Capacitors

Discussion in 'General Electronics Chat' started by tmac, Nov 17, 2009.

  1. tmac

    Thread Starter Member

    Oct 28, 2008
    10
    0
    Hi all!

    I would like to learn more about the differences between ideal and real-world capacitors, for audio applications. I know about the concepts of ESR and ESL and the non-infinite resistance between the plates in a real capacitor. From given values of these quantities I can calculate their effect on the impedance as a function of frequency, but I would like to see results from actual measurements. I have never seen anything about capacitor non-linearity, so any insights about that is also interesting.

    Also, does anybody know how well the concepts of ESR/ESL reprecent the "non-ideality" of capacitors? For the capacitors I'm looking at for the moment, the ESR is given at 100Hz. Should I expect the ESR to vary as a function of frequency?

    It would be very nice to see impedance measurement plots for commonly used capacitors (cheap and expensive) used in tube audio amplifiers, for the range 20Hz-20kHz, both high voltage electrolytics, polypropylen and ceramics, as well as lower voltage electrolytics. Actually, I am mostly interested in guitar/bass amps, so 30Hz-10kHz is a more relevant range.

    I want to know at which frequencies the non-ideal aspects of these capacitors come into play, if this is inside or outside the audio spectrum, and if there is a difference between the impedance of cheap and expensive capacitors within the same spectrum.

    This would involve e.g. electrolytics around the capacity of 47uF, and polypropylen capacitors in the region of 22nF, all with voltage tolerances above 400V. Also, e.g. electrolytics at around 22uF but with lower voltage rating, e.g. 25V.

    Does anyone know of such studies that are available on the net? I have not found anything yet, but not everything is easily found with google, so I'm trying my luck here :)

    I would appreciate any references that go beyond the mathematical model using the ESR/ESL concepts. Do any of the manufacturers present impedance plots of their products?

    Thanks,
    Torquil Sørensen
    Norway
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    As far as non-electrolytic capacitors are concerned, at audio frequencies secondary effects such as ESR are many orders of magnitude smaller than straight out capacitor reactance and have no noticeable effect on amplifier performance. So as long as it's not leaky and is the correct value any capacitor will do.

    Electrolytic capacitors can be a problem and you should probably use low ESR versions for best audio performance.

    Have a read of http://sound.westhost.com/articles/capacitors.htm for a good run down of facts and myths about capacitors in audio amplifiers.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Hope you are looking after Santa well for us in Lapland.

    I'm sure there are some here who would like to discuss the relative merits of different capacitors but also try

    http://www.diyaudio.com/forums/

    Alex, sorry Capacitors were not all born equal. In its heyday Wireless World - later Electronics World - had lots of published articles and measurements by the many of the greats of audio amplifier, crossover and loudspeaker design.

    So a visit to a good engineering library would not go amiss.
     
  4. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
  5. tmac

    Thread Starter Member

    Oct 28, 2008
    10
    0
    Thanks Alex, I just read it through. It was very interesting, and was quite thorough, although I did not double check any of the calculations :)

    After reading it I tend to agree that the uncertainty and temperature dependence of the capacitance would probably have a larger effect than what is obtained by switching between different capacitor makes/models. I still want to see some more sources of objective information.

    That reference did not really address the (slightly) higher voltages used in most tube amp circuits, but I do not expect that big of a difference. I only did a quick check, but at least for one type of electrolytic cap that I looked at, at least the ESR did not increase with increased voltage tolerance. I guess that was expected.

    My idea of a great gitar/bass amp certainly includes a fair bit of distortion, so the distortion of coupling caps, however small, is irrelevant in this case I would say :) This contribution would be very small compared to the distortion coming from the tubes, even when using a "clean" sound.

    Torquil
     
  6. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Yes, ESR definitely varies as a function of frequency, and as a function of temperature, especially for electrolytics. If you do Spice modeling of audio circuits, you might be able to include those effects in your models. But just knowing how they vary might be helpful, too. Read my post, here:

    http://www.diyaudio.com/forums/powe...lm-caps-electrolytic-caps-14.html#post1332738

    And then also read the rest of that whole thread. And there are other threads, there, that discuss capacitors' non-ideal characteristics.

    There is a link, in the linked thread, to a Cornell-Dubilier capacitor characterization Java applet that is very useful, for what you want to know. It plots various cap parameters versus frequency and temperature.

    I believe that that linked thread also has links to sets of plots of impedance versus frequency, for some common makes/models of capacitors.

    There were also some sets of capacitor impedance vs frequency plots posted at diyhifi.org, if I recall correctly, along with relevant discussions.

    - Tom Gootee
     
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