(Originally published in Musician Magazine, August 1991)
All Rights Reserved © 1991 Thomas W. Day
When I started this article, I had been an engineer with a pro power amp company for almost ten years. This article was originally a promotion piece. It was so promotional that the first chapter was published in first draft form in the company's newsletter. I'm not in the industry now. I don't have any compulsion to be kind to my employer or to the industry in general. I spend my days teaching docs and salespeople how to keep abused American hearts pumping. Nights, I dream of music and less stressful days. As the man said, "the hours are good, but the minutes are awful." But, before it all drifts away, I want to pass on what I learned about buying and using audio power amplifiers and professional sound equipment, in general.
Theory First: a perfect power amplifier would only make the original signal, louder. It wouldn't add noise or distortion. It wouldn't break. It would protect your loudspeakers from abuse. It would only cost pocket change. It wouldn't waste energy. Real power amplifiers are a long way from perfect. The real thing is expensive, complex, and too often the least reliable component in your sound system.
A big part of the reason power amps aren't perfect is that they have to work so hard. Power amps are easily the hardest working electronic component in the audio chain. Only electro-mechanical loudspeakers work harder. Anything that does work wears out. That's why marketing guys last longer than engineers. And engineers last longer than janitors. Stress from overwork causes a power amplifier's parts to fail more often than the less exercised parts of other types of audio equipment. Think of equalizers as the audio equivalent of a marketing manager. They ought to last forever; equalizers and marketing managers.
But even with the laws of physics beating on their heads, some power amplifiers do very little damage to your music, rarely ever break, and protect your loudspeakers from almost everything but bad music. If you follow a few rules, you can buy a power amp, use it for a big part of your career, and expect the finest sound quality possible with the reliability that keeps the Maytag guy unemployed. Stay with me, I'll tell you what to look for.
Accuracy is an important feature in a power amplifier. The most accurate power amps add the least '"distortion" to the amplified signal. Harmonic inaccuracy, frequency response inaccuracy, and noise are all forms of distortion. The bad news is that an accurate sound system may not sound good to you. We all tend to like hearing what we are used to hearing. You may be used to hearing a particular kind of distortion and lots of it. You may have a hearing defect that your system compensates for.
Accuracy and sound quality should tie together. It is hard to be objective about "sound quality" because many of the standards we use to measure sound quality are personal preferences. When you are designing a system, it is important that you train yourself to listen for what is accepted as an accurate sound. Listen to acoustic instruments for a reference, then listen to those same instruments amplified. You can always alter the sound with signal processing equipment, but start with as accurate a system as you can afford. And go downhill from there.
Reliability, to most sound companies, is much more important than accuracy. The most reliable power amplifiers are designed to minimize component failures (mechanical and electrical) and to survive extreme conditions. A technical discussion of a reliable design is beyond the scope of this article, but there are ways for a user to get a grip on product reliability. A reliable amp should have good references. If you don't have the technical ability to determine which amplifier has the cleanest output and is the most durable, get help.
Find a dealer who sells several brands. A dealer who has this kind of independence has little to gain by selling one brand over another. (This is assuming the dealer isn't more concerned with selling what he has on the floor than with your satisfaction and return business.) A dealer who is in for the long haul will be more likely to give you information that will bring you back for your next purchase. Beware of dealers who play used car games with you. If the salesperson doesn't know or care enough to carefully explain the reasons for his opinions, look elsewhere for advice.
If you have a professional sound company in your area, get the touring manager's opinion. This source probably has nothing to gain by recommending a poor product, unless you are likely to become competition. One drawback may be that a tour manager won't say anything good about anybody. Talking to a little sound company owner won't tell you much. That source won't have had much exposure to a variety of manufacturers and won't have used many pieces of the brands they have been using. What you want is a company that has a bare minimum of 50-75 power amplifiers in their working system and has been around for at least a decade. If you get to talk to someone like this, take names and numbers. You won't get a lot of time and the information you get will be valuable.
