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Watts and Power Ratings

by August 30, 2004

Q: Are all wattage ratings dealing with sound equipment the same? Does my 25x4 car audio receiver put out as much power as one of my channels in my Sherwood 100x5? If so or not, why?

A: Wattage ratings can be very misleading when comparing amps. For example, many lower priced amps rate their power as listed below:

  1. RMS Power at 1KHz as opposed to full bandwidth
  2. RMS Power at 6 ohms as opposed to 8 ohms. Doing so allows the manufacturer to boast larger power figures.
  3. RMS Power at high distortion ratings (IE. THD > 1 %)
  4. Same as #s 1 to 3 but Peak Power Ratings as opposed to RMS.

Note: Power rated as seen above is dishonest and misleading. Power should be rated such as the example below:

Example: 200 watts RMS into 8 ohms from 20Hz to 20KHz with THD < .01%

Helpful Guides to Determining Power Potential
In order to assess an amp's potential, ignore power ratings for the moment and focus on the following:

  1. Capability of the amplifiers power supply (IE. Xformer ratings, Power Capacitance).
  2. Current rating of amplifiers output devices.
  3. Size / Surface area of heat sink containing the output stages.
  4. Amplifier topology (IE. # of Discrete Power Devices / Channel)

The above information helps determine the capability of the amplifier's ability to provide continuous real power to a complex load such as a speaker. A speaker is much harder for an amp to drive than a simple resistor. For example, the magnitude of a speakers impedance may vary from 2 ohm's to 15 ohm's as a function of frequency. An amp with a capable power supply and good discrete power output section, will handle these types of loads with no problem as the amp will provide more current under these low impedance conditions.

Never trust a manufacturers power rating on their amps unless you have a way to qualify it's legitimacy. Make sure they rate RMS power at full bandwidth (20Hz to 20KHz) with acceptable itolerances ( +- 0.5 dB) for a given load impedance (IE. 8 ohms) and THD figure < 0.1 %. When evaluating different brand amps power capability, make sure you are comparing apples to apples (in other words, make sure the way the specs are rated between each amp is similar.)

Note: PMPO Power measurements usually found on computer speakers or Car Receivers are nearly useless. The figures are usually exaggarated by a factor of 10 with respect to real RMS values.

 

About the author:
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Clint Deboer was terminated from Audioholics for misconduct on April 4th, 2014. He no longer represents Audioholics in any fashion.

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