When an amplifier reproduces the entire musical range, passive crossovers made of coils (inductors) and capacitors are used in the speaker cabinet to separate the high and low frequencies so that each may be supplied to the appropriate loudspeaker driver. A loudspeaker can be a difficult load for any amplifier, but when additional inductance and capacitance enter the equation, this only makes matters worse. Add to this the fact that all passive crossovers introduce some degree of loss (in some cases as much as 3dB - which means that they are "stealing" half the available power), and one can see that getting rid of them cannot be such a bad thing.
Bi-amplification (or sometimes tri-amplification) helps to relieves such issue. What exactly does the term "bi-amplification" and "tri-amplification" mean? It means that there are separate amplifiers to drive individual woofers, tweeters, and in the case of tri-amplification, midrange speakers. In its simplest form, a two-way system, two amplifier channels would be required: one for the woofer (low frequency or LF) and one for the tweeter (high frequency or HF). For stereo, four amplifier channels are required, since there are four loudspeakers involved (2 LF and 2 HF).
Here are some of the attractive advantages of these systems:
- Better overall sound quality.
- Electronic crossover (required) allows for easy adjustment of crossover frequency to optimize system performance.
- Non linear and lossy effects of passive speaker crossover networks are eliminated
(big plus for low frequency drivers). - Electronic crossover allows excellent performance with very high filter slope cutoff
(keeps unwanted energy out of speakers that can't deal with it). - If one amp clips, the other amps in the system are not impacted (resulting in less degradation of the overall sound).
- Increase in audio output capability as compared to one large amp of equal
power driving a full range speaker. - Audio level to each frequency band can be easily adjusted (to compensate for different speaker sensitivities).
- Lower intermodulation distortion.
- Increased damping factor at woofer.
Unfortunately, nothing is free, and of course there are some non-trivial disadvantages associated with multi amp systems:
- Expensive: one amplifier is needed for each frequency band, also an electronic
crossover is required. - Overall system is quite a bit more complicated compared to a single amp full
range system. - Must be careful of tweeters (there should be some kind of DC block to protect
tweeters from low frequency "thump" at amp power on/off). - If not properly wired, ground hum and other noise can be a problem due to so many components to connect up.
An example of how the bi-amping increases the power output capability
The ROSSO 460 disposes of two 60W rms power amplifier (LF and HF) for each monophonic channel. Let's suppose that the low frequency signal is just barely strong enough to cause the amplifier LF to clip and the high frequency signal that is just barely strong enough to cause the HF amplifier to clip (the two signals lie on opposite sides of the crossover frequency). That means:
So, in this particular case, in order to reproduce the same overall signal with a single
full-range amplifier, it would be requested a power capability of about 240W rms.
What is the difference between bi-amping and bi-wiring ?
Some higher performance speakers include dual sets of connectors, usually the type known as "binding posts" . Models with dual connectors almost always also feature a special type of crossover with separate "high-pass" and "low-pass" sections. Speakers with dual sets of connectors will also work fine when used with a single set of speaker cables. In fact, they usually come from the factory set up for conventional operation with "jumpers" installed between the two sets. These jumpers can be easily removed for bi-amping or bi-wiring.
Bi-amping means that instead of driving a speaker full-range with a single channel of amplification, through a single set of speaker cables, you actually connect two sets of cables, with each set driven by a separate amplifier, or separate channels of a multi-channel amplifier. This way, low frequencies and high frequencies each receive dedicated amplification.
Bi-wiring involves connecting two sets of cables to your speakers, like bi-amping, but both sets of cables connect to the same set of output connectors on your receiver or amplifier. Bi-wiring doesn't deliver more watts to your speakers, so it doesn't offer the dramatic sonic improvement and higher loudness capability of bi-amping. Still, many audiophiles find that it adds subtle improvements in imaging and detail.
Bi-amplification isn’t new. It’s been with us for quite some time...easily since the days of vacuum tubes and mono. For many applications, the extra expense and complexity aren’t worth the improvement. If you want the best, bi-amplification is de rigueur.