To get really unbiased advise, talk to a repair technician who specializes in professional audio equipment. These guys have nothing to gain from selling you a bad product. They know what breaks and what doesn't. And it won't hurt to get to know the people who can bring your system back to life in a crisis. A good technician is often more technically competent than the engineers who design the equipment. They see all of the faults of the products in the field, put the pieces together, and can give you the least biased reliability report of any source.
Although this article is being published in a pro audio equipment magazine, I can't recommend using magazine articles as valuable references. From my own past experience, I don't think magazine reviewers have an unbiased perspective or any other qualifications of an accurate source. I take everything I read with a block of salt.
LIFE ON THE SHELF (What to look for when you are buying an amplifier)
Once you get the advise, there are a few logical steps to follow in purchasing a power amplifier. A sound system is called a sound system for a reason; all of the components must be selected to work together as a system. In selecting the components to your system, you have to honestly evaluate how the system will be used. This isn't as easy as it seems. We all have "golden ears" and our systems "never clip." In reality, some systems never clip more often than others. If you expect your system to have a long life, you have to be honest about how it will be used.
First, figure out how much power you need. Contrary to some folk tales, you can buy too much power. It is possible to destroy speakers with too much clean power. I'm not contradicting the fact that distorted power will quickly destroy high and mid drivers, but 1000 watts into a 100 watt driver will do the job just as quickly. Buy a reasonable amount of overhead power for your system, run the system within the limits of available power,
An "honest" rule of thumb is "whatever power your guitarist's amp has, you need a minimum of ten times than power that in your sound system." A margin with headroom would be 100X the guitar amp's power. If you have two guitarists, double the calculation. It may sound ridiculous to have to buy 1000 watts of PA because the guitarist has a 100 Watt Marshall, but it isn't. If the systems have equal efficiency (a big, highly unlikely "if"), this will buy your system 6-16dB of practical headroom over the guitarist. That isn't huge, but it's far more than most rock bands ever approach. Yes, I realize that my math appears to be overly conservative. I'm comparing a (hopefully) clean 1,000 watt system to a very distorted 100 watts of guitar amp output. The reason 100 watts of guitar power sounds so loud is that it's distorted and is often averaging a lot more than rated power.
The dynamic requirements of a system also determines the amount of power a system needs. An concert orchestra system has considerably different requirements than a heavily compressed disco system. Those requirements determine the amount of excess power the system will need. In a wide dynamic range application, like a classical piano concert, the continuous system power will be considerably less than peak power. Normally, quite a bit less than one tenth of the peak. In this application, you may want the power rating of the amplifier to be considerably above the speaker power rating. Since the system will rarely be using the extra power for short duration, the speaker system is unlikely to be damaged. This is a trade off between the speaker safety margin and your budget. You have to make the call.
In a limited dynamic range rock and roll or disco system, the average power is often very close to the peak power of the system; probably about one third of peak power. If the music is heavily compressed or the system is run near or beyond clipping, the power amplifier and speaker power ratings should be very close to each other. To protect the speaker system components from overheating or excessive driver excursion, you may even limit the output of your power amplifiers to a value considerably lower than what the speaker system can handle.
To get the information you need to design your system, you will have to interpret manufacturer's specifications. When you are evaluating any product's specifications, remember this: designing an amplifier is a lot different than advertising one. Anyone can write a spec sheet. Nothing protects you from advertising lies, not the FCC, the FTC, or the FDA. You are on your own.
Here are just a few specifications to distrust.
Continuous Power. Most power amps can live up to their "continuous power" spec at 8W, but, as the load drops and the power climbs, "continuous" takes on new meaning. It means "continuous until the circuit protection trips." Could be five minutes, could be five seconds, could be five milliseconds. Technicians are used to making this test after bypassing the circuit protection; or not bothering with it at all on many amp brands.
Distortion. You will probably be disappointed to learn that this is a "fudge-able" specification. Very low distortion specs are often quoted at a single frequency or output level. If the distortion percentage isn't from 20Hz to 20kHz and from full output to at least 20dB below full output, it isn't saying anything significant. If distortion is quoted over those ranges, it could still be untrue.
