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How powerful does my AVR need to be?

post #1 of 16
Thread Starter 
So I've read through the stickied posts and searched for others, but basically what I've been able to take away is that every brand rates their watts/channel differently, and unless it's HK you don't know what you're getting.

My question is - what kind of power rating should I be looking for in order to push a constant 100W to my 6ohm fronts (5.1 setup)? Is it possible to get an AVR that will do it for <$500?

I keep looking at receivers advertising 100W+/channel, but then it always turns out that's with 2 channels driven, and it drops to around 80W/channel with all driven.

Thanks for any help
post #2 of 16
Quote:


My question is - what kind of power rating should I be looking for in order to push a constant 100W to my 6ohm fronts (5.1 setup)?

If you push 100w/ch continuous at any speaker, the only question is whether you destroy the speakers or your eardrums first. My bet would be eardrums.

Amps do not "push" anything continuously. The odds are that 99% of the time your AVR will be pumping out no more than a couple of watts. The only time it will exceed that will be an transient peaks (which could quite conceivably require well over 100w for a fraction of a second). If you think about that for a bit, you'll realize just how irrelevant those continuous power ratings are, even if they are derived honestly (which, as you note, most are not).

Now, does that mean you can ignore them completely? Not quite. Some speakers are more difficult to drive, either because of impedance or sensitivity. In particular, many AVRs have trouble with low-impedance (think 4 ohm) speakers. You say you have 6-ohm fronts. What about the other three? Two 6s and three 8s probably won't tax any AVR you'd consider. Two 6s and three 4s would shut a fair number of them down.

Most AVR manuals will tell you what impedances they can handle, and it's easy to download manuals before you buy.
post #3 of 16
Thread Starter 
Got it. Thanks for the explanation. My understanding was that it's bad to 'underpower' the speakers, which is what I was concerned about for my fronts.

The fronts are both 6ohms (older bookshelf speakers), the other 3 are 8 ohms. You say it probably won't tax any AVR - even a budget receiver like the Denon AVR-1612?
post #4 of 16
Quote:


You say it probably won't tax any AVR - even a budget receiver like the Denon AVR-1612?

What I said was, RTFM.

But in general, I don't think that would be a problem for even budget AVRs.
post #5 of 16
How sensitive are these speakers? How far away do you intend to sit? etc
post #6 of 16
Unless you hear distortion on peaks, you don't have a problem.

If your receiver will actually put out 60 watts per channel with all channels driven, that is plenty for most speakers.

What receiver and speakers do you have? Without that information it is hard to say much.



Quote:
Originally Posted by blakeyj08 View Post

So I've read through the stickied posts and searched for others, but basically what I've been able to take away is that every brand rates their watts/channel differently, and unless it's HK you don't know what you're getting.

My question is - what kind of power rating should I be looking for in order to push a constant 100W to my 6ohm fronts (5.1 setup)? Is it possible to get an AVR that will do it for <$500?

I keep looking at receivers advertising 100W+/channel, but then it always turns out that's with 2 channels driven, and it drops to around 80W/channel with all driven.

Thanks for any help
post #7 of 16
This is a pretty good thing to play with: http://myhometheater.homestead.com/splcalculator.html

Underpowering was a much bigger problem 20 - 30 years ago when 25 - 50 W/ch was typical and speakers were generally less efficient.
post #8 of 16
Quote:
Originally Posted by commsysman View Post

Unless you hear distortion on peaks, you don't have a problem.

This is really the ultimate test. Play something with the good dynamics and start at low volume. Then gradually increase it until you can't tolerate it anymore. Does the sound change? Listen to it getting distorted, the bass changing, getting brighter or any other characteristic change. If not, then you are good with what you have, assuming your AVR does not go into protection and shut off (either immediately or after some use).
post #9 of 16
Quote:
Originally Posted by blakeyj08 View Post

So I've read through the stickied posts and searched for others, but basically what I've been able to take away is that every brand rates their watts/channel differently, and unless it's HK you don't know what you're getting.

That seems narrow and harsh. In the US there are laws related to how power ratings are determined, and they are often overkill.

Quote:


My question is - what kind of power rating should I be looking for in order to push a constant 100W to my 6ohm fronts (5.1 setup)?

Where does this 100 wpc criteria come from?

Quote:


Is it possible to get an AVR that will do it for <$500?

Yes.

Quote:


I keep looking at receivers advertising 100W+/channel, but then it always turns out that's with 2 channels driven, and it drops to around 80W/channel with all driven.


So what? 80 watts is only 2 dB less than 100 watts, and it takes a whopping 10 dB of power difference to create the impression of just twice as loud.

