The various features of a power amplifier can be broken down into several sections in order to simplify explanations. The sections break down as follows:

General Information | Features & Specifications

POWER AMPLIFIERS - GENERAL INFORMATION

MONO OR STEREO?
Mono power amplifiers are something of a vanishing breed these days, however they do exactly the same job as one channel of a stereo amp in terms of basic sound quality and are still employed in a variety of applications from live PA to installations. They usually have two Speaker Outputs wired in parallel and may have two Inputs also wired in parallel. Dual parallel Inputs are not intended to accept stereo mixer signals. Instead, the extra input can be patched to feed some of the (mono) mixer signal to the input of another power amp - rather like using a "Y" adapter. If you do connect a mixer's stereo outputs to these inputs there can be phase cancellations causing a change in the sound, and possibly distortion.

• { TIP - To connect a stereo mixer to a mono power amplifier or one channel of a stereo amp, find a mono output on the mixer and use that. If the mixer does not have a mono output, i.e. one representing the sum of the left and right channels, simply pan all the channels left or right and use the output you've panned to.}.

Stereo power amplifiers can be viewed as two mono amps in the same package. As a result, you can connect two 8-0hm speakers, one to each channel, and the speaker impedance encountered by the amplifier will still be 8 ohms. But connect another 8-ohm speaker to one of the channels and that channel will encounter 4 ohms. And, because each channel can operate quite independantly of the other, channel one can power a (mono) FOH (Front Of House) speaker system while channel two powers monitors or anything else - perhaps subwoofers. Stereo FOH PA systems are becoming more common these days, however a mono system works as well in most situations (more about this later too).

POWER - Which Watts are Which?
Amplifier power ratings these days tend to be in watts expressed as "continuous average" or "burst average" or "peak" or "music power" or "continuous music power", etc. In the old days, the nomenclature was "RMS" which stands for "root mean square" and reflects the results of a test for the amp's long-term, continuous output capability. Other, more modern tests tend to net fairly similar results but are more comlpex and require more sophisticated equipment. Two ratings which are worth looking for are Continuous Average Power and Burst Average Power. The first rating will be similar to what an RMS test would net and the second one will be higher, reflecting the amplifier's ability to repeatedly produce clean peaks which last for at least one complete wavelength.

HEADROOM
A good amp will have 3dB of headroom at its maximum, continuous output. This means that the amp can deliver double that output on frequent, full-wave peaks. That's what really helps to make those deep "thuds" and "rumbles" shake the floor when you are powering subwoofers.

SLEW RATE
Slew rate is basically a measure of the amplifier's ability to supply voltage in response to fast, short-duration peaks and is measured in volts per microsecond (one millionth of a second). A slew rate of "30VUsec" - 30 Volts per microsecond - is considered good, but anything down to 20 VUsec is also considered good. High frequency reproduction is thought to be better in amplifiers with a high slew rate hence, if you do hear a difference between two amps with radically different slew rates it would be in the highs. However the truth is that slew rates have to be unusually low - well below 10VUsec before any highs are likely to be muted. Few amps like that are produced today.

DAMPING FACTOR
Damping factor is usually, but not very accurately, linked to low frequency reproduction. The popular thinking goes as follows; although an amplifier may have flat frequency response all the way down to 20Hz, its ability to make speakers reproduce low frequencies with maximum sound pressure depends to a certain degree on its damping factor. This is supposedly because amplifier outputs encounter a certain amount of "Counter-EMF" (electro-magnetic force) from the speakers, especially woofers. These induced signal voltages tend to be out of phase with the amplifier's output and can cancel some of it, especially on low-frequency peaks where the cone is travelling very far in and out causing the voicecoil to cut more lines of magnetic force hence generating more counter-EMF voltage.

Damping factor, in its popular perception, reflects an amplifier's potential ability to counter or "damp" the effects of this process thus permitting more power to be delivered to the woofers. There is some truth to this, but not as much as you might think. Damping factor is a matter of impedances. As long as the amplifier's output impedance is lower than the overall speaker impedance you have damping. In fact the method of calculating an amplifier's damping factor is based on the difference between the two impedances. If, for example, an amplifier's output impedance is 0.01 Ohms and it is designed to operate into a 4-Ohm impedance, the damping factor would be 4 / 0.01= 400.

