The little PA system that could, I hope.
March 13, 2010 7:51 PM   Subscribe

Will I blow up my PA system? A question of ohms, watts, speakers, amps and the love affair that almost tore them all apart.

I'm trying to figure out if I can use two sets of speakers with my little PA system without overheating or blowing the poor thing out. I've done some internet reading on similar questions but, being a little slow in the electrical engineering department, I'm having trouble definitively figuring out my particular situation.

Here are the specs for my PA head:

It's a Crate PX-600DP for anyone who may have heard of it. The electrical specs given on the back are thus:
Line: 120V, 60Hz
Power: 750 VA

There are two sets of input jacks on the back. One set for "Amp 1", which I believe are generally used for the main speakers, and one set for "Amp 2", which I believe are for the monitors. There are two quarter-inch jacks for each amp. Each set of amp inputs has the following specs written next to them:
150W @ 4 ohms
4 ohm min. load

I believe that each set of inputs does indeed have it's own separate amp within the PA head.

Now, we have two different sets of speakers with different specs that we want to plug into the PA head. The first set of speakers has this stamped on the back of each of them:

Power Capacity: 200 Watts
Nominal Impedance: 8 ohms

The second set of speakers has this stamped on each one:

Continuous: 150 watts
Program: 300 watts
Impedance: 8 ohms

I am getting hung up on whether or not plugging each speaker separately into the jacks on the PA head counts as running them in parallel which, unless I am totally off the mark, would give the speakers a 4 ohm rating, which the amp should be able to accomodate. Is that right or dead wrong?

Also, since the wattage given for the PA head is less than that given for the speakers, does that just mean that the speakers will be a little underpowered from where they could be (which would probably be fine for my purposes, unless there is something else bad about that)?

Anything else I should be aware of? I'm hoping to use this set-up to run sound for a long-ish (6-7 hour) concert, do you think I should be worried about destroying it? I can shut it off between sets of different bands to give it some downtime.
posted by otolith to Technology (12 answers total)
 
I don't know the particular amp you're describing, but the chances are good that if you're plugging a speaker rated at 8 ohms into an amp where the output is rated at 4 ohms, you should be fine.

Running an 8 ohm loudspeaker on an amplifier rated at 4 ohms will not damage the amplifier, though you may not get as much volume (sound pressure) out of the speaker/amp combo as if they were matched properly.

If you're concerned about the speaker/amp combination, listen for distorted sound and watch for excessive heat from the amp, both of which are signs of trouble.
posted by quidividi at 8:07 PM on March 13, 2010


I agree with quidividi
posted by sanka at 8:09 PM on March 13, 2010


Is there any way you could post pictures of the amp? I haven't found any manuals on-line, so it's hard to know what's going on, exactly.

The jacks on the back are probably the OUTputs of your amps. The two jacks for each amp are also probably in parallel.

If you plug two 8 ohm speakers in parallel (say, in the two outputs of amp 1), you'll get a 4 ohm load (1/(1/R1 + 1/R2). Your amp should be able to handle 4 ohms. Each speaker will "see" the same voltage. The current will be evenly divided (since they have the same impedance). At the maximum, each speaker will get 75 watts.

This should be fine, as long as you don't need to push the speakers farther. Trying to get more volume out of your setup might push the amp into hard clipping and damage your amp/speakers.
posted by Monday, stony Monday at 8:20 PM on March 13, 2010


I am getting hung up on whether or not plugging each speaker separately into the jacks on the PA head counts as running them in parallel which, unless I am totally off the mark, would give the speakers a 4 ohm rating, which the amp should be able to accomodate. Is that right or dead wrong?

That's correct, two 8 ohm speakers in parallel is a 4 ohm load, so you can run two 8 ohm speakers on Amp 1 and two on Amp 2.

Also, since the wattage given for the PA head is less than that given for the speakers, does that just mean that the speakers will be a little underpowered from where they could be (which would probably be fine for my purposes, unless there is something else bad about that)?

That's actually one thing to be careful with. More speakers have probably been destroyed by an underpowered amp clipping than being given too much power. Clipping the output drives the speakers with a signal approaching a square wave which is really hard on speakers. So watch out for that. You don't want your PA amp to clip. If you hear distortion, turn it down. Some amps will have built in limiting circuitry to prevent this or a red light that will come on when it clips. That aside, as long as the PA is staying clean you won't have any issues driving higher wattage speakers.
posted by 6550 at 8:25 PM on March 13, 2010


All good advice above. The only thing I would add is that the converse of what your doing WOULD be a problem. That is, if you connected 4 ohm speakers to an 8-ohm rated amp, you would be taxing the amp too much. The maths works out that you'd be sucking more current out of the amp (twice as much) than it was designed to deliver.
posted by intermod at 9:05 PM on March 13, 2010


Is there any way you could post pictures of the amp? I haven't found any manuals on-line, so it's hard to know what's going on, exactly.
The jacks on the back are probably the OUTputs of your amps. The two jacks for each amp are also probably in parallel.

I don't have any pictures, but the set up on the back of the PA head is basically physically identical to the XR 680E Powered Mixer on this page. And you are right, I said input jacks when I really meant output jacks.
Thanks for the tips about not overdriving the speakers, everyone. I will keep a sharp ear out for distortion. Otherwise, I am heartened that it sounds like this set-up should work.
posted by otolith at 9:11 PM on March 13, 2010


@6550
Citation please. I have been unable to blow my 1000 watt speaker cabs with a 1 watt amplifier.
posted by yeoldefortran at 12:04 PM on March 14, 2010


JBL's Danger: Low Power is the classic source.
posted by Monday, stony Monday at 1:18 PM on March 14, 2010


yeoldefortran: "@6550
Citation please. I have been unable to blow my 1000 watt speaker cabs with a 1 watt amplifier.
"

That's not what was being said. The point is a square wave is sqrt(2) the power of a sine wave so, close to the power capacity of a speaker, clipped waves will fry drivers.

OP, what are the models of the speakers and what drivers do they have in them? Do they have passive crossovers?
posted by turkeyphant at 2:47 PM on March 14, 2010


Not to totally derail, but the problem is the rating. The amp isn't actually underpowered, it's overpowered as stated in the jbl doc referenced above. The other legitimate issue is how the power is being distributed to the speakers. The HF driver, if not on it's own amp, is likely being loaded by a passive crossover, and so equalization alone can push far more than the expected or tolerated wattage (assuming the source is feeding it frequencies that will hit the HF driver. Maybe I'm being too uptight here but the underpowered is worse than overpowered thing drives me nuts. That's why I gave the example of the 1 watt amp, because, as the JBL article states, your peak output is not likely to be more than double your rated wattage, and there aren't even many HF drivers that will be blown by two watts.
posted by yeoldefortran at 5:06 PM on March 14, 2010


One set of speakers are EV Electrovoice S-152

The other ones are Stageworks VP15 (sorry, can't find a better link).

I think they all have passive crossovers.
posted by otolith at 6:44 PM on March 14, 2010


Smashing success. We had seven bands play an outdoor show over the course of an afternoon with this setup. Worked perfectly.
posted by otolith at 12:27 PM on April 13, 2010


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