shavlogo2.gif (3085 bytes)     Choosing a Power Amp

11/04 - Larry Mundy -

Note: Larry Mundy is the author of Design and Build Your Own Live-Sound Speakers, now available from Amazon, and an occasional contributor to this site.

You know those naval movies where you see the engine room of a large ship?  It’s full of gigantic mechanical things straining and chugging and burning up huge amounts of coal or diesel or something, and when these gigantic machines break, the ship is dead in the water.  Power amplifiers are the “engine rooms” of a PA system and do the heavy work, taking a line-level signal and boosting it so it can come thundering out of your speakers.  They are heavy, dense boxes of iron and silicon and suck enough power to dim the lights.  They can radiate enough heat to cook food.  But for all their muscle, they’re actually fairly simple.  I won’t go into how they work, but rather how you use them.

Power Ratings

Power amplifiers can be accurately rated in the decibels of gain they’re capable of producing, but more often (and often less accurately) they’re rated in “watts.”  A watt is simply a unit of electrical energy that measures the power used to accomplish work over a given interval of time.  It is variously, one ampere (amp) of current flowing at a pressure of one volt; one seven-hundred-and-forty-sixth of a “horsepower;” or the amount of power needed to accomplish work at the rate of one “joule” per second.  If none of that makes any sense to you at all, you’re reading the right article.

For our purposes, a “watt” is simply an indication of how much gain (think of that as “boost in signal level”) a power amplifier can supply.  All other things being equal, a 200-watt amplifier can play more loudly than a 100-watt amplifier (but not twice as loud, the power-volume relationship is not “linear”), and it costs more because of that.  So for a louder PA, you generally need more “watts” of power amplification.  The problem is that one manufacturer’s “500 watt” power amplifier may actually be inferior in capabilities to another manufacturer’s “200 watt” model.   If you didn’t know anything about amplifier-power specifications, you might as well buy a power amp based on its shipping weight, since many of the aspects of good amp design add pounds here and there.  But you really should be choosing based on actual as-tested specifications.

Distortion.  Watts don’t care what they sound like, but you do.  Every power amplifier will put out a certain number of watts’ worth of sound, but as the power is increased, a number of factors (heat, circuit design) make the amp introduce progressively more distortion into the signal it’s amplifying.  At some level, this distortion ruins the sound.  So amplifiers are rated by wattage at stated distortion levels.   Usually these levels of distortion are denominated “THD,” for “total harmonic distortion.”  It’s a tradeoff between power and distortion – the same amplifier that is capable of 100 watts’ output at a fraction of 1% in distortion of the signal, may be capable of 200 watts’ output if you are willing to put up with 5% or 10% distortion (trust me, you’re not).

Stamina.  Also due to circuit-design and heat-dissipation issues, an amplifier may be able to put out a powerful and relatively undistorted sound for a fraction of a second, but introduce excessive distortion if asked to put out its rated wattage for hours on end – which is exactly what you want it to do in a performance situation.

Load impedance.  The output wattage of an amplifier is also dependent on the impedance of the speaker load it’s driving.  That wattage will be greater if your speakers have lower impedance.  There is a danger if your speaker impedance is too low, because the amp “works harder” and produces more heat driving a lower-impedance load, but generally a well-designed amp will drive a 4-ohm speaker with far more power than an 8-ohm speaker.

Frequency response.  It’s easier for an amp to deliver a certain number of watts at a midrange or high frequency, than at a low frequency.  Any wattage measurement should be made over the entire span of audible frequencies, which by convention is 20 to 20,000 hz.  And over that frequency span, the amp should be designed with relatively “flat” response, not exaggerating one frequency over another.

So let’s put all these principles together and do a comparison of those two theoretical power amps.  Amp #1 has specifications of “500 Watts peak @1khz into 4 ohms, 3% THD.”  Amp #2 says “200 watts RMS into 8 ohms, 20-20,000hz, +/- 1 db, less than .01% THD.”  Which is the better and more powerful amplifier?  It’s Amp #2, and here’s why.

“PMPO” or “peak” power means that an amplifier can attain the stated performance only on “peaks,” or loud signals of short duration.  This rating can be two, five or even ten times greater than “RMS” ratings, which indicate an amplifier’s output capability over hours or even days of continuous sound testing.  Amp #1’s true “RMS” power rating is probably less than that of Amp #2 for this reason alone.

