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Old 27th January 2013, 05:11 PM   #11
DF96 is offline DF96  England
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That may be a reasonable estimate. You can calculate quiescent current by using Ohm's Law on the cathode resistors. A typical push-pull Class A output will increase its current draw by 10-20% at full output, but that will only be on music peaks so can almost be ignored. Class B will increase more, of course.

A design should say what current it draws. The designer must know, because it is part of the design process.
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Old 27th January 2013, 05:21 PM   #12
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with PSUD2, instead of an end resistor, you can use a constant current load. It's much handier than a final resistor - especially if you really do know what the "average" mA will be. Obviously easier with a Class A circuit.
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Old 27th January 2013, 06:28 PM   #13
surfstu is offline surfstu  United Kingdom
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Great guys thanks...

So for the circuit posted below I can assume that V across the cathode of final stage is 10.5 V and the cathode R is 390ohms...

Therefore I = 10.5 / 390 = 0.0269 or 27mA

This is the idle current which would be the same as the quiessent current seeing as this amp operates mainly in class A (is this correct?)

and.... I,m still trying to figure out how the first stage only draws around 1mA and has a constant current source to determine this...

my limited background in physics always taught me that the current was the same for the entire circuit?
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Old 28th January 2013, 07:18 PM   #14
surfstu is offline surfstu  United Kingdom
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Default Source resistance calculated 1

Ok, thanks to all here - I'm making headway!

primary resistance = 9ohms

secondary resistance = 63ohms (CT to Sec)


Source resistance = (primary resistance / (Vpri/Vsec)2) + Secondary resistance

Vpri = 230
Vsec = 270

(Vpri/Vsec)2 = 0.726

therefore Source resistance = 75.3ohms
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