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Old 9th April 2013, 09:13 AM   #11
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Values aside, I would add that it is supremely important that the zobel goes back to signal ground for all sorts of reasons - that it is not put across the speaker terminals. One simple reason is the inductance of the additional path to ground. Another is that any sort of shared path is a bad idea. In purist terms, if you send it directly back to ground you have simply added two components (perhaps 3 including the L). Across the terminals you have changed the topology of the circuit hugely. R & L to terminals + C to Gnd, same on the way back with an RL and C in between.
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Old 9th April 2013, 09:28 AM   #12
AndrewT is offline AndrewT  Scotland
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Chris,
I think you explain it badly.

The Zobel is an HF load. The series capacitor ensures it can only be an HF load.
HF signals do not pass easily through inductors as loads .

The first priority for an amplifier output Zobel is that it must be capable of loading the output stage at HF. This demands that the decoupling ground on the PCB that is wrapped around the output devices and the supply pins is the RETURN point for the output Zobel.

If you want to analyse the layout effectiveness, then look at your layout like this.
Remove the main PSU. Remove the MF decoupling.
This leaves the HF decoupling as the ONLY source of current to feed the output devices. Draw the route that the HF transients must follow from decoupling to output pin/terminal. Now add on the HF Zobel load.
Where is the lowest impedance route for the HF load?
In my view it can only be from the output pin to the Decoupling Ground. That whole group of power input and power ground and output devices and Zobel and HF Decoupling must be laid out in a very tight and compact layout that minimises trace lengths to make the HF route for least inductance.

Finally, I think too many layouts concentrate on the inputs at the expense of what needs to happen at the output.
Since the output is the biggest stage, it should become the major layout target. Only after the output devices are laid out to minimise inductance is it then time to add on the earlier stages to optimise them.
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Old 9th April 2013, 09:30 AM   #13
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Quote:
Originally Posted by Analogbuff View Post
The zobel that fits one may be grossly wrong for another. It pays to look carefully at this, or you may have unexplainable failures with IC amps.
0.1U and 10R as a standard cure is no cure.
& here is why:
IC Audio Power Amplifiers and Zobel Networks: One Size Does Not Fit All | Digital Home DesignLine

Another matter is that the standard loudspeaker cable and loudspeaker termination can have some pretty wild resonances in the low MHz range, killing any kind of amps like flies.. A HF load like the zobel may help.



But
I'm sorry to say but that article is pretty much bo***cks. The zobel has nearly nothing to do with compensating the inductance of a driver. To do so would require a uF or two even for a tweeter. It is much more what you said about the impedance of cables and out of audio band Zs. In short, it gives a largely resistive load for the amplifier to look at above a certain frequency, usually starting at 70 or 80kHz so that it can be flat when getting into the tricky regions.

Cordell has some rather marvellous graphs of what the impedance can look like and even says that the impedance of a loudspeaker in the MHz range is anyone's guess. For those who don't have the book, those graphs look like a mountain range!
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Old 9th April 2013, 09:40 AM   #14
AndrewT is offline AndrewT  Scotland
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A Member posted that link to that SPEAKER correction paper.
It was clearly discussing SPEAKER correction and it's effect on the amplifier.

An amplifier OUTPUT Zobel is a completely different animal.
The amp Zobel is what the AMPLIFIER needs to see as an HF load to maintain stability if there is NO LOAD at the speaker terminals.

If one wants good load tolerance from the amplifier it is usually addressed by converting the Output Zobel to an Output Thiele Network. Part of that Output Thiele Network can be located on the amplifier's speaker terminals. Cherry discusses this at length.
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Old 9th April 2013, 10:11 AM   #15
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Andrew

Part 1. I think we are on the same track here, though I'm at a loss to understand why your explanation is clearer than mine . I think what you are doing is introducing the power supply impedance as a necessary part of the circuit (and I do agree with you here in general) and saying that really the only low impedance (or dominant) route, at the frequencies we are concerned with, is from the local bypass caps. This is probably true, though the extent would vary with the type of power supply (in a normal linear supply the impedance will eventually be dominated by the ESL of the reservoir caps, and you are saying that it is important to keep this circuit tight. This is probably generally correct, notwithstanding that some of us join these two caps up before taking them back to the main ground.

On the more complicated networks I'm with you there too. There is a good argument for having something across the terminals and reducing the amount of RF that comes back in. I'm just saying that it's not a good place to put a standard zobel, even though it looks pretty harmless at first glance.
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Old 9th April 2013, 10:18 AM   #16
AndrewT is offline AndrewT  Scotland
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We are on the same wavelength.
Maybe Members will read both versions and come to understand what is important.

Thiele's original has two options.
R+C to ground before the series connected R//L, where some values are either open circuit or short circuit.
R//L in series followed by R+C to ground. Again some component values are zero/open.
Both these work and Cherry's paper is well worth reading to gain a fuller understanding of the infinite series of component values that can be used.

Again we are in agreement that an EXTRA Zobel across the amplifier's Speaker Terminals offers some extra RF attenuation. Cherry's alternative NFB tapping point improves this even further. I started a Thread many years ago showing this, that developed into a grounding topic.
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Last edited by AndrewT; 9th April 2013 at 10:22 AM.
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