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5. Technical Matters

Unfortunately, even the leading manufacturers of power amps have yet to make it easy for non-technical musicians to know what’s right for their needs. Here, we take a look at some of the terminology used and give you some idea of what it means, so you can be sure that you’re comparing like with like.

RMS

RMS stands for ‘Root Mean Square’ and is the official, international and only really meaningful indication of the power capacity of an amp. The RMS value of an amp is expressed in Watts (W) - 200 Watts RMS, 1 kilowatt RMS and so on (1 kilowatt (kW) = 1,000W, often referred to simply as ‘K’), and indicates that the amp will continuously maintain a stable, consistent signal across all frequency ranges, up to the specified power value. You should always consider buying an amp with a slightly higher RMS value than you need, so you can operate it comfortably within its specified range.

Sine Wave Power

Another measurement of power output and quality, largely superseded by RMS, but still used occasionally. If you come across it, it is broadly interchangeable with RMS, providing it is ‘Continuous’ Sine Wave Power.

Peak Power

Beware of this measure - it always looks higher than RMS, but it is only an indication of the maximum output capacity of the amp, or indeed the maximum loading of the speakers. It gives you no real indication of the amp’s ability to perform throughout its range or over time.

Peak Music Power Output (PMPO)

Again, a measure sometimes used to suggest more power than can realistically be delivered. Unlike RMS or Sine Wave Power, PMPO measures the power according to the varying frequencies of actual music playing through the system, and produces a measure of the maximum capacity. Like Peak Power, this measure is an absolute maximum, which may actually damage the system if it is sustained for any length of time. Approach this rating with caution - always use RMS or Sine Wave Power.

Rated Output

A measure used in the electrical industry that is occasionally applied to power amps. Broadly, it is similar to RMS, but since they do not use the same methods of calculation, it is not safe to directly compare the two.

Energy Dissipation

This is the measure of the amount of electricity that is converted to heat energy by the amp. The higher the value, the more cooling is required.

Impedance

This is the electrical measure of resistance, measured in Ohms. It is a critical specification when matching amps and speakers. If there is a bad match, at best you’ll get a bad sound, at worst you’ll damage amp, speakers, or both. Note the following key considerations:

  • Low impedances place a heavier load on the amp, which will generate more heat. Make sure the cooling system is good enough, and allow for plenty of circulation.

  • Very low impedances may also overload the amp to the extent that its circuit protection kicks in an switches it off.

  • You need thicker and/or shorter cables when attaching low impedance speakers, because the quality of the signal deteriorates rapidly along longer cables.

  • Usually, higher impedances lead to better sound because they allow for easier matching of amps and speakers, although modern power amps are often capable of driving down to 2 Ohms.

Impedance essentially works like the narrowing of a stream of water - the higher the impedance, the narrower the gap, so it follows that an amplifier will deliver more power at lower impedances, and in fact their ratings are generally given at several different impedances, typically 16 Ohms, 8 Ohms, 4 Ohms and 2 Ohms. In short, always be careful that you are comparing apples with apples in terms of power and impedance, and that you are looking at the measurement per channel, not ‘bridged mono’ - some amplifiers are capable of joining both channels together to form a more powerful but single channel amp. Try to work within the maximums set out in the data sheets - just as you wouldn’t rev a car at its peak for long periods, so your power amp is not designed to work flat out continuously.

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