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Old 9th June 2004, 07:35 PM   #1
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Default Calculating values for input coupling capacitor

I havenít found a discussion on this really. The few things I found while searching was that really anything will work, as long as it's not to small, and too large will take longer to become stable. Not really any math or reasoning behind that.
To my understanding an input coupling capacitor seems to look a bit like a first level crossover. But that canít be right since the resistance is also in the signal path, and not to ground. So could this be like a Ĺ level crossover? How do you calculate the minimum value of the capacitor, or calculate what the crossover point will be for a given capacitor?

I was just going to use the standard 4.7uf, but that bothered me. I need to know why that value is recommended or used.
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Old 9th June 2004, 07:41 PM   #2
Nisbeth is offline Nisbeth  Denmark
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An input coupling capacitor acts as a high pass filter together with the input impedance.
The formula for the calculation is C = 1/(2pi*f*Zin) where C is the capacitance, f is the corner frequency and Zin is the input impedance. You should aim for a corner frequency of 5 Hz or below to avoid any phase shift in the lowest bass region.


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Old 9th June 2004, 08:51 PM   #3
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So in the case of an lm3875, or 3886, is Zin just the value of the Rb resistor? Or is there resistance in the chip and it's load calculated into that figure? And is the C value in uF?
And I come up with values then that are in the .00X range for capacitors. Why are values as high as 4.7uF recommended?
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Old 10th June 2004, 10:13 PM   #4
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So after a few hours of playing with LTspice and excel. I've found that the calculation "C = 1/(2pi*f*Zin)" gives C in F, so multiply it by 10,000 to get uF.
Using for example Peter Daniels simplified gainclone (22K feedback, 220 input, 680 feedback to ground and 22K ground ref) an input cap of .0001 (1uf) gives a -3dB of 7.23hz. The "magic" 4.7uf gives 1.53hz.

I'm not sure how to calculate if there is no resistance between the capacitor and the op-amp (like shown on page 13 of AN-1192.pdf) Would 7.12hz be right for that, or is it 0.07hz?
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Old 10th June 2004, 10:29 PM   #5
Nisbeth is offline Nisbeth  Denmark
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In that figure, Zin is = Rin = 47.5 k and so f = ~7,1 Hz (meaning that using a 1uF cap here wouldn't hurt )



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Old 10th June 2004, 10:30 PM   #6
sreten is online now sreten  United Kingdom
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Farads x 1,000,000 (10^6) gives you uF's.

0.0001 = 100uF.

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Old 10th June 2004, 10:37 PM   #7
Nisbeth is offline Nisbeth  Denmark
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Sorry I forgot to mention that in my original post


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Old 10th June 2004, 10:44 PM   #8
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Ok, I think it got it.

Thanks guys..

Now to fix the amp I just put together.
My subamp seems to have a 71hz highpass filter.. and I don't think that's gonna work.
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Old 10th June 2004, 10:49 PM   #9
Nisbeth is offline Nisbeth  Denmark
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Naahh, that's probably a little on the high side


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Old 11th June 2004, 06:28 PM   #10
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Quote:
Farads x 1,000,000 (10^6) gives you uF's.
I believe that microfarads is 10^-6 farads, one millionth of a farad, not a million farads.
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