with parallel bridging you can drive lower impedances of speakers.
With serial bridging you will drive higher currencies through the speaker.
So, imagine you have a 8 ohm speaker and one single 100W into 8 ohm amp (200W into 4 ohm), and the minimum load that may be connected to the amp is 4 ohm. When you bridge 2 of those amps in series, the fictive load per amp is 4 ohm when a speaker of 8 ohm is connected. But now, we have twice the voltage and twice the current of one single amp with the same speaker. We all now that P = U * I, so the new maximum power is: P(new) = 2*U(1amp) * 2 * I(1 amp)
=> P(new) = 4*P(1 amp).
When those 2 amp are connected in parallel, you will be able to drive a minimum load of 2 ohms (200W) instead of 4 ohms (200W).
You want to watch out when you start paralleling
amplifiers directly. Unless they are matched in gain
and DC offset, they can get into quite an argument.
The lower the output impedance (the more feedback)
the bigger the problem.
You can parallel Zens without a blink, and generally
you can parallel the Alephs and a lot of tube amps
If you have any doubts, use some resistance at the
output of each amp channel, at least initially, and measure
the voltage differences between the channel outputs
during operation. I suggest maybe .47 ohms as a
I knew quite a few people who bought two Conrad Johnson MV-75s (tubes) and paralleled the outputs. Worked very well, indeed. Killer 150W per channel monoblocks for a relatively reasonable price, particularly if you beefed up the power supply with some extra capacitance.
Check out Rod Elliots ESP pages, he gives a complete explanation of bridging, pitfalls etc, as well as a circuit to use to get two inputs for the amps, one inverting another not. I have built it and it works.
Keep in mind if one amp is not playing for some reason, but on, the other will be effectivly dead shorted. If one amp happens to be off, it risks damage. Make sure each amp is playing EXACTLY the same level -- they should be measured with a multimeter before paralleling them! Connect the AC voltmeter to the positive terminal of each amp, and while playing at any volume the reading should be zero, or millivolts at most.