Originally Posted by gcoulson
Hi, I have an absolutely naive question to ask. I have a Denon 1613 receiver. It is rated at 75W per channel (5.1). If I am only running two speakers, does it still push out 75W to those speakers, or does it push out more since the other channels are not being used?
Here's what actually happens - I have measured this many times.
The unused channels draw minimal amounts of current - usually just the bias current which is less than a tenth of an amp with most amplifiers. In comparison an amplifier channel running full tilt may draw over 10 amps.
The current drain of channels that are actually being used is highly dynamic, varying over a wide range from millisecond to millisecond.
Music has what is known as a high crest factor
or peak to average ratio. The peaks are high and the average is often far lower.
IOW music's peak values can be very high while the average values are very low. Some of this happens because different notes are being played with different loudness all along, and some of this happens simply because music is not composed of pure tones. Most musical sounds have a characteristic envelope with an attack, a peak, and a decay. But musical sounds will have a high crest factor even if they are played steadily. For example if a musician plays a steady note, there is still a high crest factor because in general musical sounds are not pure tones. One reason might be that they are chords, composed of multiple notes played at the same time. Even if a single note is being played on a single instrument, the crest factor is still high because the note is composed of both a fundamental and harmonics, often along with some tones that are not exactly pure harmonics.
To summarize, music has a high crest factor
(1) Different notes are being played with different loudnesses at different times. if you look at a music waveforms they are sprinkled with peaks and even short periods of silence.
(2) Musical sounds have acharacteristic envelopes with an attack, a peak, and a decay. The peak is high but in particular the decay may have very low energy. Classic examples: A piano note or a drum beat.
(3) In general musical sounds are not pure tones and only pure tones have high average energy. The laws of physics assert themselves!
(4) Musical sounds may not be pure tones because they are chords which are multiple notes being played at the same time.
(5) Musical sounds may not be pure tones because musical instruments generally make single note sounds that all by themselves aren't pure tones, but usually have a lot of harmonics and even non-harmonic components.
One consequence of all this is the fact that bench tests with pure tones, which are used to characterize the amplifiers in receivers etc., are very misleading since we use them to amplify music. Pure tones such as are used in bench tests have a very low crest factor
but music generally has a very high crest factor
(100:1 is not unusual).
I have recently seem some papers by some very smart people that I generally respect that completely ignore the effects of the high crest factor of music. Yet I'm sure they know about the crest factor of music, they just haven't put it togoether.