#1
I read when setting up a PA system it's good to have 2x the WATT comming out of the Power Amp that what the PA Speakers require, for example:

1x Power AMP: 500W 4ohm

2x Speakers Parrarell connected: 125W 8ohm each

now this would be good right?

Why is it that on my 100W peavey amp that my head is 100W and my cabinet is 300W. Shoudlnt this overheat the head and kill it?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
#2
I think you have it basically backwards. You DON'T want to have double the power in your power amp. If you have an amp rated at 100w you want your speakers to be able to handle at least 100w. Your speakers aren't going to blow your head, if anything it will be the reverse.
IdirtI

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#3
Why do ppl keep saying diffrent things :S Whats "headroom" then?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
#4
"When you're matching a Power Amp to a PA Speaker, a good rule of thumb is to pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means that a speaker with a "nominal impedance" of 8 Ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8 Ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 Ohms. A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion."

is this true?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
#5
I don't know much about PA but this quote looks like bullsh*t, but again, maaybe I'm wrong
and headroom is the amount of volume that an amp can take before starting to distort
#6
300Ws is how much power a speaker will take before it blows up. I don't know anything about PA systems, so I can't help with that part.
#7
Quote by imicius
300Ws is how much power a speaker will take before it blows up. I don't know anything about PA systems, so I can't help with that part.
I'm running a Peavy XXL 100Watt and the Cabinet is 300Watt. Why is this? Why ain't the cabinet 100W?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
#8
Quote by FlakKer
I'm running a Peavy XXL 100Watt and the Cabinet is 300Watt. Why is this? Why ain't the cabinet 100W?

You could do that; if you dime a 100W amp running through a 100W cab you'll get both kinds of distortion, tube and speaker.

With a 300W cab, your speakers won't distort, so it's kinda like headroom, but a little different.
#9
Because that's how they made it? Higher wattage of speakers usually means more headroom and bass, but the bass part isn't alwasys true. The speakers don't require 300w to run, that's the most they can take before they start to get damaged.
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#10
ahh ok, doesn't that count as overpowering though, the clipping things? also

1. why do people do the complete opposite (underpowering?) 50W amp to a 100W speaker

2. when doing a live concert, is it better to have this headroom?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
Last edited by FlakKer at Feb 4, 2009,
#11
Quote by FlakKer
ahh ok, starting to get a picture, 2 questions though.

1. why do people do the complete opposite (underpowering?) 50W amp to a 100W speaker

2. when doing a live concert, is it better to have this headroom?

Some people want to dime amps and still have no speaker breakup. If you want that, get speakers rated for more then the amp is.

If you decide to not get higher-powered speakers, keep in mind that when power tube saturation occurs, the tubes will put out more power then they advertise (I think up to 50% more?). So a 50W amp could potentially actually run at 75W when overdriven. So if you are looking for more headroom, you need speakers rated for significantly higher wattages then the amp is.

This is all moot, really, unless you have a deaf audience (or an attenuator).

As to if it's necessary/better to have it, it's personal preference. As long as you don't blow out your speakers, either way is perfectly fine, as long as you like how it sounds.
#12
The answer to your question is quite complex. The rated output of an amp is usually the power it can put out without distortion under specific conditions. When an amp distorts it creates extra power and the maximun theoretical power it can generate is the rated power times the square root of 2, or 1.414 times. Hence the advice for speakers 1.5 times the amp.

The big problem is heat and excursion for speakers (heat and current for amps.)

Touch a hundred watt light bulb and you will burn your fingers, loudspeaker coils get just as hot.

Protecting speakers is a matter of how they are used. Guitar speakers are driven into distortion and should always be over the amps rating at least one and a half times two times is better. Some makers under-rate their speakers though which is why the advice can be confusing.

PA speakers are not meant to be driven into distortion so they can be matched to amps if your sound engineer knows their business in fact having a powerful amp can mean less distotion and less power for the same sound level. Confusing! Most PA's are run by non -experts however so it is a good idea to over rate them too.

Amps do not give more power into big speakers the only thing that affects them are the ohms with too low a resistance being the killer.

Have a look at my article in gear maintenance on matching amps and speakers.
#14
so jsut to get some ground rules. The thing that breaks AMPS are if they OHM is too low. And they way to break the speaker is if you have a speaker which WATT range is under 1.5 times that of the amp WATT range.

And the thumbrule of mathing one speaker to one amp is to have the same ohm and the speaker watt range at least 1.5 times more.

Ok, one little question, why does this not affect PA system? If I rig up my cabinet with a microphone so that my guitar comes out the PA won't it distort the PA speaker? Or is the distortion of the cabinet speaker a special thing that only happens with a head and cabinet? (should I be worried about blowing out the PA speakers if they aren't 1.5 times more than what the power amp is squeezing out)

also the volume button. does that control how many watts are let into the speaker? if so.. then can't you have a 500W AMP and a 100W speaker and have the volume at 1/5th of max?
• Ibanez RG1570
Dunstan Distortion (Bridge)
Standard V7 (Middle)
Air Norton (Neck)
011 Strings Drop C
• Peavy XXL 100W
• SM58 Mic
Last edited by FlakKer at Feb 5, 2009,
#15
Please be careful with your PA speaker. The distortion could mean you are delivering more power than you think. I'm afraid that the answer to this first question is that it depends. NOrmally you push a guitar amp to its limits to get a deliberately controlled distortion. If you mic this up and take it to the PA then you will retain the distortion, but you don't want the PA to add any extra distortion so it is normally working within its limits. But there is nothing to stop an inexperienced sound engineer from overdoing things and then the same rules apply to any speaker.

I think you have this nailed though. You are quite right that the number of watts an amp produces is dependant upon the volume setting. The problem with judging this is that our hearing is very subjective, if I'm in doubt I actually put a meter across the speaker terminals and measure how many watts I am delivering. Valve amps in particular can be surprising one amp rated at 120W actually measured at 300W for a few seconds, though this is unusual.

By and large if you keep the volume low or only try things for short periods you should be ok it is only high power for long periods that will heat your gear to the point where it fails.

Good Luck
#16
Quote by idirti
I think you have it basically backwards. You DON'T want to have double the power in your power amp. If you have an amp rated at 100w you want your speakers to be able to handle at least 100w. Your speakers aren't going to blow your head, if anything it will be the reverse.

actually, when it comes to PAs, he is right. i dont know why, but doing it your way will result in the blowing of the amp.
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