How to choose speakers to go with PA amp.

[thos]

Hi,

I'm a keyboardist getting ready for some gigs, and I picked up a QSC RMX 1450 Power amp (280 watts per channel @ 8ohms). Now I want to find speakers to match.

Besides the obvious things (like listening to them, pricing, and size/weight), how do you check that speakers have the appropriate specs to go with a particular amp?

thanks,

Tom

[yoyo]

Watts, size of the speaker. Same impedance

[thos]

thanks, can you be a little more specific? you mean the speakers watts and impedance are the same as the amp?

[pandamonk]

Originally Posted by thos

thanks, can you be a little more specific? you mean the speakers watts and impedance are the same as the amp?

It's best to get speakers that can handle more than the amp, so look for something 400w 8ohm, imo. If you want subs as well then go for 10-12" woofers but if these are your only speakers go for 15".

[stevieb]

First, amp output is rated at 8 ohms- observe that as much as possible when buying speakers. Lower inpedence (say, 4 or even 2 ohms) will lead to overheating your amp- not good. If you know you will always be driving only two speakers, get 8 ohmers. If you know you will be chaining speakers (two hooked up to amp, one more hooked up to each speaker, for a total of 4 speakers) get 16 ohmers- total load will be 8 ohms. You can probably do that with 8 ohm speakers, too, for a total load of 4 ohms, but most amps are not happy with a 2-ohm load.

Keep your speaker power handling rating a bit BELOW your amp's output rating, but not by much. Over powering the speakers, while not advised, is not as bad for them as driving them with a seriously clipped signal. A 100-watt amp straining to drive 500 watt speakers does more damage to them than an amp driving them with 600 watts. Overdriving can burn out coils, but clipping causes them to just come all apart. Sounds like crap, too.

For a relatively low-power rating (280 watts can get lost pretty quick in a venue) I would go with high-efficiency speakers.

For your amp, I would try speakers with about a 300 watt/ 8 ohm rating. If your pre-amp device (keyboard or mixing board) has a sub-woofer out, you might want to get a self-powered sub of about 500 watts (guessing, here) which will take care of the lows and allow your QSC to deal with mids and highs. Using that self-powered sub would also allow you to go with much smaller speakers- 2-ways with 10's or even 8's might carry all you need.

If you are coming directly out of the keyboard to the amp, a small mixing board might come in handy- it will allow you to hook up a second or third keyboard, and a CD player to keep the music going when you take a break, and allow you better control of what goes to the sub (if it has a sub-woofer out).

There may be others who can give you better advise, but this is a good start.

[Simman]

Originally Posted by stevieb

Keep your speaker power handling rating a bit BELOW your amp's output rating, but not by much. Over powering the speakers, while not advised, is not as bad for them as driving them with a seriously clipped signal. A 100-watt amp straining to drive 500 watt speakers does more damage to them than an amp driving them with 600 watts. Overdriving can burn out coils, but clipping causes them to just come all apart.

This is complete nonsense and poor advice.

Whatever speaker you choose, if you keep the wattage rating of the speaker greater than the output of the amp you will rarely ever have an issue.

[Lt. Bob]

Originally Posted by stevieb

Keep your speaker power handling rating a bit BELOW your amp's output rating, but not by much. Over powering the speakers, while not advised, is not as bad for them as driving them with a seriously clipped signal. A 100-watt amp straining to drive 500 watt speakers does more damage to them than an amp driving them with 600 watts. Overdriving can burn out coils, but clipping causes them to just come all apart. Sounds like crap, too.

.

This is just not correct ..... I don't know where this idea came from and I see it all the time but it's wrong.
Don't misunderstand me .... I agree that driving the amp into clipping is worse than putting too much clean watts to a speaker in general.
But this has nothing at all to do with the speakers' power rating.
It is because clipping puts out so much energy that it damages or burns out the voice coil of the horn usually.
But a speaker that's rated higher is better able to withstand it.
So the higher the speaker is rated .... the better as far as not blowing it.
There's no such thing as a 100 watt amp straining to drive a 6oo watt speaker as if that's harder on the amp or something. The amp clips when it's turned up too loud regardless of the rating of the speaker it's driving.
Now, conceivably a speaker might get a high power rating by being very inefficient and that may cause you to turn the amp up more trying to get it louder.
But, in general ...... the more power your speakers can handle, the better.

[boingoman]

Originally Posted by Simman

This is complete nonsense and poor advice.

Whatever speaker you choose, if you keep the wattage rating of the speaker greater than the output of the amp you will rarely ever have an issue.

Nope, sorry. Stevieb may have some facts wrong but the recommendations from every major speaker maker is to choose an amp that equals or exceeds the power handling rating of the speaker.

[Simman]

Originally Posted by boingoman

Nope, sorry. Stevieb may have some facts wrong but the recommendations from every major speaker maker is to choose an amp that equals or exceeds the power handling rating of the speaker.

Okay, so let's just say these speaker manufactures are tell you to choose an amp that exceeds the power ratings of their speakers; why do you think that is exactly?

Dah...Could it be so you might fry the speaker and have to buy more...go figure.

No offence dude that is complete bull shit.

