Choosing the correct coupling capacitor

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warpie

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What is the criteria for choosing the correct coupling cap in a circuit.

I realise that the value depends on the impedance of the circuit and on the required Fc, which should be well below 20Hz (is around Fc=5Hz low enough)?  However, I've seen circuits where (let's say) a 10uF cap will provide a cutoff @ 5Hz but instead a much larger cap is being used (22uF, 33uF or even higher).

So, what is the correct procedure for choosing the right cap?

I somehow feel that ESR and self resonance play a role here but could someone explain it to me?

Many thanks!
 

abbey road d enfer

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warpie said:
What is the criteria for choosing the correct coupling cap in a circuit.

I realise that the value depends on the impedance of the circuit and on the required Fc, which should be well below 20Hz (is around Fc=5Hz low enough)?  However, I've seen circuits where (let's say) a 10uF cap will provide a cutoff @ 5Hz but instead a much larger cap is being used (22uF, 33uF or even higher).
In jurassic times, engineers aplied the rule of 10, i.e. they calculated the value for the desired -3dB frequency and  multiplied by 10, because when you cascade 10 stages, the resulting cut-off is about right.
In addition, since electrolytic capacitors tended to degrade considerably, they took a margin.
Today, the quality of electrolytic caps has improved considerably, but we are also more concerned with other aspects than basic frequency response, particularly distortion.
In order to minimize distortion, the voltage across the caps must be kept very low. I apply the rule of 100, so I calculate the value for 0.02Hz, which results in 80uF for 10kohm load, that I round to 100uF.
For a 600 ohm load, it results in 1500uF. Hower, due to the bulk of such a cap, I prefer using a THAT1646, that uses small bipolar 'lytics.
This was almost impossible in the 70's but today, the size of electrolytics has decreased enormously.
Film caps' distortion is at least an order of magnitude lower, so you can safely apply the rule of 10.

I somehow feel that ESR and self resonance play a role here
Not so much. ESR just adds a small loss and self resonance appears well beyond audio frequencies.
 

warpie

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That's a nice and easy explanation, thanks.

So, if my math is right for 2Kohm impedance a 47uF would be about right? If so, is there any benefit in going a bit higher like 68uF instead?
 

abbey road d enfer

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warpie said:
That's a nice and easy explanation, thanks.

So, if my math is right for 2Kohm impedance a 47uF would be about right? If so, is there any benefit in going a bit higher like 68uF instead?
If you followed my advice you would use a 400uF electrolytic or a 40uF film cap. So 47uF for a film cap is correct. Beware it's huge.
 

warpie

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abbey road d enfer said:
If you followed my advice you would use a 400uF electrolytic or a 40uF film cap. So 47uF for a film cap is correct. Beware it's huge.

Yes sorry, I meant 470uF not 47uF.

BTW,
I apply the rule of 100, so I calculate the value for 0.02Hz, which results in 80uF for 10kohm load, that I round to 100uF

I believe you mean 0.2Hz?
 

JohnRoberts

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More is better (right?).... Some of those capacitor values sound excessive but 2k seems a little low for nominal line level loading. Some op amps will barely support 2k feedback Rs.

Note Abbey's 10x rule of thumb assumes 10 blocking caps in series.

My personal design philosophy was to set crude electrolytic cap DC blocking HP poles well below the path's dominant HPF. Most electrolytic capacitors non-ideal behaviors are related to changing terminal voltage (like from very low frequency signals). For high fidelity paths I would use a film capacitor in an active HPF, and set passive DC blocking poles at least an octave lower, more often a decade lower to keep them well behaved.

JR
 

warpie

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squarewave said:
Use an online calculator like this one:

  http://sim.okawa-denshi.jp/en/Fkeisan.htm

It's simply C= 1/ (2pi*Fc*R) isn't it?

