monkeytree said:
so to bridge it i link "in" "A" and "out"?
with mono input?
You have two amplifiers, lets call them 'master' and 'slave' - your input signal goes to 'master', on the 'slave' the input is grounded and point 'A' is connected to the output of the 'master' amplifier. You connect the 8 ohm speaker between the outputs of the two amplifiers.
Unless you have some specific reason for doing this I generally find it less than useful? - if you have a single 8 ohm speaker rated at 400W then it's fine!. However, this is probably not the case?.
A more usual situation (PA speaker wise), is that you might have two cabinets, each containing two 8 ohm speakers wired in parallel, this gives 4 ohms per cabinet, and the two amplifiers will give 2x200W stereo in to these speakers.
If you bridge the amplifier, you could then connect the two 4 ohm cabinets in series, giving 8 ohms, and producing 400W mono.
So you get exactly the same amount of power, into exactly the same speakers, but only in mono rather than stereo - and reduce your reliability as a single amplifier failure will result in a complete loss of sound, stereo is particularly useful for the extra redundancy it gives.