Amp Head Peak Wattage or RMS

[dincz]

RMS power is understood to be based on RMS voltage and current. Though not correct by definition, it's a pretty easy concept to sort out in reality.

It's a slippery concept. Certainly it's based on RMS values of voltage and current but it seems that some manufacturers use it to mean continuous average power (a more meaningful and understandable term) while others may well be using it for expressing burst power capability - also based on RMS voltage/current - which is arguably a rating with more relevance to a musical signal. But how long is a burst? About the same as a piece of string it seems.

 

[strictlybass_ic]

Answer to your question as written:
I'm a big fan of getting an amp that can dish out more than your cabinet can take. It allows you to get a lot out of your cabinet without straining the amp (and you can keep a cleaner tone). BUT, if you go that route you have to listen closely to your cab for signs of overloading. A bit of carelessness can lead to blown speakers.

What you actually wanted to know:
Power ratings on amps and cabinets are a reference at best. Your cabinet's true power handling is dependent on a wide variety of factors within (speaker design, cabinet design) and outside (frequency range, compression, filtering, EQ) the control of the manufacturer. Basically, get yourself an amp rated at 500+ amps that sounds good to you. That should be enough juice to cover typical gig setups without overworking the amp. When you're playing listen for sounds of distress from either the amp or the cab and adjust knobs accordingly. If you find you're overworking the amp look into a different cab setup to get more air movement or get the bigger brother to your current amp.

 

[calypsocoral302]

This may have been asked, if so just drop a link but I couldn't find it.
Anyway I am planning on buying my first non-combo amp soon. The cab I am planning on getting has an 400 watt RMS and an 800 watt peak. Should I get a head over 800 watts for the peak?
Oh and if you could suggest a cheap head in said wattage that would be great
Thanks

agedhorse gave you the best answer (he's an engineer for Mesa and previously for Genz Benz, so he has a ton of cred), but it is important to learn how to determine with your ears when your speakers are sounding "stressed" from too much power. As previously stated, the power ratings of amps and cabs are somewhat elastic, especially if you are mixing and matching amps and cabs from different manufacturers.

 

[beans-on-toast]

Let's all step back for a little dose of reality please...

1. RMS is a measurement based on the RMS voltage of the output of the amp into it's rated load and has nothing to do with continuous. The "RMS" power is also considered the thermal power because it's the area under the power curve.

2. Peak power is exactly 2x the "RMS" power because it is generally (there are some exceptions) based on the peak voltage of the undistorted sine wave used for the thermal or "RMS" power defined in #1.

3. Once you recognize the above, any comparisons made must be with equivalent units... ie. peak to peak or RMS to RMS... EXCEPT that the speaker industry has chosen another arbitrary rating called "program power" which is also defined as 2x the "RMS" power, so then USUALLY BUT NOT ALWAYS, the peak power is taken to be 2x the program power or 4x the RMS power.

4. Now that you are aware of the totally screwed up (different) rating methods used by the two industries, there's another "gottcha", that is most speakers give thermal ratings (RMS) that do not consider the mechanical limitations of the speaker... which DECREASE a lower frequencies (ie. the lowest octave of its rated bandwidth).

With all of that said, I would in general recommend roughly matching the RMS rating of the head with no more than the RMS rating of the speaker, that will get you reasonably close, reasonably safely. Yes, you may be able to drive the speaker harder IF you know more about the specific products, but I wouldn't feel comfortable making that recommendation without knowing a lot more about the amp and speaker involved.

Also, note that the amp's ratings apply only to the speaker impedance presented to it, so those units must be the same also (ie. compare the 8 ohm rated power of the amp to an 8 ohm speaker and the 4 ohm rated power of the amp with a 4 ohm speaker (or 2 paralleled 8 ohm speakers).

Hope this helps.

Here are two related references from JBL.

 

[agedhorse]

RMS can be continuous, time limited or burst.

If time limited, the time limits should be specified. If burst, the burst specs (ie. burst time, low level, low time and repeat period) should be stated.

 

[karl_em_all]

OP is probably ready to switch to piano by now.

The cab you have your eye on is 400W @ 8 Ohms. Get yourself an amp with about a 500W rating and you'll be fine and plenty loud.

Also do a search on Ohms in relation to hooking up your potential new rig.

Good luck!

 

[beans-on-toast]

RMS can be continuous, time limited or burst.

If time limited, the time limits should be specified. If burst, the burst specs (ie. burst time, low level, low time and repeat period) should be stated.

Yes, an important distinction. In the conclusions of the JBL tech note they state that the pairing of the amp and speakers depends on the application. They suggest that musical instrument applications require the most conservative approach. Never hurts to be safe if you want a long service life out of the speakers.

 

[dincz]

RMS can be continuous, time limited or burst.

If time limited, the time limits should be specified. If burst, the burst specs (ie. burst time, low level, low time and repeat period) should be stated.

If only! I wonder if some manufacturers' power specs based on very short burst capability are the reason some players find lightweight amps disappointing. For example, 500W continuous average power compared to 500W based on a 20mS burst.

This only a gut feeling and I'd be interested to hear your thoughts.

 

[agedhorse]

I've always used much longer burst times myself during the development process of bass amp products. The required burst to rest ratio depends on the low frequency performance that is desired. 20msec isn't going to do much at 50Hz for example.

 

[Crazyman582]

Hate to ask another question , but if I say had a 2000 watt head for a amp with a 1000 watt peak, if I never went over half volume would I be good? These numbers are just examples don't think I need that kind of power

 

[agedhorse]

Doesn't work they way. If the "RMS" power is 2000 watts, then the peak would by definition be 4000 watts peak.

The volume control only changes the amount of signal required to achieve rated power. For example, with the master volume at "one half", if you doubled the signal (say with the input gain/volume control), the end result power would be exactly the same.

 

[dincz]

Hate to ask another question , but if I say had a 2000 watt head for a amp with a 1000 watt peak, if I never went over half volume would I be good? These numbers are just examples don't think I need that kind of power

Do you mean a cab with a 1000W peak? You'll probably hear the cab struggling if you overpower it. The problem with half volume is knowing when you'running at half volume. Knob position won't tell you that.

 

[beans-on-toast]

The volume settings are usually not linear. On some amps, 50% of travel is already full volume, even though you haven't turned it up all the way through it's travel.

You really have to listen carefully. A slap can put out a transient that will overpower the speaker. So it also depends on how you are playing. It depends on the tone settings, too much bass is like turning up the volume. So it has to be done VERY CAREFULLY and you have to listen for signs of distress. Not always easy to do.

 

[Slevage]

Now to really confuse you.

The wattage rating for a speaker is the power it takes to melt the voice coil, NOT the power it takes to cause physical damage.

Depending on your use of low end EQ, *most* commercial cabs can only handle around 1/2 their thermal RMS rating before farting out (don't even worry about "peak" numbers, they are BS).

Melted voice coil sounds like physical damage to me

 

[CallMeAl]

Related Q on parallel speakers:

Say I have a 500w @ 4 ohm head, and want to run 2 8 ohm speakers. Does the amp send 250w or 500w to each?

(Using the cab rating x2 rule)
Should each cab be rated 500w rms, or 1000w rms?

Thx

 

[JimChjones]

If the cabinets have equal impedance (best not to mix them if they don't unless you have a detailed understanding of the underlying science) then they will get an equal share of the power from the amp.

Two 8ohm cabs make a 4ohm load, so your amp will be capable of developing it's max 500ws and sending 250 to each cab.

That means 500w cabs would be reasonably safe. Or, of course, you could get your band to cultivate a quiet stage, and not drive the kit hard...