Have you ever heard an actual 2 dB difference in power being delivered to a speaker? Remember that over just a few seconds the power being delivered to a speaker can change as much as 60 dB or more.

The hidden gotcha is that music is far easier to amplify than steady tones. because it has a constantly varying amplitude. Power supplies, heat sinks and output transistors are far more stressed by the steady grind of a sine wave.

Music is also constantly varying in terms of frequency content which means that the effective impedance of your speakers is also constantly varying because their impedance is frequency dependent. While your speakers may be rated at 6 ohms, their effective load on the amplifier is probably significantly lower with real world music as compared to the sine waves used in power ratings.

The power supply and heat sinks in even very low cost receivers is usually overkill because advertised ratings have to be based on sine waves while music is what we usually listen to.
post #10 of 16
Quote:
Originally Posted by arnyk View Post


That seems narrow and harsh. In the US there are laws related to how power ratings are determined, and they are often overkill.

Where does this 100 wpc criteria come from?

Yes.

So what? 80 watts is only 2 dB less than 100 watts, and it takes a whopping 10 dB of power difference to create the impression of just twice as loud.

Have you ever heard an actual 2 dB difference in power being delivered to a speaker? Remember that over just a few seconds the power being delivered to a speaker can change as much as 60 dB or more.

The hidden gotcha is that music is far easier to amplify than steady tones. because it has a constantly varying amplitude. Power supplies, heat sinks and output transistors are far more stressed by the steady grind of a sine wave.

Music is also constantly varying in terms of frequency content which means that the effective impedance of your speakers is also constantly varying because their impedance is frequency dependent. While your speakers may be rated at 6 ohms, their effective load on the amplifier is probably significantly lower with real world music as compared to the sine waves used in power ratings.

The power supply and heat sinks in even very low cost receivers is usually overkill because advertised ratings have to be based on sine waves while music is what we usually listen to.

100W in 5.1 usually means 200W in stereo. I would also look at official statement about capability to drive 4 Ohms load. Also look at specified power consumption, real total output power with all channels driven is usually 50% of total consumption.
post #11 of 16
Quote:


100W in 5.1 usually means 200W in stereo.

Usually? Have you got some basis for this statement? I don't know that it's wrong, but IME power specs are all over the map. Drawing any conclusion about them is dangerous.
post #12 of 16
Quote:
Originally Posted by mcnarus View Post

Usually? Have you got some basis for this statement? I don't know that it's wrong, but IME power specs are all over the map. Drawing any conclusion about them is dangerous.

I got it from whatever test data I saw. "Usually" means exactly that - in most cases, with few exceptions.
post #13 of 16
Quote:


I got it from whatever test data I saw. "Usually" means exactly that - in most cases, with few exceptions.

And what kind of test data was that? Who has tested most of the AVRs on the market to confirm such a thing?
post #14 of 16
Just drawing from some published test data, the claim that "100W into 5.1 equals 200W into stereo" is likely false. I'm not pretending to speak for all equipment, but there's a lot of equipment that absolutely does not live up to that burden.

Here's a few examples:
http://www.hometheater.com/content/s...-labs-measures
http://www.hometheater.com/content/i...-labs-measures
http://www.hometheater.com/content/s...-labs-measures

They're all fairly close to their power ratings, and even if they were hitting 200W, it's only 3 dB, and negligible in most cases. It's not really worth having an intense debate over.

I don't think there's any fair rules of thumb or generalizations that can be drawn here.
post #15 of 16
Quote:


Just drawing from some published test data, the claim that "100W into 5.1 equals 200W into stereo" is likely false. I'm not pretending to speak for all equipment, but there's a lot of equipment that absolutely does not live up to that burden.

Thanks for the links. 2:1 seemed rather high to me, and your examples suggest that 1.5:1 might be more in the ballpark. But this is assuming you're dealing with honest, consistent measurements. When it comes to spec sheets for AVRs, you can assume no such thing. The FTC rules might as well not exist anymore.
post #16 of 16
Quote:
Originally Posted by mcnarus View Post

Thanks for the links. 2:1 seemed rather high to me, and your examples suggest that 1.5:1 might be more in the ballpark. But this is assuming you're dealing with honest, consistent measurements. When it comes to spec sheets for AVRs, you can assume no such thing. The FTC rules might as well not exist anymore.

What's FTC?

No but in all seriousness, I think it's relative. There are some Denon AVRs that can break 200W into a single channel, and then there are some AVRs that will basically give you X into one, two, four, five, etc until they overload. It's all over the place.
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