Home Theater Systems-wattage

In My Humble Opinion
21

My Brother-in-law has one of those Sony all-in-one-box systems, the silver one with the 4-inch cube speakers and the shoebox subwoofer. It sounds just great in his smallish living room (+/- 10’x15’). He has a townhome so his neighbors don’t mind that he doesn’t have 1000 watts.

I have the more snazzy I-put-it-together-over-six-years-with-a-speaker-here-and-there-a-receiver-upgrade-etc. type setup. It’s now a Sony DE545 100x5 watts, Yamaha powered subwoofer, all Bose speakers, etc. I spent twice as much money on it than he did - maybe three times - and it sounds only slightly better.

The phenomenon described by Dropzone is right on - between the kids going to bed early and my wife’s preferred anemic volume levels, We rarely get anywhere close to using the theorical output.

Take a favorite DVD/CD to the store and try it out. I’m discovering that clarity & crispness is very important to the perceived quality at normal listening levels.

22

So it sounds like they’re doing the following (for each channel):

  1. A 4 or 8 ohm resistor is connected to the amplifier’s output terminal.
  2. A voltmeter (that measures true RMS) is connected across the resistor.
  3. A signal is fed to the amplifier.
  4. The voltage is recorded.
  5. P[sub]RMS[/sub] = V[sub]RMS[/sub][sup]2[/sup] / R

Is that correct? If so, then what they’re really measuring (by definition) is average power, not “RMS power." (More specifically, it is the average power over one period. Peak power will be higher.) And when they crank up the gain & input voltage for maximum clean output voltage (no clipping, tolerable distortion, etc.), then I guess it would be called the maximum continuous average power rating.

[sub]I apologize for the geeky nit-picking here. I guess I have a knee-jerk reaction to technical misinformation…[/sub]

23

Somebody wrote:

"Next make sure you go buy some Monster speaker cable to connect the speakers to the receiver, because it makes a hell of a difference (they dont lose as much signal and have a bigger frequency range which equals better sound). "

No. Don’t waste your money on oversized cable, this has been thoroughly hashed out on the SDMB, there are no sonic differences in speaker wire, unless one insists on using 22 gauge wire for several hundred feet or something equally absurd. Save your money for better speakers, or more tunes.

24

INAEE, OK. But as I said the term has been in use since the early 70’s. It’s not my term. What you said is totally correct AFAIK. I’ll just add that “average power” is not a correct term either. The average would be zero, right? If you average the + / - voltage,by golly it equals the big zero. Even using some other definition of average power, it’s not the same as “RMS power”. BTW, the term is also used in regards to AC power transmission.

You mentioned technical misinformation. Since I’m not an engineer that is possible, but what did you find that was inaccurate?

25

Average power is the correct term. And no, the average power would not be zero.

If you hooked a resistor to the output of an amplifier, all current that passes through the resistor, no matter what direction it’s going, constitutes positive power dissipation. In other words, the power never dips below zero when sending a current through a resistive load, no matter what the waveform looks like. (This almost true for a speaker, but not quite, since a speaker has a reactive component to the impedance.)

The use of the term “RMS Power” is simply wrong, no matter who uses it. If Crutchfield uses it, then they’re wrong. If the power company uses it, then they’re wrong.

26

To continue the hijack…

[sub]Standard disclaimer: I don’t know what I’m talking about. :)[/sub]

I’ll just show you where my confusion arises and then I’ll shut up. I promise!

I see some sites that say the “RMS” power is the same as “average” power as you say. But the sites that go into a bit more detail seem to make a distinction.

http://www.eatel.net/~amptech/elecdisc/voltages.htm

So maybe their is no difference when talking about a purely resistive load? But if that is the case, what does this mean:

http://www.electronics123.com/amazon/catalogue/permanent/watts.pdf

So for a sine wave it looks like the RMS value is not the same as the average value (.707 peak vs. .637 peak).

As for the terminology, is Psub[/sub] acceptable to distinguish from Psub[/sub] and Psub[/sub]?

Thanks for your patience.

27

In the first link the author basically says that RMS Power = Average Power. This, of course, is incorrect in the strictest sense, since taking the root-mean-square of the power will not equal the average power. But then he goes on to say that “RMS power is the power you get when you use a true RMS voltmeter.” I understand what he’s saying, but why not just call it average power and leave it at that?

Suffice to say, no one calculates the root-mean-square of a power signal, which is why you should never use the term “RMS power."
In the second link the author is talking about RMS voltage, RMS current, average voltage, and average current (not average power). These are all valid quantities, but average voltage and average current can be quite confusing.

Let’s say we have a voltage signal that looks like a sine wave. Let’s also say there is no DC component, which is usually the case. Examples include a sine wave on the output of an audio amplifier and the 120 VAC at your outlet.

Question: What’s the average voltage?
Answer: Zero.
Question: Does this mean the power equals zero?
Answer: No. If you connect a load, and the load has an impedance w/ a resistive component, then you will get positive power. This is true even though the average voltage is zero.
Question: I’m confused. How can that be?
Answer: Because a resistive load doesn’t care about polarity, and it doesn’t care about current direction. As long as there is voltage & current, the resistor will be dissipating power, no matter what the directions are. (When the voltage is positive, the resistor will dissipate power. When the voltage is negative, the resistor will dissipate power. The resistor doesn’t know and doesn’t care about polarity.)
Question: I’m still confused.
Answer: That’s probably because average voltage doesn’t have a lot of meaning. Think about it: 99.999999% of the time, the voltage is not at zero, right? And during this time, the load is dissipating power. So although the mathematical average may be zero, the voltage is at some value other than zero 99.999999% of the time.
Question: O.K., I understand that the average voltage is zero, and that average power is not zero. But why does it say that the average voltage is 0.637peak?
Answer: Because they’re trying to confuse you! Actually, here’s their reasoning: Let’s say you put a voltage sine wave on a resistor. If you look at the voltage on an oscilloscope, you will see alternating positive swings and negative swings. But guess what? The resistor doesn’t know the difference! For all it knows, all of the swings are positive. So even though the oscilloscope shows alternating positive and negative swings, you should pretend that the negative swings are really positive. (It would look like a train of humps, like you would find on the back of a dinosaur.) Now calculate the average voltage… you’ll find that it’s 0.637peak.
Question: O.K. I understand. So this average voltage can be used to calculate average power?
Answer: No! By definition, RMS voltage is used to calculate power, not average voltage.
Question: So why calculate average voltage?
Answer: I don’t know. I’ve never really had to use it for anything, but I’m sure someone has. I ignore it for the most part.

28

Nothing to add except thanks to everyone for their assistance and reccomendations.