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Importance of the quality of the DC-blocking feedback capacitor?
Importance of the quality of the DC-blocking feedback capacitor?
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Old 25th June 2019, 04:02 PM   #231
N101N is offline N101N
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leadbelly,
the Sumerians insightfully implemented a sexegesimal number system long before the Babylonians. The French obsession of decimalizing time was clearly not an award winning approach. Mathematics had never been used in a more superstitious, religious, magical, paranormal, holy or weird way than it is in our time.
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Old 25th June 2019, 04:57 PM   #232
alayn91 is offline alayn91  France
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Hello,
And, what about the inverting input resistor to ground ?
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Old 25th June 2019, 06:32 PM   #233
cbdb is offline cbdb  Canada
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Quote:
Originally Posted by jan.didden View Post
Got in touch with Kendall but he feels it is an exercise for the reader. :cool
Now what?

Jan
Now we look at the circuit at the freq extremes, where C is ( close to ) a short and an open.

At low f a gain of 1, at high f 1+ R5//R3 / R4 and since R5 is large 1+ R3/R4.

And I would GUESS the corner freq is at around Zc=R5 which is way up there. So a small cap will do. ( but Ild worry about parasitics at these high impedances?)

As the author says, its not HiFi.
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Old 25th June 2019, 08:01 PM   #234
knutn is offline knutn  Norway
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Quote:
Originally Posted by cbdb View Post
As the author says, its not HiFi.
Why not? Increase the capacitor to 2.2 uF and the pole is at 1.6 Hz. I think the problem is the DC resistance in Mohm range which demands a FET input.
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Old 25th June 2019, 08:13 PM   #235
steveu is offline steveu  United States
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I can't imagine what you did wrong but op27 works fine for me. The DC resistance presented to both inputs is 100k in this example in order to cancel any DC offset. The op-amp is a surrogate for a power amp where 20K to 50K is typical.
In any case this is about AC response and impedance, not DC. If you are not familiar with circuit dynamics then "don't worry about it."
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Last edited by steveu; 25th June 2019 at 08:16 PM. Reason: missing bits
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Old 25th June 2019, 10:44 PM   #236
cbdb is offline cbdb  Canada
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Quote:
Originally Posted by knutn View Post
Why not? Increase the capacitor to 2.2 uF and the pole is at 1.6 Hz. I think the problem is the DC resistance in Mohm range which demands a FET input.
Even with fet inputs you might have trouble with offset. High resistance usually means high noise, and if the cap leakage resistance is in that mega ohm range you have to consider it. And somtimes with that high resistance other parasitics may effect things. Personaly i try to avoid extreme values, you have to start to consider stray inductance and capacitance as well as leakage, ESR, ect.
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Old 26th June 2019, 03:10 AM   #237
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by steveu View Post
I can't imagine what you did wrong but op27 works fine for me. The DC resistance presented to both inputs is 100k in this example in order to cancel any DC offset. The op-amp is a surrogate for a power amp where 20K to 50K is typical.
In any case this is about AC response and impedance, not DC. If you are not familiar with circuit dynamics then "don't worry about it."
Sorry about that - the OP27 test was run on a single +15V supply whereas the LT1057 used +/-15V which makes a world of difference regardless with both op.amp results.
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Old 26th June 2019, 09:02 AM   #238
knutn is offline knutn  Norway
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Quote:
Originally Posted by cbdb View Post
Even with fet inputs you might have trouble with offset. High resistance usually means high noise, and if the cap leakage resistance is in that mega ohm range you have to consider it.
The point with this arrangement is to reduce the capacitor value without having to increase the noise. Remember that the resistor in the megaohm-range is shunted by the capacitor. The problem could be the 1/f-noise, but with a proper value of the capacitor, this is not an issue in the audio frequency range.

And I think capacitor leakage is not an issue for most film types.
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Old 26th June 2019, 09:17 AM   #239
knutn is offline knutn  Norway
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With R5=1000k, R4=10k, C2 =1u and R3 removed, we get the same LF rolloff, but the noise is higher in the audio band.
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Old 1st July 2019, 10:02 PM   #240
audiofan is offline audiofan  Canada
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If we look at an audio amplifier as a subtractor amplifier it is quite simple to show that the noise ( and residual signal ) at DC blocking capacitor will reach output the same way input signal does ... this is wy this capacitor is so important ... and also layout of the circuit is critical around '' lifted ground ''
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