how much class A power can be expected with a Pair of 2sc5200 1943 pair at +/-52V - Page 2 - diyAudio
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Old 20th December 2012, 03:09 PM   #11
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I think the OP was looking to compare the classes for his own interest. Without going to high power, Douglas Self's Trimodal amplifier offers A, B, and AB classes at the flick of a switch. A full schematic article and description is carried in his "Audio Power Amplifier Construction Handbook" and "Self on Audio" books, for anyone interested.

This is a worthwhile, finessed project delivering 20-25W/8R in all modes and is more like the full deal for the OP.
http://www.douglas-self.com/ampins/ampins.htm
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Last edited by Ian Finch; 20th December 2012 at 03:18 PM. Reason: add link
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Old 20th December 2012, 03:18 PM   #12
jcx is offline jcx  United States
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for more fun you need the calcs for realistic loads

loudspeakers aren't pure resistors, even "polite" multiway often have |Zmin|< Rnominal/2

then you can adjust power dissipation for phase angle

add reserve for "pumping" resonace, sudenly reversing phase - the current peak can be >5x the Rnominal calc
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Old 20th December 2012, 04:32 PM   #13
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Rule of thumb is that the quiescent dissipation for class A is about 4 times the max output power (assuming the power supply voltage is dimensioned for that but not higher).
So that's 400W for 100W class A in 8 ohms.
If you want to keep the temp rise down to say 25 degrees C, which means a heatsink temp of 47 degrees at room temp of 22 C, you need a sink with no more than 25/400 = 0.0625 deg/W.
That will need a lot of (forced) air cooling.

jan
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Old 20th December 2012, 07:07 PM   #14
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ClassA at high power is a terrible waste.
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Old 20th December 2012, 08:24 PM   #15
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If I was to design a 100W into 8ohm class A, I would consider to make it a balanced design. This reduces the rail voltage to half, but off cause doubles the peak current needed. (Amplifiers see 4ohm in BTL)

Rail voltage on output stage can be reduced to a bit above +/- 20volt.
The bias current needed in each Pushpull stage would be 2.5A (20V into 4 ohm=5A or 40V into 8ohm=5A). Power dissipation in each BJT device will be 2.5Ax20V =50W and a total of 200W for the four transistors. (One balanced channel)

I like that both rails of power supply is equally loaded with a balanced BTL output stage.
No nasty rectified signal currents flow in the PSU. This would also be almost hum free, even in a non negative global feedback amplifier. Also the high bias current gives a lower output impedance, compared to a Class AB. But in BTL mode the output imp. is twice compare to one stage.

Cheers
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Old 20th December 2012, 08:53 PM   #16
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yeah i thought it for a pair of output devices..

LME49811 is said to be built especially for class A amplification more for class A designs..
even if im able to get 30W in 8 ohms is still more than required for my application...

I was going through the site

Gain-X2 - LME49811 15-150Watt Gainclone amplifier

what should be the VA ratings of the transformer to get the 20W of class A power?
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Old 20th December 2012, 09:51 PM   #17
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The PSU schematic is silly.
Big resistors into the rails!!!
100mH 6A coils!!!

Edit: This project is a joke.

Last edited by bobodioulasso; 20th December 2012 at 10:02 PM.
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Old 21st December 2012, 07:20 AM   #18
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i know psu is crap but the amplification stage is common. I have a question that can LME49811 can be biased in 2Amps?
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Old 21st December 2012, 07:30 AM   #19
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The biasing is all about the output stage.LME itself runs little current.
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Old 21st December 2012, 09:54 AM   #20
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yeah the chip delivers very little current but with one 2sc1517 transistor it gives enough current but the collector current at max is 2A but how to get more current?
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