What the heck is an Active Crossover?

A crossover is the circuit that separates the audio signal into the appropriate frequency bands for the woofer, midrange, and tweeter. Normally, the crossover is inside the speaker box, and consists of high voltage capacitors, huge inductors, and sometimes resistors. When you plug the wire into your speakers, you are plugging into the input of the crossover. The output of the crossover is connected directly to the drivers. This is what is known as a passive crossover.

An active crossover is different in that it is placed in the signal path before the amplifier. An active crossover is powered, and is made up of op-amps, capacitors, and resistors. It takes the audio signal from the preamp, and splits it into the desired frequency bands. The drawback of this is the need for a separate amplifier for each frequency band.

The trouble with passive crossovers

The design equations for passive crossovers are based on the assumption that the drivers act as purely resistive elements, with resistance independent of frequency. Having done the measurements, I know that this is blatantly false. Matters are complicated further by inductors which have DC resistance, which must be accounted for. Therefore, getting a passive crossover right becomes a matter of testing, tweaking, and re-testing. I wasn't interested in attempting this without lots of free time and extensive testing facilities, neither of which I had.

What makes active crossovers so nifty

On the other hand, active crossovers are made from components which are as close to the "ideal" components assumed by the theory. As a result, when you design an active crossover, you get exactly the frequency response you expect. In addition, by splitting the signal up before the amplifier, the high frequency signal (in which distortion is most audible) is separated from the bass, which is the most demanding in terms of power. This results in reduced IM distortion, higher sound levels before clipping, and reduced risk of frying your tweeters due to bass-induced clipping.

My Active Crossover

At the top is a photo of my active crossover. The only parts not pictured are the power supply (bought from Marlin P. Jones Surplus) and the input and output jacks. The input from the preamp comes in on two white wires from the left. On the board, the tweeter filters are at the top (obscured by the power wires), the woofer filters at the bottom, with the midrange filters in the middle. The white wires coming off the right edge of the board go to the output sockets. The blue wires carry signal. The red and green wires are positive and negative power lines, and the entire top surface of the circuit board is a ground. The big reddish brown rectangles are capacitors, and the little blue blobs are resistors. The chips you see are dual op-amps. Go back to read about the speakers I built using an active crossover. For more detailed information, you can read the specifications for my crossover, or you can see my suggested references.

John Stimson / Art Rock Cafe, New Armageddon / john@idsfa.net