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Combining Zobel network with feedback network
Combining Zobel network with feedback network
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Old 9th December 2019, 04:23 PM   #1
Mark Tillotson is offline Mark Tillotson
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Default Combining Zobel network with feedback network

An idea I had while doing noise analysis on an amp design:



Click the image to open in full size.
The idea is that capacitive divider provides a low-noise alternative to resistive feedback network at higher audio frequencies, yet capacitively loads the amp output - however a Zobel network is already present, so it can share that duty as part of a capacitive feedback network.


Note R1 is a major contribution to output noise which the capacitive divider bypasses at higher frequencies.


This relaxes the need to use low value feedback resistors for noise-performance (except perhaps for LF).
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Old 9th December 2019, 04:48 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Surely you will try it in real world, but I'm skeptic. Zobel net is a kind of snubber, to equalize high frequency loading to those of the lower ones, mainly with loudspeaker loads that increase their impedance merced an inductive load. But this increasing voltage as frequency tends to infinite, inserted into the NFB loop don't seems to be a good idea, in my opinion. Think what happen, for example, in the amp must drive a line transformer, where the impedances are different (and more complex) than simple voice coils. And the behavior with frequency divider networks?
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Old 9th December 2019, 06:54 PM   #3
MarsBravo is offline MarsBravo  Netherlands
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R2-C1 have a zero at 7Hz, so net amplification will be 1.
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Old 9th December 2019, 07:23 PM   #4
Elvee is offline Elvee  Belgium
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OK, C1 and C2 form a capacitive divider, but R3 and Ceq. create a major pole in the FB response, and this will impact the stability.

Some amplifiers might tolerate it, but even so the stability margin will be curtailed.

Placing R3 directly in series with C1 (= connecting R2 directly to the output) could solve this issue
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Old 9th December 2019, 07:25 PM   #5
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Surely you will try it in real world, but I'm skeptic. Zobel net is a kind of snubber, to equalize high frequency loading to those of the lower ones, mainly with loudspeaker loads that increase their impedance merced an inductive load. But this increasing voltage as frequency tends to infinite, inserted into the NFB loop don't seems to be a good idea, in my opinion. Think what happen, for example, in the amp must drive a line transformer, where the impedances are different (and more complex) than simple voice coils. And the behavior with frequency divider networks?

The snubber is to prevent oscillation - how does combining it with the feedback network change that? The amp oscillates the feedback network will couple that to the input stage, however the divider is implemented.


Quote:
R2-C1 have a zero at 7Hz, so net amplification will be 1.
Huh? the time constant is 2.7ms, about 60Hz, marking the switch over from capacitive to resistive divider, which only affects the noise, assuming the IS isn't loading the network significantly.
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Old 9th December 2019, 07:27 PM   #6
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Elvee View Post
OK, C1 and C2 form a capacitive divider, but R3 and Ceq. create a major pole in the FB response, and this will impact the stability.

Some amplifiers might tolerate it, but even so the stability margin will be curtailed.

Placing R3 directly in series with C1 (= connecting R2 directly to the output) could solve this issue
R3 and Ceq are the Zobel network. The resistive divider can be ignored at the frequency of interest for the Zobel. It seems to simulate OK, but the phase shift might be an issue. The 10R prevents the capacitive divider from capacitively loading the OS so it seemed natural to use it.

Last edited by Mark Tillotson; 9th December 2019 at 07:31 PM.
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Old 9th December 2019, 08:13 PM   #7
Mark Tillotson is offline Mark Tillotson
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I've realized there's a simple way to cancel the Zobel pole from the POV of the feedback, see the new R4 I've added to the circuit Click the image to open in full size.
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Old 9th December 2019, 08:28 PM   #8
Elvee is offline Elvee  Belgium
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You posted the wrong picture, but I now realize that my fix is not going to solve anything.
A correctly scaled resistor in series with C2 would do the job, and I imagine that this is the R4 you mention.
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Old 9th December 2019, 10:37 PM   #9
Mark Tillotson is offline Mark Tillotson
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Ah, the image gets cached, try this: Click the image to open in full size.
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Old 10th December 2019, 01:08 PM   #10
sgrossklass is offline sgrossklass  Germany
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Novel idea, but having to rely on capacitor tolerances for gain accuracy, channel tracking and FR flatness sounds like a bit of a nightmare to me tbh. C2 sounds like an electrolytic, too, so aging may be another issue.
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