Author Topic: EEVBlog #1116 - The Capacitance Multiplier  (Read 9536 times)

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Online EEVblog

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EEVBlog #1116 - The Capacitance Multiplier
« on: August 28, 2018, 09:07:28 am »
Circuit building block time. The capacitance multiplier and how it gives almost negligible power supply ripple compared to a voltage regulator.
Whiteboard theory and then some bench demonstrations and experiments. Plus a twist at the end that proves that the "Capacitance multiplier" is perhaps one of the most mis-named circuits of all time.

 
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Offline chickenHeadKnob

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #1 on: August 28, 2018, 01:55:22 pm »
Good to see you doing this type of content again. I know you have expressed doubts about  whether the viewer-ship justifies the effort, I merely wanted to offer encouragement that's all.
 
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Offline boB

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #2 on: August 28, 2018, 04:06:34 pm »
TAPCO, a microphone mixer company in the 1970s that I used to work at used this same C multiplier circuit that drove the microphone preamp.

Audio preamps are one circuit that benefits highly from this type of thing.

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Offline sibeen

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #3 on: August 28, 2018, 04:06:42 pm »
Yeah, another shout out for Fundamentals Friday. Thoroughly enjoyed that one.
 

Offline wilfred

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #4 on: August 28, 2018, 08:07:32 pm »
Good to see you doing this type of content again. I know you have expressed doubts about  whether the viewer-ship justifies the effort, I merely wanted to offer encouragement that's all.
I'll take quality content any day of the week. It justifies itself for that reason alone.
 

Offline fmiz

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #5 on: August 28, 2018, 09:54:16 pm »
This video gave me an idea...
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?
Adding an inductor in series with R to get a 2nd order filter for the base current... bad idea?
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #6 on: August 28, 2018, 10:04:21 pm »
DAMN ! Now I have to try it on 5A+ linear P/Supplies. I've resorted to experimenting with switching in/out series caps / inductors with a CPU etc
I'm going to be pi$$ed off if it works !! :-)
Hello <tap> <tap> .. is this thing on?
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #7 on: August 28, 2018, 10:08:37 pm »
Very interesting Dave, thank you.
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #8 on: August 28, 2018, 10:17:44 pm »
Dr. Phil Hobbs (of ElectroOptical Innovations) loves them for cleaning up dirty supplies (in the frequency range that takes the most space to filter otherwise -- say kHz to low MHz), say for low noise optical front-ends.  Particularly making multi-stage ones, where the base filter RC is cascaded, so each subsequent transistor only incurs a minimum Vce(sat) drop, rather than a full Vbe (if you were to chain stages naively, supplying the next base divider from the emitter of the proceeding stage).

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Offline David Hess

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #9 on: August 28, 2018, 11:25:33 pm »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #10 on: August 29, 2018, 12:20:09 am »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

Sort of.  Check the PSRR of the regulator in question.  LDOs are notoriously poor, while HDOs like the old fashioned LM7805 are okay (only "okay", I wouldn't call them "great").

All(?) active regulators have the problem that PSRR is proportional to loop gain, so PSRR is always dropping at high frequencies.  The C-mult. avoids this, being limited instead by Early effect and strays.

Related:

Some LDOs and references (usually low power kinds) have a bypass pin on them, which is the internal gain node of the voltage reference.  You attach a meaty bypass cap, to give much better noise rejection than would be possible with any on-chip solution.

But, because it's the reference, not the output driver, it's not technically a C-mult. circuit, and PSRR won't improve beyond what the output stage and control loop are capable of.

Tim
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Offline David Hess

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #11 on: August 29, 2018, 12:51:43 am »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

Sort of.  Check the PSRR of the regulator in question.  LDOs are notoriously poor, while HDOs like the old fashioned LM7805 are okay (only "okay", I wouldn't call them "great").

All(?) active regulators have the problem that PSRR is proportional to loop gain, so PSRR is always dropping at high frequencies.  The C-mult. avoids this, being limited instead by Early effect and strays.

Output impedance of the error amplifier driving the pass element increases with frequency for the reason you identify but this does not have to be the case.  A parallel capacitor could be added to ground and appropriate frequency compensation adjustments made if necessary (1) producing a "regulated" capacitance multiplier.  In the primitive form, this could be the old ubiquitous zener series regulator in the form of a capacitance multiplier with one added zener diode.

