[quote author=geewhizbang link=topic=68709.msg1188348#msg1188348 date=1170715756]
Bridging can done many different ways. The important thing is that the amplifier has enough impedance to be stable.
Parallel loads:
Load Impedance = 1
1/Load1 + 1/Load2 + ...
Series Loads:
Load Impedance = Load1 + Load2 + ....
Bridging load = Total Load / number of bridges (usually=2)
Example: You have two four ohm loads, and if you run them in parallel, you get a 2-ohm total load.
The bridging load in this case would be: 2 ohms total load / 2 bridges = 1 ohm
A very few (very pricey) amps can handle 1 ohm loads. Most auto amplifiers are designed for 2-4 ohm impedance. So it would be dumb in this situation to run the two subs in parallel / bridged with most affordable amps. It would make more sense in this example to connect each voice coil to a separate amp channel without bridging them, giving a 4 ohm load per channel.
There are too many permuations of amps bridges, and loads to cover them all. But in general if you use the three equations above, you can find an arrangement that gives you 2-4 ohms load per amp channel, you will have a good setup.
The other rule of thumb is to keep it simple. I would prefer NOT to bridge amps if you don't need to, but on the other hand 2-ohm loads usually give you a free 20% or so more amplifier wattage over 4-ohm loads. Eight ohm reduce the amplifier wattage of most car amplifiers by 30% or more.
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Many car audio amplifiers will give you nearly double the output into a 2 ohm stereo load vs. a 4 ohm stereo load.
The popularity of dual voice coil subs and lower impedance (2 ohm, especially) subs are what have spurred the increased popularity of mono sub amps. Most of these are optimized to run a 2 or 1 ohm mono load.
The amp I linked above will handle a 2 ohm mono load and provide plenty of power (500 watts).