Damping Factor. This specification is a marginally useful measurement of the amp's output impedance vs. the speaker impedance. Since a bigger damping factor is better, some manufacturers quote numbers that would require superconductors to achieve. If you figure 20 feet of 14 gauge wire will reduce a perfect damping factor to about 150, damping factors of much larger values are meaningless in the real world. Amplifier manufacturers often measure damping factor at frequencies that get the best numbers out of their designs, not where the figures would mean the most to your sound system.
Slew Rate. This is a 70's holdover. Some manufacturer's claim slew numbers that are faster than the individual components they use can produce. The whole can't be better than the parts. Slew is very hard to test on some of the more "sophisticated" designs, because the design won't pass a square wave without tripping the circuit protection.
Power Bandwidth. This is the last spec I'm going to whine about. Sometimes called "frequency response" at a specified power rating. You really have to watch this spec. Be wary of bandwidth response specs at very low power output, unspecified power output, and numbers that are a lot better than competitor's products for the same money. If it seems unreal, it probably is.
The disclaimer at the bottom of nearly every product's specification information tells the story best: "The manufacturer reserves the right to change the product specifications at any time." This means that the product does not have to do what the spec sheet says it will do. On top of that complication, the tests listed in the spec sheet are selected to idealize the product. They are usually "best case" tests; or pure fantasy. Since the specs are suspect, you will have to find another way to determine if the amplifier does what you want it to do.
You can make a basic, logical, inspection inspect of a power amplifier without a cart full of test equipment. Some simple points of inspection are:
An amplifier should have good manners. It should not make power-up or power-off thumps, pops, or explosions.
When you drive the amplifier hard, it should not sound drastically more distorted at the point of clipping (when the clip lights light). The amplifier should not shut off or make weird noises when it is clipping.
The amplifier should have faultless short circuit protection. You should be able to short circuit the output of the amp at full power without damage to the amp, or the speakers, after you remove the short.
The amplifier should have a decent sent of connection options. I'd recommend Speak-Ons and barrier strip outputs and XLRs and barrier strip inputs. Balanced inputs are a non-negotiable requirement for professional live sound systems.
You may need the assistance of a technician to perform these tests safely. Most dealers have someone capable of running these kinds of tests and that person may even help you interpret the results of the results.
Obviously, you should listen to the amplifier, but listening tests can be deceiving. When you are evaluating the sound quality of a group of amplifiers, be certain that the test is honest. My rule is "ABX or it doesn't count." If you can't do a double blind test (one where no one knows which amp you are listening to), don't waste a lot of time with the test. I've sat in on too many listening tests where "golden ears" who could easily point out the faults in amps they could identify suddenly couldn't tell anything about the same amps when ABX was applied.
But before you start listening there are a few things to set up. Power amp manufacturers don't agree on how much "gain" an amplifier should produce. This is important in a listening test. Our ability to hear the upper and lower frequencies varies with volume. If you compare two identical amplifiers, the louder of the two will sound the best; bass will sound stronger and highs will sound brighter. Take the time to match the output gain of your test amplifiers before you try to make any listening tests. Use a test tone as an input signal so you can accurately measure the output signal of each amp. If you have a voltmeter, you can precisely calibrate the test amplifiers' gain by applying the same input to both amps and setting the amp gain controls for equal output voltage (measured on a power resistor, don't blow up your test speakers with test tones).
Make your listening comparison tests quickly. Most people have a very short aural memory. It is hard for most of us to hear the difference between two very different amplifiers, even when there is nearly instant switching. If you have to fiddle around with wiring between each test, you probably won't learn anything in your test.
Make your tests at a volume level that you expect to use the amplifier. A quiet listening test in a music store is not valid. You are interested in the characteristics of the amplifier when it is driven hard. You want to know what its dynamic ability sounds like. The ability of the amplifier to control large speaker drivers is not tested without serious driver movement.
