Crossovers Demystified

[flemingmras]

For those of us who run electronic (active) crossovers and either biamp or triamp our setups, this is a must read topic.

The crossover is one of the most critical components in live PA setup. Most people don't even bother with them. They're the ones running maybe 250 watts of power and full range mains without subs. Full range mains usually consist of either a 12" or a 15" speaker with a horn. These cabs have an internal passive crossover built onto the backside of the input connector plate. Most subs have an internal crossover coil as well. However, low frequency crossover coils don't have a very steep rolloff unlike that of an electronic crossover.

Passive crossovers are made to go inline with the speakers themselves. The drawback is that these components rob power from the amp, and there's no way to adjust them either. The other drawback is that most crossover components can't take the full amount of power that the speaker can, and when you're running your speakers at program power (1/2 the peak or 2x the continuous rating) these components will fail. If you're running 2 way biamp(subs on one amp and mains on the other), remove the jackplate and bypass the rolloff coils on all low frequency drivers. On the mains, the mid freq driver will receive some highs, but this won't hurt anything. In a 2 way biamp setup, the horns however MUST be hooked up to their passive crossover units as sending low frequencies to a horn will blow it. In a 3 way triamp the horns will be on their own amplifier and the electronic crossover will be doing the crossover duties so these components can be removed also.

Electronic crossovers are the last thing in the chain before the amp inputs. They seperate lows, mids and highs in a 3 way setup and lows and highs in a 2 way setup, and send each band of frequencies to the amps designated to run the speakers that are made to work in those bands. Much more efficient than passive crossovers. However, if the crossover is not set up properly, you can and will blow speakers by trying to make them reproduce frequencies that they weren't designed to reproduced. I've already done this once.

Most electronic crossovers are of the Linkwitz/Riley type and roll off the crossover frequencies at 24dBu per octave. This means that on the low band, at frequencies one octave up from the crossover frequencies, the signal level has been knocked down by 24dBu, whereas on the high band at the frequency that is one octave down from the crossover frequency the signal level has been knocked down 24dBu. For example, if we have a crossover frequency of 100Hz, the low band output at 200Hz(one octave up from 100Hz) will be 24dBu less than what it's putting out at 100Hz. Given that 0dBu(what the crossover is putting out at frequencies within each band's range) is 0.775V, -24dBu would be about 0.048V. Go up another octave(400Hz) and it's putting out about 0.003V. So you can see that at 24dBu per octave it's a pretty steep rolloff curve. On the high band you're going down in octaves, so for a crossover frequency of 100Hz, at 50Hz(one octave down from 100Hz) the signal level is 0.048V and at 25Hz...you get the idea.

Most higher end crossovers feature an adjustable rolloff on the lower side of the low band. Frequencies below about 40Hz you can't hear anyway, and most direct radiating subs can't reproduce frequencies below that anyway. So by rolling off the low side of the low band at 40Hz, you're not wasting amp power trying to amplify frequencies that can't be reproduced, minimizing the risk of driver failure and freeing up some headroom from the amp, giving you better performance from your subs.

Most good crossovers also feature time alignment. This is mainly for 3 way systems, and it allows you to delay each band by X amount of milliseconds(usually anywhere from 0-10mS). What this does is it virtually pushes the speakers for each band back so many feet, which makes the point source part of each speaker vertically aligned with each other. You would typically delay everything to the horn, since the length of a horn puts the horn drivers magnet behind the magnets of the other drivers. For 2 way biamped systems it's unnecessary to even mess with time alignment since you cannot delay the mids.

Also, most good crossovers feature a multiband limiter as well. This is a necessity for driver protection. I'm sure those of you who are sound engineers know about the dreaded moment the vocalist drops his mic, sending that ungodly loud "POP" though the system, which can and will very easily take out your speakers. The hard limiter in the crossover is there to protect against this and other potentially damaging peaks.

For most 2 way systems, I typically set the crossover up as follows:

Crossover Frequency: 100Hz
Low Band Low Side Rolloff: 40Hz

Each band's output level is set up during the gain structure setup with pink noise. On my crossover, the input and high band are set at 0dB and they both clip at the same time. However, the low band on my crossover is boosted at about 5dB in order to get it to clip at the same time the input and high band clip.

On 3 way systems, you'll need the specs of your horns to figure out where to crossover between mids and highs. My horns recommend no lower than 2kHz as a crossover frequency. I would probably cross them over at 2.5kHz to be on the safe side.

I then set up the limiter with pink noise. With each band just going into clip, I rotate the amp's input attenuators until they start going into clip. I start dropping the limiter threshold until I reach the point where the amps clip indicators JUST go out. This way no matter what I do at the board, the amps will never clip, yet I'll be getting max power from them.

[Eoin]

Great write up.


I've been thinking lately about what would happen if you put a passive crossover into a HF driver after an electronic crossover. Has anyone done it? It's something I'd like to A/B - not at all intending to try and improve performance, literally just in the hope that it makes no difference - it'd cost a bit but might save the driver if something goes awry with the poweramp. I've lost a set of HF drivers in my time as well, and it wasn't pretty. They weren't mine... which in some ways made it worse.

[flemingmras]

Well typically most passive crossovers in most PA cabs have fused protection for the horns. As far as an actual crossover section, I would think that there would be a tonal change since coils and capacitors add some phase shift to a certain degree.

I would just take out the passive crossover components altogether while leaving the fuses in line with the horns.

Jon

[Eoin]

Hmm... I haven't seen any kind of fuse protection in any of the tri-amped systems I've looked inside here.

[flemingmras]

Most full range with a built in crossover usually have fuses on the crossover circuit board itself. It may be an older cabinet.

What make/model are those cabs out of curiosity?

Jon

[Eoin]

Yeah, I've looked at a few passsive crossovers alright. But like I said, the couple of systems with electronic crossovers that I've poked about with seemed not to have any kind of protection in the driver electronics.

Now that I think of it, that particular system was some kind of custom job by a sound company I'd never heard of at the time, and haven't heard of since. The HF drivers were Beyma CP850NDs. I haven't worked there in about a year anyway (for reasons unrelated to the destruction of 4 compression drivers ). The venue was a hell hole, and I gather that the sound system had been imported whole from another venue some years before I came to be cursed with having to work there.

I live in a half-arsed country.