Bi-amplification of Loudspeakers

Many audio enthusiasts see bi-amping as an intelligent upgrade for their hi-fi systems. Bi-amping is, simply, the process of "doubling up" on amplifiers in a system and hence doubling available power. This article seeks to point out some important considerations to bear in mind when bi-amping speakers.

Don't Mix & Match
It is crucial when bi-amping speakers that you not mix and match amplifiers. One current trend among audiophiles is to use a solid state amplifier for the bass frequencies, and a tube amplifier for the mids and highs. While this may initially seem logical, there are several reasons why this is a bad idea. Let's take a look at each one independently...

Rise Time
For one thing, two different amplifier designs will inevitably have different rise time specifications (sometimes referred to as event time). For the purposes of this discussion, we will define rise time as the time required for a signal to travel through an amplifier. Depending upon the amplifier circuit, the rise time will vary. Even a small mismatch in rise time will significantly degrade the sound of a bi-amped system.





Consider that in a bi-amped system, there are two possible configurations. The first, pictured above left, is more common. In this scenario, one stereo amplifier is dedicated to the high frequencies, and one amplifier to the low frequencies. The other scenario, pictured above right, is referred to as vertical bi-amping. In this scenario, one stereo amplifier is used to run both the high and low frequencies on one loudspeaker. (Note: In each of these scenarios, obviously two monoblocks could be used in place of one stereo amplifier).

In the case of "standard" bi-amping, any discrepancy in rise times would result in timing mismatches between the low and high frequencies. This sort of mismatch between frequency ranges will degrade the sound substantially because the precision in timing is important to maintain the integrity of harmonics. When a hammer strikes a string on a piano, it is not one frequency which is reproduced, but several - the fundamental and each of its harmonics. In a loudspeaker, it is likely that the fundamental will be reproduced by one driver, and the harmonics by at least one other driver. Hence any timing errors will result in a degradation of tonal balance, and a smearing of the soundstage.

In the case of "vertical" bi-amping, discrepancies in rise times will result in mismatches between the left and right channels. The results will obviously effect imaging, especially soundstage depth. A simple analogy clarifies the scenario: using amps with the same rise time is important when vertically bi-amping for the same reason using cables of the same length is important.

Gain
Another critical consideration is the gain of each amplifier used in a bi-amped system. The greater the gain of an amplifier, the more quickly the amplifier's output will increase as the preamplifier's volume control is augmented. Conversely, as the preamplifier's volume is attenuated, the amplifier's output will decrease at a rate directly proportional to its gain. One simple way to think of the net effect of an amplifier's gain is to associate high gain with bigger "steps" in volume and lower gain with "smaller" steps in volume.

What this means in the case of "standard" bi-amping, is that if the gain of the amplifiers is not matched, the volume of the highs and lows will be changing at a different rate as the volume on the preamplifier is adjusted. In the case of vertical bi-amping, the volume of the left and right speakers will change at a different rate. The effects of these volume discrepancies should be fairly obvious: in the first scenario tonal balance is altered, and in the second scenario the stereo channel balance is altered.

Concluding Remarks
Bi-amping provides many benefits, especially when using lower powered tube amplifiers where the system's overall power output can be doubled easily by simply bi-amping the loudspeakers. Many of our customers at Symphony Sound bi-amp with SET tube amps (2A3's, 300B's, etc.) and enjoy wonderful sound as a result. However, it is important to bear in mind that if certain guidelines are not followed, bi-amping can actually do more harm than good. If careful attention is paid to gain and rise time, it is possible to mix and match amplifiers, but only is unusual circumstances do we feel this is a sensible and prudent approach.

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