PAPERS

Active Crossover Applications

Not a Straight-forward Issue . . . .

We've been asked several times about the application of the AKSA to active-crossover, multi-amplifier systems. The most recent suggestion was use of an AKSA as the bass amplifier in a tri-amped, active crossover. Other enquirers have suggested the addition of passive first or second order crossovers preceding their AKSA in attempts to achieve the same result. The following discussion considers the issues. There are couple of points to make about active crossovers which are quite significant. This is not a straightforward area, unfortunately, and one must consider the options carefully since the extra cost and complexity is appreciable.

To create a low pass filter on the front of an amplifier you need additional circuitry which cuts the input level to the amplifier drastically as the frequencies pass the specified crossover point. Let's choose a corner frequency of 100Hz; a simple, 1st order crossover made up of a single resistor and capacitor would, however, only give you flat response at 100Hz (no attenuation), 6dB down at 200Hz (very little attenuation, you could hear it as about three quarters of the original loudness), and 12dB down at 400Hz. It would be 18dB down at 800Hz, and 24dB down at 1600Hz (just audible at 24dB down). Depending largely on your drivers, this is generally not adequate for a domestic hifi. Too many higher frequencies would come through the bass driver, very likely far beyond its engineered range, and the quality of this sound would ruin the wonderful contribution from your mid-range driver.

To fully exploit the advantages of an active crossover, you need more attenuation as the frequencies rise above the crossover point. 6dB/octave is not enough, but neither is 12dB/octave. For example, speech can be heard very clearly through a woofer with crossover of 100Hz and a slope of 12dB/octave. You don't want this to happen, since it affects intelligibility and adds 'mud' to the sound. Woofers are usually poor at reproducing speech frequencies and beyond...
Now we strike a different problem. A taller slope on the crossover curve brings very considerable attenuation of the signal at all frequencies; it is very lossy. Let's say we use a 3rd order, of 18dB/octave. If this were made up of three capacitors and three resistors, sitting at the very front end of the amp where the input impedance is 47kohm, the attenuation up to 100Hz would be about 10 times. Thus you would need 10V input to get 1V output, even before the signal enters the amplifier. This complication points you in yet another direction.

Electronic (or 'active') crossovers are widely use in pro-audio. A stadium rock concert might use around 150,000 Watts of amplification, with at least 120 amplifiers, and usually with a four way crossover system, using balanced lines throughout and 24dB/octave slope. These systems really work, very well in fact, and they ensure huge amounts of power go only to the intended drivers over a specified narrow frequency range. It is almost impossible to discern human speech on a sub-woofer with a 24dB/octave slope at lowish volume. This is the best option with an active crossover; 18dB/octave is not favoured because it does not permit full exploitation of the high power, narrow frequency range of the drivers. No, the 24dB/octave system is the best because the slope is quite abrupt and the phase output can be arranged to linear throughout all passbands.

However, the attenuation problem is a serious issue with a passive 24dB/octave system. This means they must be built with integrated voltage amplifiers so that 1Vrms in gives 1Vrms out. This is the 'active' in the active crossover description. The gain blocks can be quality integrated circuits (good ones like the OPA275!) but can also be done with tubes. The tube option is quite complex and expensive, however, and inclined to create noise unless very low noise tubes are used; many designs we have seen use the ECC88/6DJ8. Surprisingly, all this low level electronics has very little effect on the integrity of the music; the real damage to the music is usually inflicted by the power amplifier stage.
An active crossover is essentially a highly specialised pre-amplifier, with a similar level of complexity. These beasts are widely available as constructor circuits in popular electronics magazines; Elektor published one in the mid-eighties and again in more recent times. The best of them are the Linkwitz-Riley circuits, which are phase linear and which deliver 24dB/octave slopes which can be individually adjusted to suit the gain of the amplifiers and sensitivity of the drivers. The circuit I have in mind has three outputs; woofer, mid-range and treble, and would be ideal for most domestic purposes. It requires a +/-15V power supply, and the PCB is about 8" square - making it markedly more difficult to construct than the AKSA, in fact.

If you should choose to go the active crossover route, you cannot effectively do it with simple 1st or 2nd order passive crossovers in front of the amplifiers. Unfortunately the issue is much more complex, largely because of the nature of drivers, human hearing, and the massive attenuation of passive filters. You really need an active crossover, it should be 24dB/octave, and it is a complex piece of equipment which requires careful assembly. Now you begin to see why passive crossovers are put into the speakers, rather than into the front ends of amplifiers....
In Australia an electronics chain is now importing a sub-woofer from Taiwan which is available with a built-in amplifier. We understand there is a crossover also available, which can be bought separately. These products are in Europe and the US, we believe, so check out some of the higher end electronics shops. It could save you a lot of hunting around for components and boards.

As an aside, and casting no aspersions on the other choices of amplifiers constructors might make in active crossover systems, we have compared the AKSA to a refurbished Leak Stereo 30 and found the bass vastly superior, the mid-range a very close match, and the treble clearer and more detailed. This comparison was made with a full range system operating into a bookshelf speaker with 6dB/octave crossover and Morel MWP142 woofers in D'Appolito configuration. While the active crossover is the esoteric way to go for domestic systems, very high levels of performance can be achieved with conventional speaker crossovers. It is true that the very high powers of pro audio mandate use of electronic crossovers, but their complexities make them a problematic choice for home systems. The secret is to work out where the damage is done to the music, and fix the problem. In general, the damage is not done in the passive crossovers used in conventional domestic speakers as long as the crossovers are 1st or 3rd order, the design is carefully crafted, and the component quality is high. There is probably more to be gained by correct choice of amplifier and cables than there is by use of a three way, 24dB/octave crossover.
Of course, last but not least, the path we advocate is less complex and a lot cheaper.

Hugh Dean

© Copyright Hugh R. Dean 2001
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