Get the amplifier to a place where you can listen to it the way you are going to use it. This is important for lots of reasons, but the main one is that high volume causes fatigue. At high volume levels, you will only have a minute or two to listen and compare before a guitar amp starts to sound as accurate as an audiophile system.
Before you pull out your cash, beware of the "buck factor." We all want to believe our money is well spent. If you are about to spend a few thousand dollars on a new amplifier, you probably expect it to sound better. Positive thinking is a great personal tool, but it doesn't have any value in a fair product comparison. Remember that when a salesperson is harping about the great things that you should be hearing in your listening tests. If you don't hear the difference, it isn't there.
LIFE IN THE RACK (How to setup and use the amp.)
After you have purchased your amplifier, you have to use it. I'd like to tell you that you can simply plug a stack of amplifiers into the wall and go for it; but I can't. You have to know a few things about power, wiring, and troubleshooting.
There is no free lunch. You may have noticed that an amplifier with a 200 watt output rating will have an input power rating of 300-500 watts listed on the serial number label. (You may have to use Joule's Law to calculate the input power: P=VI or Power = Voltage times Current.) The extra wattage is not a spare tire waiting to carry on when the original watts get tired. This unaccounted-for-wattage is the price for using a less than perfect power supply and output circuit. You can find the missing watts in the heat you feel on the face plate, heatsinks, and chassis of your amplifier. Instead of converting all of the electrical energy to mechanical energy and loudspeaker motion, a lot of the power used in an amplifier produces thermal energy. This thermal energy can cause problems.
You will need to allow for moving this energy away from the amplifier. The amplifier designer probably tried to help you with this. If the amp is fan-cooled, make sure the air flow is unobstructed. Don't put obstacles in front of, or behind, the amplifier. If you can keep the amp cool, it will last a lot longer. The amp will stay alive longer if you protect it from dust and other "foreign materials." Most kinds of contamination act as a thermal insulator. The blanket of dust that builds up on the amplifier's components holds the heat to the components. The components get hotter when they are insulated, the same way blankets work on humans. Some not-so-bright designs route the hot air into closed places; like the sides or top of the amplifier. When the amp is sitting on your kitchen table, this might work. Rack cases, generally, don't open on the top or sides. If your amp vents in those directions, plan on doing some carpenter work to your rack.
This may seem like dumb advice, but I have to say it, "Don't put your beer cans on top of the amplifier!" For that matter, keep the beer away from any other electronic gear. I can't count high enough to figure out how much money I have charged to clean beer muck out of expensive electronic gear. Liquids and electrons don't mix. Friends don't let friends do stupid things. And so on.
You will also need to make sure your amplifier gets enough to eat. The power output of your amplifier is dependent on the power input. The amplifier's power supply components are designed to provide optimum output with a good, "stiff" power line. If your local power company is prone to "brown outs" (low power line voltage), your amplifier is not going to be as powerful as the manufacturer designed it. If the supply voltage is especially low the amplifier may become unstable, the protection circuitry may not work correctly, or the amp may not work at all. Most power amplifier performance specifications are measured with a line voltage of 120VAC (in the United States). If the power line is 10% low, or 108VAC, an amplifier rated at 200 watts will probably only provide 162 watts. If the power line is 15% lower, or 102VAC, the 200 watt amplifier will only be good for 145 watts. From a bottom line perspective, you will probably pay at least $0.80 per watt for a professional power amplifier. On a 200 watt per channel stereo amplifier, the 15% low line will waste $100 of your investment. It is worth your time to make sure your amplifier is getting the power it is designed to use.
If your power source is good but your wiring to the amplifier is bad, you still lose. Your AC power wiring must be large enough to get the power from the power company to the amplifier. A 150-foot 16-gauge extension cord is not going to cut it for a multi-kilowatt concert system. If you don't have the technical knowledge to make the calculations for a serious power distribution ("distro") system, hire someone who can. This is serious business. A poorly designed power system is not just going to cause problems with your amplification system, it could burn the house down. Seriously. Undersized power-wiring can get hot enough to melt insulation and start fires. Improperly grounded power wiring can kill you.
