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Old 09-27-2002, 01:18 AM   #1
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OHM's and bridging.

can someone explain these terms for me. Really would like ot understand them better. more terms and definition would be appreciated. thanks
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Old 09-27-2002, 10:49 AM   #2
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Ohm = a unit used to measure the electrical resistance of a circuit, called it's Impedance.

The higher the number, the more resistance the circuit has.

Speakers have a set impedance based on how their voice coil is wound. It can't be changed.

For an amp, an 8 ohm speaker is easier to drive than a 4 ohm speaker. The resistance is more in the 8 ohm, so the amp doesn't produce as much power. With the 4 ohm speaker, the resistance is less, so the amp produces more power.

If it was a perfect world, the amp would produce twice as much power every time you cut Impedance in half, but things like regulated power supplies, cheap components, and battery voltage prevent this.

All amplifiers have a limit, though. If you cut impedance down to 2 ohms in an amp only designed for 4 ohm loads, bad things happen. The components of the amplifier are only designed to produce so much power, and making them produce more will cause overheating and eventually fry the amp.

Bridging is simply combining two channels of an amp into one to provide more power. The old rule is that bridging an amp cuts the effective impedance in half. So a bridged amp powering a 4 ohm speaker means the amp is "seeing" a 2 ohm load. This isn't really accurate, but it's a good thing to keep in mind. When an amp is bridged, it puts a greater demand on the components. That's why many 2 channel amps are rated at 2 ohms in stereo operation, but only 4 ohms when bridged.

If you have dual voice coil speakers, or multiple speakers, you can wire them differently to achieve different Impedance for the circuit. Wiring two 4 ohm speakers in parallel with yield a 2 ohm load, and wiring them in series will yield an 8 ohm load. There are diagrams floating around on how to do this.
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Old 09-30-2002, 12:47 PM   #3
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Great write up JRod! I have always wondered why most amps are rated at 4 ohms for bridged channels and 2 ohms individually.

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