Receiver Amp Specifications - THD and RMS Power

Q: A quick question. Isn't THD important in the sound quality of a receiver? The lower the better? Also, when you see the 20 - 20,000 Hz versus 40 - 20,000 Hz, the higher number 40 will affect the sound quality won't it? So, if you have a receiver with .08 THD and a 40 - 20,000 Hz, will you not get as good of a sound out of a receiver as say the one with .04 THD and 20 - 20,000Hz.? In essence, is it something your average audiophile should look at when buying a receiver? Thanks.

A: Receiver specs are mostly a useless and misleading as companies do not measure them with any consistency from manufacturer to manufacturer. Just because one is measured from 40Hz - 20KHz does not mean it will not produce frequencies below 40Hz. They rate the amp at this limited bandwidth to give the illusion of more RMS power as RMS power measurements are based on the bandwidth they are measured. Many mass market Home Theater Receivers rate the amps at 1KHz in multi channel mode. This allows a RMS power measurement of about 20% more than a more honest measurement at full bandwidth (20Hz to 20KHz).

Bottom line, when playing the numbers game of Receiver Specs, make sure the models you are comparing are rated with the same constraints. Look for:

1. RMS Power (20Hz to 20KHz) in 8 ohms with all channels driven with < .1% THD.
2. RMS Power (20Hz to 20KHz) in 4 ohms with all channels driven (most Receivers do not rate specs like this because the power supply limitations would yield poor measurements or cause the amps to go into thermal overload.
3. Dynamic Power (little meaning as it is not a steady state parameter but its fun to compare)
4. Weight!!!! Usually the heavier the Receiver, the better the power supply section and heat sink area on the amps.

Please refer to the Receiver buying guidelines that I wrote in the "Tech" section for further information on these topics.