How to read power specifications on amplifiers.

Author: Toby  

How to read amplifier specifications and tell how powerful an amplifier truly is!


1.  The honest power rating

There are no official industry regulated power ratings, this means that the companies regulate their own product specifications. Companies will use all kinds of sneaky techniques to advertise an amplifier to be as powerful as possible. The most honest rating is a full frequency reading, using all channels powered at once (or at least two channels if it is an AV receiver) with around 70 to 80% of full power available. Traditional hi-fi brands use this rating system as they make decent quality components and do not need to mask the fact that the amplifier is weak. This is what the honest rating looks like:

e.g. 1  < 0.001% THD at 1 kHz

This means less than 0.001% Total harmonic distortion at 1kHZ

e.g. 2  < 0.005% THD at 20 Hz - 20 kHz

This is a measurement of distortion when the amplifier is outputting the full frequency of human hearing (20hz – 20,000hz).

e.g. 3  80W per channel into 8 Ohms at 80% all channels driven, 100W per channel into 4 Ohms at 80% all channels driven

This measurement indicates how much power is being used and what output wattage is achieved at these levels. Keep in mind, 80% power is generally around the highest most people will play an amplifier. At 100% full power, many amps THD increases exponentially. It is also a good sign that the manufacturer specified that all channels were used, as the amplifier becomes weaker when more channels need power.

2.  High distortion, single channel driven & single frequency ratings

      e.g. 4  <0.5%THD, 80W RMS into 4 Ohms @ 1kHZ

This is often used to mask the ways a brand has measured the power of its amplifier. The measurement means the amplifier was rated at a lower ohm to appear more powerful. You can also see that the THD is really high compared to the previous example. Also the amp was only rated at 1kHZ rather than a full 20khz to 20hz sweep. There is also no percentage of how much power was used to achieve the rated wattage. On the positive side, you can see that the manufacturer has listed the power being RMS, which means continuous power output (read more on that next page). So we can see that both examples 3 and 4 are listed as 80W however the amplifier in example 3 is going to be more powerful as it has lower distortion and more accurate rating of power. There is also no note of how many channels were used so you can only assume it was 1ch driven.

3.  PMPO – Peak Musical Power Output.

You will find this rating used on mostly cheap amplifiers that claim absurd power outputs like 3000W etc. This is not realistic power as most people in a home environment at normal listening levels use less than 10W RMS (RMS = Continuous average power output). They achieve this rating by measuring a momentary peak in the playback of audio to fluff up the power of a weak amplifier. In other words, pay no attention to this measurement when buying amplifiers however it is good to have knowledge that it exists