# ???watts per channel???



## Audiodreamer (Jan 11, 2009)

I have a couple of thoughts/ questions floating around in the ole brain housing! If a receiver has 135w/ 2 channel and it says it has dynamic power of 150w, is this what is meant by reserve or head room?

If so this does not seem like a lot. I know that I am probably never using much more than 10w, however I would think this would still have an effect...??

Based on the numbers above, if driving 5 channels how is the watts per channel computed? Or shall I say for each additional channel how much does that wattage drop?


----------



## vidiot33 (Dec 12, 2013)

The numbers depend on the manufacturer. Most receivers only quote the wpc with 2 channels driven, and not all 5 (or 7). And yes, the higher number will be headroom for peaks. As a practical matter, unless you're using very inefficient speakers, these numbers are not particularly important, as a barely discernible 3db is all you get if you double the output. If you're using a powered subwoofer, these numbers are even less meaningful. In my view, the amp, provided it's not driven to clipping, is the LEAST audible component in the chain. Not a good idea to get caught up in power specs.


----------



## chashint (Jan 12, 2011)

Amplifier power is one of the hot button topics and there are a lot of very divergent opinions on it.
It is highly unlikely an amplifier that outputs 135w/2ch driven could be identified from an amplifier that outputs 200w/5-7ch driven using real world media.
As was mentioned it takes doubling up on power to produce 3dB increase in SPL.
Real world media does not simultaneously produce max output on all channels and very seldom (maybe never) even on two channels.
Low frequencies require significantly more power than high frequencies and true full range speakers are few and far between so the subwoofer takes a lot of load off the amplifier too.
If your maximum peak output power does not exceed your amplifiers maximum output power capacity having additional unused reserve power cannot help since it is unused.


----------



## AudiocRaver (Jun 6, 2012)

Audiodreamer said:


> I have a couple of thoughts/ questions floating around in the ole brain housing! If a receiver has 135w/ 2 channel and it says it has dynamic power of 150w, is this what is meant by reserve or head room?
> 
> If so this does not seem like a lot. I know that I am probably never using much more than 10w, however I would think this would still have an effect...??


Headroom exists in different amounts at many points in a system. Then the headroom bottleneck or constraint becomes the system constraint. Amp headroom is often the system constraint, not always.

Then you have to ask how much it matters. With rock & pop, pretty much anything but classical, nearly every part of every track is running with peaks at full scale on both channels, so no need for headroom beyond that. Movies have more dynamic range and more unrelated impact events on one or two channels and not on others, so the need for peak power on all channels at once is questionable - the need would be rare - but it CAN happen. HOW much do you want to pay to handle that rare event? Only you can say.



> Based on the numbers above, if driving 5 channels how is the watts per channel computed? Or shall I say for each additional channel how much does that wattage drop?


It is complicated, depends on room, setup, speakers & efficiency, desired volume level. We figured out a formula some time back, I will try to find it. Re-creating it would be WORK!


----------



## lcaillo (May 2, 2006)

Audiodreamer said:


> I have a couple of thoughts/ questions floating around in the ole brain housing! If a receiver has 135w/ 2 channel and it says it has dynamic power of 150w, is this what is meant by reserve or head room?
> 
> If so this does not seem like a lot. I know that I am probably never using much more than 10w, however I would think this would still have an effect...??
> 
> Based on the numbers above, if driving 5 channels how is the watts per channel computed? Or shall I say for each additional channel how much does that wattage drop?


Wayne's statement that it is complicated is most accurate. Many factors are involved and it is hard to make many conclusions with the kind of info that you will have available in most cases.

That said, when you see a statement like 135w/ch and dynamic power of 150w, you can conclude that they dynamic power rating is pretty much meaningless. Virtually any amp can provide short term bursts of power above its continuous rating. When a power amp is designed, there are two primary limiting factors to power output. One is the voltage swing and the other is the available current available and that can be tolerated by the output devices. Headroom requires greater voltage swing than needed for the continuous power rating. It typically does not require more current, as short term peaks are often not what stretch the current or thermal capacity unless low impedance or complex loads are involved.


----------



## AudiocRaver (Jun 6, 2012)

FWIW, here is a link to the end of a thread where we tried to settle on a somewhat useful formula for determining needed amplifier power. the final post even had a downloadable spreadsheet submitted by another user to help make that calculation. I recall it was fairly complete and based on solid principles, but make no guarantees whatsoever. Consider it an educational tool.

Reading through that thread will give you a pretty good idea what factors are involved.


----------



## istvan (Jul 5, 2013)

There are two factors when it comes to quality audio amplifiers. Low distortion and High slew rates. !000watt amplifier delivering .5 percent distortion is much worse than a 50 watt amplifier with .01. Slew rate is basically the speed at which an amplifier can respond to a change in amplitude. I rarely see slew rates on amp specs these days. Sudden changes in volume, like in an action movie or music with many dynamics require high slew rates at low distortion. To answer your original question - if your playing white noise into 5 channels that are all the same and at a steady volume, you can basically divide your power with the number of outputs. If however the channels have different frequency characteristics and and responses and you playing music through them using a single amp then you will need a supercomputer to determine the actual power at any given speaker at any given time. If you are expecting a precise answer when it comes to real world power and amplifiers, you will need provide much more than "Watts" and "Channels" when posing a question.


----------



## AudiocRaver (Jun 6, 2012)

So true. Back when Transient Intermodulation Distortion (TIM) was "discovered," you heard about slew rate all the time, now not so much. Power Bandwidth is another way to get there, and ability to deliver full power at 20 kHz without distortion (hard to measure distortion at 20 kHz, though). But you are right, it is very important and seems to get little direct attention these days.


----------