I just wasn't sure if the Fc is the only criteria for selecting the capacitance value. and also what would be an appropriate Fc. For example, here they suggest Fc=2Hz

https://www.v-cap.com/coupling-capacitor-calculator.php
 

abbey road d enfer

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warpie said:
For example, here they suggest Fc=2Hz

https://www.v-cap.com/coupling-capacitor-calculator.php
They are only considering the frequency response aspects there. They totally neglect the effects of voltage-related distortion across capacitors. Typical for audiophools.
 

warpie

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abbey road d enfer said:
They are only considering the frequency response aspects there. They totally neglect the effects of voltage-related distortion across capacitors. Typical for audiophools.

oK fair enough  :D

Well, they say that bigger caps cause more distortion.

The higher the capacitor value, the more material in the signal path. The more material in the signal path, well, you know where this is going... the less transparent the signal becomes. When it comes to output coupling caps, less is more, as long as you've achieved a reasonable -3db point.
 

john12ax7

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10x is an engineering rule of thumb in order to ensure your passband of interest is undisturbed. You could simulate or calculate the amplitude and phase response yourself and see.

If you want good response 20-20k you design for 2-200k, it keeps the funny business outside the bandwidth you care about.

If your components are not well behaved,  like for example an electrolytic cap, then you need to extend things even further.
 

Bo Deadly

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abbey road d enfer said:
They are only considering the frequency response aspects there. They totally neglect the effects of voltage-related distortion across capacitors. Typical for audiophools.
Really? We've talked about this. The distortion of a modern vaguely good electrolytic is too small to be audible. There was a recent post about this referencing some paper that showed distortion at some freakishly high level at high frequency was elevated but still too low to be audible.
 

JohnRoberts

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warpie said:
oK fair enough  :D

Well, they say that bigger caps cause more distortion.

The higher the capacitor value, the more material in the signal path. The more material in the signal path, well, you know where this is going... the less transparent the signal becomes. When it comes to output coupling caps, less is more, as long as you've achieved a reasonable -3db point.
Huh...Sounds like mumbo jumbo.

JR
 

abbey road d enfer

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oK fair enough :D

Well, they say that bigger caps cause more distortion.
Who are "they" ?
The higher the capacitor value, the more material in the signal path. The more material in the signal path, well, you know where this is going... the less transparent the signal becomes. When it comes to output coupling caps, less is more, as long as you've achieved a reasonable -3db point.
It's such a one-dimensioned and simplistic claim I won't even start to discuss it. The person who wrote that needs a complete education.
 

warpie

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Who are "they" ?

It's such a one-dimensioned and simplistic claim I won't even start to discuss it. The person who wrote that needs a complete education.

Well, "they" :)

I agree. The "more material" thing is not very explanatory/ convincing.
 

Bo Deadly

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Is it really? I don't remember it was the conclusion of this discussion. If it was, I have forgotten, and would like to have hard proof of it.

This is the article:


linked to from this post.

Looking at it again I see the paper is strictly about coupling and not using an electrolytic small enough to cut lows. Certainly the voltage across it would be a lot higher when used as a filter. But still, I don't see that making a massive difference in the results. The article shows triple-zero THD through TEN (10) electrolytics at +20 dBu util it gets to 10kHz where it bumps up a bot to 0.002. If the THD becomes 0.002% at the corner, no one is going to hear that and below the corner the signal is attenuated so no one is going to hear that either.

This is something that I would really like to study one day. Specifically, take a 10uF film and 10uF electrolytic and maybe a 10uF bipoloar electrolytic and null test them against each other. If the added distortion is really negligible as I claim, then it should really not be taboo to use electrolytics in filters.
 

JohnRoberts

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Using a capacitor for an inband audio filter makes all the difference in the world. The "voltage coefficient" term quantifies an obvious nonlinearity (capacitance changes with terminal voltage).

It should not be difficult to find good quality dielectric capacitors for most filters but passive loudspeaker crossovers require higher values than cheap film caps. Back last century I approved an engineering change order for switching to a film (polypropylene) cap inside a passive speaker crossover. It was a few cents cost increase so the engineer's actual boss refused to sign the ECN because of the cost increase. I signed it because it was the right thing to do.
 
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