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Old 23rd July 2002, 04:13 PM   #1
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Default Sound-relevant effects in decoupling caps for analog nodes/measurement methods

The idea for this thread was born from the discussion on ftorres' excellent thread in the digital forum on decoupling caps for digital ICs.

While it is almost universally accepted that electrolytics, ceramics and some plastic film caps make poor coupling and analog filter caps and it is at least partially clear what effects are detrimental to sound quality in those application, the situation is no so clear for decoupling caps on analog nodes such as supply rails of power amps, voltage reference for discrete constant current source or reference decoupling pin of a DAC.

Many believers advocate a certain line of caps in this application but fail to explain the theory behind their reasoning or preference.

Clearly, the caps should have a very low impedance to ensure a good decoupling. However, this is not necessarily true in all cases. If the impedance between the rectifier diodes is low enough, the ripple current will increase if more caps are added or low-ESR caps are used. This will put a certain strain on transformer, diodes and caps which might in some cases degrade the sound. Also, if the ground layout is poor, it might induce 100 Hz noise plus whatever harmonics are on the mains.

Then there is microphonics. DAC and CCS references would be most susceptible to this.

Finally, there is the question of how the caps react to a sudden current demand. Assuming the voltage does not sag too much, capacitance vs. voltage is not relevant. It boils down to the question of how smoothly the current is delivered, if it triggers strange effects in the electrolytics and if it starts sound waves within the cap. A CCS reference will not have a modulated current draw. A DAC reference, if there is no further internal buffer, may see a capacitor charging current which would come in short spikes at some 100 kHz to MHz frequency. This may or may not be dependent on the audio amplitude that must be converted.


As a starter, I shamelessly copied one of my own posts from the digital forum. It deals mainly with ceramics, but the questions raised are also relevant to electrolytics.


microphonics and nonlinear properties

quote:

Originally posted by ftorres
Last thing, just to relaunch the debate : What about the piezo-electric properties of Y5V
compared to X7R ? Ceramic layers are so thin in these caps that vibrations may induce some
unwanted noise. So we will have to damp them. Any suggestion for a damping material have
nice HF dielectric properties ?


reply:

That question would almost merit a new thread. Obviously, other factors than ESR, ESL and DA may influence the audio quality of a cap.
The most obvious is microphonics, as pointed out by you. My guess is that a dielectric with a high dielectric constant would be more susceptible. But it would also depend on the mechanical properties of both the dielectric
material and the overall design of the cap.

Most sensitive application in my eyes is not rail decoupling but reference decoupling, i.e. voltage reference in a DAC or zener diode in discrete current sources.

Then there is the question how a cap reacts to voltage drops and current draw. ESR, ESL, DA are, after all, just linear measurement methods, i.e. they are done with a huge DC offset and a small AC test signal.

Capacitance vs. voltage is specified but measurement is carried out again only by changing the DC offset from measurement to measurement. How is linearity with voltage drop spikes such as may occur when a DAC charges its internal capacitors? Again, this is not only a question of the dielectric properties alone but also of the mechanical properties as the electrodes may "move" when voltages change or currents flow.

Listenting tests might help but might only produce new myths. We should try to come up with tests for nonlinear capacitor properties....
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Old 23rd July 2002, 04:46 PM   #2
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Default low frequence effects in the electrolyt -> red herring?

another self-quote from digital:
"Now for something that confuses me: BC components (formerly Philips Passive) type 136 low impedance cap specs give ESR at 120 Hz and impedance at 100 kHz. For 1000uF/25V, ESR is 0.22R and impedance is 0.034R.

To my understanding ESR (equivalent series resistance) is a real impedance. A HF complex impedance can be no smaller than the real series impedance. Strange, uh?"

I took a closer look at the BC data sheet and I am beginning to get the idea although the data sheet does not explain the measurement method.

At 100 Hz there seems to be a procedure to measure and compute the resistive or real (i.e. no phase shift contribution) part of the impedance, i.e. get rid of the 1/(i*omega*C) contribution of the capacitance.

At 100 kHz they seem to report the amplitude of the complex impedance of the series connection of the capacitance and a parasitic inductance and resistance, i.e. Z_data sheet = real(Z(100 kHz))=real(R + 1/(i*omega*C) + i*omega*L).

R(100 Hz) is roughly a factor of four higher than real(Z(100 kHz)), showing a slight dependence on the rated voltage of the cap.

Looking at the plot of ESR(f) on page 13 of the 136-series data sheet (download e.g. from www.schuro.de) and assuming ESR=R in my equation, there seems to be an additional resisitve contribution at 100 Hz that deminishes with increasing frequency until R becomes constant in the low single digit kHz range.

