Bi-Amping: Pleasure or Pain?

In the world of high end audio, bi-amping is (or has become) a very misunderstood concept. In an attempt to clarify the situation, we’ll have look at the ideal model, define it, and explore the advantages and disadvantages of implementation in a modern audio or video system. First, the definition and background.

Using two separate amplifiers, one for the bass frequencies, another for treble, is an idea that has been around for many, many years. In the professional audio industry (where the concept originated), using multiple amplifiers to serve different frequencies is more than just an option; in many cases an absolute necessity. Audiophiles and videophiles look at bi-amping (or tri-amping) as a way to improve performance, the pro audio industry sees multi-amping as a basic requirement.

Bi-amping or multi-amping (splitting the signal into more than two frequencies) is not only far more efficient, it allows a large sound reinforcement system to be scaled to meet the needs of the application. A traveling show, as an example, encounters many different venues, each with different requirements for good sound reproduction. Large settings demand more speakers to fill the space; some locales require a different mix of drivers to achieve the correct tonal balance (e.g. outdoor venues). A multi-amped modular system allows the flexibility required to fulfill the diverse needs.

The large scale systems for concert venues, auditoria, churches and stadiums require huge amounts of power to accomplish their given tasks. The number of speakers and amount of amplifier power necessary to fill a large auditorium or arena is mind boggling. Without multi-amping, the amount of power necessary would be exponentially greater. Why is this? The answer lies is an important part of multi-amping, and indeed the part of the puzzle most often overlooked by audiophiles; the use of the electronic crossovers.

Passive crossover networks found in consumer speakers waste an enormous amount of power. The often complex network is made up of large coils, chokes, capacitors and resistors. The circuit splits the full range signal into different frequencies (low, mid and high) appropriate for the different drivers in the speaker. Further, a crossover network compensates for efficiency differences in the drivers; woofers demand mode power than midrange drivers which in turn demand more power than tweeters, etc. Further, each of the drivers has different sensitivities, with some requiring far more (of far less) power than other drivers in the same speaker system. In a passive crossover, the excess power not required is dumped into resistors and burned off as heat. This makes for an incredible waste of power.

in addition, passive crossovers do much to degrade the signals that pass through them, and wastes a good deal of amplifier power, so bi-amping is an attractive idea. But there are pitfalls to be recognized before one embarks on that journey.

As we have seen, the key part of the equation is the electronic crossover. Splitting the signal at line level allows us to bypass the lowly passive network. So, buy another amp, an electric crossover and you are off to races. Ah, but it's not that simple. Now comes the task of calibrating the crossover to your speakers; making sure that the drivers are sent the specific frequencies their designers intended, and that  slopes (the rate at which the transition between the frequencies occur, and how much they overlap) are correctly set.  These adjustments are key to not only optimum performance, but system safety; operating a driver beyond its range will likely result in its failure.  Maybe you've read the book, "Poof the Mangled Driver"?

Ok, so what about just using two amplifiers and forgetting about the electronic crossover? Simply using two amplifiers is not true bi-amping and does not offer the same advantages; we still face the limitations of the passive crossover. What about the notion that bi-amping reduces stress on the amplifiers since they are powering only limited frequency ranges? That would be true in a true bi-amp configuration where the frequencies are split ahead of the amplifiers, but in a passive environment both amplifiers receive a full range signal from the preamp and dump that power into the speakers, regardless of whether one is connected to the tweeter or woofer inputs. The only benefit (and it marginal at best) is simply the additional power offered by the second amp.* 

Lastly (and maybe most importantly), the idea of using different types of amplifiers is a real issue. It has long been thought that the ideal situation was to use a sweet, refined low powered amp in top (tubes, for instance), teamed up with a powerful  (usually solid state) amp to control the bass. This may indeed produce nice extreme top and bottom, but rarely did the two disparate sonic characteristics of the two dissimilar amplifiers mesh well in the critical midrange area. Further, matching signal level between both amplifiers extremely difficult, maybe impossible without sophisticated measurement equipment. So, more often than not, pseudo bi-amping, or poorly executed true bi-amping causes more problems than it cures.

In most cases, I am not a fan of bi-amping a high end audio or video systems. As we have seen, it can be a fairly complex (not to mention expensive) modification. Proper implementation requires the use of multiple amplifiers and an outboard electronic crossover. This mandates bypassing the internal passive crossover, which requires work inside the speaker, and will clearly void your warranty. There is significant expense in hardware: the additional amplifiers and crossover, not to mention the extra cables required. Further, we dramatically add to the complexity of the system. Though it can offer substantial benefits in the right set-up, in most every instance the listener is better served by using the funds to upgrade components in the system.