How Many Watts am I using?

[yepimonfire]

And again, you're ignoring the reflections. Unless your speakers are far from the walls, floor and ceiling or their dispersion is narrow, the walls will affect the SPL.

I'm not arguing that having extra power is useful- I'm arguing that 100W IS loud, especially when more than one channel is operating.

I'm not arguing it isn't either.

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[lovinthehd]

So there's no argument? Let me add...there's no guarantee just because your avr reads a volume of "0" post-calibration that your speakers aren't going to need to exceed 105dB peaks (even without consideration of the LFE channel); it's a recording reference that isn't necessarily adhered to let alone what happens in your system during playback of various sources/levels.

 

[Johnny2Bad]

The short answer is: way less than most people think.

Now, we are talking average levels ... what the amp is putting out more than 95% of the time while listening to music.
At most listening levels, almost certainly less than 1 watt, and possibly less than 1/10th watt.

Note: I'm using watts, which is customary, but we can define that as 2.83V into 8 ohms. Into half that load, the wattage would double but the voltage would remain constant, and almost all power amps are voltage amplifiers (a very, very few are current amplifiers, but they are a completely different kettle of fish).

The Crest Factor (CF) is the way we define the difference between average and peak levels in music signals. For most Classic Rock, the CF is about 14 dB (average level is 14 dB below peak level).

With modern Rap / HipHop, and pop music such as Taylor Swift or ... well, name your poison ... the CF might be as little as 1 or 2 dB. But, remember, that is above the average level, so it's not as dramatic a power requirement as you might at first think.

You need clean peak power for quality reproduction. So despite the fact that we are listening at very low power outputs most of the time, we absolutely need that peak power capability.

So we use that level to base our expectations on how large an amp we need, with our speakers, with our choice of music format, with our listening room and typical (or desired maximum) Sound Pressure Levels (SPLs).

If that were not the case, we could probably get by happily with 1 watt amps. If you ever sit in a classic automobile ... one where an aftermarket stereo hasn't been fitted, and one that pre-dates the modern high power car systems from the OEM; definitely anything from the 70's and most vehicles made into the 1990's ... the maximum output from the available car power supply is about 4 watts (@ 8 ohms) per stereo channel. Yes, I know that is an unlikely speaker impedance, but we already know how to translate that into watts/4 ohms, so feel free to exercise your brain.

At an SPL in-room of perhaps 70 dB, which isn't loud but certainly loud enough for background music ... average power into an 8 ohm load with speakers of average efficiency, is about 0.01 watt.

 

[Johnny2Bad]

... (snip) ...

BTW, and this is really for anyone who may try it- I hope you know not to use a Simpson or any other analog VOM to read resistance when it could be connected to an amplifier's output or to any other sensitive input jacks or devices. For those who don't know why, set the VOM to read Resistance X1 and connect it to a 12V light bulb.

Note that the 12V bulb (or any light bulb) is electrically a variable resistor. With no power driving it, it has near infinite resistance (unless your meter has enough power to light it brightly). But the filament is purposely built to generate light with minimal (waste) heat, so as soon as it lights, the resistance falls to near zero as the filament heats up.*

But the point is well taken, and in particular, to measure very low resistances, you need specialized measurement techniques. DMM's and VOMs are not reliable at low impedances. Even when fitted with Kelvin probes the procedure is fraught with potential errors.

* The math? Assuming 12V and a 60w bulb ... well, there's the values you need to calculate resistance when lit. Oh, and please don't go around trying to measure working bulbs in your home, or, put another way, "can I have your gear when you die?"

 

[ski2xblack]

I think your light bulb analogy is backwards. Resistance rises with the temperature of the conductor.

 

[Johnny2Bad]

I think your light bulb analogy is backwards. Resistance rises with the temperature of the conductor.

D'oh! Bad brain. Bad!

Right ... resistance is near zero when not energized and rises to the rated load of the bulb when energized (eg 60w incandescent bulb).

Thanks for noticing; we don't want to lead people astray here.

 

[mtrycrafts]

I think your light bulb analogy is backwards. Resistance rises with the temperature of the conductor.

Yep, that is why the big $$ spent on trying to get superconduction above absolute zero as high as possible; not practical at 0.