The reason this is rarely done (2) is that it can only equal the performance of a capacitance multiplier and if better high frequency rejection is required, there are better ways like adding LCR input decoupling to remove high frequency ripple so each circuit element is applied where it works best.  Now you have a regulator which may approach the performance of a capacitance multiplier at high frequencies but does not need to.

More interesting I think are the capacitance multipliers which used a shunt element instead of a series element.  The audio guys like these but I am unconvinced.

(1) If the capacitance is large enough, then this becomes dominant pole compensation and the circuit becomes a capacitance multiplier with low frequency feedback to produce better load regulation.

(2) Some rail splitters operate this way.  If you squint a little at their schematic, it shows a capacitance multiplier with a class-ab output stage driving an output capacitor.
 

Offline chriswebb

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #12 on: August 29, 2018, 01:47:27 am »
Thank you so much for this video, Dave! Always happy to see more fundamentals videos, since I am still so new to all of this. Felt good being able to completely understand one for once!
Always learning. The greatest part of life is that there will always be more to learn.
 

Offline luismh

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #13 on: August 29, 2018, 04:04:16 am »
Thank you Dave!
I was just dealing with this noisy power supply. Impossible to use a bigger capacitor due to the lack of space.
I will try this immediately!
Thanks!
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #14 on: August 29, 2018, 04:21:25 am »
Output impedance of the error amplifier driving the pass element increases with frequency for the reason you identify but this does not have to be the case.  A parallel capacitor could be added to ground and appropriate frequency compensation adjustments made if necessary (1) producing a "regulated" capacitance multiplier.  In the primitive form, this could be the old ubiquitous zener series regulator in the form of a capacitance multiplier with one added zener diode.

Yup.  Of course, as history has shown, designers are loathe to touch anything that needs extra capacitors, let alone capacitors that have confusing, complicated requirements, like... "compensation".

I would say the other reason this is rare, is more because you need a big stinking cap to do it, in semiconductor terms.  So you'd be asking for another pin (ain't gonna happen on a TO-220 device, say), plus inviting that opportunity for the user to get it wrong.  But yeah, not having all that much benefit is another drawback.

On that note, there are a few LDOs out there, targeted for RF applications, with impressive PSRR up to and beyond 1MHz (where LC filtering is quite economical to take over at).  But they aren't cheap.


Quote
More interesting I think are the capacitance multipliers which used a shunt element instead of a series element.  The audio guys like these but I am unconvinced.

Mmmh, shunt is fundamentally limited by the gain of the device.  Say you do a BJT shunt amp, collector to supply, emitter to ground, with some base bias, and a coupling cap from supply to base.  This has exactly the impedance 1/Gm.  So, for 10mA bias, say, only 2.5 ohms.  A small electrolytic is better, and by a MHz say, a modest ceramic cap more than dominates.

Whereas a pass device is limited by Early effect and Ccb vs. Cce, which can probably give a much higher ratio.

I suppose you could combine both, so that the capacitor being multiplied, is multiplied twice, in a sense.  You could supply the whole thing from a gyrator or something, so that the source impedance is higher than an equivalent resistor, giving more stopband width and depth for the same bias current.  Doing this for just the base RC filter network, you wouldn't be improving the main pass transistor at all, but you could improve its filtering with respect to bias current, economizing it a little.

Tim
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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #15 on: August 29, 2018, 06:05:14 am »
Dave,  Thanks for the great video on the capacitance multiplier.  I was not aware of this circuit before.  After watching the video, I decided to try and simulate with Qucs.  It seems to work very well in simulation.  Looking forward to trying it in a real design. 

[edit]  removed link to simulation due to suspect component models
« Last Edit: September 02, 2018, 10:47:31 am by radioactive »
 

Offline exit_failure

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #16 on: August 29, 2018, 06:29:08 am »
First of all, thank you a thousand times for that video. This is something that I hadn't seen before but actually very nicely solves a problem I have with my current project.

Not being a native speaker of English, one thing I would appreciate very much is if you would wrote down some of the more unusual or not as easy to understand terminology. For instance, it took me a bit of googling until I found out that the one configuration you mentioned was spelled "Sziklai pair".

Anyway, thanks a bunch again.
 
