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Old 14th May 2005, 04:10 PM   #1
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Default Phase Accuracy

Mr. Graham Maynard mentioned the importance of minimum phase shift (at 20khz being benchmark) for good sounding power amp. But he always use JLH topology.

I read this app note from Analog Device, the number is AN-107, title "Active Feedback Improves Amplifier Phase Accuracy". The "Active Feedback" here is using opamp, not like Scott Wurcer's thread.

In this app note, it shows that if we put 1 or 2 opamps in the feedback path, it can reduce phase shift dramatically, like in table 1 of the app note (from -5.7deg to -0.006deg at 50khz)

If this is the same thing with what Mr. Maynard saying, then this trick can be used to any power amp, not necessarily using JLH topology. Anyone has tried this? Does it making better sounding power amp?

PS. I have the copy of the AN-107, but the pdf is sized of 778kb, how to show it here?

I also wonder, why AN-107 disappears from Analog Device's website. Does this AN-107 defective/not working?
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Old 14th May 2005, 04:24 PM   #2
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For my experience, this parameter doesn't change by himself the audio performances. There are amp. "top" with very tall phase shift ( depends simply from the technique of internal compensation ).
I believe that the better audio quality in an encircled with little phase shift are creditable to the better distribution of the BW of gang in operation of the OPen loop gain.

Mauro
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Old 14th May 2005, 11:44 PM   #3
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Putting aside implications of more than a few deg phase shift at audio frequencies (and these are important implications regarding bandwidth and more complex issues if NFB is used), the proboem itself is not of phase shift as much as of it's linearity and repeatability between many, and at least two examples of the same amp.
Obviously, because in a stereo system there are at least two amplifiers, in order to have accurate stereo reproduction, both amps have to be as much as possible the same within the audio bandwidth, including phase shift. If a phenomenon is present naturally (as opposed to deliberate), tolerances of a parameter tend to be proportional to the magnitude of the parameter, and this includes phase shift.
It is exceedingly difficult to produce an amp with i.e. quickly increasing phase shift with frequency, where the relative phase shift between two such amps remains negligible (unless we are diong this deliberately, in which case, it's a filter not merely an amp!).
Therefore, the easyest way to ensure negligible phase DIFFERENCES between channels, is to ensure negligible phase shift of all channels by themselves. Otherwise you get phase differences between channels where they were not supposed to be. This very problem was the reason digital filtering and oversampling were introduced to digital audio, as phase shift differences between analog brick-wall filters proved themselves very (and anoyingly) audible. Of course, the problem is not nearly as pronounced in power amplifiers, but it does exist, and designers need to be aware of it.
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Old 15th May 2005, 01:21 AM   #4
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This is what it can do
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Old 15th May 2005, 01:23 AM   #5
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This is what it looks like. For gain of 10x
In the paper, it is stated the importance of resemblence of both opamps block.
What if A1 is any arbitrary audio power amp, and A2 is opamp, will it still work?
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Old 15th May 2005, 01:26 AM   #6
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This is how to do it with 2 opamps. It's called "Third Order Compensation CCT"
The response seems flat up to 200khz
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Old 15th May 2005, 01:55 AM   #7
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And how does it affect transient response? In my opinion, a more important factor than phase shift good damping. If reducing phase shift causes ringing then I would go with the higher phase shift myself.
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Old 15th May 2005, 03:17 AM   #8
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Quote:
And how does it affect transient response? In my opinion, a more important factor than phase shift good damping. If reducing phase shift causes ringing then I would go with the higher phase shift myself.
hi, EVIL

I think current feedback amplifier also has less phase shift at high frequencies. what are your thoughts??

regards,
Kanwar
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Old 15th May 2005, 07:00 AM   #9
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Hi,

I am not familiar with the appnote, but I have somewhere copy of magazine article coping with the same thematic. IIRC, prerequisite is that all opamps have their phase characteristic matched, so you must use quad opamp package for that. I doubt it will work with different amps.

Best regards,

Jaka Racman
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Old 15th May 2005, 07:39 AM   #10
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Hi, Jaka,

You are very familiar with classD. I read somewhere that classD can reach 20deg shift at 20khz, is this true?
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