PMPO - A Marketing Gimmick!

Have you ever wondered how that little set of powered speakers on sale at the store could contain a 120 watt or otherwise large amplifier? Likely, these large numbers are describing PMPO or "music power" figures rather than sustainable wattage.

PMPO, Peak Music Power Output, music power, or peak power describe the ability of an audio amplifier to provide very short but large bursts of power. As music is typically quite dynamic with alternating loud and soft portions, an amplifier can be built to provide very high power to the loud bursts while falling back to safe levels for most of the song. PMPO is the amount of power which is momentarily provided to these loud bursts in music.

Marketing departments at electronics companies like PMPO because it allows them to build a cheap product with minimal power supplies and amplifier components yet still display big numbers on the box. Naive consumers who couldn't care less about sound quality will purchase the product because they think it is cool to have a 3600 watt amp that draws power from an 1800 watt maximum electrical outlet.

Because the power supplies are often undersized as well as the amplifiers' active devices (chips, transistors), there is likely to be extreme distortion produced when the PMPO power is reached, as the power supply voltage will drop and the filter capacitors will be drained in the process of producing large peaks of audio output. Low-impedance speakers such as 3.2 ohms are commonly provided with PMPO-rated devices to push the amplifier as far as possible.

PMPO Standards?

PMPO values seemed to vary between manufacturers and even products themselves, so there appear to be no universal standards used in defining PMPO. Typically, the true sustainable power handling ability of an amplifier is stated in "RMS Watts", the root mean square power of a constant sine wave (e.g. a 1 kHz tone) signal that can be amplified without overheating the amplifier and/or grossly distorting the signal.

RMS power ratings are more legitimate because they require the amplifier to have an adequate power supply and adequate heat sinking to remove any waste heat that is produced as a resulting of producing the output power. Attention must also be paid to the amount of distortion the amplifier produces when producing a given power output.

An Example...

I have personally seen a stereo system which claimed a 50-watt PMPO power output. I found the unit in a dumpster at a college apartment complex, beat up pretty badly but still retaining the fluorescent yellow marketing stickers with PMPO ratings on the front (never removed by the owner). On the back of the stereo, the power input rating from the AC line is 24 watts.

The transformer core looked to be rated at most 20 volt-amps (~16 watts)

The power supply was 12 volts, single-ended - one "rail" supplying only a positive voltage, using a large-value electrolytic capacitor to AC-couple to speakers to the output stage. With the low-impedance 3.2-ohm speakers supplied with this unit, these capacitors would have to be over 6800 µF in order to get any kind of halfway-decent bass response. They appeared to be 1000 µF.

There was a 2-ampere fuse in line with this 12 volt power supply, which means a maximum power to the entire stereo (amplifier, tuner, and CD player) or 12 * 2 = 24 watts.

There was 1000 uF of filter capacitance in the power supply. Even for a 2-amp supply this is under-rated, which means there will be ripple (noise) entering the amplifier when it is operated at high volumes.

Power Isn't Everything

The quality of an audio system is not determined by its power output. Large wattage figures in bright bold letters on audio equipment packaging should be an indication that the manufacturer is looking to sell the product based upon quantity rather than quality, and you can expect to get poor quality speakers and amplifiers in such packages.

There is NO mathematical formula that will convert an arbitrary PMPO to RMS values or vice-versa.

CREATED/WRITTEN: 2004-07-12 00:00