All well and good, but the truth is, a high damping factor does not guarantee better bass response from the speakers. That characteristic can be chalked up to large amounts of power "headroom". In fact a really high damping factor can have an effect on the woofers which makes their lowest frequencies roll off more abruptly than if they were being powered by an amp with low damping, the net result being a reduction in deep bass response. Once again, check the specs for headroom figures. A good amp will have 3dB of headroom at its maximum, continuous output. This means that the amp can deliver double that output on frequent, full-wave peaks.

POWER AT IMPEDANCE
An amplifier varies its maximum output capability in accordance with the overall speaker impedance it is driving. Solid state amplifiers do this inversely to changes in the speaker impedance (yes, speaker impedances actually change with the frequencies they are reproducing - more about that in the Speaker section). In other words, as the impedance decreases, a solid-state amp is able to put out more power, and as it increases the opposite happens. [For what it's worth, tube amps have a "favoured" impedance above or below which their maximum power cabability decreases].

It would seem to follow that solid state amplifiers might be able to deliver more than their rated power if you connected them to very low impedances. But we all know that this is not the case - especially if we've had an amp shut down or, Heaven forbid, blow up because the speaker load was too low. What happens is, the amp actually tries to put out more power than it was designed to deliver. It overheats in the process and will self-destruct if it has no thermal protection circuitry to shut it down or otherwise limit its activity. Conversely, amps are not harmed when connected to high load impedances; their maximum output is simply reduced. And when a solid-state amplifier is connected to no speakers at all, in other words an infinitely high impedance, it won't put out any power at all, it just takes a holiday.[Tube amps are very different in this regard. They will self-destruct if run with no load connected].

• { TIP - Be sure that the overall speaker impedance being driven by each ampifier channel is not lower than the manufacturer's specification. If the rating is much too low for your amp to handle it may cause serious problems for your power amplifiers. On the other hand, if it is only a little lower than the amp's minimum rating, you might be alright.}

Oddly enough the answer is, not necessarilly. You could, for example, run as many as four 8-ohm speakers from one channel of an amplifier, assuming it has a minimum load rating of 2 ohms per channel. And by making a special series/parallel wiring rig, you could theorectically run as many speakers as you like from one amplifier, however it could be a wiring nightmare.

Before we leave this subject it's worth mentioning that a rough rule-of-thumb regarding power at impedance goes as follows: double the speaker impedance and you cut the amp's output in half - cut the impedance in half and (assuming it's not too low now) you double the amp's output. Again, this is a very rough rule because all amplifiers react differently to speaker impedance changes and almosts none of them puts out exactly half or double power into double or half the impedance. If you are in doubt about what an amplifier delivers into a certain impedance, check the manufacturer's specifications.

PASSIVE & ACTIVE COOLING
In the beginning, all power amplifiers were passively cooled. With passive cooling, the output transistors are tightly fastened to metal fin clusters called heatsinks attached to the outside of the amp. As air passes over the fins, heat from the output transistors is radiated away. If there is restricted air flow, the result can be overheating and possibly damage. Passive cooling is common on units rated at under 800 watts; however, in power amplifiers over 800 watts, active cooling is the norm. Although it is theoretically possible to passively cool a one-thousand or two-thousand-watt amplifier, it is impractical. The heatsinks, whose size largely dictates their thermal transfer ability, would have to be huge and so would the amplifier.

• { TIP 1 - Although proper ventillation is important to all amplifiers, it is critical to those which are passively cooled. Racks or cases should always be open-backed and placed well away from walls or other obstructions.}

• { TIP 2 - Clean or replace the air filter regularly. This will prevent it from getting clogged with dust, choking off cool air.}

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Some amplifiers such as the old Beta-800 have an input feature which utillizes internal wiring and a special switching jack, usually the B channel input, to patch the two channels together. The amp operates with both channels reproducing the same (mono) input signal until a jack is inserted in the B input which breaks the internal patch so that the channels will be independant. The emphasis on designing features into stereo amplifiers which make them easy to convert to what might be called "dual-channel mono" operation futher illustrates that this does tend to be the way they are often employed outside the living room.