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Offline AviZiv

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #17 on: August 29, 2018, 12:26:58 pm »
Hi Dave - I was so excited to see this video! About a year ago one phono channel of my NAD stereo preamp stopped working. I found the problem to be a dead transistor on the power supply path and could not understand what it was doing there. I even tested bypassing the transistor and observed that the preamp still worked. NOW I UNDERSTAND :) I'm linking to a partial scan of the schematics and you can see the exact configuration that you showed on the 26V supply going to the MM (moving magnet) module. It always bugged me that I made a repair without understanding it completely. Thank you!

https://flic.kr/p/2axJ7ka

Avi
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #18 on: August 29, 2018, 01:13:39 pm »
Not being a native speaker of English, one thing I would appreciate very much is if you would wrote down some of the more unusual or not as easy to understand terminology. For instance, it took me a bit of googling until I found out that the one configuration you mentioned was spelled "Sziklai pair".

Yep, noted, I'll write down such thing in the future.
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #19 on: August 29, 2018, 01:17:39 pm »
DAMN ! Now I have to try it on 5A+ linear P/Supplies. I've resorted to experimenting with switching in/out series caps / inductors with a CPU etc
I'm going to be pi$$ed off if it works !! :-)

Be sure to report back!
 

Offline ali_asadzadeh

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #20 on: August 29, 2018, 03:57:43 pm »
Thanks dave for sharing ;)

I would suspect that what happens if we just put the cap Multiplier circuit between the R1/R2 that would set the regulators output voltage, does this solve the regulation problem too?
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Offline Dubbie

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #22 on: August 29, 2018, 08:07:37 pm »
I really enjoyed this video a lot as well. I hadn’t come across this concept before, and it’s good to hear a nice rundown of the technique.

These fundamental themes are by far my favourite of your videos. Please keep them coming! There are probably hundreds of nifty tricks hidden in application notes or in your brain (behind the boxes of couch feet) that you could turn into episodes.

Good job.
 

Offline AviZiv

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #23 on: August 29, 2018, 10:51:05 pm »
https://flic.kr/p/2axJ7ka
Whats up with C422?

If you are asking why does C422 not go to ground, but rather to the output of the transistor, my own little research suggests that this is a bit more than a simple RC filter. It looks to me like the topology of a Sellen-Key filter like this: https://en.wikipedia.org/wiki/Sallen–Key_topology

Do I have it right? I can't explain exactly how it works though. Need to study this.

It was interesting to note that the board designers placed this filter right up against the module that used that power - probably to clean up the power line as close to the target as possible.

Avi
« Last Edit: August 29, 2018, 10:53:48 pm by AviZiv »
 
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Offline P_Doped

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #24 on: August 30, 2018, 01:56:55 am »
I think a better name for this circuit topology would be resistance multiplier instead of capacitance multiplier.

The reason is that the presence of the current gain element allows the use of a resistor in the R-C filter larger by the current gain and still get the same drop across the R-C network.

Dave finally got to this aspect towards the end when he pointed out the R-C network itself isn't doing anything magical, the f_-3dB is still what you expect by looking at the R-C values, but the current going through the R is reduced by the current gain.

In a similar vein, this brings to mind the approach of using a resistance divider in series with another resistor (call it a compound resistor) as the feedback resistor in an op amp gain circuit.  Dave could talk about that, pros and cons.
 

Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #25 on: August 30, 2018, 02:16:13 am »
A Sziklai pair doesn't work here at all :rant:
You don't want ripple at the emitter- that's the opposite of how a capacitance multiplier works.

edit: added url
« Last Edit: August 30, 2018, 04:03:30 am by floobydust »
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #26 on: August 30, 2018, 02:52:03 am »
If you are asking why does C422 not go to ground, but rather to the output of the transistor, my own little research suggests that this is a bit more than a simple RC filter. It looks to me like the topology of a Sellen-Key filter like this: https://en.wikipedia.org/wiki/Sallen–Key_topology

Do I have it right? I can't explain exactly how it works though. Need to study this.

Quite right, it doesn't have the unity gain of an op-amp so the stop-band attenuation won't be perfect, but the transition band can be peaked for whatever filter characteristic you need, and the asymptotic slope can be -40dB/dec (until leakage takes over).

