver bridged to mono, is to take the amp's 4 ohm (not 8 ohm)
power rating per channel and double it. That number is the
amount of watts into 8 ohms (not 4 ohms) you can expect in mono.
If the manufacturer doesn't rate their stereo amp into 4 ohms,
it may not be safe to bridge that amp and play at loud levels,
because bridging might ask the amp to exceed its safe maximum
Another interesting consequence of bridging is that the amplifier
damping factor is cut in half when you bridge. Generally, if you
use an 8 ohm speaker, and the amplifier is a good amp for driving
4 ohm speakers, it will behave well bridging.
Also consider amplifier output protection. Amps with simple
power supply rail fusing are best for bridging. Amps that rely
on output current limiting circuits to limit output current
are likely to activate prematurely in bridge mode, and virtually
every current limit circuit adds significant distortion when it
kicks in. Remember bridging makes an 8 ohm load look like 4 ohms,
a 4 ohm load look like 2 ohms, etc. Also, real speakers do not
look like ideal resistors to amps. They have peaks and dips in
impedance with frequency, and the dips can drop below 1/2 the
nominal impedance. They also have wildly varying phase with
Finally, some amplifiers give better sound when bridged than
others. Better bridging amps have two identical differential
channels with matched gain and phase through each input, left
and right, inverting and non-inverting. Simpler bridging
amplifiers have one or two inverting channels, and run the
output of one into the input of the second. This causes the
two outputs to be slightly out of phase, which adds distortion.
There are also other topologies. One uses an additional stage to
invert the signal for one channel but drives the other channel
directly. Another topology uses one extra stage to buffer the
signal and a second extra stage to invert the signal. These are
better than the simple master/slave arrangement, and if well
done, can be as good as the full differential power amp.