# Push-pull output impedance

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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.

#### lcsaszar

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.

#### revintage

Do not forget to add the secondary RDC to the above mentioned Zout

#### yaron

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.

#### cerrem

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

#### weinstro

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.

Rob,

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

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