Note the feed-forward path through the first resistor and capacitor, so that at frequencies where the transistor gain is dropping off, this path dominates, limiting stop-band attenuation.  Admittedly, this is still a higher roll-off than for a typical op-amp, so it might not be too bad.

Here's an example I made, with vacuum tubes for audio cred:



The 6V6 model is actually pretty close (when the model's bugs have been avoided..) to what was actually used, a 5702 submini pentode.  The simulated response:



You can see it's much sharper and tighter than anything you can do with resistors and capacitors alone, and smaller than most things you'd do with inductors (I know, you can get pretty small ~1H chokes, though the Q is pretty bad).  Plus you'd still need the buffer to drive an LC filter, so might as well do it in one. :)

I don't have a picture of the actual filter, but it went on this radio, just after the detector.  The construction is similar, just imagine it with another submini and more capacitors and resistors around. :P

Also, incidentally, I finished up that radio by powering it with two SMPSs, in a hybrid configuration.  One AC to 12V 2A supply (Meanwell) just to get things going, then a discrete 12 to 6.3 and 100V supply (a slight variant of this) for the tubes.  I added shielding around, and an extra stage of LC filtering outside the discrete module, and that was all that was needed to keep the thing dead quiet on the 20m (~14MHz) band.

Tim
« Last Edit: August 30, 2018, 02:59:09 am by T3sl4co1l »
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Offline Cerebus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #27 on: August 30, 2018, 03:18:05 am »
Not being a native speaker of English, one thing I would appreciate very much is if you would wrote down some of the more unusual or not as easy to understand terminology. For instance, it took me a bit of googling until I found out that the one configuration you mentioned was spelled "Sziklai pair".

Yep, noted, I'll write down such thing in the future.

Having watched, with occasional amusement, your regular mailbag frustrations and wrestling with mittel-europa place names may I offer a little pronunciation help here. Sziklai is a Hungarian name and like all Hungarian words with 'sz' or 'cz' phonemes is unpronounceable by anybody who isn't a native Hungarian. The nearest I've managed to get transliterates as 'shik-laii' with the 'sh' sound being something between the English 'sh' and 'z' sounds - shape your mouth to say 'z' but instead say 'sh'. I got this pronunciation years ago from a Hungarian descended German colleague.
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Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #28 on: August 30, 2018, 04:08:53 am »
Thanks, here I thought it was a Japanese name  :-[  Let's get our Sidney Darlington and George Clifford Sziklai right.

But Sziklai pair still won't work in a capacitance multiplier.
 

Offline ggchab

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #29 on: August 30, 2018, 09:03:45 pm »
Thank you for this very interesting video  :-+
 

Offline IanMacdonald

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #30 on: August 30, 2018, 09:10:14 pm »
But Sziklai pair still won't work in a capacitance multiplier.

BS. PNP series pass, NPN driver.

Though, I wonder if we could achieve time travel this way with less flux.

Incidentally, the way this circuit works is that the cap will try to charge to Vin, and thanks to the light loading of the emitter follower it will mostly succeed. However, when Vout gets too close to Vin the transistor becomes unable to draw collector current because the difference is too small, and the transistor then effectively becomes a diode between the cap and Vout. In which case the cap voltage is rapidly dragged down until the collector can draw current again. Hence Vout tends to stabilise at the lowest instantaneous ripple voltage.
« Last Edit: August 30, 2018, 09:29:06 pm by IanMacdonald »
 

Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #31 on: August 31, 2018, 04:13:06 am »
With all due respect, you are mistaken. Unless you have a sketch to clear up misunderstanding on how the NPN driver works at multiplying capacitance.

Once you have high ripple on either transistor's emitter, you are modulating conduction at the E-B junction.
This breaks the basic principle of the an emitter follower here in the capacitance multiplier that relies on pure DC at the base and the resulting near constant collector current (in both transistors) to filter ripple.

People arbitrarily change from Darlington to Sziklai, NPN/PNP etc. and let's not be sloppy.
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #32 on: August 31, 2018, 04:27:24 am »
With all due respect, you are mistaken. Unless you have a sketch to clear up misunderstanding on how the NPN driver works at multiplying capacitance.

