5 Power amplifiers

5.1 Specifications

There's much more to a power amplifier than its power rating. On top of that, even a simple, basic specification such as power can be measured in many ways. Not every company will specify the maximum output power of their amps the same way. There are no laws that dictate how power should be measured. Some will specify max power at 1% THD, some at 10% THD (true for most car amps), some will measure short-term power, others will specify long-term burst power, etc. etc. There's no right or wrong, they're just ways to measure. Often companies that offer low budget equipment choose a method that boosts their figures. Reputable manufacturers will often tell the blunt truth, even if it makes their figures look less appealing than their cheaper competitors.

Below is a table with the most common specifications on power amplifiers, what they mean roughly and "good" and "bad" values.

Term Unit Description Good Bad
Sensitivity dB Input voltage needed to achieve full output power. Useful for level matching. - -
Noise dB Noise produced by the amplifier itself with master volume set to 0 -80dB -50dB
SN ratio dB Signal to Noise ratio 100dB 60dB
THD % Total Harmonic Distortion; describes how much the output signal resembles the input signal 0.05% 0.25%
IMD % InterModulation Distortion; describes how large signals (low frequencies) influence smaller signals (higher frequencies) 0.05% 0.25%
Slewrate V / μs The maximum rate at which the output voltage is able to change 50 V / μs 20 V / μs
Channel separation dB Crosstalk between channels of a stereo amplifier 60 dB 30 dB
Voltage gain dB Input-to-output voltage gain with master volume at maximum - -
Power consumption W Mains power consumption. Typically an amplifier requires twice its output power from the mains when operating at full power. 1.5x output power 3x output power

© Joris van den Heuvel 2001-2009