Power Amplifiers are the “no respect” part of any live sound system, and they tend to be procured in such manner. Often audio power amplifiers are shopped in used sections of music stores, on-line auctions, or scavenged from musician to musician. I really want to break you of this habit, and learn to purchase new power amplifiers whenever possible.
Chicken/Egg OR Amplifier/Speaker
A lot of bands mire themselves in the speakers-first or amplifiers-first quandary when upgrading a sound system. Let’s face it, we buy the speakers first, and then figure out if the existing power amplifiers are good enough. And in general, ignorance is bliss, until some over-worked power amplifier blows a driver in the new speakers. Then hopefully the education comes from the repair shop, or from this magazine.
Most speakers kill their drivers from “under-powering”. This comes from too small a watt rating in the power amplifier for the speaker to be driven. Small amplifiers are then pushed into clipping, and the clipped waveform over heats the voice coils of the speaker drivers (usually the high-frequency horn first) to the point of failure. Now a larger amplifier will pump out the extra watts cleanly, which correlates directly to more loudness out of a given speaker.
Rules of Thumb
When choosing the right size power amplifier for a given speaker, first look at the speaker power ratings. Typically, you will be given two or three power ratings to work with; the continuous (RMS) rating, the program (music power) rating, and the peak power rating. As a rule of thumb, these ratings approximately double from each other. For, example a 200 watt RMS rated speaker will likely have about a 400 watt program rating and an 800 watt peak power rating. The RMS rating is basically a torture rating of the voice coils given a pink noise test signal and waiting for the coil wire to melt like a fuse. The music power rating is a more realistic rating using test signals that emulate the peaks and averages of real live sound sources. And the peak power rating is more a mechanical breakage specification where cones or diaphragms will rupture if provided this kind of power or more.
To choose the right size power amplifier power rating, choose one that applies maximum power in the speaker’s continuous to program rating range. In the above example, a 200 watt RMS speaker with an 8-ohm nominal impedance, needs a power amplifier in the 200 to 400 watt per channel rating at 8-ohms. If two of these speakers are to be driven from one channel of a power amplifier, then the load impedance presented to the amplifier is 4-ohms (typically parallel connected). But the wattage provided must double to evenly divide the power into each speaker, so a 400 to 800 watt per channel rating is needed at 4-ohms.
Most amplifiers aimed at live sound reinforcement for bands have power ratings from 300 to 1200 watts per channel into 4-ohm loads. Today power amplifier offerings are at an unprecedented low price for the many watts per channel they offered. I remember laying out $800 in the mid-1980’s for a power amplifier that could not keep up with today’s amplifiers in the $400 price point. Just about all the power amplifier offerings today are of good quality, but I want to recommend some amplifiers that come to mind when comes to value for the gigging musician. These amplifiers at the top of my list of value rated power amplifiers would be Peavey’s PV and CS series, QSC’s GX and RMX series, Crown Audio’s XLS series, and Yamaha’s P-series amplifiers.
Most of the power amplifiers today come in two rackspace sizes (3.5” by 19” rack panel sizing) and have a modest feature set such gain/volume controls, LED signal metering, fan-cooled ventilation, and plenty of input and output jack options suitable for most cabling. I highly recommend using the twist-loc “Speakon” connections in the back of today’s power amplifiers. Not only are they almost fool proof to ensure good connections, but they tend to force the usage of heavier duty speaker cables with Speakon plugs with 14 or 12-gauge wiring. And another tip is to always use balanced XLR patch cables for signal inputs to power amplifiers from speaker processors, equalizers, or mixing consoles. This is done to prevent hum intrusion to your sound system.
Power Amplifier Specsmanship
Before scrutinizing things like power amplifier distortion ratings, frequency response, or damping factors; let’s get down to what really counts, price and output power. To me power amplifier value still comes down to usable watts for the price. And unless you plan to wire four 8-ohm speakers together or two 4-ohm speaker, do not pay attention to the 2-ohm output power ratings. Use the 4-ohm and 8-ohm ratings, since that is the most typical usage configuration for bands playing live today. And while distortion, frequency response, and damping factor are interesting specifications, as long as you have less than 0.1% THD, about 20Hz to 20kHz flat response, and greater than 100 damping factor; you pretty include all power amplifiers made today.
Since I crawled out of the garage band scene in the 1980’s and earlier, power amplifier power ratings and what is assumed necessary have changed dramatically. In my youth, a hundred watts of PA power was very good for vocal mic reinforcement, but today we mic up everything from vocals to drums to keyboards to guitar amplifiers (yikes!). And it is not uncommon to toss a thousand watts into a mid/high frequency sound system speaker (a.k.a. top boxes) for medium club size rooms. And stage monitors can consume many hundreds of watts safely these days as well. A good starting out tactic is to buy a power amplifier that just hits the top box RMS rating, and later demote it to stage monitor duty as finances allow you to buy bigger wattage amplifiers for the main speakers.
Originally posted 2009-01-16 00:36:33.