how much class A power can be expected with a Pair of 2sc5200 1943 pair at +/-52V - diyAudio
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Old 20th December 2012, 02:24 AM   #1
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Default 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?
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Old 20th December 2012, 06:58 AM   #2
tinitus is offline tinitus  Europe
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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')
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Old 20th December 2012, 07:08 AM   #3
knutn is offline knutn  Norway
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Quote:
Originally Posted by rhythmsandy View Post
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?
The important thing here is the bias current. With a bias of 500mA, Class A power into 8 ohms is 4 Watts, increasing to 16 Watts with 1 A bias.
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Old 20th December 2012, 07:44 AM   #4
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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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Old 20th December 2012, 09:12 AM   #5
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Without forgetting that disipation is proportional to V supply.
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Old 20th December 2012, 10:34 AM   #6
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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.
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Old 20th December 2012, 11:14 AM   #7
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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....

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Old 20th December 2012, 11:15 AM   #8
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Originally Posted by bobodioulasso View Post
Without forgetting that disipation is proportional to V supply.
Indeed.
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Old 20th December 2012, 12:56 PM   #9
knutn is offline knutn  Norway
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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...
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Old 20th December 2012, 01:23 PM   #10
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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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