Bi-Amping

Bi-amping is the process of taking a line level audio signal and splitting it into two signals; one signal containing the high frequency components and the other the low frequency components of the original signal. The two signals are then amplified separately and sent to designated loud speakers.

The line level audio signal is split using an electronic (or active) crossover. A conventional loud speaker enclosure contains a passive cross over that performs the frequency split after the signal is amplified to full power. The active crossover performs a cleaner split between bass and treble than a passive cross over. The passive crossover contains an inductor which subjects the amplifier to a variable resistance load. The combination of clean split between bass and treble and elimination of passive crossovers produces a noticeable improvement in loudspeaker performance. In fact it is awesome!

The following diagram compares bi-amping to a passive crossover network. The line level input signal is the sum of a 200 and 2000 Hz signal of an amplitude of 0.5V each resulting in a combined signal of 1.0V. The split (or filtered) signals each contain a small component of the other frequency. These signals were generated with Audacity using the Mazoni high and low pass filter plug in. Audacity is a good alternative for generating signal sources if a hardware generator is not available.

The active crossover and power supply used in this amplifier were purchased from Elliot Sound Products (ESP) as PCB blank (Projects 05-RevB and 09-RevB). Mr. Elliot provides an Windows compatible executable that will calculate the required resistor and capacitor values for a given crossover (or splitting) frequency. A detailed explanation of crossovers and bi-amping can be found through these links to the ESP website.

In order to select the optimum crossover frequency it helps to know the frequency response range of each driver. Manufactures (reputable ones anyway) provide a frequency response curves such as the ones shown below (I used these drivers to build a pair of three way enclosures using an active crossover between low and mid + high range drivers and a passive crossover between mid and high range drivers.

The graph on the left, below, is the 8" woofer and graph on the right is the 5" midrange. The frequency response overlap between the two drivers is large so picking the exact crossover frequency is not that critical.

If the 8" woofer, characterized in the frequency response curve above, were paired with one of the tweeters, characterized in the frequency response curves below, to make a two way enclosure, nailing the crossover frequency is more critical because of the smaller range of frequency overlap.

If the 8" woofer were matched with either the tweeter on the left or right, which tweeter would be the best choice? If a crossover frequency of 1500 Hz is selected, the tweeter on the right has a more consistent response at 1500 Hz than the tweeter on the left.

Given a crossover frequency of 1500 Hz, the ESP Crossover Calculator is used to select the resistor and capacitor sizes. Advice for acceptable capacitor and resitor values is provided. For drivers discussed above, one possible R-C combination was calcluated and is shown below (1.34 KR and 56 nF).

For an explanation of how to hookup an active crossover see Amp Hookup