Once you have high ripple on either transistor's emitter, you are modulating conduction at the E-B junction.
This breaks the basic principle of the an emitter follower here in the capacitance multiplier that relies on pure DC at the base and the resulting near constant collector current (in both transistors) to filter ripple.

People arbitrarily change from Darlington to Sziklai, NPN/PNP etc. and let's not be sloppy.

I'd be interested to see a simulation illustrating this.

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Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #33 on: August 31, 2018, 06:21:35 am »
With Szikilai the pass transistor appears to not be contributing, its (emitter) in-phase with the ripple so the result is slightly worse? Only the driver is doing the work.

For discussion I did a basic run in LtSpice and also in Paul Falstad's Circuit Simulator of the capacitance multiplier using a Darlington and Szikilai.

It's kind of a nothing burger circuit because it wastes a fair bit of voltage as heat, and adding a zener turns it into a voltage regulator anyhow.
I've rarely encountered capacitance multipliers. Only seen in esoteric audio gear.
 
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Offline JS

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #34 on: August 31, 2018, 04:43:03 pm »
Thanx for coming back to fundamental, bummer it wasn't a friday :D

Someone please run a SIM on the shizaki pair and end our missery, that and a darlington to compare. My money is lower ripple for the darlington, lower drop out for the other.

JS

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Offline JPortici

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #35 on: August 31, 2018, 06:19:50 pm »
I just saw this that seems kinda related :)

MIC94305
500mA Switch with Ripple Blocker Technology
http://ww1.microchip.com/downloads/en/DeviceDoc/MIC94305-500mA-Switch-with-Ripple-Blocker-Technology-DS20006029A.pdf
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #36 on: August 31, 2018, 06:43:30 pm »
I think a better name for this circuit topology would be resistance multiplier instead of capacitance multiplier.

I tend to agree, but capacitance multiplier is the industry term like it or not.
 

Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #37 on: August 31, 2018, 10:43:09 pm »
I ran a couple more Qucs simulations comparing Darlington vs. Sziklai.  Sziklai ended up being lower voltage drop as well as slightly better ripple rejection.
 
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Offline Fungus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #38 on: August 31, 2018, 11:50:50 pm »
I just noticed that "InfiniVision" is spelled InfiniiVision.  :popcorn:

 

Offline JS

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #39 on: September 01, 2018, 02:32:59 am »
@radioactive: thanks, I guessed one and missed the other...

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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #40 on: September 01, 2018, 02:48:17 am »
I ran it again with just a single frequency (vs swept frequencies) and just a pure sine wave of 1KHz for ripple noise.  The output for both look nearly identical except for the difference in voltage drop.  These plots might be easier to look at.  Let me know if you want me to run with a different configuration.
 
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Offline JS

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #41 on: September 01, 2018, 03:21:57 am »
Looks like Sziklai wins this time. Same rejection but with lower dropout. One thing that might worth checking is high freq to see how it will work in a SMPS.

Excelent you are running this! Thanks so much, shacky days here or I would running them myself...

JS

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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #42 on: September 01, 2018, 03:54:52 am »
Good suggestion.  I changed to 10khz sine and got some interesting results.  I was going to try 280khz, but the transient sim was going to take forever.  Looks like sziklai is the clear winner here ( at least in a simulation with these component models ).
 

Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #43 on: September 02, 2018, 01:17:41 am »
I forgot to post a reference to the data sheet for the transistors used in the simulation for comparing Darlington to Sziklai configurations.
2N3055 /2955 complementary power transistors:   https://www.mouser.com/ds/2/68/2n3055-1108522.pdf

 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #44 on: September 02, 2018, 04:33:58 am »
Datasheet not very useful -- you're assuming the datasheet is even descriptive enough (it's not), or that the models are anything like an average part (who knows). :-//

What's the library entry look like for those parts, anyway?

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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #45 on: September 02, 2018, 05:50:47 am »
I'm not assuming anything.  I was just providing more info about the simulation and the fact that they are supposed to be complementary.   I'll attach the parameters from the Qucs models that were used for reference as well.  I agree that it is just a simulation.  Let me know if anyone is interested in trying a different configuration.
 

Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #46 on: September 02, 2018, 06:14:21 am »
I tried your simulation in LTSpice, much different than what I'd drummed up.
2N3055 Bf=73 with 1.5A load and 10kHz input ripple compared to my 2N3904 Bf=300 with 42mA load and 120Hz input ripple.
I could not get the same results in favour of the Sziklai for ripple reduction.