"Bridging" (not to be confused with dual-channel mono operation) is another way of turning a stereo amp into a mono amp, in this case with the power of both channels combined into one output signal. Although modern PA ampifiers tend to have a Bridge switch, some may not in which case it is necessary to connect a mono mixer signal to both channels either using a "Y" adapter or by patching to the other channel via the additional input if there is one. Then you reverse the phase of the signal going into the other channel. This is accomplished by taking the cable-end apart and resoldering the leads in reverse order. If you're using balanced patch cables, reverse the "phase" and "reverse phase" wires. In an unbalanced line, reverse the hot and ground wires.

• { TIP - Be very careful about bridging - it can be difficult even for people who are familiar with it. Read the "Bridging" section below.}

The 1/4-inch jack, for example, only has around one square millimeter of contact area at the tip - enough to carry no more than a few hundred watts before heat and then resistance begin to build up at the jack tip thus deceasing power delivered to the speakers. Post terminals carry plenty of power and have been the standard speaker output connection for many years, however there are two problems with them. First, cables have to be either bare-ended and wrapped around the terminals or equipped with "banana" connectors which go onto them. In either case the other end of the cable will need to have whatever connector matches the ones on the speakers - until recently, XLRs. This lack of standardization has been a sore point with pro audio users for years.

The second problem has to do with user safety; you can receive a very nasty shock by touching post terminals while a kilowatt-plus amp is in use. Most people don't do things like that, but consumer safety groups usually have their way, hence legislation is looming somewhere in the future to remove binding posts from power amps, at least the high-powered ones.

This leaves XLRs and Speakons, both of which appear on a few amps at present. XLRs work fine, but due to the cables' resemblance to mic lines, people ocassionally use the wrong ones to connect speakers, a practise which nets less than desirable results - MUCH less than desirable (mic cables just heat up and waste power, assuming they don't melt). Speakon connectors were specifically engineered for the application which their name implies and may be destined to eventually become the standard connector for high-powered amplifier outputs and speakers alike. They even have accomodations for bi-amping built into the basic design, something XLR's do not have.

LEVEL CONTROLS
The function of power amp Level controls is simple enough, the only question is how and why they should be used. For most PA applications you would run them at maximum, reason being that you want the amplifier's full output capabilities available. There are exceptions of course; for example if one channel is powering the subwoofers and the other the full-range cabinets in a club system, you would probably have the subwoofer channel at maximum and the other at a lower setting. Also in a club PA, you may be using a single monitor mix for everyone and thus the amp channel driving the dummer's monitors could be at max and the other at a lower setting (drummers aren't deaf, just noisy).

MONO STEREO SWITCH
In the "mono" position, this feature ties the two channels' inputs together via internal wiring so that they operate in unison, a.k.a. mono. Mono is a common amplifier operating mode for PA applications involving multiple speakers. If, for instance, you needed to power four subwoofers and four full-range enclosures, you would employ two amplifiers in mono mode, one fed by the low frequency output from the electronic crossover and the other from the crossover's high-frequency output. You would then connect half of the subwoofers to one channel of their power amp, and the other half to the other channel. The full range speakers would be similarly connected to their amplifier. In the "stereo" position, the switch bypasses the internal input patching so that the channels function independantly.

BRIDGE SWITCH
This feature does almost exactly the same thing as the Mono/Stereo switch but with a major difference (in fact, it is often combined into a mono/stereo/bridge switch); its input-to-input internal patch wiring has the positive and negative leads reversed so that one of the channels is 180 degrees out of phase with the other.

