In theory, bi-wiring separates the crossover legs, preventing any bleeding of the back EMF from one driver, through the crossover to another. Given that the motor on the bass driver is (in most cases) the most powerful in a speaker system, thus possessing the greatest back-EMF, separating it from the other drivers by splitting the crossover makes sense. Moving from bi-wiring to bi-amping takes that a whole step further.
Now think about the topology of bi-amped systems. Most use a pair of stereo amps and it quickly becomes obvious that they can be arranged in one of two ways: you can use one amp to drive the bass leg of both speakers and the other to drive mid and treble; or you can use one channel of one amp to drive the bass leg and the other channel of the same amp to drive that speaker’s mid and treble. These two options are generally referred to as ‘horizontal’ or ‘vertical’ bi-amping. If you are using dissimilar amps then the former (horizontal) bi-amping is your only option, but then, as we’ll see, you shouldn’t be using dissimilar amps to start with. But assuming for the moment that you are using a pair of identical stereo amps, then vertical bi-amping will deliver by far the most musically engaging results. Why? Because you are spreading a single pass band across two channels and a single power supply. Each channel has a reduced bandwidth and reduced challenge to deal with, while the power supply doesn’t see simultaneous peaks in both channels, at the same frequency and at the same time. If that proposition strikes you as odd, just consider where most pop and rock recording place the bass guitar and drums, let alone those slabs of synth bass that populate everything from dance tracks to soundtracks. Of course, for those using four, identical mono-blocs, whether those mono-blocs are in two chassis or four, the argument becomes moot. But all the time you’ve got one transformer and two channels, then splitting the amp so that one channel works at low frequencies and the other higher up, so that signal peaks rarely coincide, delivers a very real benefit.
Bi-amp operation eases the amplifier’s load in another way too, by effectively eliminating the need to drive at least one crossover point. In a three-way speaker, there are two crossover points, where an amplifier will have to drive different drivers, with different electrical characteristics and demands simultaneously, while also dealing with the filter characteristics of the crossover itself. Split the crossover between bass and mid/treble and the channel driving the bass sees one frequency range and a roll-off – rather than multiple frequency ranges, multiple different drivers and two simultaneous and overlapping filter slopes, while the same simplification applies to the mids and up. Mmmm… guess which it prefers?
None of this means that horizontal bi-amping is a complete bust, or that it won’t/can’t offer improvements over a single amplifier set up. But it isn’t the slam-dunk upgrade that vertical bi-amping represents and if it involves different amplifiers then it risks introducing a major discontinuity into the system or at the very least, a challenge to its overall integration.
The righteous path…
In order to bi-amp vertically, you clearly have to have four identical channels of amplification, normally divided across two chassis. But the demand for consistency/symmetry goes further than that. Using the same amplifier top and bottom on a single speaker gives you a real advantage in terms of musical coherence. Arguably the hardest part of designing a multi-way loudspeaker is to seamlessly combine the output of the different drivers. It’s something that is reflected in the often brutally severe slopes used in crossovers, with their equally brutal impact on the speaker’s phase coherence and electrical characteristics. Start driving the different drive-units with different types of amplifier and you are reinforcing the problem. But by the same token, start mixing cable lengths, terminations or types and you are introducing further discontinuities into the system. The AC supply to the amplifiers and the use of identical power cords and grounding arrangements are just as important. Keeping everything as identical as possible, not just left and right but top and bottom is crucial to achieving the best possible results.