In LTSpice, I noticed the 2N3055 Darlington has a higher voltage drop, I'm getting 8.5V out, compared to your 10.4955V so the base current is way different. 0.85mA across the 5.4k
 

Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #47 on: September 02, 2018, 07:16:16 am »
I downloaded your LTSpice simulation and noticed a few differences from the Qucs simulation
1khz vs 10khz
1uF cap vs 4.7uF cap
The PNP model in LTSpice is not 2N2955, but D45H11
Bf was significantly different.  70 vs 360.

I adjusted the Bf/Rf in Qucs transistor models to match the LTSpice as well as the Cap value and input sine frequency.  I still get same results in Qucs (same difference in output ripple) , but with an expected higher voltage drop.  The voltage drops are still significantly different between LTSpice and Qucs.   I may dig into it some more later, but the models definitely don't agree even on the Darlington where they are both supposedly 2N3055 models.  After looking at some other datasheets for these transistors, I suspect that the Qucs models are incorrect (at least the forward beta parameter).  I apologize for not doing more to verify the models before posting on here. 
 

Offline David Hess

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #48 on: September 02, 2018, 09:24:34 am »
The difference in performance between the Darlington and Sziklai pair comes up in audio power amplifier design so there might be something to learn there.  Local feedback designs with a third transistor are much better than either.

Another thing to learn from audio power amplifier design is Ft and hfe droop at high currents which are going to be present in a high current capacitance multiplier.  There will be considerable advantage to using RETs (ring emitter transistors) or equivalent like the D44/D45 series instead of common power transistors.

I myself have only used capacitance multipliers like this at 10s to 100s of milliamps for sort of super power supply decoupling and surge protection.

 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #49 on: September 02, 2018, 10:17:29 am »
Wow, are all the parameters identical for those models, except for type (NPN/PNP)? Whose idea was that? :-DD

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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #50 on: September 02, 2018, 10:44:56 am »
@T3sl4co1l,

I just did a diff on all the parameters (after removing NPN/PNP) and yes they are!  I normally don't use Qucs for these type of simulations (utilizing 3rd party contributed component models.  Guilty parties provided e-mail address).  After this embarrassment,  I probably won't again without checking much closer.  It is great software for many things, especially s-parameter simulations.  Again, sorry for wasting everyones time in this thread.  No good deed (attempted) goes unpunished... why is that almost always true?
 

Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #51 on: September 02, 2018, 10:45:04 am »
My intuition squawked because of the difference in the two configurations, reasoning the PNP transistor does not contribute with pulsing DC on its emitter, so the Sziklai is using beta of only the NPN for multiplying capacitance.

I went down to 1kHz to see if speed is an issue knowing that both transistor configurations are relatively slow. Very rare to see a Darlington used as a SMPS switch for example.

Perhaps small signal AC analysis is better? If we assume the driver's base is an AC ground, it's acting as a low-pass filter and attenuation would show differences.

No need to apologize, I don't think electronics is about being correct or perfect.
For me the forum is about sharing and learning, I don't care if I'm wrong aside from the troll attack.

Similar transistor models would help in the comparison, I was just lazy and using the ST 2N3055 that comes in the library.

Rod Elliot from Elliot Sound compares the two configurations in audio and switching.
 
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Online T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #52 on: September 02, 2018, 10:56:09 am »
@T3sl4co1l,

I just did a diff on all the parameters (after removing NPN/PNP) and yes they are!  I normally don't use Qucs for these type of simulations (utilizing 3rd party contributed component models.  Guilty parties provided e-mail address).  After this embarrassment,  I probably won't again without checking much closer.  It is great software for many things, especially s-parameter simulations.  Again, sorry for wasting everyones time in this thread.  No good deed (attempted) goes unpunished... why is that almost always true?

No worries man, I appreciate taking the time to try. :)

Me, I'm just incessantly paranoid about everything I have to work with: models, simulator quirks, datasheets, tolerances...

