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Rectification in aluminium electrolytic capacitors

Discussion in 'Electronic Components' started by Eeyore, May 1, 2007.

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  1. Eeyore

    Eeyore Guest

    I've just been having a discussion elsewhere about the performance of aluminium
    electrolytic caps for audio coupling in a zero bias situation. This is a method
    very widely used nowadays.

    I've observed, indeed I first heard of it decades back, that provided the signal
    *across* the capacitor doesn't exceed about 100mV ac there is no measureable
    non-linearity. I've confirmed this using Audio Preciosn test gear which measures
    down to 0.0008% THD (-102dB). Hence if the value of the cap is such that 100mV
    is never exceeded, there will be no distortion contribution from the cap. I've
    also confirmed that if the 100mV is exceeded, you do indeed get distortion.

    The mechanism seems to be electrolytic rectification AIUI. I presume that the
    absence of any effect below ~ 100mV is due to the 'forward' voltage of this
    rectifier, and current only flows when it's exceeded.

    Any more thought on this ?

    I did find this page about electrolytic rectification btw.
    http://home.earthlink.net/~lenyr/borax.htm

    The oscillograms are rather interesting.

    Graham
     
  2. Damon Hill

    Damon Hill Guest

    Was this tested with non-polarized electrolytic capacitors as well?
    Any of the more esoteric capacitors like Black Gate (graphite mixed
    with the electrolyte) or polymer-based caps?

    It would seem to confirm that electrolytic caps should ideally be
    biased.

    --Damon
     
  3. Eeyore

    Eeyore Guest

    I did test one example of a non-polarised type. Interestingly, once that 100mV
    threshold had been exceeded it produced twice as much distortion (at low signal
    levels) as a polarised type. It would seem that's simply because it is
    internally 2 such caps back to back. I've therefore concluded that for 'low
    level' signal coupling the NP types offer no advantage and in fact degrade the
    signal more.

    No, I haven't tested any more esoteric types. The last I heard, Sanyo ? were
    stopping manufacture of the Black Gates.

    Ideally. I have used them 'back to back' with the junction polarised to rail via
    a very high value R. It's cumbersome though.

    The zero bias method works well provided you don't exceed ~ 100mV across the
    cap. A value of 100uF deals with this in most real world audio circuits.

    Graham
     
  4. Damon Hill

    Damon Hill Guest

    Rubycon, actually. They manufacture Black Gates for a third
    party, and a new agreement was negiotiated recently. For better
    or worse, that brand will continue for a while. I use them,
    sparingly, for input stage bypassing and feedback circuits.

    The chemistry of electrolytics has always seemed like a black
    art and the Black Gate web page itself doesn't help things a
    bit. I can't blame professional engineers for being skeptical,
    though even some major audio manufacturers use them.

    http://www.blackgate.jp/

    ....if you'd like to take on their bafflegab.

    --Damon
     
  5. tempus fugit

    tempus fugit Guest

    Hey Graham

    I just want to clarify a thing or 2 if its OK.

    By no signal across the cap, you are referring to AC, correct?
    If that's the case, aluminum electros would be the cap of choice for a mic
    pre where the 48v from phantom power would need to be blocked, right? But
    only Aluminum electros, or do other electros also exhibit this trait?

    Thanks
     
  6. I'm no electrochemist, but perhaps it takes something in excess of 100
    mV or so of reverse polarity to begin the reduction reaction that
    damages the oxide layer.

    If the peak voltage is kept low, no oxidation or reduction occurs and
    the cap is linear.

    Did I guess right?
     
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