Still more wires! The wiring at the output end of your amplifier is as critical as the power wiring. Very long or very small speaker cables waste power and impair the amplifier's ability to control the speakers. Wire resistance is determined by the diameter (or cross-sectional area) of the wire and the wire length. In other words, if the wire is very short it doesn't have to be large. On the other hand, if the wire diameter is large it still shouldn't be long. If your cables are long, you will introduce other types of problems to your system. As the cable lengths increase, the cable inductance and capacitance increase. Both of these effects reduce your system's performance. The cable capacitance acts as a frequency dependent current path across your wire, bypassing your speakers. This effect worsens as the frequency rises. The cable inductance acts as a frequency dependent load in series with the speakers, increasing the cable reactance and reducing the amount of signal available to the speakers.
"It ain't what you don't know that hurts you, it's what you do know that ain't right." Will Rogers was talking about politicians, but he could have been talking about damping factor. I have heard damping factor blamed for everything from speaker cone damage to Republicans in the White House. The loss of control produced by long or undersized speaker wires is often noticeable on large low frequency drivers where the amp's "damping factor" can be important. Damping factor is one indication of how well an amplifier will control your loudspeakers, but the effect of damping factor is often overrated. Poor damping factor can produce a bass response sometimes described as "flabby" or "loose." Practically, you will probably have to limit yourself to a system damping factor of 30 to 50 (including the impedance of the amplifier's output, but not including the wiring, and speaker connectors).
The speaker connectors can be as difficult to figure out as the rest of the wiring. Many amplifiers have a collection of output connectors for output connectors. You may find "5-way connectors," XLRs, barrier strips, and 1/4" phone connectors. The old guitar standard, 1/4" phones, is rated for max of 1 amp and is not useful for high power speakers. XLRs are no better. A brand new dual banana connector is a high current connector, but banana connectors fall out easily and wear out fast. The ideal speaker connector is large (to carry high currents), mechanically secure, and gas tight (to prevent corrosion). If you use the barrier strip's screw terminals or the terminal connector section of the "5- way," you will get all of the above advantages. Don't try to shove the bare wire under these terminals. Crimp a fork tongue connector to your wire ends and you will have a relatively removable wiring setup that will rarely require maintenance, unless you move the system a lot. The current power amp speaker connection standard is Neutrik's "Speak-On" connector. This is a high current, high impact, polarized connector. The old amp speaker connection standard is the "twist-lock" power cord connector, found at your local electrical equipment supplier but not found cheap. I'm still partial to these connectors at the speaker end of the cable, but old habits die hard.
Now that you have the amp hooked up and you are ready to use it, you will need to pay attention to the information the amp's indicators provide. Most pro power amplifiers have "clipping" indicators. Most pro users ignore this indicator and everybody else wonders why the "little red light is always flashing." The clipping indicator is telling you that your output signal is exceeding the amplifier's power supply voltage. Kind of an electronic, "You are too loud, Bozo." When the amp runs out of voltage, it can't get any louder without getting distorted.
Normally, most of the power content in music is concentrated in the low and mid frequencies. When an amplifier is clipping, it is producing lots of harmonics of the original signal. Those higher frequency harmonics get directed by passive crossover networks to the high frequency drivers. An overdriven power amplifier can produce much more power than the advertised power rating. In fact, an amplifier that is driven into heavy distortion can often produce nearly 1.5 times its rated maximum output power. A good dose of this clipped signal will quickly smoke your wimpy little tweeters.
Some amplifiers have indicators to let you know when an output signal is present. Some amps have VU meters. These indicators are troubleshooting tools. If you are not getting anything out of the amplifier, but the signal presence indicators say something should be coming out of the amp, you should suspect the speakers or output wiring.
Many modern amplifiers have circuitry to protect the amp from overheating. When an amplifier is overheating or tripping circuit breakers, it is being worked too hard. Believe this warning! You are going to have to reduce your system volume or remove some of the speaker load. If your ignore this symptom you may damage the amp.