Amp #1’s ratings were derived at unacceptable (3%) levels of total harmonic distortion.  Distortion becomes audible to different ears at different thresholds, but by the time it’s at 1% or more, most people can notice that the sound is somewhat unclear.  Amp 2’s wattage rating is based on a negligible distortion level.  Don’t trust any power rating that cites a THD level greater than one-half percent (.05%).

Amp #1 can only attain its rated power at 1,000 hz – a midrange frequency that’s fairly easy to reproduce compared to the power required for deep bass notes.  Amp #2 can sustain its rated power over the entire audible spectrum.  We also don’t know whether Amp #1 emphasizes some frequencies over others, the equivalent of a “tone control” stuck in some possibly-unwanted position.   Amp #2 delivers its power with no more than 1db of deviation over the entire frequency range.

Finally, Amp #1 was tested with a 4-ohm speaker load.  Most reputable amplifier manufacturers give specifications for the more normal 8-ohm load, or even better, specify wattage outputs for a variety of loads.

TIP: An amp whose output into a 4-ohm load is approximately twice its output into 8 ohms, all other specifications being equal, is responding “linearly” to the impedance change.  That’s a hallmark of a rugged and well-designed amp.  If the 4-ohm rating is only marginally greater than the 8-ohm rating, something is limiting the expected greater power output – perhaps an overtaxed power supply, perhaps higher levels of heat and distortion.  That shouldn’t keep you from buying the amp if it otherwise does what you need, but it is some indication that you should pay special attention to the impedance of the speakers you hook to it.

So Amp #2 is a far better unit, and probably more powerful, than Amp #1.  It will deliver good sound at its rated wattage for hours on end at the entire range of audible frequencies.  Once you understand what the ratings mean, you can further narrow your choices by looking at features. 

Attenuation Controls

If you have an integrated amplifier or mixer-amplifier, all the “volume” controls will be in the preamplification stage.  A separate power amp, however, will usually have a pair of knobs (one for each channel, since most such amps are stereo) that control “volume.”  Their actual job is to turn the amp’s gain down (“attenuate”), rather than up; unless you’re matching the power of two dissimilar power amps or doing some sort of testing, these controls should normally be set “wide open” so the entire capacity of the power amp is available to your system.   Nevertheless these can be handy things to have to balance a mono input signal going to dissimilar speakers, to isolate a channel for testing without disturbing mixer settings, and so forth.

Protection Circuits

There are two types of these, and both can be useful.  As an amplifier is called upon to deliver more and more power, especially into lower-impedance speaker loads, it also generates more and more heat, and eventually limits in circuit design and heat-dissipation capability cause a particularly bad type of distortion in the sound output called “clipping.”  Severe clipping makes even the most beautiful music sound like it’s being performed by a chainsaw.  This is dangerous to both your amp and your speakers.  Some power amps contain a circuit to guard against clipping distortion; every amp maker calls this circuit by some different acronym but they all function similarly.  The circuit compares the input and output signal and when it observes the output is “clipped,” it limits or reduces the gain.  It’s an electronic way for the amp to say “that’s all I’ve got” rather than simply put out more power, overheat, distort and risk damage.  If your power amp has such a circuit, you may be able to turn it off with some switch or setting.  I don’t know why you’d ever want to do that.  You paid for that protection, use it!

Another, simpler circuit is the “thermal protection” or “thermal overload” type.  Since distortion and eventual amp failure are by-products of excessive heat, this circuit functions like a thermostat and when the temperature of internal components reaches some preset level, the power supply to the amp is disconnected for some cooldown period.  This is very effective at protecting your amp.  Unfortunately, it also stops your performance while the amp cools down to the point it will operate again.   If you have such a feature in your amplifier and it “trips” repeatedly, you need a bigger or better amp, or you need to reduce your system gain.  You are asking your amp for more than it can deliver.

The operation of either type of protection circuit should be indicated by some sort of light on the amp’s front panel.  This is particularly true for thermal-overload features which cut power to the amp during cooldown, because otherwise you’ll be checking power cables and mixer settings and whatnot trying to figure out where all the sound went.

