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coupling caps 3 approaches
coupling caps 3 approaches
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Old 23rd June 2019, 03:37 PM   #1
Ajcrock is offline Ajcrock
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Default coupling caps 3 approaches

This post is to present a 3 design alternatives to the coupling cap approach that I would like to share.

The traditional coupling cap is a single cap based on frequency response and DC offset. For high voltages this means physically large caps. I call this One Big Cap in the attached PDF.

A couple years ago I move to 2 smaller caps in parallel which increased speed, reduced cost and somewhat reduced the space needed. This is the two in parallel approach.

Lately I have been using WIMA SMD films. Small in size but also small values. So while this ticks off the space and speed boxes. There still can be some leakage. Stacking the caps help reduced the DC offset but did not fully get rid of it. Since the output was going to another device this would not do.

Then it occurred to me to use a cap resistor cap resistor series approach. I call this the two in series approach. The first cap has to be 400 volts which the .1uf SMD WIMA Film fits the bill. The second cap only needs to meet the amount of initial DC surge and leakage. So a nice small .15 or .22uf WIMA through hole of 63 or 100 volts fits the bill. This allows me to add color as needed, immediately cut off the DC surge / leakage, have fast charge and discharges, while maintaining a smaller footprint (5 by 12mm ish) at a lower cost.

True this means a -6dB pole at 10 hertz. But from an overall system approach this is not an issue as most speaker are 30 hertz.

If you are wanting to reduce cost and the size of your design the series coupling cap approach is something to consider.
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Old 23rd June 2019, 03:46 PM   #2
VictoriaGuy is offline VictoriaGuy  Canada
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coupling caps 3 approaches
I don't understand your ideas about speed and color.
Haven't single caps been used successfully at a wide range of frequencies (audio up beyond RF?) in the past?
Also, why didn't you change the cap values when using series connection to give the same overall capacitance?
(0.1 uF + 0.22 uF in series = 0.07 uF)
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Last edited by VictoriaGuy; 23rd June 2019 at 03:50 PM.
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Old 23rd June 2019, 06:30 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Very odd thoughts indeed.
A single capacitor of the correct value is adequate and always has been.
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Old 23rd June 2019, 06:32 PM   #4
Mark Tillotson is offline Mark Tillotson
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For a given dielectric the volume of a capacitor is closely proportional to CV^2, paralleling caps won't change the overall size needed (or leakage) for a given value and voltage. The price may be more or less depending on the manufacturer's pricing structure (which depends on volume of sales as well as amount of stuff - common E3 values tend to sell in greater numbers).

Paralleling typically reduces ESL and ESR, sometimes useful.

Caps in series don't change the overall size or leakage either, its still the same amount of stuff. This will increase ESR and ESL typically, and you have to worry about voltage distribution across the series chain - for DC-blocking this is particularly difficult as you don't want a balancing series resistor string increasing the leakage.
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Old 24th June 2019, 03:16 AM   #5
PRR is online now PRR  United States
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Quote:
Originally Posted by Ajcrock View Post
...a cap resistor cap resistor series approach...
RCA used that ploy in some industrial audio gear.
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Old 24th June 2019, 01:41 PM   #6
DF96 is offline DF96  England
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A coupling cap is not supposed to "add colour". It is supposed to pass AC and block DC. A single cap is almost always the right solution.

Paralleling caps may reduce ESR and ESL, but for an audio coupling cap this is of no consequence so talk of "speed" just demonstrates lack of understanding.
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Old 24th June 2019, 01:53 PM   #7
Ajcrock is offline Ajcrock
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My post was to present an alternative. We used to use mail for everything in the past and that worked well. We now use email and the internet. Times, ideas and approaches change. The circuitry following the coupling cap as well as the chassis size needs to be taken into account. For many tube circuits the old way of having some leakage and passing that on to a tube and an output transformer may be fine. And I am not saying that you cant continue with your 1 cap approach.

But take a headphone circuit. That 100mv of leakage may be passed into a solid state output stage and into the headphones. Now the 100mv of leakage and the initial startup surge is unacceptable. With the series approach the initial surge and DC offset is immediately shut down and there is no speaker pop.

Size is reduced by the usage of 2 smaller caps. A 400v .33uf WIMA is 26 by 11mm
MKP1J033305I00MSSD WIMA | Mouser

The series approach reduces the size to 1 10 by 7.6mm SMD
SMDIG03100VA00KR00 WIMA | Mouser

And 1 through hole that is 7 by 5.5mm
MKS2F032201H00KI00 WIMA | Mouser

Speed
Smaller caps charge and discharge more quickly than larger caps. Therefore, the transient response of a smaller cap is quicker. Which in turn affects your frequency response. This goes for both the parallel and series approach.

Color.
I like PCB caps. But they are no longer made and high voltage values are hard to find. With the series approach I get to sound coloration of a PCB cap without needing the high voltage rating as it is only dealing with the leakage from the first cap.

Overall capacitor value
What was described is not 2 electrolytic caps being combined to get a higher voltage rating. The series approach to blocking DC with 2 smaller caps results in 2 CR high pass filters and should be thought of as such. It is the effect on the low end response of the circuit that you should be concerned about and not overall capacitor value. Now if the next device in the signal chain is another tube then its characteristics need to be taken into account. At any rate the smaller the capacitance the better.
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Old 24th June 2019, 02:03 PM   #8
Ajcrock is offline Ajcrock
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As to color. All parts add color. Carbon resistors sound different then metal film. Polyester caps different than Polypropylene. By understanding how parts color the audio you can change the sound of your design.

As to speed the capacitance size greatly affects the the ability to respond. Take for example a switching power supply. The 100kHz frequency means that a 100uf cap is useless as a filter. But a 1000pf to .01uf cap is effective because is can respond quicker.

The same goes for a coupling cap. As the audio signal moves up and down the cap must also respond with charges and discharges. The smaller the cap the more quickly it will charge and discharge. But it will be less effective at blocking DC. This is why you don't see 10uf coupling caps which are great at blocking DC but are sluggish to respond.

Again I am just proposing an approach. You can try it or not.
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Old 24th June 2019, 04:56 PM   #9
merlin el mago is offline merlin el mago  Europe
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Ajrock, now will come the official discourse: if sounds different there is something bad in the design, that's the argue they uses to justify the unmesurable.

Last edited by merlin el mago; 24th June 2019 at 04:59 PM.
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Old 24th June 2019, 05:04 PM   #10
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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You can also put 1000 caps of 330pF in parallel with same final value. :-)
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