If you are moving your equipment a lot, you may be shaking it to death. Vibration is the enemy of any equipment that has more than one part. If you can shock-mount your rack, do it. Otherwise, handle all of your equipment like you had to pay for it.
LIFE AND DEATH (Finding and fixing problems with the amp and your system)
Eventually, something in your system is going to die, or act weird and then die. The best way to troubleshoot anything is to eliminate unnecessary variables. Simplify the testing of your system by removing anything that isn't needed to produce a test signal. The most simple system I know of is a signal source (a CD player, cassette recorder, or signal generator), a power amplifier, and one speaker. If you connect the headphone output of a portable cassette or CD player to the input of a power amplifier, you will be able to control the volume of the system with the player's gain control. If the problem you are trying to locate is missing with this simplified system, add your other system components; one at a time.
Start with one speaker to completely eliminate a second set of speaker cables and speaker from confusing your troubleshooting. If the single speaker works in both channels, add the second speaker. Keep adding components until you locate the component that causes the problem. This technique will work for locating ground loop, distortion, and loss-of-signal problems.
Ground loops are among the most common problems you may have to troubleshoot. Ground loops can appear any time you change your system. Your first setup may be the quietest system in history; and you may never have it that good again. The large transformers in high powered amplifiers often produce a magnetic field that can induce ground currents in the chassis and wiring of other products mounted in the rack. The large 60Hz AC currents required by power amplifiers can cause ground currents to flow through input cable grounds and chassis grounds. These ground currents can produce "hum" (60, 120, or 180 Hz noise) in your signal output. You may find that moving an input cable a few inches will cause a major difference in the system noise.
Usually, ground currents can be eliminated by establishing a single ground path for all of the components in the system. Occasionally, the design of a particular product will require it to be isolated from the rest of the system. Preamplifiers and power amplifiers are examples of products that should be physically isolated from each other. High gain, preamplifier circuitry is particularly sensitive to the large magnetic fields produced by a power amplifier's power supply transformer. Leaving a single rack space between a preamp and a power amp is often enough isolation to eliminate this induced noise pickup.
The phase relationships between each of your system's electronic components is another major troubleshooting hassle. System phasing and connector polarity is made difficult by the lack of industry standardization for amplifier and connector polarity. The international standard for XLR-3 connectors is pin 2 "+" ("non-inverting", "hot", or "high"), pin 3 "-" ("inverting", "cold", or "low"), and pin 1 ground. The terms "inverting and non-inverting" describe the phase of the output signal compared to the phase of the input signal. A non-inverting input does not change the phase of the signal, inverting does.
Some American and English manufacturers build equipment with the phase of the input XLR wiring reversed (pin 3 "+" and pin 2 "-"). If your system uses a single power amplifier or all of your amps are the same brand, this is not a problem. If you are mixing amplifiers brands, you will have to watch for phase differences.
Phase problems can do more to your sound system than just make it sound funny. A sound system operator may drive the speakers hard enough to fry speaker coils because of the output cancellation caused by two speakers reproducing the same signal, but out-of-phase with each other. The reduced system sound pressure level (SPL) from the out-of-phase speakers is the clue for troubleshooting phase problems. An easy way to determine if two amplifiers are in-phase is to listen to the output of the amplifiers with the same input signal. Compare the system output with one amp and then with both amps. The system should be about 3 dB louder with both amplifiers operating. If it is slightly quieter, or "hollow" sounding, the amps may be out-of-phase. When a system gets large, this procedure is not very efficient. There is test equipment available for testing system and component phase. It isn't cheap, but if you have a lot of equipment to test it might be a good investment.
If your amp dies, the work you did in researching manufacturers will pay off. If you didn't do that work, you will be doing the paying. All manufacturers are not created equal. Some companies offer transferable warranties (if you bought the amp used), some don't. Amp warranties run from one to twenty years. Some amp companies haven't lasted long enough to cover their first year's products. Some companies fix everything that breaks for an indefinite period, no matter what the warranty. Others pretend that anything that broke was your fault. Some companies fix what's broke, no questions asked. Others never stop asking questions. Don't leave finding out how service is performed until you need service. Repairs can cost you the purchase price several times over in a very short product lifetime. This is where a company's reputation means the most to you.
