These are good yet basic questions that many audiophiles do not know or understand. As such, it makes a great platform for all to learn and share. Thanks for posting your questions. Hopefully we can answer them in a manner that you and others can understand.
When you "bridge" a stereo amp, you are combining the two channels together electrically. As such, an amp rated for 100 wpc @ 8 ohms and 200 wpc @ 4 ohms would be rated at 400 watts into a single channel @ 8 ohms.
Due to how the amplifier circuitry works ( i'm keeping it simple here ), it sees half the impedance of what the speaker is really rated at. Since an 8 ohm speaker would look like a 4 ohm speaker to a bridged amplifier and you've tied both channels together, you get two times its' rated power into that 4 ohm load. Hence, your 2 x 200 @ 4 ohms is now 1 x 400 @ 8 when bridged.
Keep in mind that most amps don't "double down" ( double power as impedance is cut in half i.e. our 100 wpc @ 8 & 200 wpc @ 4 example). Many amps rated at 150 wpc @ 8 will do appr. 225 wpc @ 4. As such, it should do appr. 450 wpc bridged @ 8 ohms, or what is two times the 4 ohm rated power.
Since most bridged amps do not want to see a speaker with a low impedance rating to start off with, you have to be careful. A speaker that is nominally 4 ohms would look like a 2 ohm load to a bridged amp. Since most speakers DO dip measurably lower than their average rating, this could go VERY low and become quite close to what looks like an electrical "short circuit" to an amp. This can cause it to run very hot and / or thermally shut down. There ARE amps that can deal with being bridged and driving very low impedances though. This is a sign that the amp is both powerful and has a very sturdy power supply.
Another factor that most people don't realize is that as impedance of a speaker is lowered, the damping factor of the amplifier is reduced. The closer the impedance of the speaker is to the output impedance of the amplifier, the more likely the bass is to become "sloppier" or "bloated". That is why some amps have a hard time with or sound "loose & flabby" into low impedance speakers. The amps output impedance may be higher AND the power supply may be current limited when really "gittin' it". Since tubes amps typically are high impedance designs on their output section AND current limited, this explains why SOME tube amps suffer from "round" or "ill-defined" bottom end, especially with low impedance speakers.
Your questions about amplifier configuration is a matter of personal choice. Running one amp for the highs and the other for the lows is called horizontal bi-amping. This can be done using two identical amps or two different amps. One benefit to this is that you can run a "soft, sweet & airy" amp for the mids and treble and a "big brute" for the bottom end. Another benefit to this is that when large bass notes pull on the amps' power supply come into action, it would only sag the woofer amp without affecting the output of the mid and tweeter amp. This can result in more stable imaging and soundstage. Of course, this would only happen if the amps were not quite powerful enough to deal with the demands being presented to them in the first place.
While running two specialized amps for each frequency range can give you the best of both worlds, there is more work involved since you now have to worry about making sure that each amp puts out the same amount of power with the same amount of "drive" or input signal applied. This requires either gain matching of the individual amps or an active crossover with individually adjustable channel output control levels.
The method that uses both channels of a stereo amp to individually run the high and low section of one speaker is called vertical bi-amping. You would obviously need two identical amps to do this if running in stereo. This avoids the need for gain matching in most instances ( as long as the amps are built to specific standards during production ) and can also be done passively ( no active electronic crossover ).
Either way, an electronic crossover with a relatively steep slope can be drastically advantageous IF properly set up and the speakers are optimized for it. Since the amp would only see a portion of the frequency range, it can now concentrate on reproducing ONLY that area. As such, it becomes more efficient. The smaller the range that it has to reproduce, the more efficient it becomes. In effect, a 100 wpc can seem as powerful as an amp rated for almost twice its' power output if the system is well assembled and carefully tweaked. This is why you can get away with running a "wimpy" amp on top in comparison to what the bottom end requires. The "wimp" doesn't have to supply or even get to see any of the high current, big power demands of low frequency reproduction.
Your other option was bridging the amps into monoblocks and running one for each speaker. Many amps do not sound as good when running bridged for some reason while others cope with this in stride. This gives you very high voltage potential compared to the other amps but your still stuck with the current limitations as if you were running only one channel of the amp. THIS is why many bridged amps get sloppier with low impedance or reactive speaker loads. Another common complaint with bridged amps is that high frquency reproduction becomes "harder" or "grainy" sounding. Of course, this is all up to individual designs and components.
As you can see, there are pro's and con's to each method. Much of what works best will depend on the specific amps and speakers in question and what you're trying to achieve. Either way, the expense of the system has drastically risen, as you now need TWO amps and FOUR sets of speaker wires AT THE MINIMUM for any of the scenarios that you presented.
Do your homework very carefully before jumping into any of the above set-ups. It can get very involved and is definetly more work to get things dialed in. Make sure you know what your dealing with BEFORE shelling out the cash. Hope this helps and clears some things up for you. Sean
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