the thread topic is cap considerations for Cdom - and presumably only those at least as "good" as silver mica
that suggests to me comparing cap quality such as DA to the magnitude of other expected errors of the VAS, in that context
the DA model is linear - long time constant parallel RC branches which would give ~ 0.1 dB fr variations of the integral gain
using high gain/"low bandwidth" loop gain the lower frequency DA effects are attenuated by the excess loop gain - easily >60 dB @ 1KHz, >100 dB for "100mS" time constant signals
at the same time the VAS Q is subject to nonlinear gm modulation thru operating current and thermal effects, Early effect and junction C modulation - causing static and dynamic errors of greater magnitude than the linear Pease' Mylar DA model
some of the VAS Q nonlinearity can(should) be addressed by circuit modification before worrying about reasonable Cdom cap "audiophile" qualities
that suggests to me comparing cap quality such as DA to the magnitude of other expected errors of the VAS, in that context
the DA model is linear - long time constant parallel RC branches which would give ~ 0.1 dB fr variations of the integral gain
using high gain/"low bandwidth" loop gain the lower frequency DA effects are attenuated by the excess loop gain - easily >60 dB @ 1KHz, >100 dB for "100mS" time constant signals
at the same time the VAS Q is subject to nonlinear gm modulation thru operating current and thermal effects, Early effect and junction C modulation - causing static and dynamic errors of greater magnitude than the linear Pease' Mylar DA model
some of the VAS Q nonlinearity can(should) be addressed by circuit modification before worrying about reasonable Cdom cap "audiophile" qualities
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Balaboo, you are more exacting than I realized. I am inclined to agree that oil and paper, even if it measures high in DA can be pleasant sounding. Kind of an 'effect generator', or 'sweetener' like Aphex properly applied.
Sometimes we have to use metalized, and certain metalized caps are better that others.
For example, the 8 uF Rel (Wonder Cap) that I 'bonked' with a piece of circuit board, behaved better than the 1 uF Mylar cap, that is so microphonic.
My concern with NPO or COG ceramic isn't high distortion, necessarily, but microphonics as well. Speaking to Ed Simon, just yesterday, I was told that NPO ceramic was indeed microphonic, yet there is nothing about it on a typical NPO data sheet, only that the NPO caps are not piezoelectric. Thank goodness for that.
For the record, I am using some NPO caps in my latest phono designs, because I need space saving too, but only where it is not in the audio path, at the moment.
I have been 'bitten' more than once, believing exclusively in data sheets, (we are engineers, are we not?) and not my own personal testing and the experience of others, with Tantalum, Ceramic, and even Mylar. That is what I am trying to get others to avoid, so that they don't make the same 'mistakes' that I have in the past. Sorry, if it sounds like preaching or parsing, but I hate to see people walk through the wrong 'door' that I did at one time.
Sometimes we have to use metalized, and certain metalized caps are better that others.
For example, the 8 uF Rel (Wonder Cap) that I 'bonked' with a piece of circuit board, behaved better than the 1 uF Mylar cap, that is so microphonic.
My concern with NPO or COG ceramic isn't high distortion, necessarily, but microphonics as well. Speaking to Ed Simon, just yesterday, I was told that NPO ceramic was indeed microphonic, yet there is nothing about it on a typical NPO data sheet, only that the NPO caps are not piezoelectric. Thank goodness for that.
For the record, I am using some NPO caps in my latest phono designs, because I need space saving too, but only where it is not in the audio path, at the moment.
I have been 'bitten' more than once, believing exclusively in data sheets, (we are engineers, are we not?) and not my own personal testing and the experience of others, with Tantalum, Ceramic, and even Mylar. That is what I am trying to get others to avoid, so that they don't make the same 'mistakes' that I have in the past. Sorry, if it sounds like preaching or parsing, but I hate to see people walk through the wrong 'door' that I did at one time.
Well said John .......................... In the end we have to be able to live with "it" ...
There's a lot of ways to go. I just want to start by duplicating what you did. Did you do anything beyond putting the cap across the scope terminals and tapping it?
For my own microphonics tests, I had a DC supply and a resistor in series with the cap, then monitored the voltage across it when I tapped. Seem also reasonable to you?
Yes, I am sure that the DC bias will add additional microphonics. However, I find the near DC bias also important, because I don't usually use Teflon, etc caps for DC blocking applications, but for EQ. This still might show a significant difference between Teflon and polystryrene for power supply bypassing applications. I think that the Mylar cap has some polar properties, and this could be significant, but a different 'generator' for microphonics.
Mr.Curl;
You are absolutely spot-on about the microphonics of ceramic capacitors in general - they do have their uses, and appreciate your insight. After all, I am a MSEE too !!