Tim
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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #53 on: September 02, 2018, 02:03:31 pm »
Quote from: radioactive
  ... Any requests from anyone for specific transistors to use? ....
Well, since you're taking orders :-) When I ever get a chance, I want to test these Power FETS AUIRFS-8409-7P at 10A (or something similar).
I can send you a couple (+) if you're interested. I use them in LT4320 bridges, up to 30A + and would love a simpler option for minimizing ripple.
Hello <tap> <tap> .. is this thing on?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #54 on: September 04, 2018, 03:31:34 am »
Hi Dave,

I do aknowledge that the cap multiplier is a common and useful circuit, but to avoid frustration it is OK to show its limitations, too:

- This circuit is strictly for low frequency ripple removal only. Supression of fast switching spikes will be poor
- For the sake of efficiency, the transistors operate at rather low Vce voltages where there collector capacitance is very large.
  This capacitance feeds the unfiltered input right to the transistor base.
- As a safety precaution, the input of the darlington base should have a current limiting resistor attached. When your output is shorted
  and you have no such resistor, the filter cap will discharge thru the base of the first darlington transistor without a current limit,
  possibly killing it.
- The input resistor is a bit tricky to choose, because a proper value is dependent on transistor beta(s). If its too small, transistors
  almost go into saturation becoming excessively slow, if its too large the voltage drop increases too much. Transistor betas
  can vary a lot from device to device and over temperature, so a resistive divider is probably a better idea (or even a string
  of two diodes from the input, with a resistor going to ground, and then a series resistor plus the cap).


 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #55 on: September 07, 2018, 11:07:43 am »
I'm using this circuit actually. When my signal into the op amp increases, the noise from the output of the transistor goes up too. Otherwise it's actually a good few mV ripple supply.
 

Offline Leo Bodnar

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #56 on: September 09, 2018, 07:39:56 pm »
This is barely scraping into the topic of the discussion but I think it might be useful for practiotioners of ripple removal.

Instead of blocking the high frequency ripple you can shunt it out. Wenzel has an apponote here.

Cheers
Leo

 
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Offline Cerebus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #57 on: September 09, 2018, 11:20:03 pm »
While some of the circuits in Wenzel's appnote might have fair to good performance the one illustrated above looks pretty mediocre. Surely the highest effective attenuation (with the shunt transistor fully on and theoretically a short circuit) would be that of the divider formed by the 15 ohm resistor and the ~1k to ground presented by the rest of the circuit - i.e. a gain of 0.985 or -0.13 dB. With that relatively huge 1k in the way of the shunt current I don't see how it could be effective under any circumstances. There must be some error in the presented resistor values.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #58 on: September 09, 2018, 11:49:08 pm »
Agreed. The 1K is probably 1Ohm in reality.
 

Offline JS

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #59 on: September 10, 2018, 12:39:14 am »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

  In this circuit it's not easy to apply superposition, and analyze bias and signal independently. Other way you can think it would be the 15Ω resistor as part of the collector resistor and picking frome where the noise is smaller, or summing cancelling out the noise using an inverted signal.

  The point is the noise rejection capability is given from the DC and the ratio of those resistors, not from the signal itself. Then it's a matter of tweaking the gain for best cancellation and bias point for best efficency, depending on the shape of the ripple.

JS

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Offline Cerebus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #60 on: September 10, 2018, 01:42:12 am »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

Into an open circuit, yes. But with any kind of load? Remember, the LM317 is a 1 amp regulator. If it was drawn with a bijou regulator and a specified load, all well and good, but into an unspecified load with an implied maximum load of 1 amp and the shunt regulator capable of diverting at most 1mA/volt? The effectiveness is going to be very dependent on load.
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Offline glarsson

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #61 on: September 10, 2018, 01:54:52 am »
The text under the figure says it us for low current loads. The resistor on the LM317 output also prevents high current.
 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #62 on: September 10, 2018, 03:18:10 pm »
how many of these problems does using an op-amp in the capacitance circuit eliminate?

how about using a full fledged gyrator with some fast op-amps? (though I had problems with this)

and he says its for instrumentation power supplies. Nothing in instrumentation is going to use 1 amps on the 15V rail unless its something really fast or there are a TON of chips.
« Last Edit: September 10, 2018, 03:40:59 pm by coppercone2 »
 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #63 on: September 10, 2018, 03:49:26 pm »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

  In this circuit it's not easy to apply superposition, and analyze bias and signal independently. Other way you can think it would be the 15Ω resistor as part of the collector resistor and picking frome where the noise is smaller, or summing cancelling out the noise using an inverted signal.