TIP: An amp needs some way to get rid of the heat it generates.  It will have either some sort of cooling fins that radiate heat into the air, or a small fan that blows outside air over the internal components.  A force-cooled amp can usually put out more power, all other things being equal, just like motorcycle engines or Pentium chips.  Amidst the thunder of a large PA system, the tiny noise emitted by a fan will not be noticeable.  Without a fan, the amp is cooled by convection as the warmed air rises away from the cooling fins and pulls cooler air up from below.  But no matter how your amp cools itself, the most critical thing is to keep cooler, outside air available to the fan or cooling fins.  Don’t smash the amp flat against a rear wall or operate it in a closed case or rack.  For outdoor jobs, avoid placing the amplifier in warm sunlight.   Whatever you can do to help the internal parts stay cooler will be rewarded with cleaner, louder sound and a longer service life.

If you repeatedly "trip" the protection circuits of your power amp in normal use, it may be that something is wrong - too little cooling or an inoperative fan, too low load (speaker) impedance hooked to the outputs, or a defective protection circuit.  It's just as likely, however, that you simply need a larger amp to deliver the power you need.

Input Connectors

An integrated or mixer-amplifier won’t have these, since the preamp/mixer stage and the power amp are hard-wired to each other inside the box.  A separate power amp, however, is likely to have multiple input connectors.  Here are the common types:

Barrier Strips.  These are simple rows of screws with some sort of insulator between them.  The input signal cable is simply wrapped around the screw, and the screw is snugged down over it.  This is a very effective type of connection for permanent installations, since there is no jack/plug combination for someone to disconnect mistakenly.   It is nearly useless for mobile installations, however, except in an emergency.

1/4” “phone” jacks.  These may be balanced or unbalanced, depending on the type of jack, but even the balanced-jack inputs will generally accept unbalanced cabling.  The differences between the two are discussed elsewhere on this site.

XLR or “Cannon” jacks.   These are designed to receive balanced input cabling, which is the best way to interconnect your system.

Hybrid XLR-1/4” jacks.   These are cleverly designed to accept either XLR or phone plugs in the same receptacle, saving space and adding flexibility.  If you have these, you’re ready for just about anything.

Output Connectors

These are where your speakers hook to your power amplifier, so no matter how “integrated” your system, it will have output connectors.  You may encounter any of the following:

Barrier strips.  Again, these are a simple row of screws, useful for fixed installations only.  If you are hauling your PA around, do you want to connect your speakers each time with bare wires and a screwdriver?  Didn’t think so.

” jacks.  These are OK for input connectors, since the line-level input signal isn’t very strong.  For speaker output connectors, however, they are only useful in relatively low-powered systems.  The tip of the ” plug only makes contact with the jack at a very tiny point, too small for efficient transfer of the larger currents going to your speakers.  If your mixer-amplifier has these, it’s an indication that the system power is not great enough for this to matter.  If a powerful separate amplifier has these, use them only if they are the last resort.

Binding posts.  These look like little red and black knobs with a hole in the middle that can be used two ways (even though they are called “5-way binding posts” by marketing types for some unknown reason).   You can unscrew the knob, poke a bare-ended speaker wire through a hole in the post, and tighten the knob.  For mobile applications, this is only slightly more convenient than barrier strips.  Or, you can use speaker wires that terminate in “banana plugs,” so named because their conductors are fatter in the middle and look like little metal bananas.  These plug right in the binding post hole, and provide a much larger area of contact than ” plugs and jacks.  Their only disadvantage is that they are easy to disconnect if someone steps on a cable – or sometimes unplug themselves from the cable’s own dangling weight.  If you use these, arrange the cable so there is no “pull” on the connection.

Speakon connectors.  These are the best way to connect speakers yet devised because (1) Speakon connectors are only used for speakers, so they can’t be confused with other types of cables; and (2) they have a secure, locking connection that resists cable-tugging, but releases easily with a button or sleeve on the connector.  Speaker cables equipped with Speakon jacks cost a little more, but if your power amplifier has these output connectors, it’s worth it.

