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Amplifier bias
Amplifier bias
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Old 13th October 2017, 10:54 AM   #11
Mooly is offline Mooly  United Kingdom
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Amplifier bias
This type of amplifier will just revert to class AB when the load current exceeds the available bias current in the output stage.

You have to decide how much current you wish to be available in class A before you can determine a suitable current. For example 1 watt rms into 8 ohms means that 500ma peak is flowing in the load. 50 watts rms is 3.5 amp peak.

So yes, 1 amp per transistor is less than 50 watts rms into 8 ohm if you want class A
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Old 13th October 2017, 11:16 AM   #12
JMFahey is offline JMFahey  Argentina
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1) for any resemblance for the stated "Class A" they mean 1A per transistor.

2) even so, they donīt meet that claim even for 8 ohm speakers, let alone the ridiculous 2 ohm ones they also show as acceptable

3) the "multi digit high precision" numbers shown make me think that designer does not have a real distortion meter either, those are just simulated numbers, may or may not apply to a real world amplifier.

4) in fact I am not so sure he ever actually *built* it, or itīs just a simulation exercise.
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Old 13th October 2017, 11:53 AM   #13
AndrewT is offline AndrewT  Scotland
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the schematic shows the Vdrop on the emitter resistors as 309mVre
for 0r33 emitter resistors this indicates that each output transistor is biased to 936mA
For a 3pair output stage that sets the total output bias @ 2.8A
The maximum ClassA output is roughly double that equating to ~5.6Apk
The maximum ClassA output power is 5.6^2* Rload / 2 = 126W into 8ohms, or 63W into 4ohms, or 31W into 2ohms, if the output voltage is high enough.
With supplies loaded down to +-38Vdc I would expect the highest output voltage into 8ohms to be around 30Vpk.
That equates to ~60W into 8ohms, or a bit less than double into 4ohms, maybe around 100W but this is above the 63W ClassA maximum, i.e. the output into 4ohms has reverted to ClassAB.

From the above I would expect the amplifier to have it's highest ClassA output power into ~6ohms and that would be around 75W into 6ohms (5Apk & 30Vpk). It might manage 85W into 5r5 in ClassA and just avoid voltage clipping.
But how do you propose to handle the heat dissipation?
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Last edited by AndrewT; 13th October 2017 at 12:02 PM.
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Old 14th October 2017, 10:13 AM   #14
rmfar is offline rmfar  Philippines
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To AndrewT, Mooly & FAHEY,

Thank u so much with your stunning, enthusiastic and vast knowledge and ideas about audio electronics you are all a truly diyAudiophiles. All your replies really keep me go on this project. I will study all your advice and ideas and the parameters. Thanks a lot. I will keep you inform about the progress. Thanks men!
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