If you find that a power amplifier is defective, you will have to find a technician. There are two sources of repair: the original manufacturer and a local facility. Generally, I would recommend you return the amp to the manufacturer. If the company has an in-house service department, that department's technicians will be intimately familiar with the company's products. That is all they ever work on and they see the product in quantity. The next best service location is the local authorized warranty station. Often, local service companies simply box the product up and ship it to the manufacturer for any serious repair. If they don't perform that service for free, call the manufacturer. If the product failed in warranty, you should be able to get the manufacturer to pay for shipping both ways. If the manufacturer won't do it for you, they may do it for your dealer.
There isn't anything that is user-repairable in a broken amplifier. I wouldn't even recommend replacing an internal fuse. Usually, the internal fuses protect the circuit board from major failures. If those fuses are gone, something major took them out. Do not replace these fuses with a fuse value greater than the original! If you insist on replacing these fuses, you might try a smaller value first. If it blows immediately, give up and take the amp to a repair shop.
If you spill anything liquid in the amp, turn it off and send it back to the manufacturer with a note explaining what happened. Tell the technician what you spilled into the amp. The printed circuit boards will have to be washed and some components may have to be replaced. If you leave this kind of contamination on the components they will quickly collect dust and they may be damaged by corrosion. Many liquids contain conductive chemicals that can cause your amplifier to do strange things to your speakers.
You may want to talk to your dealer about any major repair job. The place that sold the amp may service it. If you are a good customer, the dealer will probably handle getting the amp to a service facility or to the manufacturer. Dealers can often get the best repair deal from the manufacturer.
If you aren't near the place where you bought the amp, call the manufacturer. Most manufacturers have service centers somewhere near just about anywhere. The Service Department of most manufacturers' will rush a repair; if you ask for it, beg, cry, scream, or whatever works.
A typical major amplifier repair can run anywhere from $75 to $500, depending on how many components have to be replaced. The kinds of things that break in power amplifiers are sometimes expensive. The components that fail the most are the output transistors. These things do most of the work in the amplifier. They are the source of most of the amp's heat. They will cost you $15-25 each and there may be 20 or more output transistors in a large amplifier.
A lot of repairs are minor. Minor repairs usually cost $30 to $75. Many repair shops estimate the cost of repairs for a small service fee. Many manufacturers will do free estimates, plus the cost of shipping.
Make sure you are not paying for a repair when the amplifier is still under warranty! Pro amplifiers have really long warranties: three years is not unusual. If you don't take the amp to an authorized service facility, the manufacturer's warranty will probably not cover the repair. Call the manufacturer first, to verify the warranty service center status of your repair shop.
Someday, when the warranty is long gone, your amplifier will die and you will have to decide if it is worth reviving. This can be a tough call. The "watt per dollar" figure is always shrinking and yesterday's technology is pretty sad by today's standards. It doesn't take a pair of golden ears to hear the difference between a mid-1970s amplifier and today's technology. You will have to make a careful comparison of the repair cost for the old amp and the purchase price of a new amp. If the two values are close, the new amp with its warranty and improved technology may be the better buy.
The future for power amplifiers holds direct digital inputs and signal processing, low weight and high efficiency power supplies, computer control, greater reliability, and more steps toward sonic perfection. Amplifiers will always get more powerful and the watt-per-dollar ratio will probably keep growing. We may see an amplifier feedback loop that includes sensors on the loudspeaker and distortion will be radically lower. Loudspeakers may radically change and this could require totally new concepts in power amplification.
Your choice in amplifiers may be what defines the reliability and sound quality of your sound system. Or your amplifiers may limit the performance of your system. With a little research you can find a power amplifier that will handle any professional requirement. With a little care and common sense you can depend on your power amplifiers for years of service.