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Old 1st January 2016, 10:37 AM   #1
ygg-it is offline ygg-it  Italy
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Default Zobel RC network and stability of GNF SE amplifier

Does a Zobel RC network at the output of a single ended tube amplifier (with global negative feedback) make the amplifier less stable?
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Old 1st January 2016, 12:27 PM   #2
Koonw is offline Koonw  Malaysia
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Originally Posted by ygg-it View Post
Does a Zobel RC network at the output of a single ended tube amplifier (with global negative feedback) make the amplifier less stable?
A zobel or capacitor in the output will counter the inductive rise of the speaker, making them look resistive, good news for speaker crossover. The bad news is if you change the speaker, the resultant may be capacitive instead, this introduces more phase shift hence less stable. By reasoning is it a good practice to install zobel net on the speaker side? In this article it's call matching network, think it's on speaker side. http://users.ece.gatech.edu/~mleach/...oads/zobel.pdf

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Old 1st January 2016, 12:56 PM   #3
DF96 is offline DF96  England
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Old 1st January 2016, 02:04 PM   #4
Ketje is offline Ketje  Belgium
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Depends, is for a violin or a cello ? :-):
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Old 1st January 2016, 04:08 PM   #5
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by Koonw View Post
A zobel or capacitor in the output will counter the inductive rise of the speaker, making them look resistive, good news for speaker crossover. The bad news is if you change the speaker, the resultant may be capacitive instead, this introduces more phase shift hence less stable. By reasoning is it a good practice to install zobel net on the speaker side? In this article it's call matching network, think it's on speaker side. http://users.ece.gatech.edu/~mleach/...oads/zobel.pdf
So in any case, even if 100% matching and make the load pure resistive, it will always add more phase shift, thus adding it will make the amplifier with GNF less stable than before in any case (I'm not saying unstable...), correct?

Why adding it close to the speaker should improve the situation? The resistance of the cable is negligible.
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Old 1st January 2016, 04:30 PM   #6
Koonw is offline Koonw  Malaysia
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Originally Posted by ygg-it View Post
So in any case, even if 100% matching and make the load pure resistive, it will always add more phase shift, thus adding it will make the amplifier with GNF less stable than before in any case (I'm not saying unstable...), correct?

Why adding it close to the speaker should improve the situation? The resistance of the cable is negligible.
A pure resistive looks just like a resistor, no phase shift, unless you mean the amp out characteristic is altered as result of driving the zobel net? That could well be, so your amp phase shift, frequency response, damping factor etc should be good to begin with. These parameters should be checked before and after add the zobel, maybe the zobel design is the problem or your amp.
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Old 1st January 2016, 05:59 PM   #7
DF96 is offline DF96  England
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The effect of adding a CR network with unknown values to an unknown amplifier design is unknown.

A Zobel on its own at the amplifier may do little good. You may need a series inductor too. A Zobel at the speaker end may help, under certain circumstances. Basically, you need to determine what the problem is before asking how to solve it.
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Old 2nd January 2016, 03:21 AM   #8
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Done right, a Zobel can add to the stability of an amp by forcing the gain to stay below 0dB after crossover.

I've seen amps that have a nice square wave/time domain response display a frequency response that goes below zero, and comes back up to positive gain at a higher frequency - trouble waiting to happen. Mr. Zobel keeps the gain down below 0 dB after crossover, giving proper gain margin as well as phase margin. Unfortunately, you need a good gain-phase analyzer to be checking this kind of thing. I use the HP4194A at work, which goes up to 40 MHz.

The usual default values of Zobel I see suggested are something like 100 nF + 10 ohms. I start with something far less aggressive these days, like 33nF + 10 ohms. This is based on results Ive seen with the G-P analyzer.
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