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Discussion Starter · #1 ·
Say you have an amp that is rated 50x2 and it is honestly spec'ed there. If you were to drive to to 60x2, would it start clipping or would there be just a little more distortion? It looks like amp makers spec a power at a certain THD ect. If it is .05% thd at rated, when you push it past there a little do you just start increasing THD or does it start to clip shortly after rated power? Here is an example of what im asking. Say you take a 50x2(something decent) bridge it and use it to drive a single sub. So you are looking at a rated bridge power of 200W. One day you get a bug and crank it up. If you were to provide 250W at peak draw, would the amp clip or would the THD just be worse than the spec'ed power. The amp im talking about is a crossfire TEK 50x2. I just threw that in there incase somebody knows alot about that particular amp.
 

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You need to find some o-scope screen shots of thd and of clipped waveforms. That might help give you a better idea of how amps work.

A typical AB transistor amp has a thd curve that is higher at low power outputs, and then thd decreases as power output increases. As the amp hits the limits, the thd then rockets upwards.

So "honestly" is a judgment. You can pick an amp to be 5W at 1% thd, or 50W at .01% or 60W at .5% or 70W at .1% or 80w at .5% or 5% at 90W. (I am estimating the curve, but look on the side of an old Alpine amp box if you want). But at some point the power output flattens and the clipping of the waveform becomes even greater and causes all sorts of distortions - harmonic, chaotic, whatever.

I don't worry about THD a bit. There are other forms of distortions, IMD, as well as non-steady-state distortions linked to peaks - which are far more objectionable. harmonics sound like music. But your speakers are a couple percent right off the top, remember.
 

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Discussion Starter · #3 ·
Thank you very much. Ill do some searching and see what i can find. So THD is not linear with power. How do you go about buying an amp based on a generic spec sheet that gives you rated power specs. Do the better amp makers (zapco ect.) provide you with specs at different power levels. This could explain why some cheap amps sound bad even know they are rated decent at rated power.
 

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Every amp maker picks one point and lists THD and power for that point. Many list output at .01 and I think CEA created another attempt at a standard a few years back.

Here is a graph as an example:

http://www.signaltransfer.freeuk.com/TrimodalClassesAB20W.gif

But you are asking a huge question. My point is that I would not use THD specs to buy an amp or to decide how it sounds.

There are some design techniques (such as specific uses of global negative feedback techniques) which allow very low THD numbers but drive up intermodulation numbers, for instance. IM is far more offensive to the listener than harmonic distortion, but few manufacturers list IMD. (I haven't heard much about IMD for a while, wonder if new faster devices may have changed the old negative feedback side effect?)

In about 1988, buddy of mine took his Fisher home rack system amp (well after the Sanyo acquisition, when Fisher was making real crap) to a McIntosh clinic. He knew the amp was crap, but wanted to see what kind of crap. The amp was listed at .01 THD at 100WPC, and it measured on the test bench EXACTLY 100W with .01% THD.

Intermodulation distortion was 13%. That's why that amp sounded like crapola.

So how do you shop for an amp with specs? I don't. At some point, I need to hear one or hear comments from someone I trust about the sound. I don't trust that the specs commercially available for a car amplifier will tell me whether or not it sounds good under load. There's no car stereo press to half-heartedly keep the manufacturers honest with their bench-testing. They might be lying.

Amps that play test tones OK - which are how specs are measured - may not play music well.

So you are asking a tough question, and it's one reason why the people on this board tend to buy old amps, many disccontinued, because they trust old proven designs more than new designs. Evaluating amps is a messy and difficult business, and for car audio, beyond a certain point, you get much bigger payoffs in other places (speakers, install, etc.)
 

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Amps that play test tones well may not sound good. True, but amps that don't play test tones well will not sound good either.

Read reviews and listen yourself. That's the best way to determine if an amp sounds the way you like it to. The only real reason to delve more into the clipping and THD is when you are setting up an amp and you want to match all your stages so the HU drives the amp to it's full output without clipping. I fiund tones or an RTA very useful for that, but 99.9% of people on this board don't have an RTA...but they can hear distortion which is all you need with a tone to set the gains.
 

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Discussion Starter · #6 ·
Okay, thanks again. I have been designing cell phone amp for almost 10 yrs now so im pretty familiar with the specs that are used. Before we release a part there is a rediculous spec sheet that need to be filled out under the most crazy conditions. It would be nice if amp makers would provide a usefull spec sheet so you could buy an amplifier taylored to the condition you are planning on using it. I could care less if i have "2000W! if im plannig on setting up a system that sounds good at normal volumes and im driving components. It would also reduce the amount of speculation of what is good and not. I have a RF T3002 amp that to me is over sized physically and produces more power than i probably need to run a pair of components. It may be a great amp but i swear it is a sub amp. I want to replace it with my crossfire TEK 35x2 becuase i feel like it will be fine and is much smaller so it will be easier to hide, but i dont know and it is driving me nuts not knowing. I would much rather have the lower power amp if the total distortions are much less at powers that im using. Until now i had myself convinced that i didnt need that extra power but there is a post on here that is called something about "75W enough" or close to that. Several people swear that there music becomes fuller or better in some way if they use the bigger amps. Would it be fair to say that other aspects of an install are more important than fussing about amp specs, assuming you have decent amps and speakers.
 
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