Now to get back into voltage terms again, we need to take the square root of the mean average power analog. That gives us RMS voltage, which is shown as a blue line. Note that it flattens out at 0.707. When you multiply voltage times current, you get power. When you multiply RMS voltage times RMS current, you don’t get RMS power – you get average power.
Therefore, what is often referred to as “RMS power” should actually be called “continuous average power.” And that’s what any decent and believable amp spec should mean when it shows a figure. And, please, don’t get me started about “peak” power specs for amplifiers.
Q.) More on power specs: what do “FTC” and “EIA” mean?
A.) Those are currently the two most common ways of measuring amplifier power.
One common method years ago was the IHF standard, which involved measuring an amp’s maximum RMS voltage, just at the onset of clipping, across a load, such as 8, 4, or 2 ohms. The tricky part was that it was measured with a 1 kHz sine wave repeated pulsed on fully for 20 milliseconds, then 480 milliseconds at 20 dB down (or 1% power, in other words).
Consequently the spec measured headroom, and not continuous power, causing many wimpy amps to boast impressive numbers as power specs.
Thankfully, the IHF spec is almost completely gone now. A somewhat more rigorous method is the EIA standard, which requires measurement with a constant-amplitude single midband frequency.
Most manufacturers choose 1 kHz. The amp drives the sine wave into a specified load resistance, such as 8, 4, or 2 ohms, and the gain is turned up until the amp’s clipping point is reached.
The clipping point is defined as the level at which the total harmonic distortion of the sine wave reaches a specified figure, such as 0.05%, 01%, or 1%. This THD figure is only a reference – it describes a repeatable means of arriving at the clipping point. Don’t confuse it with the amp’s actual THD performance. At this clipping point, the RMS voltage is used to calculate the power delivered into the load resistance.
One shortcoming of the EIA spec, aside from its limited frequency spectrum, is that it allows a manufacturer to drive and put under load only one channel at a time. Not all manufacturers take advantage of this loophole, which can be used to conceal a weak power supply.
Far more rigorous is the FTC spec, which has been around since May 1974. Established as a consumer protection measure for home entertainment equipment by the Federal Trade Commission (FTC), an agency of the U.S. government, the FTC spec requires measurement using a sine wave over a specified range of frequencies, instead of the single mid-band frequency required by EIA or IHF.
Usually the frequency range used is 20Hz to 20kHz, although it can be different as long as it is specified. Also, all channels of the amplifier are driven and put under load at the same time, which would tend to expose any weak power supply.
