2. amps-preamps
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Watts and power

Can somebody break it down in layman's terms for me? Why is it that sometimes an amp that has a high watt rating (like, say, a lot of class D amps do) don't seem to always have the balls that much lower rated A or AB amps do? I have heard some people say, "It's not the watts, it's the power supply." Are they talking about big honkin' toroidal transformers? I know opinions vary on a speaker like, say, Magnepans - Maggies love power, right? A lot of people caution against using class D amps to drive them and then will turn around and say that a receiver like the Outlaw RR2160 (rated at 110 watts into 8 ohms) drives Maggies really well! I'm not really asking about differences between Class D, A, or AB so much as I am asking about how can you tell the POWER an amp has from the specs? 
redstarwraith
optimize
164 posts
 
Optimize, yes we do know what you are asking if say they give a spec into 8, 4 and 2 ohms which many do..
No they do not.
No manufacturer what I have seen have been given what ampere (current) spec into 8, 4 and 2 ohms.

Only wattage and maybe the distortion. We are in the dark of how much current (and voltage) there were when we got the power (wattage).

Internet know hows and reviewers say now and then. "The speaker X need a high current amplifier"..
How would they know?! They do not have any other specifications that we all get..
There is specific amplifier designs that are primarily designed to be current sources. So they can have relatively low wattage but for producing that wattage it gives high amount of current and low voltage..
Or are missing something here? Were is the ampere and the voltage in the specifications?
georgehifi's avatar
georgehifi
10,312 posts
 
Class-D and complimentary mosfets poweramps fall down miserably when asked to deliver current into 2ohms so they can double the wattage all the way.
That's why they never advertise "true" independently tested  8, 4, 2ohm wattage's just before clipping, because the 2 usually goes backward in wattage, indicating sever current starvation or limiting.

Cheers George  
audiozenology
362 posts
 
Oh come on George, I don’t know any amplifier vendor that publishes independent data, only reviewer data.


Your comment about MOSFETs is ignorant. It’s easy to drive into 2 ohms with complimentary MOSFETs. Purely an architecture thing. I don’t know why you insist on this repeated ignorant. Please learn more about amplifier design before posting on this. You know the Behringer is rated into 2 Ohm ... Why would you say what you do?

Optimize,
  • P = v * I
  • P = v^2 / R
  • P = i^2 * R

If you know P and R, then you can calculate I and V.


I am not aware of any commerical audio amps designed as current sources ... Not practical as it would not work with off the shelf speakers as it would screw up the crossover operation.
georgehifi's avatar
georgehifi
10,312 posts
 
Oh come on George, I don’t know any amplifier vendor that publishes independent data, only reviewer data.
When I say that it’s Stereophile, Miller Labs, Newport Test Labs ,ect ect and like that are independent that give you the truth. If you want to believe the BS many manufacturers give you that’s your problem.

As more and more these days the manufacturers are "grossly understating their 8ohm wattage" to make the the 4ohm look like it’s closer to doubling, same goes for the 4ohm to the 2ohm, and Stereophile test confirm it.
And if you don’t know this then you’ve been hiding under a rock, and can’t see the forest for the trees. But I already knew this.

It’s easy to drive into 2 ohms with complimentary MOSFETs.
You will not get anywhere near the current performance what same amount of complimentary BJT’s can do into 2ohms. That’s why all the very best amps that deliver huge current into 2ohms are all BJT (bi-polar) again your under a rock


audiozenology
362 posts
 
Oh George, maybe someone will give you a new record for Christmas. Yours appears to be broken. 


You have never actually designed an amplifier. That much seems obvious to me. Guessing you don't know much about speaker performance versus damping factor either.


Your holy grail wrt power doubling from 4 to 2 ohms ... Says more about the power supply AND feedback level than the output devices. An amplifier designer would know this. 



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