Push-pull output impedance

SpreadSpectrum · 6th March 2008, 12:57 AM

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.

oshifis · 6th March 2008, 10:59 AM

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 · 6th March 2008, 01:21 PM

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

yaron · 6th March 2008, 08:14 PM

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 · 6th March 2008, 08:23 PM

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 · 6th March 2008, 09:40 PM

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.

SpreadSpectrum · 6th March 2008, 11:38 PM

Rob,

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

6th March 2008, 12:57 AM	#1
SpreadSpectrum diyAudio Member Join Date: Nov 2007	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.

6th March 2008, 01:21 PM	#3
revintage diyAudio Member Join Date: May 2007 Location: Eskilstuna, Sweden	Do not forget to add the secondary RDC to the above mentioned Zout __________________ Brgds Lars

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6th March 2008, 10:59 AM	#2
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	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.

6th March 2008, 08:14 PM	#4
yaron diyAudio Member Join Date: Mar 2008	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.

6th March 2008, 08:23 PM	#5
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	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

6th March 2008, 09:40 PM	#6
weinstro diyAudio Member Join Date: May 2003 Location: California	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.

6th March 2008, 11:38 PM	#7
SpreadSpectrum diyAudio Member Join Date: Nov 2007	Rob, Thanks for the tip on the tutorial. It all makes sense now.

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