“Bridging” capability

Most modern power amps have two identical “stereo” channels.  For PA use, you are probably feeding your amp a mono signal anyway, so both channels of your power amp are doing the same thing (DJ’s may want to play with the “pan” controls on their mixers to get a little stereo separation, but since your crowd is all over the place, the mix will change depending on where someone is standing – not always a good idea).  Some power amplifiers have a “bridged mono” mode that allows them to have greater monophonic output to a higher-impedance load – a subwoofer, for example.  Whether this capability is important to you depends on whether you anticipate using multiple power amps and, probably, a biamplified system, so I figure you know who you are.  There is more technical information on bridging amplifier power outputs on this site at  But running power amps in bridged mode, even when they are designed for that, entails greater risk and requires careful attention to load impedance and speaker-lead isolation from ground.   Don’t do it unless you understand it, and unless the manual that comes with your amp clearly states that it will operate in “bridged” mode.

“Bridged” mode is not the same as “mono input” mode, which is a feature of some power amplifiers.  That means simply that you can plug an input signal into one channel of the amp, throw the “mono input” switch, and the same signal is routed to the input stage of the other channel.  If you have only a mono output from your preamp or mixer, this can save you a Y-connector or cord.  So, it can be a handy feature, but it’s one whose function can be replicated with a $5 Y-cord.

Filters and crossovers

Some fancier power amps are equipped with “subsonic” filter switches that can prevent low bass (i.e. below 30 hz or so) from coming through the amp, on the theory that unless it’s being used to amplify a bass instrument, signals at such frequencies are probably some sort of noise anyhow.   If you have a problem with your microphones picking up bass sounds, these filters may be useful, but I’m hesitant to recommend any sort of “band-aid” approach that stops an unwanted signal here, at almost the final point in the reproduction chain.  If you have a problem with subsonic “rumble” coming through your system, find the source and fix it!

Some stereo power amplifiers are capable of being used in a “bi-amped mono” configuration, with a built-in electronic crossover (usually at a fixed frequency) splitting the signal between one channel and another.  This can be useful in a bi-amped system, but since fully adjustable electronic crossovers are relatively inexpensive, there’s no reason to pay a lot of extra money for this feature.

There may be other filters, buttons, knobs and “features” built into a power amp you’re considering, but think of these as you would think of hubcaps on a car – pretty cosmetic things that do absolutely nothing to improve performance or longetivity, and one more thing to go wrong.   And no one will admire your beautiful power amp in the driveway.

Using many High Power amplifiers from a single power source is not always a good idea.

In the USA (120V AC), most home electrical outlets are 15 amp circuits (1800 watts maximimum). In peoples kitchens, this is 20 amps (2400 watts maximum). On some stages, you will find 30 amp (3600 watts) circuits, however, 15 amp or 20 amp are the most common. What this suggests is that buying 10,000 watts of power amps does not not mean that you can use them all at the same time on the same 15, 20 or 30 amp circuit. Some stages have multiple 20 or 30 amp circuits, others only have a single 15 or 20 amp circuit.

While your power amp may be rated at a high wattage, its often the case that you are using only 10% to 40% of its capacity. If you have a lot of deep bass, you may see peaks of up to 80%. Keep this in mind as you connect your Power Amps, Guitar Amps, Rack Mounted Effects, Lighting Systems and any other gear that you bring along - if you exceed the power availability, you will either 'pop' a circuit breaker, or encounter a 'brown-out' (where the voltage drops well below the Normal AC line voltage). Brown-outs can greatly increase harmonic distortion levels. Knowing what is available (AC Power) will determine how much power is available for your needs and will affect how much gear you can use. If possible, make sure that you are not sharing your power with some other function (like a food service using electric hot-plates).

Use your Power Amplifiers effectively - using high SPL speakers will get the most out of your amplification systems - please see the discussion of SPL to get an idea how your speaker choices effect things.

Overall, the best advice I can give is to buy the most powerful power amp you can afford, so it will be there to deliver loud, clean sound whenever you need it.  If you don’t need its full output capabilities at that small indoor gig, that’s fine – it will sit there and deliver low-level sound with negligible distortion year after year, happy as a clam.  But when you need to really crank up the system in a large hall or outdoors, you will be happy you paid for those “extra” watts.

Questions? Comments? .

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