Whether the crossover is set up for two, three, four or more bands will be largely pre-determined by the PA’s speaker system. Here’s an outline of what the different numbers of ‘ways’ generally accomplish, and how they’re placed.
Two-way crossover systems require a minimum of two amps, so they’re referred to as bi-amped. The split signal feeds just low frequency (LF) and high frequency (HF) drivers. Such a simple set-up is typically restricted to monitors or mini PAs, because for most kinds of music it’s not possible to cover the audio range fully enough (particularly with low-enough bass) with only two types of drive units.
Three-way systems (sometimes called ‘tri-amped’) feed low-, mid- and high-frequency drive units, and are the most widely used configuration for ordinary FOH PAs and more elaborate stage monitor cabs.
If you require a three-way PA system on a tight budget, you can use set-ups such as ‘bi-amped + passive split’ or ‘two-way active with passive split.’ All that’s required is a single two-way stereo crossover, a minimum of one stereo power amp per side and three-way cabs wired for this approach. The system is still bi-amped, but is also three-way. It’s achieved by ‘splitting’ the HF band’s signal (really mid + high) using an additional passive crossover in each mid/high speaker cab. Hence ‘passive split’. The active crossover splits the bass from everything else. The passive then splits the HF from the midrange – like a sequence of two forks in a road network.
Despite the limitations of passive crossovers, passive splitting of HF (only) can work quite well.
One restriction of the ‘passive split’ configuration is that the top-end’s level can’t be limited with any discrimination – any protective limiting is ‘lumped in’ with the midrange (we’ll cover limiting in more detail shortly). But this only affects the ultimate sound level capability. The excess levels in one or the other will turn down both. Still, the limit has to be set low enough to protect the tweeter, rather than the mid, which would handle more power.
If higher sound levels are really needed, more active ‘ways’ are simply going to be required. But you can still add a passive split to these.
Typically this is done for occasions when super-tweeters (working at frequencies above about 14-16 kHz) might be used – for example providing a PA in a small venue where high frequencies won’t be largely absorbed before they’ve reached the audience.
Four-way crossovers (quad-amped) will either send signals to a sub-bass driver, plus low, mid and high units; or else to low, low-mid, high-mid and high-frequency speakers. It’s mainly used for more up-market FOH PA.
Five-, Six- and Seven-way
These were more common in the past, when PA developers experimented with different schemes – and before accountants worked to prune tour costs to the bone. The frequency ranges became further sub-divided and the speakers used were increasingly specialized. These higher-way systems are more complex, but offer potentially higher sound quality and ‘maximized’ power handling.
But the ‘law of diminishing returns’ sets in fairly steeply once the crossover has split-up the audio range into four fairly equally-sized bands. For one thing, the weight of amps and cabling, as well as the wiring and rigging complexity, is bound to start increasing substantially, but without much worthwhile increase in sound level or quality. There’s also an acoustic trade-off, in that it can be increasingly hard to synchronize larger numbers of separate sound sources.
Despite this, you will occasionally still meet five-, six- and seven-way systems. There are commercial analog (active) crossovers with five and more bands, some of them flexible modular types. Or else they’re bespoke (custom-made), way crossovers. The more upmarket digital crossovers also typically offer up to six bands.
This article is reprinted with permission from The Live Sound Manual, published by Backbeat Books, www.backbeatbooks.com. All information is copyrighted and cannot be reprinted without the permission of the publisher.