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Old 6th March 2008, 01:57 AM   #1
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Default Push-pull output impedance

I want to calculate the output impedance of a class AB push-pull output stage from the speaker's point of view(without GNF loop). I have estimated rp for the output tubes but I'm not sure what to use for RL, the class A load for each tube? the class B load? the combined plate to plate load? Do I halve the impedance because there are two tubes? I just need a little direction here.

Then do I divide by the impedance ratio of the transformer to get the output impedance at the speaker?

Forgive me if this was covered in another thread, but I could not find it.
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Old 6th March 2008, 11:59 AM   #2
oshifis is offline oshifis  Hungary
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You can calculate for low-level signal around the idle operating point. Rp can be derived from the tube characteristics at Vp and Ip. The two tubes in push-pull are connected in series with the primary. Rsec = (2 x Rp)/T^2
At high level the output impedance will be a nonlinear function of drive level, but I don't think it will differ much from the low-level value.
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Old 6th March 2008, 02:21 PM   #3
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Do not forget to add the secondary RDC to the above mentioned Zout
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Old 6th March 2008, 09:14 PM   #4
yaron is offline yaron  Jersey
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The ampliofier see the impedance of the speaker multyplied by the tranformer ration with power of 2, and the speaker see the output impedance of the amplifier divided by the transformer ratio with power of 2.
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Old 6th March 2008, 09:23 PM   #5
cerrem is offline cerrem  United States
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Output impedance in AB is going to be between Class A and Class B..... This depends on where you are biased, or how much of the time one side is in cut-off... This is when there is no GFB and there will be inconsistancy due to the variance in valves..
With GNF you can can provide consistancy ....

Chris
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Old 6th March 2008, 10:40 PM   #6
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Try a Google search for Patrick Turner audio. You will find a number of tutorials on loadline analysis that are very helpful, and follow RDH process.
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Old 7th March 2008, 12:38 AM   #7
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Rob,

Thanks for the tip on the tutorial. It all makes sense now.
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