Polarized Capacitors for Audio Lines

Discussion in 'General Electronics Chat' started by AzifchiK, May 29, 2007.

  1. AzifchiK

    Thread Starter New Member

    May 29, 2007

    I been adviced to use polarized capacitors for AC coupling of Audio signals.
    Not convinced with this I started searching for reasons.
    Unfortunately everythng i found just doesnt go by this advice.Please see my findings below:
    You'll ruin a polarized cap by passing AC through it".Polarized capacitors use a very thin oxide layer as the insulating medium between the plates, so that the capacitance per unit volume can be made very high. They usually require a dc polarizing voltage to stabilize the oxide layer, and being a chemical process, either very high or very low temperatures can be a problem. Another difficulty is that the actual value of capacitance may be neither very accurate or very stable.Polarized capacitors are usually electrolytic, I.e., part of the capacitance has to do with chemistry, which is slow, so they can introduce phase errors"
    To some people, 3kHz is a fast signal. I have a friend who's into music in a big way. He has replaced all the electrolytic in the signal path of his audio equipment with non polarized caps. I think I can hear the difference.
    Together with it my comments:
    I can understand earlier it was tough to get high value ceramic capacitors.Now that too being easily available (of course at a trade-off of cost)should it be a problem ?
    Also the various attributes of non polarized ceramic capacitors like temperature stability should give it an upper hand .
    Please comment

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    One way around the problem is to use a pair of electrolytic caps. They are placed in circuit in series, so the value of each must be twice the proper value for the coupling capacitance. Further, they are arranged so the polarities are in opposition, say +_-_-_+. It seems strange, but this arrangement relults in a non-polar equivalent capacitance.

    Another dodge is to use a bipolar capacitor of the proper value. They are expressly designed to pass AC signals with both positive- and negative-going excursions.
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    It depends of what is coupling. Polarized electrolytic caps can be used if the sound output is polarized, that is, the "live" is always positive or always negative in relation to the return pole (normally ground). If not, you should really use unpolarized capacitors.
    Also, mylar capacitors, although small, can be used in paralell with electrolytic caps, so the response will be fast.

    Now there are multilayer ceramic capacitors, having big capacities, but they have resonance problems.
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    Quite true. Aluminium electrolytic capacitors have an anodic film formed on their anodes and that constitutes the dielectric. Anodes can be made from etched film to increase the surface area and it would be impossible to keep the aluminium cathode plate in intimate contact with the dielectric. A conductive electrolyte is therefore used as a current carrier. The dielectric film has a temperature coefficient that will change the capacity, but the biggest change is brought about when the capacitor is used in relatively high temperatures and with a polarising voltage less than the original forming voltage. Some of the dielectric then tends to get dissolved in the electrolyte and the capacity increases.

    Operation of the capacitor is not affected by chemistry. But by its construction it cannot be a pure capacitor. The electrolyte between the cathode plate and the dielectric has a finite resistance and that is in series with the capacitance. This property used to be defined by ‘power factor’ but now seems to be called Equivalent Series Resistance.

    It the capacitor is feeding into a resistive load, phasing will not be affected – the ESR will be in series with the load resistance, so there will be a marginal attenuation of signal.

    Needless to say, they should never be used as a critical component in filters.
  5. AzifchiK

    Thread Starter New Member

    May 29, 2007
    Thanks Guys !!
    I guess I can prefer film ceramic capacitors for the AC lines !!
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    you mentioned that your friend has replaced all electros in the signal path of his sound system and improvement has been quite audible. however you did not mention what he had used as replacement.

    sound quality would audibly improve with the following:
    metallized polyester
    metalized polycarbonates
    metallized polypropylene
    teflon (very expensive)

  7. cumesoftware

    Senior Member

    Apr 27, 2007
    Ceramic capacitors, especially multilayer, may resonate like a crystal when connected to audio lines (especially in crossovers). I advise the use of mylar capacitors, or at least, teflon or polyester.