Q: What are Peak and RMS (Root Mean Square) power ratings? What's the effect on audio output?

A: RMS power is continuous power that the amplifier is capable of outputting over long periods. Peak power is short-term power that an amplifier is capable of briefly outputting when faced with sudden, high-energy signals. Another power measurement that is very important for audio output is the Average Power Per Cycle of sine wave.

  • Vpeak = Peak output voltage of amplifier
  • R = Speaker load in Ohm
  • Peak Power = (Vpeak)2 / R
  • RMS Power = Power supply's RMS power
  • Max Power Per Cycle = (Vpeak)2 / 2R

Headroom is referred to an amplifier's ability to go beyond its rated average power (RMS or continuous power) for a short time in order to recreate loud or explosive audio signals that rise very quickly. In order to have high headroom (an ability to achieve loud peak levels without distorting), an amplifier must have a stiff power supply with a good amount of reserve energy on which it can call (stored in capacitors). The ability of the power supply to quickly recharge the capacitors is also very critical. A typical low frequency (50Hz for example) explosive sound consists of an attack followed by a series of rapidly decaying lobes of a 50Hz sine wave. Therefore, an amplifier with 140W peak power with 20ms holdtime is capable of providing 140W power to the 50Hz musical note (full cycle of 50 Hz is 20ms). A series of successive explosive sound might sounded very close to human ear, but their base notes are far apart enough (1 second = 1000 ms) for the amplifier to provide the required peak power. For most typical home listening, hardly more than 20W of power is consumed. Therefore, a good amplifier should provide sufficient RMS power and a high peak power with low distortion (NuForce's Amplifier is capable of generating the peak power at ultra low distortion of less than 0.05% THD+N).

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