So some process in the electrolytic with a time constant well in the audio range inhibits the flow of current.
If I am not completely mistaken, an electrolytic is not a true dielectric in which the polarisation is only a function of the electric field. Rather, it acts like some kind of battery, i.e. there are chemical processes which store and release the charge. If this ~ms time constant was a typical time for the reaction to take place, one could understand the inhibition of the current flow. However, my reasoning is that this should also reduce the effective capacitance at low frequency.
Looking at page 12, however, capacitance (again, they don't explain measurement/computation) seems to be fairly constant up to 20 kHz where I assume the parasitic inductance begins to have a relevant contribution. There is no sign of a reduced capacitance below 1 kHz!

Can anybody make sense of the data?

Aren't effects in the Hz- 5 kHz range likely to have some audible impact?
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Old 23rd July 2002, 04:47 PM   #3
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Default Sound-relevant effects in decoupling caps for analog nodes/measurement methods

http://www.national.com/rap/Applicat...570,28,00.html

http://www.capacitors.com/pickcap/pickcap.htm

http://www.capacitors.com/consider/consider.htm

http://www.rbtech-inc.com/learn/Capacitors.html

http://www.atceramics.com/pdf/techno...ric_effect.pdf

http://www.faradnet.com/deeley/book_toc.htm

ESR Defined
ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a capacitor (thus the name Equivalent Series Resistance).
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Old 23rd July 2002, 04:57 PM   #4
tiroth is offline tiroth  United States
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Microphonic effects can be quite substantial, as seen here. I've been unable to find much information on the piezoelectric qualities of different ceramic dielectrics, although NPO is supposed to be non-microphonic.

I do have some Taiyo Yuden caps that were designed for mobile equipment (cell phones, etc) and that are supposed to be less susceptable to shock-induced noise.
Attached Images
File Type: gif ceram_piezo.gif (5.5 KB, 384 views)
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Old 23rd July 2002, 04:58 PM   #5
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Send a message via Yahoo to Elso Kwak
Lightbulb Decoupling Caps

Hi Eric,
<I>If it pleases you,
sounding fluent
What matters measurements.......
If sound is OK for you....??</I>[Joke]


I am sorry, back to <B><I>normal</B></I>
Routinely I was using ceramic bypass caps in all of my DIY equipment: preamp, DAC, etc.
Later I tried polyester caps; all 0.1F in my DAC and the sound was slightly more fluent, less harsh to me.
Later I switched to polypropylene for bypassing and that was much better. I found the measurements by Francois verry interesting but a little hard to follow.
I also tried polyester caps in the powersupply; Jung like regulator but that modification gave rise to oscillations.
Those ceramic caps are certainly microphonic: If you press on them while measuring capacitance you see them change capacity!
Also they have a high temperatur coefficient. Measured them while heating with my daughters hairdryer. I believe they are called KDPU Conrad part# 453358, if this helps.


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Old 23rd July 2002, 05:11 PM   #6
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Harry,

thanks for the links. On first glance, they appear to be an excellent primer on capacitors and common capacitor parasitics such as ESR, ESL, dissipation factor and dielectric absorption.

I am familiar with Walt Jungs publications on capacitors in Analog Dialogue, and this seems to be an expanded version. I have made a thick print-out for bedtime reading...

DA should be relevant in coupling and analog filter apps, but not necessarily in decouling apps.

ESR, ESL and DF are important in decoupling apps but as I tried to argue, they cannot make up the whole story. In particular, they are part of a linear analysis where we should be concerned about nonlinear effects.

Is there any passage in your links that you feel might address my rather specific topic?

Regards,

Eric
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Old 23rd July 2002, 05:14 PM   #7
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Default Electrolytic article

http://www.faradnet.com/deeley/book_toc.htm
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Old 23rd July 2002, 05:30 PM   #8
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Default Gromanswespeak

The answer of course,
is all in the conductor noise,
the noise to shunt.
or the noise to pass,
this combination,
is in the resultant.

It's all in the vibes, man,
Eric.
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Old 23rd July 2002, 08:53 PM   #9
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Default Re: Gromanswespeak

Quote:
Originally posted by mrfeedback
The answer of course,
is all in the conductor noise,
the noise to shunt.
or the noise to pass,
this combination,
is in the resultant.

It's all in the vibes, man,
Eric.

*LOL* *ROTFL*
Goodday me namesake from down under,

having suffered groman's pseudo-sophisticated blabber throughout the better days of audioasylum only to find after a couple of calm months that he has decided to get his daily dose of negative attention from diyaudio, I find your impromptu emulation of his style vexing, to say the least....
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Old 23rd July 2002, 08:57 PM   #10
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Hey Harry,

that ain't fair play, expanding your four links to six while I asked about their relevance ;-)

So what addresses my questions? Carried home a pound of printouts from your links, but bedtime's yet to come....
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