I originally asked this question on audioreview.com but haven't gotten any answers yet. I thought maybe you guys would be more knowledgable.

My question is this, what's the correlation between watts, amps & THD. My assumption is this, watts are directly proportional to amps and volts. That is given a fixed voltage, as amps increase so will watts. And with relations to THD, as watts increase, the THD also increases. These conclusions are based on things I've read and deduced from lab reports seen that shows brand "X" receiver will have (for example) .01 THD at 50 watts, .10 THD at 65 watts and 1.0 THD 83 watts. It seems to me any receiver will have increased THD as the amount of wattage applied increases. The reason I ask is that I've heard so many complain that the mainstream brands, i.e. sony, pioneer, technics, etc have "horrible" THD. Usually .8 THD 5 channels driven. But I've also heard people complain that their power ratings are terribly exagerated. Usually they are only 2/3 of the listed rating. So is it correct to assume that if brand "X" does indeed have .8 THD at 100 watts, but the actually amount of power that is actually applied is 66 watts max, that the THD will, in general, remain significantly below .8 THD, making arguments like, brand "Y" has much "cleaner" power because it has only .04 THD at 60 watts of power, moot. And what is a "high current" amp? If watts and current are directly proportional relative to voltage, what's the diff? Can a manufacturer then toute a "High Voltage Amp?"

Bottom line, are amp specs marketing hype or can you really determine something from them?

MOst people that are looking for a hi-end amplifier. not meaning sony, yamaha, denon or any of the main stream stuff don't even look at the THD, hi-end amps being Mark Levinson, Krell or the old Carver often the THD spec is higher that you would anticipate, watts being made of volts X ampers of current, a watt is not a watt, a hi-end watt is made of more current then voltage. And I would never buy a amp on Peak power you should look for RMS per channel, also slew rate and damping factor are important.

If your interested in what makes a good or bad amp look at this [Link: britannica.com]

Thanks for the feedback Sean. Your information was very enlightening.

If I understand what you're say, a "high current" amp refers to a higher ratio of amps to voltage producing a higher quality watt. Watts ratings in and of themselves have less meaning. As for THD, in your opinion, it doesn't have that great a bearing on sound quality per se. However, you are talking about high end equipment, which I have high regard for but, unfortunately, they are way beyond my means. My question originally meant to focus on the "main stream" low end receivers, the sonys, pioneer, yamaha, denons, and onkyos (I'm sure this statement will piss off the denon, yamaha, and onkyo owners who consider their stuff high end and take offense that their equipment is mentioned with larger brands like sony and pioneer. I don't mean to imply their equipment doesn't sound excellent for their price). Does the information you provided still remain relevant to them? Is THD still irrelevant? Are the one's that claim to be "high current" better than the ones that don't? If that's true, does that mean a high current receiver has a much lower voltage than its competitors? what's a normal current or voltage suppose to be?

The truth is, I've heard mainstream manufacturers claim "high current," but I've never heard them state what that current is reletive to the voltage and they never even mention RMS power (actually they don't say whether watts are peak or RMS), slew rates or damping factors.

So, is the specs published by "mainstream" manufacturers marketing hype or can you really determine anything from them?

When comparing the power of Amps make sure that they are using the same power measurement methods. There are a number used, RMS, peak, IHF Dynamic Headroom Measuring method, and DIN Standard output power method to name a few. If a amp is rated to drive both 4 and 8 ohms loads the power rating into 4 ohms should be twice that of the power into 8 ohms. If the rating is not twice the amp has too high of an output impedance. The output impedance relates directly to damping factor, however most manufactures will not publish the damping factor because most consumer amps have such a low damping factor. For comparison Crown amps have a damping factor of over 1500, my profession grade Yorkvill amp has a damping factor of over 600 and my consumer grade Yamaha RXV-995 has a damping factor of over 160. Sony and others do not publish this number. THD is an important number, despite what some people say. It is a measure of distortion. Most harmonic distortion is caused by an amplifier clipping.

Here is a link to the specs on a cRown amp damping factor is way below 1500, it says greater than 400.
[Link: fullcompass.com]

I have 3 CARVER Professional PM-600 amps for my DD set up, and there damping factor is around 250, I would consider anything over 200 to be good.

I would not let THD make my mind up on wheather to buy a amp or not, the best thing you can do it to listen to different amps in the same setting with the same speakers, pre-amp, etc...THD is not measured at cliping it is measured at idle. What sounds good to one person might not be what someone else likes. Clipping distortion is when the amp runs out of power and clips the peaks off the sine-wave and start to send DC instead of AC to the speaker.

I stand corrected on the damping factor of the Crown Amps, the macro series is rated for a damping factor of over 1000. However on the measurement of THD you are not correct when you state THD is measured at idle. From the Crown MACRO TECH specs:

"Harmonic distortion at rated power into 8 ohms per channel (stereo mode): less than 0.05% from 20 Hz to 1 kHz and increasing linearly to less than 0.1% at 20 kHz."

Harmonic distortion occurs when clipping occurs and it does not produce DC instead of AC. When clipping occurs the result is odd harmonics of the original wave form. This is why it is dangerous to drive a speaker with an under powered amp. The high power low frequency signals can be clipped which results in a lot of energy at higher frequencies that can result in damage to tweeters.

When the clipping occurs it produces a flat on the sinewave which is DC, not a sharp oscilating peak like AC does.

Clipping of a sinewave causes odd harmonic distortion and has no DC component if the original waveform had no DC component. Any first year electrical engineering text book will explain this in more detail. See for example:
Digital Signal Processing: Oppenheim and Schafer
Principles of Communications: Ziemer and Trabter
Modern Electrical Communications: Stark and Tuteur

To prove it yourself just take the Fourier Transform of a clipped sinewave and you will see it is made up of the fundemental and odd harmonics, No DC whatsoever.