dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
4281
share rss forum feed


disconnected

@snet.net

Ceramic vs. Polystyrene Capacitor Distortion Analysis

Click for full size
Ceramic vs. Poly Spectrum Analyzer Plots
This is some very interesting results that I obtained while testing capacitors in interstage op-amp circuits.
I breadboarded a quad of inverting op-amp stages and intercoupled them with capacitors for this experiment.

The first trace with the green callouts is four polystyrene capacitors intercoupling. (These are from old stock I acquired in 1984 in bags of 100.) The second is four ceramic disc caps in place of the poly caps. No other changes. I performed this test yesterday at 1KHz and found small differences that only my Tektronix AA501 Distortion Analyzer could detect reliably, but today I used 100Hz as the test frequency and the differences amplified 100-fold.

The ceramic caps produced .216% THD while the poly caps produced .0031% THD at 100Hz.

I also did testing at 10Hz and this produced enough distortion that I could see it on an oscilloscope and see what exactly was happening. The waveform with the ceramic cap became almost triangular with a notch at zero crossing. The poly produced a clean sine wave. If I substituted a smaller poly, the sine wave amplitude would decrease, but no increase in distortion resulted.

Here's a superimposed snapshot of the 100Hz spectrum analysis. This is 100Hz per division. 2nd, 3rd (dominant) and 5th harmonics are evident here. It proves that I was not measuring noise, but actual harmonics. Note that the ceramic has a dominant THIRD harmonic above.

So this tends to suggest that the grade of interstage coupling capacitors used in audio amplifiers, particularly at bass frequencies, is rather important.


shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
Reviews:
·EarthLink
·Comcast
·Atlantic Nexus
Interesting but you did not say what type and the dielectric used: disc, Monolithic, multilayer , Z5U, X7R, X5R, C0G?

Sounds like non-linearity and that would appear most at low frequencies where they start getting a bit more voltage on them.

What is the low-freq cutoff of the amp design?

Edit: ah, see they were disc.
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
reply to disconnected
I don't remember seeing ceramics in hi-fi analog signal path much -- mostly films for lower and/or precise values, aluminium or tantalum for the rest.

What causes that distortion? Dielectric absorption or loss?
--
Wacky Races 2012!


shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
Yes, All audio stuff I saw used polystyrene, metal film, or polyester.

Would be interesting to compare polyester and some of the old ones like paper/film.


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Reviews:
·Verizon FiOS
reply to disconnected
For you old timers that remember Walter Jung and his Cookbook Series of electronic books.

"One thing seems quite clear, however, and that is the simple fact that you cannot "work around" the distortion problem in ceramics. Our feeling is that they should simply be avoided anywhere near an audio signal path and probably just avoided altogether for audio". -Jung (1980)

Do a search on DA (Dielectric Absorption)

Edit: Sorry I see aurgathor See Profile already mentioned DA
--
--
--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"How many assholes do we have on this ship anyhow?" - Dark Helmet


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
said by SparkChaser:

and probably just avoided altogether for audio". -Jung (1980

Would that avoidance also apply to power supply lines? It's pretty normal to have a 100nF discs in parallel with aluminiums.
--
Wacky Races 2012!


disconnected

@snet.net
reply to SparkChaser
I first thought it was dialectric absorbtion, but that doesn't explain why electrolytics in place of the ceramics don't display nearly as much distortion. Electrolytics have the highest DA of all capacitor types and they store residual energy after discharging. I noted some of this behavior at 10Hz on negative peaks.
poly and paper film comparisons may require more stages of amplification to make the distortion easy to measure.


shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
Is there any DC offset on the caps?

Usually electrolytics have a DC potential across them, that should reduce the DA.

As electrolytics are DC only (except for NP ones), I don't think they can compare to ceramics in use.


cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to aurgathor
said by aurgathor:

Would that avoidance also apply to power supply lines? It's pretty normal to have a 100nF discs in parallel with aluminiums.

That's because aluminum capacitors tend to act more like an inductor at high frequencies, while ceramics will cut them effectively. The main purpose is to improve the transient response.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to aurgathor
said by aurgathor:

Would that avoidance also apply to power supply lines? It's pretty normal to have a 100nF discs in parallel with aluminiums.

