Two amplifiers driving a pair of speakers has got to be better than one, right? Well everything's not as logical as it first sounds in the world of audio. In many cases running two amplifiers (bi-amping) is a complete waste of an amplifier.
There's lots of information out there, but most of it is in long winded articles. Let's see if we can get the basics a little more clear with some diagrams.
I'll diagram out the three most common ways of connecting bi-ampable speakers: Bi-Wiring, Bi-Amping with a passive or no crossover, and Bi-Amping with an active crossover.
I'm going to assume you have a set of speakers that can be bi-amped or bi-wired. How do you know if you do? Look for two sets of terminals on the back of your speakers:
Bi-amp terminals on the back of Polk Monitor speakers.
Each set of terminals connects to either the highs in the speaker (tweeters) or the lows in the speaker (woofers). In the picture above you can clearly see the terminal bridges installed. Terminal bridges allow you to connect bi-ampable speakers via a single speaker wire to your amplifier. Just as you would if you had non bi-ampable speakers.
First up we have bi-wiring. Bi-wiring is simply running two sets of wires to each speaker. Your first logical thought is that adding more wire will reduce resistance of the wire connecting speaker to the amp. Well yes, but the resistance of the wire is so small as to be negligable. Assuming you're using decent speaker wire; Bi-wirings only benefit is wasting a length of wire.
Bi-wiring uses twice as much wire compared to a normal connection.
Next up Passive Bi-Amping. Here's the real waste of power with no benefit. Hows that you ask? I mean double the power right? How can that go wrong? (Jeremy Clarkson voice) POWEEEEERRRR!!!
In this case both amps amplify the full range of frequencies, and that's the problem. As we talked about above one set of terminals is only wanting high frequencies for the tweeters. The other set of terminals only wants low frequencies for the woofers. To ensure that your tweeters aren't blown by low frequencies resistors and other electronics are installed to block all low the low frequencies on the tweeters connections. Half those amplified frequencies are never going to reach a speaker. The amplifier handling the tweeters is having all low frequencies blocked before they make it to the tweeter. The same goes for the amplifier handling the woofers, half of its energy is never used, being blocked before the woofers. And I use half loosely. Woofers use more power as a percentage than tweeters, but the analogy stands (note the math on the diagram).
Passive biamping is the waste of a perfectly good amplifier.
Lastly we have Active Bi-Amping. Yes this one actually has a use, but the benefits may not be as great as you would hope. If your amplifier is powerful enough that it can cause the speakers to distort before the amp itself distorts, then you have zero reason for this setup. On the other hand if, in a normal wiring setup, your speakers can handle your amplifier at max level then you may see some benefit. But I'd argue the case of why purchase two weak amplifiers and not just buy one powerful amplifier in the first place?
The full power from each amplifier is being seen by the speakers.
And in this setup we come across the problem with the fact that tweeters need less power than woofers. So assuming you're using matching amplifiers (and they should match for even sound) for both the tweeters and woofers. When the amplifier on the woofers is at max levels, the amplifier at the tweeters will be at a considerable less level; a waste of power. But you say, "I'm not planning to max my amplifier out. That would be too loud..." then whats the point of having double the amplifiers if a single one supplies enough power for your comfortable listening level!?
If you still say (Jeremy Clarkson voice) "POWEEER!!!!" then by all means, bi-amp away.
I hope this sheds a little light on a sometimes counter intuitive concept. Comment below with your thoughts.
Looking for a super in-depth conversation? Try this AVSForum thread. 35 pages of discussion.