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20th December 2012, 02:24 AM  #1 
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Join Date: Aug 2012

how much class A power can be expected with a Pair of 2sc5200 1943 pair at +/52V
I would like to bias LM4702 into class A with Bias current of 500mA or may be 1A but will have very large heatsinks measuring 300mm x 200mm and fins will be about 50mm length.
I would like to know that the supply voltage be if its driven in class A. the idea is to put a switch where we can switch the amp in Class A and Class AB but the question is when the amp is delivering about 100W in 8 ohm with 2sc5200 1943 pair then how much it can deliver in class A 20W? 
20th December 2012, 06:58 AM  #2 
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why drive this amp in classA ?
do you think this amp will sound better with higher bias ? (bias adjusting a classAB amp is not like 'the more the merrier') 
20th December 2012, 07:08 AM  #3  
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Join Date: Mar 2006
Location: Oslo

Quote:


20th December 2012, 07:44 AM  #4 
diyAudio Member

Absolutely right Knuth!
The definition of class A is that the output devices are not turning off during the signal cycle. With 500mA bias, max output peak current in class A is 1A, which is 0.7A RMS (1A * sqrt of 2). Power in 8 ohms is then 0.7^2 * 8 = 4W. The supply voltage is irrelevant as long as it is high enough for the required class A output level. 1A peak in 8 ohms is 8V, so this 4W class A can be delivered with a supply voltage just above +/8V... jan
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20th December 2012, 09:12 AM  #5 
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Without forgetting that disipation is proportional to V supply.

20th December 2012, 10:34 AM  #6 
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Join Date: Dec 2007

100W in 8 ohm requires a bit more than +/ 40V rails.
If your heatsinks are big enough, then a rough guess is that it should be possible to have a bias current between 1  2 amp. 2 amps of bias current with no input signal, gives a power dissipation of 80 watt in the NPN and 80 watt i the PNP transistor with +/ 40V rails. There might be some problems with derating at high Tcase temps: http://www.st.com/internet/com/TECHN...CD00249630.pdf I assume that you only use one 2SC5200 and one 2SA1943 in your pushpull output stage correct ? Last edited by BachAudioDK; 20th December 2012 at 10:39 AM. 
20th December 2012, 11:14 AM  #7 
diyAudio Member

2A bias gives 4A peak in class A which is 2.8A RMS which is 2.8^2 * 8 = 62W in 8 ohms in class A....
jan
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20th December 2012, 11:15 AM  #8 
diyAudio Member

Indeed.
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20th December 2012, 12:56 PM  #9 
diyAudio Member
Join Date: Mar 2006
Location: Oslo

So even if you are using large heatsinks (0.3 K/W), the heatsink temperature would become nearly 80 degrees for a mono 100 W Class A amplifier...

20th December 2012, 01:23 PM  #10 
diyAudio Member
Join Date: May 2008

Anyway, Lme chip doesn't need to run class A for nice sounding .
It will bring nothing but heat. Last edited by bobodioulasso; 20th December 2012 at 01:33 PM. 
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