Power Amplifiers

Power amplifier choice is critical within any live sound system. The high power outputs involved in modern PA systems often mean the amplifiers are highly stressed. So design, component quality, and cooling and protection systems are all important factors when it comes to their choice. And in this respect, and in keeping with most pro audio equipment, quality costs.

Here are some key factors when choosing power amplifiers:

Power Output: A sometimes contencious area. At The Noizeworks, we recently conducted some not particularly scientific experiments. We took a rather old, but, in our opinion, great sounding power amplifier from a well respected and established manufacturer. The amp was dated from the late eighties and was rated at 150W/channel at 4 ohms. We wanted to see how this amp compared in sound and power to a modern brand new chinese built amplifier that seemed to offer great value for money. It was rated at 450 watts/channel at 4 ohms. So we set them up to a couple of 12" wedge monitors, put some music through them (probably Steely Dan. Everyone uses Steely Dan for sound tests.) and listened. In all respects our old little 150w/ch amp was superior. It was cleaner, the bottom end was warmer, cymbals sounded less 'splashy', etc. Well OK we thought. Although it's an old amp, they were quite expensive at the time and taking into account inflation, much more expensive than this 450w/ch chinese competitor. But the real surprise came in terms of just how loud the amps were before the clipping. Despite being rated at a third of the power, our plucky ageing little amp as well as sounding cleaner, also sounded louder than it's cheap far eastern competitor. In fact it drove our 12" monitor to deafening levels before it's clip lights flashed.  We think there's been a tendency of late for power amplifier manufacturer to'sex-up' their power amplifier specs a little. It's always worth reading specs in detail and taking note of distortion figures. An amp rated at 600 watts 0.01% distortion will be a louder amplifier than one rated at 600 watts 0.1% distortion, etc.

Matching amplifer outputs to speaker ratings is also an interesting area. You'll often hear handy phrases like 'your amp should be twice the power of the speakers', or 'the amp should be smaller than the speakers.' Ultimately the amp should be capable of delivering enough power to the speakers for the required level without distortion. Amplifiers tend to sound better when their not run close to or at their limits, so within a given range of amps, we'd probably recommend buying the one with the largest power output within your budget. But, with the previous paragraph in mind, not resorting to cheaper products claining enormous power outputs. We'll be happy to offer all the guidance and assistance you need.

Slew Rate/Damping Factor: These specs kinda measure how 'fast' the amplifier is. Slew rate measures how fast and accurately the amplifier responds to a signal and damping factor measures how much control an amp exerts over a speaker cone. To use the analogy of a car, slew rate is how quickly it accelerates, and damping factor is how good the brakes are! A high damping factor is something to look for if your selecting an amplifier to drive sub bass speakers in a larger system.

Protection Systems and Cooling: An amplifier that develops a fault could end up destroying many thousands of pounds worth of speakers, so it's important that it has effective short circuit and overload protection systems. Heat is an inevitable by product of amplification, so it's vital that amps have effective cooling systems (which may effect how physically noisey they are) and protection systems against thermal overload.

Efficiency: Without going into the details of amplifier designs and circuit types, some amplifiers are more eficient than others. This may be a major factor in club installations where running costs are a major concern. There are now types of installation power amplifiers specifically designed to keep electricity bills to a minimum  whilst still offering great sound and performance.

Weight: Power amplifiers can get heavy, so there's no point in selecting large, heavy amps if your a solo artist and can't shift the amp rack on your own! Some of the latest designs using 'switch mode' power supplies which don't use heavy transformers, offer high outputs with reduced weight.