Hello, first post, noob question... - Amplifier Power Ratings

Steve Smith Jr

New member
Apr 30, 2020
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Florida
Hello. So, I know this is a really dumb question, but I've literally been searching google for an hour or so now and I can't find the answers I'm looking for. Anyway...

If something says "2x120w" I'm going to assume that means the amp has 2 channels and each channel can put out a max of 120 watts, correct? So on each channel I can wire up a 120w speaker, or a combination of speakers that combined equal 120w, with the correct impedance, right? Or am I totally wrong? I'd really appreciate the help!
 
Oct 25, 2018
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Bideford, Devon. UK
Hi Steve Smith,

In essence you are absolutely correct, however it is important to be aware that quoting a power output without stating conditions is open to outlandish advertising claims. I'm sure you will have come across the terms 'Peak' and 'RMS' in connection with power ratings, and whilst these narrow down Marketing Departments' creativity and abuse somewhat, they can still be misleading. Further, power output generally increases with reducing load impedance until the amplifier shuts down/blows a fuse/goes up in smoke...
A better specification would read something like: 120Wrms continuous per channel into 4ohms, 20Hz - 20kHz, both channels driven, max. 0.1% THD.

Matching amplifier power to speaker rating is a considerably more grey area! Your 120W amplifier could be used with care driving 5W speakers; equally it could be used to power 500W speakers, just not to their full potential. In a nutshell, the speaker power does not have to precisely match the amplifier output. I always use an amplifier of a higher rating than the speakers (400Wrms on an 80W tweeter for example, and 2000W on a 1200W sub) to maintain as much dynamic headroom as possible - not easy as the frequency goes lower! (I like to use the car-on-the-motorway example: Both a 1300cc VW Beetle and a 4.2l turbocharged Porsche can achieve 70mph, but the latter has considerably more power in reserve to overtake).

Stated simply, The 120W amp would blow the 5W speaker, but the very same amp might fail if running continuously flat out for hours into a 500W speaker - both extremes could be damaging with no understanding of the setup.

Returning briefly to impedance, you are also quite correct in stating: '... a combination of speakers... ...with the correct impedance...'. You could connect 8 x 8ohm speakers to give 4ohms impedance, or 16 x 4ohm to also give 4ohms; the quantity of speakers per se is irrelevant to the impedance which the amplifier sees at its output terminals.

Good luck with your project and feel free to ask away - no question is foolish if you do not know the answer! Cheers, Carl.
 
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Steve Smith Jr

New member
Apr 30, 2020
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Thanks! Is there a rule of thumb to match power output to speaker rating? I'm guessing you'd want more "max speaker wattage" than amp output power, correct? What's a good ratio?
 
Oct 25, 2018
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Bideford, Devon. UK
Steve, if you asked five different engineers, you would get five difference answers, but no, it's the other way around. I always try to use an amplifier of about double the speaker rating. eg. I use a 1000W amp on my 450W 10" mids, and I have never used an amplifier turned up to '10'. My experience is that amplifier output is usually optimistic, so as in the old engine tuning adage 'There's no replacement for displacement'!
 

Tim McCulloch

Graduate Student
Jan 11, 2011
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The "watt" rating for loudspeakers and output transducers is a THERMAL RATING. It has nothing to do with how much power is converted to SPL, only how much power can be dissipated as heat (for a usually undisclosed amount time) before the voice coil fails catastrophically... and that's if the manufacturer is being honest about the rating.

Watts as a spec for amplifiers essentially rate how long the voltage (potential) applied to resistance (voice coil) will be sustained before the amp power supply collapses.

Presuming the loudspeaker still has excursion capability and thermal capacity, and your personal reaction to seeing clip lights on the amplifier... you can use an amp smaller or equal to the *continuous* rating of the loudspeaker and ignore the clip lights if you don't perceive audible clipping. If you have an allergic reaction to clip lights, buy a bigger amp, but realize that if you use a larger amp that is genuinely capable of exceeding the loudspeaker rating on a continuous basis, you'll eventually run them harder and possibly blow speakers or passive crossover components.
 

Steve White

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Jul 6, 2018
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Fort Worth, Texas
^^^ What Tim said. Why my initial response to the question was 25 words or less as one could easily write a doctoral dissertation on the topic. (Not being sarcastic at all - fact)

In general what Tim alluded to is the correct way to do it. Amplifier ratings, given an ethical manufacturer of professional grade equipment ( a whole chapter could be written on this) rate power on the amps is in continuous. However, it's not continuous "continuous", it's continuous over a specified period of time - generally, there again would probably be exceptions.

Simply put it works pretty much like this. Go past continuous power ratings of speakers with them amp with the mind set of "peak" the speakers will handle and put an amp that will deliver that means you're letting the amp somewhat cruise and putting the drivers are their maximum.

What happens, when we go up the hill past "nominal" power input to a speaker/amp. Well for the speaker it just shortens the lifespan. Depending upon how hard it's driven with music program. Then can be literally beat to and early death. Meaning mechanical failure in the voice coil bond to the cone or diaphragm. Or, mechanical failure due to over-excursion in LF drivers at the lower end of the bandwidth.

Drive them harder and longer and it typically becomes a contest of which failure mode will manifest first, typically it will be voice coil burnout.

We haven't mentioned "power compression" which is simply the fact that electrical input -vs- acoustic output is not linear throughout it's operating range. At the high end they manifest power compression, whereas output power is no longer tracking with increases of input power.

So, for optimal performance over lifetime, and getting the most bounce for the ounce on what comes out of the back of the truck, a close to even match between amplifier and speaker is usually the best solution.

In a system under construction I am going to drive JBL 2269 18's with Crown MA-5000i's. The lower end of the power rating for the JBL subs, however they will be at optimal efficiency, not as subject to over excursion at the low end - AND, the price I got the like new condition Crown MA5000i's makes the business case solid. Pushing the subs with that power range they will live long and prosper.

That choice when posted, there are those out there that will claim they are under-powered and can handle more. Probably so, at the trade-off of a shorter lifespan and more amplifier expense. Business case model for me say's I hit the sweet spot.

It took me a few trips to the technician's service bench with amplifiers with wasted output devices and speaker recone jobs to learn this.

The other part is system setup, gain structure and limiters to optimize performance for maximum safe output levels. Building the Ferrari is one thing, then you need to learn how to drive it to get the best lap times and not destroy it (into the wall or another car, wear out tires, burn through fuel, blow engine, destroy transmission) before the race ends.

The seasoned audio engineer is just like a crew chief on a race car.

As Carl stated, he buys at the higher power rating for the amps and doesn't drive them as hard. Solid practice, he knows not the drive the amps full tilt - doing that will shorten lifespan on the drivers. My case was the other end and I looked at the business case as well and the bang-for-buck deal on the MA-5000i's and went that way. I'll drive the amps up to the region of rated power. They're solid amps and will handle that just fine over the long haul.

Basically, when doing system design and matching amp power to drivers - just know what choices you made and why, and sometimes it will influence system setup decisions. There is no single "right answer". It can be approached from a "run what you brung", "optimum design" or "business case" perspective that will all yield good results - the best choice being the one that fits the situation.
 
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