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