How Do You Bridge An Amp?

You have to put a "Capacitor" between the amplifier and the pre amplifier or between the microphone and the re amplifier to prevent the direct currant from flowing back. If you don't put a capacitor the direct current from the Power Amp can go to the Pre Amp and burn the circuit soon (or later).
When you choose the capacitor you have to consider many things. Through a capacitor only an alternating current flows. A capacitor can show different resistances to different frequencies of current. The capacity of the capacitor you chose depends on the current that will be given out from the pre amp. To find that you have to know the power out (wattage) of pre amp and the operating voltage.

Calculation:
Then use,
P=IV
I=P/V
Divide the power out by voltage and get the current output.

Then use,
Q=CV and Q=It
therefore CV=It
CV=I/f
C=I/(f x V)

Don't worry if you don't understand. Just put the values to the following equation on a calculator. Or email me these parameters to vishva8kumara@gmail.com

C = Capacity in Farads, I = Current in Amperes, V=Voltage in Volts, f = Frequency in Hertz, x = don't ask me, that's the multiplication sign :)

                     C = I / (f x V)

Faraday, Ampere, Volta and Hertz are real people (scientists). I got to honor them for this equation.

Oh I forgot to say that frequency should be between 100Hz and 15,000Hz. Put 120Hz for "f" (frequency)
P.S. Audible range for human ear is between 16Hz and 22,000Hz.
Putting 120Hz will give you a maximum pass at 120Hz (and a natural distribution through frequencies lower and below / not sure about this < citation needed). Don't put capacitors parallel to get the full range flat, that will only increase the capacity and will pass very low frequencies and some DC backwords (to pre amp).