• { TIP - If your system is sounding mysteriously "weak", especially in bass response, check the Bridge switch. If it's "on" and you haven't performed all the following procedures, switch it "off". That weird, weak sound would be caused by acoustic phase cancellations between your speakers.}

• a) Feed the same (i.e. mono) mixer signal to both inputs, but reverse the phase of the signal going into one of the channels. This is not necessary if you have a Bridge switch, it takes care of phase reversal automatically. But if you don't have a Bridge switch, you'll need to do this by taking the stereo (i.e. balanced) plug apart going into the "other" channel's input (whichever channel you've decided to "flip"), unsoldering the ring and tip wires, reversing those leads, resoldering and then plugging it in. This trick does not work with unbalanced cables & connectors.
• b) Check the impedance of the speaker (yes, possibly only one speaker, read on) and divide it by two. This is very important because, unlike an amp running in normal stereo mode, a bridged amplifier reacts to the connected load at half its value. A 4-ohm load, for example, will seem like 2 ohms to a bridged amplifier and the amp will react accordingly.
• c) Having divided the speaker load by two, check the resulting impedance against the amplifier's minimum, per-channel load rating. Never connect a load which is lower than that rating.
• d) Check the resulting impedance against the amplifier's minimum load rating. Never connect a load which is lower than that rating.
• e) If the amp can handle that load, connect the speaker as follows: take the "hot" (+) signal only from each channel and connect your speaker so that the cable end from one channel goes to the "+" speaker terminal and the other cable end goes to the other speaker terminal. Note which channel is driving the "+" speaker terminal so that you will be able to rig another bridged amp and speaker the same way and they will be in phase (VERY important, especially for subwoofers).

If your amp has post terminal outputs this is easy; simply connect the two RED terminals, no black ones. If it has 1/4-inch jack socket outputs, you will need to rig a split speaker cable with two 1/4-inch plugs at one end, each plug connected only by the tip tab (the shorter of the two tabs inside the plug). Each plug would then be inserted in a single output jack socket from each channel.

If the amp has XLR outputs, rig a split cable as above but with two XLR ends. Check the manufacturer's output code in the amp manual to find out which XLR pin is "+" and solder the cable ends to those terminals in the connectors, once again, only one connector per cable end. Then plug a cable-end XLR into each channel's output XLR and connect the speaker at the other end of the cable.

If the amplifier has Speakon outputs and it doesn't have a separate "Bridged" output, follow the same basic procedure as with XLR connectors (except, of course, using Speakons). In any case, attach whichever connector is needed to the speaker end of the split cable. Again, be sure to note which channel is driving the "+" speaker terminal if you will be bridging another amp and speaker, so that they will be in phase.

• Set both level controls at MAXIMUM.
• DO NOT ENGAGE THE BRIDGE SWITCH UNLESS YOU HAVE PERFORMED THE ABOVE FUNCTIONS. Once again, the Bridge switch only patches the two channels' Inputs together and puts one of them out of phase. It does NOT somehow "double your power". In fact, with the two channels out of phase and just running speakers in the normal manner, you would be WASTING power because the speakers connected to them will ALSO be out of phase and CANCEL EACH OTHER OUT acoustically. This can cause up to a 3dB loss in sound pressure which is equivalent to LOSING HALF THE POWER from that amplifier.
• And if, after all this, you still think that bridging two 1,000-watt amps is preferable to using one 2,000-watt amp unbridged, you might consider seeking psychiatric help. The practise of bridging harks back to a time when the most powerful stereo amps only put out around 700 watts in total and had to be bridged in order to get all of it into a speaker. This is no longer a problem.

Compressor/limiters are a Godsend (see Signal Processors). They can be set so that the amplifiers never receive enough mixer signal to drive them into distortion. The ability to get this feature built into an amp, especially at little or no extra cost, should make such amps virtually irresistable. Some people think that limiting has an audible effect which can be true when an external unit has been adjusted the wrong way, but the limiting circuitry built into power amplifiers has been optimized specifically for them and is most often sonically transparent.

• { TIP - If you have an amplifier or amplifiers with this feature, do yourself a favour; just switch it on and leave it on. A good, sonically transparent, built-in limiter will save you untold speaker failures and cost you absolutely no performance. In fact, limiters can actually maximize clean power.}

"Clip" LED
Most manufacturers set the threshold of their Clip LED's to fire at 3 decibels (dB) below the point of distortion. As a result, small amounts of Clip light activity are likely to be acceptable, however check your manual to be sure. In amplifiers with built-in limiters, Clip LED activity can indicate that the limiter is working.

