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20th March 2002, 06:30 PM  #21 
The one and only

I have a nice rule of thumb evaluating claims of "pure
class A operation": Take the continuous idle power draw from the wall and divide it by the total rated continuous output per channel. If it is a mono amplifier, divide the number by 2. If it is a stereo amplifier, divide it by 4. Now square the number and multiply it by the rated output per channel. This gives the most generous estimate for "pure" Class A operation, and twice this number is the instantaneous point at which it leaves Class A. Example: A stereo amplifier draws 200 watts continuously from the wall and is rated at 100 watts per channel: 200/100 = 2 2/4 = .5 .5 squared = .25 .25 times 100 watts = 25 watts. This is the rms Class A figure. 2 times 25 watts = 50 watts. The amp leaves Class A at the instant of 50 watts, and probably operates AB to a 200 watt peak (the peak of a 100 watt rms sine wave). All this assumes no weird circuits, and also assumes no other losses. In real life, the numbers will be somewhat less. 
20th March 2002, 09:29 PM  #22 
diyAudio Member
Join Date: Feb 2002
Location: Charleston, SC

Thanks for the explanation Joe, that makes sense.

21st March 2002, 05:52 AM  #23  
diyAudio Member

Quote:


22nd March 2002, 01:49 AM  #24 
diyAudio Member
Join Date: Mar 2002

My amp, the X 150, is rated at 150 watts per side into 8 ohms and 300 into 4 ohms. Its power consumption
is 200 watts idle, 600 watts max. Pass claims it runs Class A to 20 watts output then switches to AB. Does the math work out, or is that not enough info? Doing Nelson's math as I read it, I get Class A to 16.67 watts. A friend, who owns the Llano Trinity 200 wrote this: When I got into audio nobody told me there would be math involved, when I did the math I got 112.5 wpc RMS and 225 wpc max class A into 8 ohms vs the claim of 200. If I understand correctly, the power draw from the wall is the total and since the Llano draws 300wpc at idle, the total would be 600watts. That is the first number in the equation. If that is wrong then every step after that will be wrong. Whereupon another aquaintance said this: Don, your AC outlets carry 110 volts. No mater what you plug in it will always be 110. If you plug something in with an impedance of say 100 ohms so much current will flow. If you plug in 50 ohms twice as much current will flow. Voltage remains the same. The amps power supply is similar to your AC outlet. It supplies so much voltage in DC. Current flow now depends on load impedance. As long as the power supply can provide the current flow it will provide what ever is needed beyond Idle. Bias voltage remains the same regardless. How that translates into power depends on the load. Mike 
22nd March 2002, 03:22 AM  #25  
Warp Engineer
On Holiday

Quote:


22nd March 2002, 08:28 PM  #26 
The one and only

Looking at the Trinity, if that's 300 watts idle a channel,
then I would expect to be able to cook a steak on them. 
22nd March 2002, 10:05 PM  #27 
diyAudio Member
Join Date: Mar 2002

It gets warm with moderate volume play. Less so than with the X150. Something to do with very efficient Mosfets, or so its been said. Thank you for your reply. I'm still looking for the right front end to feed the X, something clean, detailed and musical.

25th March 2002, 08:34 PM  #28 
The one and only

Oh! Efficient Mosfets!

25th March 2002, 10:45 PM  #29 
diyAudio Member

I think they use a secret package with a looooow!! thermal resistance and a special coating on their heatsinks.
Back to reality : you can not get pass the thermal resistance who exists from the "die" through the package .. thermal compound .. and at last the heatsink... High effiency MOSFET are only high effiency when shifted "on" where it is possible to use their low RDS... Like they do in Class D amps. In a "linear" amp there is no miracle!! Sonny 
25th March 2002, 11:27 PM  #30 
diyAudio Member

This is hilarious....I can't believe (well, given what some other manufacturers do, I guess I can believe.....) what this guy is saying. That's absolutely hilarious to use "high efficiency" mosfets as the reason a class A (not switching) amp doesn't dissipate very much power.
You've got to be kidding. As Sonnya just stated, this attribute of FET's only applies when they're fully on, not while they're switching. 
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