Balancing Servo

Jeb-D. · 10:46 PM

My next build is going to be a fixed bias class AB using toroidal OPT's. I would like to use a balancing servo circuit to ensure matched DC current at all times. Can anyone recommend a servo circuit for this? I was thinking of designing one which uses current sense resistors and a high voltage Opamp to bias 1 of the tubes to match the other. But wanted to see what other ideas people have come up with first.

Another possibility would be to use a current mirror on the cathodes, but this would make the amp combination bias, which I would like to avoid if possible.

SpreadSpectrum · 6th January 2010, 02:04 AM

I have a board drawn up implementing a bias servo for balancing two tubes (You set one tube with a pot and the other follows). I used a Broskie circuit. It is similar to the "The Emperor's New Amplifier" servo, but Broskie correctly balanced the time constants in the two low pass filters on the inputs of the op-amp.

Broskie did a couple of autobias articles and then a balance servo for a push-pull output stage. Let me know if you can't find it.

TheGimp · 6th January 2010, 03:15 AM

Amplifier auto bias circuits: Alan Dower Blumlein's garter circuit

He continues for a couple of blog entries before getting to the class AB circuit.

5th January 2010, 10:43 PM	#1
Jeb-D. diyAudio Member Join Date: Nov 2005 Location: SoCal	Balancing Servo My next build is going to be a fixed bias class AB using toroidal OPT's. I would like to use a balancing servo circuit to ensure matched DC current at all times. Can anyone recommend a servo circuit for this? I was thinking of designing one which uses current sense resistors and a high voltage Opamp to bias 1 of the tubes to match the other. But wanted to see what other ideas people have come up with first. Another possibility would be to use a current mirror on the cathodes, but this would make the amp combination bias, which I would like to avoid if possible. Last edited by Jeb-D.; 5th January 2010 at 10:46 PM.

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6th January 2010, 02:04 AM	#2
SpreadSpectrum diyAudio Member Join Date: Nov 2007	I have a board drawn up implementing a bias servo for balancing two tubes (You set one tube with a pot and the other follows). I used a Broskie circuit. It is similar to the "The Emperor's New Amplifier" servo, but Broskie correctly balanced the time constants in the two low pass filters on the inputs of the op-amp. Broskie did a couple of autobias articles and then a balance servo for a push-pull output stage. Let me know if you can't find it.

6th January 2010, 03:15 AM	#3
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Amplifier auto bias circuits: Alan Dower Blumlein's garter circuit He continues for a couple of blog entries before getting to the class AB circuit.

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