Equal power per channel - AVS Forum
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post #1 of 7 Old 03-06-2014, 08:29 AM - Thread Starter
 
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Please correct me if I'm wrong here.

So you have 5.1 or 7.1 channels of information in a movie soundtrack. If I have a power amp that delivers 100W into the front two channels and another that delivers 70W into the remaining 3 channels, then conventional wisdom would say that each speaker is not being driven equally.

That got me thinking, so please correct if I'm wrong here. The spectral content in the center speaker will not be the same as in the front channels? So the power demands will not be the same then in the center, vs the front left and right because the information will not the same?

Is the center speaker more demanding in terms of power than the left and right, or does it just depend on the mix?
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post #2 of 7 Old 03-06-2014, 08:55 AM
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Quote:
Originally Posted by Heinrich S View Post

Please correct me if I'm wrong here.

So you have 5.1 or 7.1 channels of information in a movie soundtrack. If I have a power amp that delivers 100W into the front two channels and another that delivers 70W into the remaining 3 channels, then conventional wisdom would say that each speaker is not being driven equally.

20 watts per channel from a 100 watt amp is the same as 20 watts per channel from a 70 watt amp. That is conventional wisdom.
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post #3 of 7 Old 03-06-2014, 09:26 AM - Thread Starter
 
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Is it true that amps can't drive all speakers equally in a multichannel system because each channel has different spectral content? So the dynamic range and signal level will be different, so having equal power per channel isn't absolutely necessary?
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post #4 of 7 Old 03-06-2014, 09:34 AM
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Quote:
Originally Posted by Heinrich S View Post

Is it true that amps can't drive all speakers equally in a multichannel system because each channel has different spectral content? So the dynamic range and signal level will be different, so having equal power per channel isn't absolutely necessary?

Yes, of course, Even in stereo the power being dissipated on each side at any given moment will be different. Having unequal power per channel if fine as long as the lower powered channel isn't dissipating more power than it has available. In other words 20 watts per channel......
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post #5 of 7 Old 03-06-2014, 10:44 AM
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Quote:
Originally Posted by Heinrich S View Post

Is it true that amps can't drive all speakers equally in a multichannel system because each channel has different spectral content? So the dynamic range and signal level will be different, so having equal power per channel isn't absolutely necessary?

what do you mean by "can't?"

More power is louder, right? So less power is less loud, right? this is really simple logic and if it doesn't make sense to you look at it until it does.

So if the sound is ever less than absolutely as loud as it can possibly be, you don't use all the available power at least sometimes, right? So if you turn down, you use less power.

The amount of power required at any millisecond depends entirely upon what's in the channel being reproduced. During the silent parts, zero power is used. During the silent parts zero power is used.

During the "loud" (say 80 to 85 dB) parts, for most of us,one or two watts are being used. One or two watts will make things quite loud with normal consumer speakers in normal home environments. And with real content the power will be different in each channel, from milisecond to millisecond, unless every channel contains exactly the same content. Which is not the way things are mixed.

So "can" the amp deliver equal power to each channel? Of course it can. Play a test signal in every channel at normal conversational volume (say about 65 dB) and you will be delivering the exact same amount of power to each speaker. And with typical speakers and setup you will be delivering somewhere around one or two one-hundredths of a watt, to each speaker, simultaneously. Turn it up to "loud" at 85 dB and you'll be delivering one or two watts to each speaker simultaneously.

Indeed, even if you drive the amp into severe clipping with almost every modern receiver or multichannel amp, you'll get equal power to every channel, assuming identical speakers. It'l be equally distorted, too.

The question that's confusing you is whether a receiver can deliver the same amount of clean power into 5 or 7 channels at once as it can into 2 channels at once. For the vast majority, the answer is no. Maxing out every power amp in a reeiver all at once will get you lower clean power per channel than maxing out only two at a time. With real content that's not mixed to be exactly the same and equally loud in every channel all at once, it doesn't matter, because the amps will never be required to deliver their fullest clean power to all channels all at once.
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post #6 of 7 Old 03-06-2014, 10:55 AM
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You are seriously confused.

The fact that an amplifier has more AVAILABLE maximum power does not mean that all of that power is actually driving the speakers at any given time (they are probably ALL just putting out a few watts each, 98% of the time).

That is sort of like saying that two cars with different size engines can't use the same lane because one will always be going a lot faster than the other.

ANY amplifier only operates at a small fraction of its rated power 99% of the time; just like a car with 200 or 400 available horsepower will probably only actually USE 10 or 20 horsepower cruising down the highway.

First of all, each of the signals is always different in each channel. That is why you have 5 or 7 channels; different content.

Secondly, the thing that would unbalance the sound would be if the GAIN of one amplifier is a lot different than the other; gain and power are two completely different things. The higher powered amplifier is not always the one with the higher gain.

If they ARE different, you can balance that out with the receiver settings. The overall system gain of any channel depends on both that amplifier's gain and the sensitivity of that speaker. Using a less sensitive speaker in a channel will require a higher amplifier gain setting to balance things out.

But in any case, the power demands for the various channels depends on the relative loudness of the signal at any given instant in each channel.

Actually, the left and right channels should operate down to at least 40 or 50 Hz in most cases, so they will probably need more power on the average than the center channel, which should not operate below 100 Hz to avoid "muddiness" (lack of clarity) in the vocal soundtracks.

The other channels should also use less power than the fronts, on the average.




Quote:
Originally Posted by Heinrich S View Post

Please correct me if I'm wrong here.

So you have 5.1 or 7.1 channels of information in a movie soundtrack. If I have a power amp that delivers 100W into the front two channels and another that delivers 70W into the remaining 3 channels, then conventional wisdom would say that each speaker is not being driven equally.

That got me thinking, so please correct if I'm wrong here. The spectral content in the center speaker will not be the same as in the front channels? So the power demands will not be the same then in the center, vs the front left and right because the information will not the same?

Is the center speaker more demanding in terms of power than the left and right, or does it just depend on the mix?
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post #7 of 7 Old 03-07-2014, 05:53 AM
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Quote:
Originally Posted by Heinrich S View Post

Is it true that amps can't drive all speakers equally in a multichannel system because each channel has different spectral content? So the dynamic range and signal level will be different, so having equal power per channel isn't absolutely necessary?

Not so much can't (as in a limitation of the hardware) as never being required to (as in a practical characteristic of the software).
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