PA Crossovers & Amps

[nocaster]

In my limited experience of live productions can I assume that....................

Signal goes from the mixer into a crossover and then split into separate amps for high, mid & low. Then out to the speakers?

So if I wanted to run highs & lows separate I'd go from the mixer into x-over and then into 2 amps and out to speakers?

Thanks in advance

Kev

[Danny1969]

Yep that's basically it, mixer into crossover and then into separate amps running dedicated boxs, highs on one amp, subs on another amp etc. It works well if you know the best crossover frequencies for the speakers, what to protect them from etc.
More recently speaker controllers like the DBX Driverack have become popular, these devices are basically a crossover, EQ, compressor and notch filter all in one. They sit in the chain in place of a normal crossover. The advantage they have is being digital, is they have ready made patches for popular speakers and can be updated as new models are made. Technically they are can also automatically "ring the room out" and adjust themselves for poor acoustics. Basically the device blasts a load of noise through the PA and listens to what comes back via it's flat response mic. If it detects the room has a huge peak at 60Hz for example then it's supposed to compensate for this in the FOH mix.

We brought one and used it only once, I thought it did what it was supposed to at the FOH position but the sound changes as you walk around the room, and it can't compensate for that. Plus blasting white noise through a PA for a few minutes tends to get on peoples nerves.

A lot of active speaker boxes have their own crossover sat between the input and it's internal amp. The active HK stuff uses it's DDO controller, the active Mackie stuff uses a similar system. Basically the controller makes sure each driver is protected against frequencies they can't handle and there's normally some form of soft clipping. Older passive systems need the right controller rather than any old crossover, Bose highs and subs for example.

[Jamieh]

You can also do it with passive crossovers, which most bass bins have installed in them.

Where basically your full-range output from the main PA amp goes to your subs, through the passive crossovers which filter out the sub frequencies, usually below 100Hz or thereabouts and shove that into the Sub speakers whilst then you take a mid/top out to your tops.

It is less efficient on the signal path this way and you get less control over it, but it is much much easier and perfectly adequate for pubs bands if you are using good speakers.

In my experience it does rather depend largely on whether you are using keyboards. They eat up signal and because they cross such a vast frequency spectrum and range of tones can drown everything out very quickly.

This is why for a 3 or 4 piece rock band I would be more than happy to go the passive route, using a suitably powerful amp, but for my Pink Floyd cover band we use firstly massive speakers and secondly a driverack as per Dannys suggestion above to allow compression, EQ'ing and total control over the crossover, which then goes into two amps... I did try it with 3 amps once, having a 3 way crossover but found it more trouble than it worth.

Of course any full range cabs with horns will have an inherent high frequency cross over for sounds above about 2kHz... or sometimes lower, 1.6kHz is quite common... so this effectively gives you a three way crossover in either case.

[nocaster]

Thanks for the responses. I'm glad to see I was right and that I wasn't going totally mental!

I Googled it and found these :

Danny's point :



Jamie's Point :



Kev

[Jamieh]

Exactly right!

[nocaster]

More recently speaker controllers like the DBX Driverack have become popular, these devices are basically a crossover, EQ, compressor and notch filter all in one.

Fook me. Just Google'd that. Some piece of kit that.

K

[nocaster]

Been thinking.......

If you have a Stereo amp, could you run (for example) highs into channel 1 (left) and lows/subs into channel 2 (right)? Alternatively, run the main house mix through 1 channel and monitors through the other?

Surley that's possible?

K

[Jamieh]

If you have a Stereo amp, could you run (for example) highs into channel 1 (left) and lows/subs into channel 2 (right)?

Yes you could, but of course you would only be able to have a mono mix.... which is fine, but also limits our FOH power. Also you'd still need the active crossover.

Alternatively, run the main house mix through 1 channel and monitors through the other?

Again, yes no probs. That Alto desk we were talking about, and most decent powered top boxes (Peavey/Yamaha etc) allow you to split the L / R so that it works in exactly that way, but the clever part with them is that they are designed to allow a separate monitor mix and graphic EQ, otherwise doing it exactly as you described would mean that you'd have to have the same mix FOH as monitors, which could be fine, but is a bit limiting and if you needed to filter out any frequencies causing feedback in the monitor mix it would also affect the FOH mix.

[nocaster]

Hmmmm cool.

Thinking up some new combinations of gear and these are all questions I had.

Thanks for the replies.

K

[smokingbeagle]

Been thinking.......

If you have a Stereo amp, could you run (for example) highs into channel 1 (left) and lows/subs into channel 2 (right)?

You certainly could (I've got a Speakon splitter box you can have if it's something you end up doing, just gives two Speakon outputs from each side of the amplifier in case the speakers haven't got parallel sockets), but bear in mind whichever side of the amp you use for the low-end will probably end up working far harder than the other.

In an ideal world I'd always look at a separate amp to run subwoofers, possibly bridged as there's no point running subs (or live sound in general, IMHO) in stereo; if you then wanted to have an amp 'split' to run the 'top' speakers from one side and the monitors from the other, that'd be good.