This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) 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

channel!

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

current.

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.

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