In that case I would think not.. since in that application they are "decoupling" caps, intended to keep as much trash off the power supply as possible. The Al are there to handle all the low frequency stuff, and the ceramics are responsible for taking out the higher (up into RF) frequency stuff.

garys_2k
Premium
join:2004-05-07
Farmington, MI
reply to disconnected
Nice work. With all of the unsubstantiated claims for audio performance by using some special component out there it's VERY refreshing to see some actual evidence.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
said by garys_2k:

Nice work. With all of the unsubstantiated claims for audio performance by using some special component out there it's VERY refreshing to see some actual evidence.

Well we just don't know how his results may have changed had he used monster oxygen free copper cables in his wiring.


iknow

@optonline.net
reply to disconnected
said by disconnected :

I first thought it was dialectric absorbtion, but that doesn't explain why electrolytics in place of the ceramics don't display nearly as much distortion. Electrolytics have the highest DA of all capacitor types and they store residual energy after discharging. I noted some of this behavior at 10Hz on negative peaks.
poly and paper film comparisons may require more stages of amplification to make the distortion easy to measure.

ceramics have the "ceramic resonator" effect. although much smaller than a true "ceramic resonator".


disconnected

@snet.net
I'm thinking this is a voltage coefficient effect I'm observing here.
I've made a video of this test, repeated with more traditional capacitors.. polyester film and ceramic. It's quick & dirty, but does reveal that even in interstage coupling scenarios, capacitors DO have their own unique distortion signature:

»youtu.be/xfAgOdOrJ1c


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Reviews:
·Verizon FiOS

1 edit
reply to iknow
said by iknow :

ceramics have the "ceramic resonator" effect. although much smaller than a true "ceramic resonator".

They work as good pizo pickups, too. We found out the hard way, one time.


cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to iknow
said by iknow :

ceramics have the "ceramic resonator" effect. although much smaller than a true "ceramic resonator".

I found it the hard way in a high voltage power supply...


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
reply to iknow
said by iknow :

ceramics have the "ceramic resonator" effect. although much smaller than a true "ceramic resonator".

If I remember right, ceramic filters are usually 455 kHz or 10.7 MHz (though other values do exist), but all of them are well above 100 Hz, so I'm not sure if the ceramic resonator principle can apply here.

And as per »www.murata.com/products/catalog/pdf/p11e.pdf , ceramic filters have a frequency range of 10 kHz .. 100 MHz.
--
Wacky Races 2012!


iknow

@optonline.net
said by aurgathor:

said by iknow :

ceramics have the "ceramic resonator" effect. although much smaller than a true "ceramic resonator".

If I remember right, ceramic filters are usually 455 kHz or 10.7 MHz (though other values do exist), but all of them are well above 100 Hz, so I'm not sure if the ceramic resonator principle can apply here.

And as per »www.murata.com/products/catalog/pdf/p11e.pdf , ceramic filters have a frequency range of 10 kHz .. 100 MHz.

ceramic capacitors are not made to be "ceramic resonators" BUT, since they are somewhat similar in construction(a ceramic wafer between 2 plates) you do get that effect.


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
said by iknow :

ceramic capacitors are not made to be "ceramic resonators"

iknow that


BUT, since they are somewhat similar in construction(a ceramic wafer between 2 plates) you do get that effect.

Which effect? Piezo? Mechanical resonance? Crystal? Something else?

My issue is that the frequency appears kinda low for piezo, which could be one obvious choice.
--
Wacky Races 2012!


iknow

@optonline.net
said by aurgathor:

said by iknow :

ceramic capacitors are not made to be "ceramic resonators"

iknow that

BUT, since they are somewhat similar in construction(a ceramic wafer between 2 plates) you do get that effect.

Which effect? Piezo? Mechanical resonance? Crystal? Something else?

My issue is that the frequency appears kinda low for piezo, which could be one obvious choice.

the reverse piezoelectric effect has been used on phonograph pickups and microphones for ages, at lower frequencies. my guess is in this case, the piezoelectric effect causes the reverse piezoelectric effect at zero crossing. you can use the formula here to calculate it. »en.wikipedia.org/wiki/Piezoelectricity