You are absolutely spot-on about the microphonics of ceramic capacitors in general - they do have their uses, and appreciate your insight. After all, I am a MSEE too !!
of course this thread was never about ceramic caps in general - but NP0/C0G were introduced as likely better than the subject line's Silver Mica for Cdom application
using the forum google search option gets a plot suggesting C0G microphonics isn't any worse than a (unspecified type) smt film cap:
http://www.diyaudio.com/forums/parts/28510-cog-capacitor-microphonics.html#post329783
the Murata newsletter can be retrieved from the "WayBack Machine":
http://web.archive.org/web/20040716043916/http://www.murata-europe.com/autumn01.pdf
the newsletter compares 100nF film vs C0G - for Cdom ~100pF we can use smaller case (less bending stress) and much higher V (thicker dielectric) both should improve performance
using the forum google search option gets a plot suggesting C0G microphonics isn't any worse than a (unspecified type) smt film cap:
http://www.diyaudio.com/forums/parts/28510-cog-capacitor-microphonics.html#post329783
the Murata newsletter can be retrieved from the "WayBack Machine":
http://web.archive.org/web/20040716043916/http://www.murata-europe.com/autumn01.pdf
the newsletter compares 100nF film vs C0G - for Cdom ~100pF we can use smaller case (less bending stress) and much higher V (thicker dielectric) both should improve performance
Thanks to everyone who puts links up to read. I have found them very informative.
Getting back to the original reason for this thread, Cap compensation, I remember using ceramic caps 40 years ago, with impunity for caps up to 500pf or so, for different things, especially comp caps for circuits. Years later, I did some researching as to whether these caps, commonly found at the local electronics supply store were NPO or a slightly worse ceramic. You see, they were available in about the same package, with just a few cents of difference per part. Well, that is what I try to warn against. Even Mark Levinson used these ceramics in our early modules, before I was shown the potential problem at Tektronix, back in 1974. Then, we switched to a film cap of some type.
It seems to me that many comp caps rely on Miller multiplication to work or at least driven from an extremely high impedance current source. Now, IF this comp cap changes its characteristics, through microphonics, voltage change, or DA, it is probably amplified 1000's of times compared to measuring the cap and its characteristics, alone driven by a conventional generator, etc. This is why I use polystyrene comp caps when I can.
Getting back to the original reason for this thread, Cap compensation, I remember using ceramic caps 40 years ago, with impunity for caps up to 500pf or so, for different things, especially comp caps for circuits. Years later, I did some researching as to whether these caps, commonly found at the local electronics supply store were NPO or a slightly worse ceramic. You see, they were available in about the same package, with just a few cents of difference per part. Well, that is what I try to warn against. Even Mark Levinson used these ceramics in our early modules, before I was shown the potential problem at Tektronix, back in 1974. Then, we switched to a film cap of some type.
It seems to me that many comp caps rely on Miller multiplication to work or at least driven from an extremely high impedance current source. Now, IF this comp cap changes its characteristics, through microphonics, voltage change, or DA, it is probably amplified 1000's of times compared to measuring the cap and its characteristics, alone driven by a conventional generator, etc. This is why I use polystyrene comp caps when I can.
as long as polystyrene stocks hold out it is a very good choice - but I don't see extended foil polystyrene in the low values for Cdom that would push up the SRF to beyond 10x the audio amp's loop gain intercept - is there a extended foil polystyrene source out there?
Cdom errors are reduced by excess loop gain - not amplified - just like any other error source in the GNF signal path at audio frequencies
Cdom errors are reduced by excess loop gain - not amplified - just like any other error source in the GNF signal path at audio frequencies
I haven't read all the links but IMO you have to be quite careful testing for microphonics since most cables are quite bad and keeping the "tap" out of the cables and test equipment can be difficult. Maybe small diameter bare wires going a distance to the scope input with some sort of a non-conductive anchor block before the scope?
Everything we do probably needs to be looked at in both voltage and current domains, essentially just high and low impedance conditions. DA is usually tested for as a voltage, but it also shows up as an unwanted current tail.
Everything we do probably needs to be looked at in both voltage and current domains, essentially just high and low impedance conditions. DA is usually tested for as a voltage, but it also shows up as an unwanted current tail.
An externally hosted image should be here but it was not working when we last tested it.
Ah, and tighten the capacitor leads under the binding posts.
It would depend on how one defines 'amplified'. Perhaps 'leverage' is a more appropriate term. What is important is that a modest cap of say 100 pf could effect even 10 Hz, where 100 pf in general is not very important at low impedances. Any dynamic change in the cap's value would cause FM distortion, I am fairly sure.
'leverage' ?
No.
The distortion (%wise) originated from Cdom as seen at the output of an amp, will never be larger than the distortion from that cap when measured in isolation (under the same conditions of course).
No.
The distortion (%wise) originated from Cdom as seen at the output of an amp, will never be larger than the distortion from that cap when measured in isolation (under the same conditions of course).
This appears to be a language difficulty problem, rather than anything else. I used the word 'multiply' but that meant 'scaling' or something like that, rather than making the percentage of the distortion in the cap higher. The main thing is that a small cap can dominate the transfer function of an entire amplifier. Therefore, if it is non-linear, then the amp can be 'non-linear' too.
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