Many amps manufactured these days are rated only for 8-ohm-and-above loads, and not for 4-ohm loads. The manufacturer does this largely as a cost savings. Amps which are capable of driving 4-ohm loads to the same output voltage require heftier power supplies, heat sinks, and (often) output-stage transistors: they'll be delivering twice as much current into the load, and will be dissipating roughly twice as much heat within their output stages.

If a manufacturer chooses to quote a power rating at 4 ohms in their advertising, the amp must be capable of delivering this much power after a 'warm up' period of operation at 1/3 power (which level actually dissipates _more_ heat in the output stage than full-power operation).

In order to save money during manufacture, manufacturers often use skimpier power supplies, heat sinks, and output stages - and as a result, the amps may have a 4-ohm power rating which is less than the 8-ohm rating. This is somewhat embarrassing for the manufacturer to advertise - and, so, they often do not quote a 4-ohm power rating at all, and state that the amp is designed to be used only with loads of 8 ohms or above.

With many such amplifiers, you can drive a 4-ohm load safely, as long as you don't try to drive it too hard. If you drive a low-Z load to too high a volume, one of several things may happen: the amp may begin to "clip" (sounds very harsh and distorted, may damage the tweeters), or may overheat and shut itself down, or may overheat and burn up (all the magic blue smoke leaks out).

		Wire a 4-ohm power resistor (10-20 watt) in series with each 4-ohm speaker. This makes the system to appear as 8-ohm load and is inexpensive. The cons are that the resistor wastes power, may cause frequency response go bad because speakers do not have constant resistance with frequency. When you play at high volumes the resistor may get hot and burn thing or itself.
		Using 4 ohm to 8 ohm matching transformer will not waste much power, but the transformer will be heavy, expensive and hard to find. Transformer has also problems in playing back lowest frequencies (saturation causes distortion in high levels) and in higher frequencies the inductance in the transformer will cause phase shifts.
		You can wire two 4-ohm speakers in series if you have two identical speakers. Problem is that if the speakers are not identical type the frequency response and power distribution will be uneven.
		Most "8-ohm" amplifiers can drive a 4-ohm or 6-ohm load as long as you don't try to get full power out of the amp (if you do, it may overheat and shut down).

Buy yourself a decent power amplifier whose output stage and power supply are capable of handling a real honest low-impedance load. Good amplifier will be expensive but gives best sound quality and reliability.

The output impedance of an amp should be extremely low. If it's .8 Ohms, then an 8-Ohm speaker has a damping factor of 10. If it's .08, then the amplifier provides a damping factor of 100, etc. Don't confuse the actual output (source) impedance with the load impedance that is recommended for the amp (4-Ohms, 8-Ohms, etc).

The idea is that if the speaker is 8 Ohms, and the amplifier has a source impedance of .08 Ohms, then the amplifier "damps" the motion of the cone by a "factor" of 100. In reality, the true damping that the cone "sees" is determined by many things, part of which is the damping limitation imposed by the resistance of the voice coil, usually around 5 Ohms or so for an 8-Ohm speaker. You can see that if the speaker has 5 Ohms of resistance, the internal (source) impedance of the amplifier (.08 Ohms for a damping factor of only 100) doesn't add much to the total resistance in the voice coil circuit, hence has very little effect on total damping. So any modest change in the amplifier’s damping factor correlates to virtually no change in total damping.

A speaker designer shoots for a certain damping (same as 1/Q) to achieve a certain desired type of low-frequency roll off. The assumption is that the source impedance of the amplifier is 0 Ohms. If the source impedance is .08 Ohms (damping factor of 100), very little error is introduced into the system. Higher damping factors are getting into diminishing returns in terms of the total damping. In practice we want a certain, relatively low damping figure for the whole speaker system, (1.414 for a maximally flat bass response).

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".

Bridging typical HIFI amplifier 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 appearing 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.

Another interesting consequence of bridging is that the amplifier’s 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.

If your amplifier does not have built-in bridging option built in you can use an additional stage to invert the signal for one channel but drives the other channel directly.

Reprinted with permission from http://www.us-epanorama.net/