This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
Basically, one channel's signal is inverted, and then the two channels
are combined to form one channel with twice the voltage of either of
the original channels.
Ohm's Law for Alternating Current states that I = V/Z where I is
current, V is voltage, and Z is impedance. We also know that P = IV,
where P is power. If we use Ohm's Law and substitute into the power
equation, we get P = V(V/Z), which can be rewritten as P = (V^2)/Z.
Therefore, power is the square of voltage divided by impedance.
Now, why do we care about all that? Because it explains precisely what
happens when an amp is bridged. I'll give a practical example and
explain the theoretical basis of that example.
Imagine you have a two-channel amp that puts out 50 watts into each
channel when driven into a load of 4 ohms per channel. Since we know P
and Z, we can plug these numbers back into our power equation and find
V. 50 = V^2/4 -> V = sqrt(200). So, we're seeing a voltage of 14.1
volts across each channel.
Now, imagine we bridge this amp, and use it to push just one of those 4
ohms loads. When the amp is bridged, the voltage is doubled. Since we
know the voltage (2*14.1 volts), and the impedance (4 ohms), we can
calculate power. Remember that P = V*V/Z. That means P = (28.2)^2/4,
which is 198.1 watts. It should be clear by now that the new power is
approximately 200 watts - quadruple the power of a single, unbridged
You can probably see that should be the case, especially if you look
back at the power equation. Since P = V*V/Z, if you double V, you
quadruple power, since V is squared in the power equation.
Now, all this assumes the amp is stable into 4 ohms mono. The mono
channel is putting out four times as much power as a single unbridged
channel, so it must be putting out twice as much as the two single
channels combined. Since the voltage on the supply side of the amp is
dependent on the car's electrical system, it doesn't change (OK, the
increased current might cause a voltage *drop*, but let's not worry
about that now). Looking at the first power equation, at the supply
side of the amp, we see P = IV. Now, when we bridged the amp, we
doubled the power, but the input voltage stayed the same. So, if we
hold V constant, the only way to double the power is to double the
That means the amp is now drawing twice as much current when it's
running at a given impedance mono than it would be running two stereo
channels at the same impedance. There are only two ways the amp can do
that - it can simply draw more through it's circuits, and dissipate the
extra heat, or it can utilize a current limiter, to prevent the
increase in current. Of course, using the current limiter means you
don't get the power gains, either! So, if the amp can't handle the
extra current, and it doesn't limit the current in some way, kiss it
goodbye. For that reason, an amp is typically considered mono stable
into twice the impedance it is considered stereo stable.