I haven’t 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 can’t be right since the resistance is also in the signal path, and not to ground. So could this be like a ½ 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.
To my understanding an input coupling capacitor seems to look a bit like a first level crossover. But that can’t be right since the resistance is also in the signal path, and not to ground. So could this be like a ½ 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.
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.
/U.
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.
/U.
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?
And I come up with values then that are in the .00X range for capacitors. Why are values as high as 4.7uF recommended?
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?
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?
Read this article, closer to the end, section called
"5.8 DISTORTION 8: Capacitor distortion".
Mind you the whole article is worth reading.
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm
/Greg
"5.8 DISTORTION 8: Capacitor distortion".
Mind you the whole article is worth reading.
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm
/Greg
Thought I would share a little excel sheet I made up to simplify the math.
And thanks for the good link greg. I never thought using electrolytics in the signal path was very good, but I do see it happen a lot, and i've done it.
It does seem like on signal lines it's not that big of a deal since generally it's pretty low voltage, I measured mine and after my gain stage I have 1.1V (from my 0.5v source) Seems like even if I had used an electrolytic the effects would be minimal.
But something to certainly keep in mind for speaker crossovers or preamps where the signal might be a lot higher.
And thanks for the good link greg. I never thought using electrolytics in the signal path was very good, but I do see it happen a lot, and i've done it.
It does seem like on signal lines it's not that big of a deal since generally it's pretty low voltage, I measured mine and after my gain stage I have 1.1V (from my 0.5v source) Seems like even if I had used an electrolytic the effects would be minimal.
But something to certainly keep in mind for speaker crossovers or preamps where the signal might be a lot higher.
officeboy said:
I guess a good thing to do in case of using electrolytic caps is to calculate the cap instead for F3dB=15 to 20 Hz, to use 1.5 to 2 Hz. This way the voltage across the cap will be much smaller at the min frequency you care about (20 Hz) thus less potential for distortions.
If you look at the link above you'll see the caps are calculated for F3dB at around 1.5 Hz
/Greg
Example 01 Clarification
Officeboy,
Thanks for posting this question. I have asked this question in past in topics of amps I am interested with much the same response as you have had. I happened across your question as usual while searching for something completely different
Nisbeth,
Thanks so much for posting the formula and suggested target corner frequency. I am to assume "corner" means -3 db point?
I have a few example input circuits I will post to clarify what resistor is Zin as well as to clarify what the other elements of the input circuit are, purpose and any related formulas and/or variations to the formula Nisbeth posted that need to be considered.
The attached example input from an amp is the most basic input configuration I have found.
The first question is is R1 or R2 Zin for purpose of calculation Nisbeth noted?
The second question is what impact, if any, does the other resistor have on the formula Nisbeth provided?
The third question is what is the purpose/function of the other resistor?
Regards,
John L. Males
Willowdale, Ontario
Canada
29 January 2006 12:43
29 January 2006 12:47 Corrected some typos
Nisbeth said: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.
/U.
Officeboy,
Thanks for posting this question. I have asked this question in past in topics of amps I am interested with much the same response as you have had. I happened across your question as usual while searching for something completely different
Nisbeth,
Thanks so much for posting the formula and suggested target corner frequency. I am to assume "corner" means -3 db point?
I have a few example input circuits I will post to clarify what resistor is Zin as well as to clarify what the other elements of the input circuit are, purpose and any related formulas and/or variations to the formula Nisbeth posted that need to be considered.
The attached example input from an amp is the most basic input configuration I have found.
The first question is is R1 or R2 Zin for purpose of calculation Nisbeth noted?
The second question is what impact, if any, does the other resistor have on the formula Nisbeth provided?
The third question is what is the purpose/function of the other resistor?
Regards,
John L. Males
Willowdale, Ontario
Canada
29 January 2006 12:43
29 January 2006 12:47 Corrected some typos
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