A "Channel" on an Amplifier refers to the individual module or circuit inside the amplifier which takes in a signal mono low level signal and boost's it to drive a loudspeaker(s).

All Stereo amplifiers have 2 "Channels", one channel handles the left hand signal and speaker, the other obviously does the same for the right. Other than some amplifiers sharing the same powersupply, the internal Left and right "channels" or circuit boards are seperate, which means in the event of a loudspeaker problem or component failure the opposite channel should continue to work.

Okay - the load which determines how much power your amplifier provides to the speakers is dependant on the actual number of speakers connected.

The load is called the impedence, and the level of impedence is measured in "ohms". Most amplifiers will only work to a minimum of "4 Ohms" however some amplifiers such as Peavey, and QSC etc will work into a 2 Ohms load (often refered to as being "2 Ohms Stable". However this is rarely encountered since it needs 4x 8 Ohm Speakers to be connected to each channel to acheive this.

Working out Ohms:-

Most Disco and P.A loudspeakers are rated at 8 ohms, so connecting one 8 ohm speaker to one channel of the amplifier will present a load of 8 ohms to it.

Plugging in an additional 8 ohm loudspeaker into either the existing connected speaker or the second socket on the same channel of the amplifier (if fitted) will equal 8 ohm + 8 ohm = 4 Ohms load to the amplifier (not 16 like maths would suggest!).

To get 2 ohms you would need to connect:-

8 Ohms + 8 Ohms + 8 Ohms + 8 Ohms = a 2 Ohm Load, so to get your amplifiers 2 Ohm rating you need to connect (4x) 8 ohm speakers to it.

Given the fact that each channel is identical and seperate, your Amplifier has the capability of running 4 x 8 ohm speakers per channel! - a total of 8 speakers - which is why I said to ignore 2 ohm ratings when buying an Amplifier and concentrate on the 4 or 8 ohm wattage ratings!.

2 Ohms of load on the Amp can also be acheived by connecting (2x) 4 Ohm loudspeaker cabs to the same channel. Although 4 Ohm Speakers are usually used for Home Cinema or in car use, but some manufacturers such as Carlsbro and Peavey offer 4 ohm versions of some speakers.

So some common speaker combinations are below

8 Ohms load can be acheived by connecting either

1 x 8 ohm speaker

or

2x 16 ohm speakers (16 ohm speakers are very rare nowadays)

4 Ohms load can be acheived by connecting:-

1x 4 Ohm speaker

or

2x 8 ohm loudspeakers

or

4x 16 ohm loudspeakers

or

1x 8 ohm and 2x 16 ohm loudspeakers

Bridge Mode

"Bridge Mode" means combining both the two internal amplifier boards together to double the rated output. By doing this, your amplifier ceases to be a 2 channel or stereo amplifier, and becomes a Mono amplifier with the combined power output of BOTH the amplifiers into half the load.

For example - An Amplifier usually provides it's full rated power into 4 Ohms of load, which means connecting 2  8 ohm speakers per channel (A total of 4 speakers) to get this.

However if you switched a compatible amplifier into "Bridge Mode", this essentially means that both the left and right channels are combined and so you would get the combined power output of both channels into 4 Ohms (one pair of speakers). However the output would be MONO.