  The point is the noise rejection capability is given from the DC and the ratio of those resistors, not from the signal itself. Then it's a matter of tweaking the gain for best cancellation and bias point for best efficency, depending on the shape of the ripple.

JS



Am I right in thinking that its biased by like 20uA in steady state w/10V rail?

I think its just a biased common emitter amplifier that uses the ac coupled noise as a input and uses the inverting output to cancel the signal? And its inverting output is current limited by 1k? and its output is isolated by 100k to the input so its not fighting itself
« Last Edit: September 10, 2018, 04:33:03 pm by coppercone2 »
 

Offline Leo Bodnar

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #64 on: September 10, 2018, 08:53:20 pm »
Wenzel circuit uses LM317 just as an example of noisy (and cheap) LDO. 
It's intended use is powering very low noise OCXOs.  The heater is powered from a separate regulator.
Leo

Here is higher current version from the same appnote

Offline b_force

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #65 on: September 10, 2018, 11:22:20 pm »
What I normally do with the standard circuit is using a extra RC filter.
That gives a steeper roll of (24dB/oct) with a Q of 0.5

Another way is actually forming a sallen-key filter.

With these methods you can even get away with smaller capacitors.
« Last Edit: September 10, 2018, 11:39:07 pm by b_force »
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Offline jasonhanjk

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #66 on: September 15, 2018, 06:07:20 pm »
MPC1700 has a ripple rejection of less than -10dB at the frequency of 10kHz. Should use LM317 for your video instead.

https://youtu.be/wopmEyZKnYo?t=352

 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #67 on: September 16, 2018, 12:34:54 pm »
the secret being unless you want to design for battery life don't use those. the mcp1700 has pretty much portable on 100% of its application suggestions
 

Offline Feynman

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #68 on: November 04, 2018, 07:13:53 pm »
Someone ever tried to tap the switching regulator's feedback signal behind a capacitance multiplier to regulate away its voltage drop? Is there any change the regulator is still stable (careful routing of the feedback signal assumed)?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #69 on: November 04, 2018, 10:12:02 pm »
I would not be so much afraid of a routing induced problem, but a problem with loop stability.
Personally, I have never seen something like this in the wild.
Maybe some SPICE could help.
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #70 on: November 05, 2018, 01:23:50 pm »
You don't want to do that within the same loop, the purpose of the C-mult is to have a very low frequency pole.  The controller won't be able to react to its own changes.

There are very few situations where you need perfectly stable output voltage AND low noise, anyway.  Better to address the failings of your circuit, than to nurse it with an ever-more-complicated power supply.

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Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #71 on: November 05, 2018, 05:16:39 pm »
A capacitance multiplier in the loop makes a lot less sense than a postregulator.

With this, you could
- kill as much ripple as a capacitance multiplier
- have a fast load response
- increase accuracy

There are some Jim Williams Appnotes describing such designs.
 

Offline MT

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #72 on: November 06, 2018, 08:50:24 am »
You don't want to do that within the same loop, the purpose of the C-mult is to have a very low frequency pole.
The controller won't be able to react to its own changes.
Quote
There are very few situations where you need perfectly stable output voltage AND low noise, anyway.

There are many situation where you need perfectly, very god output stability and low noise.
Quote
Better to address the failings of your circuit, than to nurse it with an ever-more-complicated power supply.
yes. Better to have a pre and post regulator and local circuitry regulators.
 

Offline VanitarNordic

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #73 on: January 25, 2019, 01:52:30 am »
How the circuit should be in case of a negative input to the capacitance multiplier?

for example from the 7660 negative voltage generator
 

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #74 on: January 25, 2019, 03:07:23 am »
Just turn the Zener around and use a PNP transistor.
 

Offline VanitarNordic

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #75 on: January 25, 2019, 05:36:24 am »
Just turn the Zener around and use a PNP transistor.

Actually, I used NPN and the results were identical with when I used an PNP in this circuit!

Besides, if I just connect something at the output (output of the multiplier), a resistor (as a load) or even a 100nF capacitor or similar, then the noise appears again!
« Last Edit: January 25, 2019, 06:31:13 am by VanitarNordic »
 


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