[Simman]

Originally Posted by Lt. Bob

This is just not correct ..... I don't know where this idea came from and I see it all the time but it's wrong.
Don't misunderstand me .... I agree that driving the amp into clipping is worse than putting too much clean watts to a speaker in general.
But this has nothing at all to do with the speakers' power rating.
It is because clipping puts out so much energy that it damages or burns out the voice coil of the horn usually.
But a speaker that's rated higher is better able to withstand it.
So the higher the speaker is rated .... the better as far as not blowing it.
There's no such thing as a 100 watt amp straining to drive a 6oo watt speaker as if that's harder on the amp or something. The amp clips when it's turned up too loud regardless of the rating of the speaker it's driving.
Now, conceivably a speaker might get a high power rating by being very inefficient and that may cause you to turn the amp up more trying to get it louder.
But, in general ...... the more power your speakers can handle, the better.

I agree with you However, not to be a stickle for detail but, the reason speakers (especially underrated speakers) can get damaged from a clipped signal has more to do with heat caused but the lack of air flow around the voice coil as a result of the lack of movement of the speaker itself. This causes the wire and/or the coating to melt which causes a short. Higher wattage speakers usually can dissipate the heat better and are less susceptible.

[boingoman]

Originally Posted by Simman

Okay, so let's just say these speaker manufactures are tell you to choose an amp that exceeds the power ratings of their speakers; why do you think that is exactly?

The reason is that the most common thing people do when they don't have enough power is to drive their amps into clipping. Clipping an amp is basically compressing the signal. Compressing the signal does what? Raises the average level. Speakers need to dissipate heat or they will fry. Raising the average level increases the heat the speaker must dissipate. Heavily clipping an amp can easily raise the average power level enough to exceed the speaker's long-term power handling requirements if the amp's rated power is around the speaker's rating. Obviously you could never fry a 500W speaker with a 1W amp, even if hard clipped and run all day long. But with a 400W amp run into clipping, it wouldn't be that hard.

Really, the recommendation should be to never clip your amps if they are smaller and keep careful watch over your levels if you use a larger amp. Companies started recommending the practice when drivers started failing even though people were using amps that put out less power than the speakers were rated for.

[boingoman]

Originally Posted by Simman

I agree with you However, not to be a stickle for detail but, the reason speakers (especially underrated speakers) can get damaged from a clipped signal...

If your speakers are rated lower than your amp, you probably wouldn't ever clip the amp. The speaker would blow from overexcursion first.

[Lt. Bob]

Originally Posted by boingoman

If your speakers are rated lower than your amp, you probably wouldn't ever clip the amp. The speaker would blow from overexcursion first.

And that's better how?

The thing you've pointed out and Simman too is what's actually important here.
DON'T drive your PA into hard clipping.
Just because your speakers have a higher rating than the amp will put out won't keep them from being eaten by hard clipping although when I've seen that happen, it's almost always the horn driver that buys it.

[karyoker]

Clipping occurs when an amp stage is hit with a peak to peak signal that exceeds the p-p of the amp At a certain point the peaks are clipped into a DC component. DC across a speaker coil will slam the cone in one direction hard. The coils resistence to DC is almost 0 thus a high current will fry the coil.


JBL

[boingoman]

Originally Posted by Lt. Bob

And that's better how?
.

Seriously? Because while almost probably half the idiots out there can hear the sound of a distorting speaker, most people can't hear that their amps are clipping, especially since the limiters on amps have gotten so much better ie transparent. And take DJs, for instance, who are mostly too...something....to realize or care that the RED LIGHTS MEAN STOP. And meters really aren't accurate enough for the most part to reliably indicate clipping.

[thos]

thanks for all your responses, i see its not so straightforward.... keep em comin' i am learning from reading this.

[stevieb]

Originally Posted by boingoman

If your speakers are rated lower than your amp, you probably wouldn't ever clip the amp. The speaker would blow from overexcursion first.

True, but ONLY if you crank the amp.

Okay, so perhaps I mis-stated some of the things I said. I still stand by my basic premis-

Driving speakers with an amp that is well into clipping is bad, bad, BAD for the speakers- not so good for the amp, either.

"300-watt" speakers with a 280-watt amp is pretty close to the same power rating, but IMHO enough "over matched" to give the user a cushion. 'Course, it's important to LISTEN to what is coming out of the speakers...

And besides, it seems my critics missed my last sentence, so let me restate it...

There may be others who can give you better advise, but this is a good start.

[Minion]

Clipping Is when the Output signal exceded the supply Voltage of the amp which causes DC to be outputed to the speaker which causes the Voice coil to overheat....

But not all amps will output DC when Clipped ,Single ended amps allways have an output capacitor (Or sometimes an Output Transformer) and a cap or XFormer will block any DC from getting to the output and hence to the speaker , these Types of amps will not destroy a Speaker when clipped ,But these types of amps are a little more rare these days (there were a lot of them made in the 60"s and 70"s) as most newer amps are symetrical Push-Pull type amps which don"t have output caps and are more suceptable to frying speakers when clipped....

I believe the recomendation of useing a Higher powered amp for a lower powered speaker is mostly related to Guitar amps/speakers ,because with guitar a Lot of the great guitar crunch comes from Speaker breakup but you can"t get speaker breakup unless you drive the speaker hard so they sometimes use a slightly higher wattage amp than speaker to get that Guitar crunch without haveing to crank the amp to max....


Cheers