Problems with ohms

[seven]

Hi,
Please can anyone help, I'm starting up a P.A business as I have been involved with the music industry for many years.
The problem I am having is that when you make up a system it is quite hard to match the impedance of the speakers to the impedance of the amps to get exactly what you want for different size venues. for example I have a Laney amp (which has 4 amps in one box) which has the following spec:-

750w @ 2ohm x4 550w @ 4ohm x 4 350w @ 8ohm x4 1500w bridged @ 4ohm x2 and 1000w bridged @ 8ohm x2.

But the speakers I am using are 4 Zeuz 300w 8ohm full range cabs and 2 zeuz 400w 8ohm bass bins. Still with me?

So what I would like to do is run the bass bins off of channel 3 & 4 ( 350w into 400w @ 8ohm x2 )
and the 4 top cabs off of channels 1 & 2 but at 2ohms thus giving the amp plenty of head room so as not to drive the amp at full power, which is the way most large pa rigs are run these days, but 2 8ohm cabs in parallel = 4ohms not 2 does any one know of a way of making a circit that will alter the impedants of a speaker without just using more speakers?
If such a box could be made you could switch from 4-16ohms to make it so you could get the best from both speakers and amps!
I hope this makes sense, and I apologise if I am ignoring the laws of physics, but any help would be very appreciated.

[Bill Fitzpatrick]

You can alter the impedance of a speaker system by putting a resistor either in series or parallel.

Putting a resistor in series is not a good idea as it will impact the performance in a negative way. So, you can scratch the series resistor plan.

Putting a resistor in parallel will reduce the impedance and allow the amplifier to deliver more power to the speaker/resistor combination.

HOWEVER, the extra power delivered will just be used to heat up the resistor. None of the extra power delivered will be available to the speaker system itself.

So, there is no secret circuit that is going to give you extra power.

[BobEllis]

why not bridge channels 3 and 4, running the bass bins off that?

It will give you a 4 ohm load, and your amp *claims* to be able to handle 4 ohms bridged. That will give you plenty of power for the bottom end. You won't be able to use it all, but better than clipping trying to drive too hard.

You'll get nearly your speakers' rated power out of the top end when you parallel a pair.

[tool49]

Just a few toughts here.

Since you have dedicated bass bins, you could put a highpass filter in front of your amp for the channels that are going to drive the full ranger. If this high pass is high enough (150-200 Hertz), your full rangers will never need that much power.

You could also parallel your bass bins and drive them in a bridged configuration at 4 ohm. Giving you a whopping 1500W into your power hungry bass region (instead of 700W). Don't forget that you need to have a high order crossover in this range of frequency or people will hear voices comming from the bass bins. You also need to make the signal mono for the channels driving the bass bins or one of the amp channel might be working much more than the other.

I'd say that driving your full rangers in pairs, 2 on each channel, feeding them a healthy 550W, even if they accept 800W, with a high pass filter, you're going to drive people deaf before you need to push the amp into clipping.

Using a variable crossover would allow you to test different crossover points.

Hope this helps!
Sébastien

[seven]

Thanks guys,

I'll try your ideas and let you know how i get on.