"Protect", "Temp." or "Fault" LEDs
In most cases these lights indicate problems at the outputs created by low or shorted speaker loads. But they are often triggered by output transistor heat levels and can therefore indicate other conditions such as inadequate cooling due to poor ventillation, dirty heatsink fins, a non-functioning fan or a clogged fan filter (clean or change these regularly). If these lights flash briefly on power up, it may not indicate a problem. Some amplifiers are designed to go through a status check when turned on - read your manual to be sure.

If these lights come on while the amp is running, you may have a serious problem. Try the following;

• Disconnect the speaker cables and see if the light goes out. If it does, the problem could be in the speaker circuit - possibly a short circuit somewhere. Use your volt-ohm meter or multimeter to check the overall load resistance. If it is very low or the numbers on a digital meter seem confused, it probably is a short. Now check everything including the cables and connectors as well as the cabinets themselves.
• If disconnecting the speaker lines does not make the trouble light go out, check the ventillation. If the amp is in a rack or a case, make sure the back is open and away from the wall or any other obstruction. If the amp is fan-cooled, make sure the fan is turning and the filter is clean. If the problem persists, get the amp to a repair shop.

Although most PA amplifiers have some form of speaker protection built in, you may inadvertantly defeat it by repeatedly trying to reset the power breaker or, Heaven forbid, trying to hold it in with something, or wrapping the blown fuse in foil (argh!). One thing which is sure to pop the mains fuse or breaker is blown output transistors and when they short out in the process of blowing they can let all the DC voltage which is stored in the amplifier's filter capacitors go straight out to the speakers. This is called "DC offset" and it instantly turns delicate voicecoil wire into a black, motionless, silent mass. Even speakers with fuses or breakers built in may succumb to DC from a big amplifier, so treat popped AC breakers and fuses with RESPECT.

POWER vs. POWER
It's worth noting that, contrary to what you might think, amplifiers do not "create" power, they only modulate it. The power that really drives your speakers is what comes out of the wall. As a result, what you get out of an amp depends entirely on what goes into it. Quite often low sound-pressure levels, distorted and/or overheated amplifiers can be the result of sagging AC line voltages caused by inadequate house wiring, high-demand electrical appliances on the same circuit as your amplifiers or just too many amplifiers on the same circuit.

If you have a lighting sytem, it MUST be on a separate AC circuit. Of course, not every power outlet is likely to be on its own circuit so always ask the propietor or manager of the venue which ones are which. Also find out which, if any, of the AC outlets have high current ratings.

For example, if you have two power amplifiers drawing 15 amperes each and you want them to be on the same circuit for common grounding, that circuit will need to have a 30 ampere ("amp" for short) current rating. But a high current rating does not guarantee that you are going to get sufficient AC Voltage out of the wall to power your amplifiers up to the manufacturer's specifications. A Volt meter can go a long way toward explaining why things may not be sounding as they should.

Then what do you do about it? Not much except perhaps to employ a "variac", a gadget which allows you to adjust the AC voltage upward when it is sagging below 117 or 120 volts, or 220 to 240 Volts depending on what country you're in. You simply plug the variac into the AC outlet then plug your amplifiers into the variac, then you adjust the variac's control until its meter reads the desired voltage. The problem is, high-powered variacs are quite expensive and the good that they do may not be necessary everywhere. For that reason they are often overlooked as part of small to medium sized PA rigs. Too bad because they can can be invaluable when you begin encountering, not only amplifier problems, but also oddball misbehavior from your digital processing equipment or keyboards, some of which require a strict "diet" of not less than 110 VAC in North America. But, whether you employ a variac or not, remember to measure the AC line voltage next time things go screwy. If nothing else, you may save an unecessary trip to the repair shop.

• TIP - When using any kind of a meter to check high voltage levels, read the meter's instructions carefully and follow them. The life you spare may be your own.