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Old 23rd September 2008, 11:30 AM   #1
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Default CI, is that RC backwards?

I was just looking at the schematics that have a capacitor at the NFB, and the positions of the cap and resistor on the "ground leg" of the NFB. . . seem to be backwards (resistor on the wrong side of the cap)--not the alignment for passing a signal intact.

The effect of adding a resistor before input to a capacitor is usually a noise--the exact same noise from putting a cap at CI. Did anyone else notice it? Well, I don't know what the "right" way is, but that's surely not it.
What are some other options for AC coupled NFB?
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Old 23rd September 2008, 11:38 AM   #2
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if a toast is roasted from both sides equally, which side do you prefer?
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Old 24th September 2008, 01:40 PM   #3
AndrewT is offline AndrewT  Scotland
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It makes absolutely no difference which order the two components are in for the same RC effect.
It can be C followed by R or just as effectively R followed by C.

The same applies to the Zobel RC at the output.

However, when a signal is tapped off from the junction of the RC then the order of component placement in the signal path is paramount. eg, See Dr. Cherry's NFB tapping point from the mid point of the Zobel.
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Old 24th September 2008, 07:18 PM   #4
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An off-topic question: why reversing signal and ground wires [of course assuming both source and amp are not safety earthed!]makes a noisy amp?

I have observed and and read about this but i look at it like if "both sides where equally roasted".
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Old 24th September 2008, 09:00 PM   #5
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Quote:
Originally posted by AndrewT
However, when a signal is tapped off from the junction of the RC then the order of component placement in the signal path is paramount. eg, See Dr. Cherry's NFB tapping point from the mid point of the Zobel.
Wouldn't you call that a tone control?
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Old 25th September 2008, 08:01 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by danielwritesbac


Wouldn't you call that a tone control?
no,
try to find Dr Cherry's paper.
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Old 25th September 2008, 01:12 PM   #7
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Quote:
why reversing signal and ground wires [of course assuming both source and amp are not safety earthed!]makes a noisy amp?
reversing shield and core on both sides of a cable, disables the shield.
Reversing it at one end only requires true floating devices.
On most audio equipment ground is connected to the case, which connects the amplifier input to an antenna, if the sources polarity is reversed.
Also in most devices case and ground are connected to mains live through a capacitor.
This works fine, if the equipment is connected together in traditional ways, i.e with coaxial cables in correct polarity.
Reversing polarity would introduce all kind of RF and hum issues.
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Old 27th September 2008, 01:03 PM   #8
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Quote:
Originally posted by AndrewT
no,
try to find Dr Cherry's paper.
Yes! Any AC on the 0v line goes through the zobel's capacitor and into the negative feedback loop--"a noise-powered dynamic tone control."

And probably. . . bog standard op amp tone control value of approximately 15nF (or smaller) polyester capacitor that's now used for the speaker zobel?

I like this a lot! It operates only in the presence of a noise--the worst noise there is in the amplifier, which is all that racket (AC) coming back from the speaker ground line. And, this works on power noise present on the 0v line too.

The more noise you make, the less noise you get? Brilliant!! Thanks man!
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Old 28th September 2008, 11:06 AM   #9
Mooly is online now Mooly  United Kingdom
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Default Re: CI, is that RC backwards?

Quote:
Originally posted by danielwritesbac
I was just looking at the schematics that have a capacitor at the NFB, and the positions of the cap and resistor on the "ground leg" of the NFB. . . seem to be backwards (resistor on the wrong side of the cap)--not the alignment for passing a signal intact.

The effect of adding a resistor before input to a capacitor is usually a noise--the exact same noise from putting a cap at CI. Did anyone else notice it? Well, I don't know what the "right" way is, but that's surely not it.
What are some other options for AC coupled NFB?
There is a "right and wrong" way. The cap is more prone to pick up noise due to it's physical size and so should be at the "low impedance" end as a generalisation.
Polystyrene caps have a red mark on them indicating the outer part of the foil. Again use this to "shield" the cap by making sure this is again connected to the low impedance point.
Worth doing if you can.
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Old 28th September 2008, 11:20 AM   #10
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Default Re: Re: CI, is that RC backwards?

Quote:
Originally posted by Mooly


There is a "right and wrong" way. The cap is more prone to pick up noise due to it's physical size and so should be at the "low impedance" end as a generalisation.
Polystyrene caps have a red mark on them indicating the outer part of the foil. Again use this to "shield" the cap by making sure this is again connected to the low impedance point.
Worth doing if you can.
I think that's what I was saying?

#1. I like this: source, C, R, load (flat response)
#2. Not this: source, R, C, load (peakish)

But, I was asking for help identifying which direction the signal is going at the low (ground-seeking) leg of the NFB.

I'm just having a hard time with the concept of negative feedback loop, relating to where the signal is coming from--either coming from the direction of the inverting input OR coming from the direction of the 0v line. But, there sure is a signal, and it came from somewhere!
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