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Discussion Starter · #1 ·
Questions related to rated power of receiver's or amplifier's power are common. I started this thread to address some common questions. To simplify this discussion I will use often use the term amp which also implies receivers.

How do I compare two amp's power ratings?


When comparing two amps make sure you are comparing apples to apples. One way is to compare the FTC or RMS rating. Make sure both amps are rated against the same bandwidth. Two common methods are 20hz-20hkz or 1hkz. Also you want to find the THD for these ratings. If the THD numbers are drastically different like 1% and 10%, you are not going to be able to directly compare the numbers. Make sure both were rated into the standard load of 8ohms.


Most Audio/Visual receivers are rated when driving a single channel even though they will say something like 100x5 watts. You can't directly compare a receivers outputs where one was rated one channel driven and the other with all channels driven.

How useful are power ratings?


Power ratings are simply one aspect of amp performance. Sound quality and the ability to handle transients are others. Ratings are done using resistive loads. Speakers are reactive loads and more diffcult to drive. Their actual impedence varies over the audio frequency spectrum. This means that real world amp performance can vary from rated power.

Are amps rated with all channels driven better?


Movie soundtracks rarely require equal power to all channels for any substantial length of time. As such, you don't often need full power to all channels. If you are usually using your multi-channel setup for movies, you probably don't care whether an amp was rated with all channels drive.

Is an amp rated for 130watts better than one rated at 100 watts?


A receiver rated for 130x5 is not going to play much louder than one rated at 100x5. A 200 watt receiver will only play 3Db(SPL) higher than one rated at 100watts due to the logarithmic response of the human ear. 3Db is accepted as the standard audible difference in sound pressure. Most well made audio/video receivers power outputs are sufficient to drive reasonably efficient speakers to levels acceptable to most people.


Why do some people say to check the amps weight?


A lot of the weight of a receiver is taken up by the power supply. More powerful power supplies need larger transformers. The heatsinks in an amp used to keep the power transistors cool are also a potential indicator of amplifier power. Heavier units are often better constructed.

Some amps can't hit their rated power on a standard house circuit

A standard 15A residential electical circuit is limited to a draw of 1800 watts total. If the amp was %50 efficient, it could turn that into 900 watts of usable power. That works out as 180 watts into five channels. At some point amps are limited by standard house electrical circuits. It does not make sense to pay extra power that you can never utilize. The All Channels Driven article at Audioholics.com contains a much more complete discussion of this issue including a much more thorough discussion of the math.

What if my receiver is not loud enough

Investing in more efficient speakers is an economical solution. A speaker rated 92Db @ 1 watt of input power measure at 1 meter plays 7Db louder than one rated at 85Db. The one rated a 92Db can hit 108Db with 64 watts of input.


Another inexpensive option is augmenting your receiver by using one or more professional amplifiers. 200 wattsx2 professional amps can often be had for under $300 USD.
 

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Talking about solutions on the cheap.


I love these Audiosource Amp/1A's running bridged. I see them factory refurbed for well under $100 each often. Makes a nice, clean, powerfull mono block. I use Qty 7 of them right now, one for each channel.


I will add to the above post, that in general separate power amps will slay on board AVR amps, no matter what the power ratings say.
 

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Discussion Starter · #3 ·
I have heard good things about Audio Source. I believe you when you say separates would usually outperform AVRs. I think that's where the interest in finding cheap pro amps came from. A year or so ago I started asking in forums why more people were not looking at pro amps. Some of the answers were:

* They usually have fans, and can be noisy

* They are less" musical"


I am sure point B is debatable, and some people have solved the fan issue with quiter replacements, or taking their chances and disconnecting them
 
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