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High voltage capacitors in audio

Discussion in 'Electronic Design' started by ectoplasm, Jun 20, 2007.

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  1. ectoplasm

    ectoplasm Guest

    In a pre-amplifier, I am replacing all relevant capacitors in signal
    lines by 100V or 250V polypropylene film capacitors (i.e. big yellow
    ones). These are much better for audio than standard alu elcos.

    My question is:

    1) the signal is only a couple of volts in amplitude. Is it bad that
    the capacitors are way beyond this value? I mean, could it be that
    these big caps handle low voltages not as well?

    2) is it actually a good idea to replace *all* of these capacitors by
    these big ones? (if you'd like, look at the circuit at <http://
    sound.westhost.com/project24.htm>) Or would some good, small tantalum
    capacitors do better in some locations?
     
  2. D from BC

    D from BC Guest

    I'd install a switch so that one can select between "crappy 10 cent
    capacitors" and "Honking Fat Ass Caps". Invite friends over and ask
    them if they can tell the difference between "A" and "B". Be sure to
    mention you have beer..

    By the way...I have a switch on my amp that selects between active
    inkwitz Riley crossover or constant power type crossover.
    I can't tell the difference but I like flicking the switch every month
    or two hoping I might. :)
    Doesn't matter anyways with all the fkn loud Harleys driving by..

    Also..forget the gold connectors...just solder the speakers wires
    directly to the PCB. :p
    Oh...use silver solder.. :p
    D from BC
     
  3. ectoplasm

    ectoplasm Guest

    I suppose it all becomes meaninglessly undiscernible with brain damage.
     
  4. Eeyore

    Eeyore Guest

    Better perhaps than *misapplied elcos*. Aluminium electrolytics used carefully
    can perform excellently for audio and in typical circuits will give better bass
    and very low phase shift at LF because you can use very large uF values that you
    simply can't get and would never be able to fit in plastic film types.

    No. It just means that the plastic film inside is a bit thicker. The voltage of
    the cap is simply the maximum rated voltage it's designed to withstand without
    dielectric failure.

    Tantalum caps used for zero bias coupling perform horribly. I expect you want a
    large value ? Use 100-220uF/16V or similar al elco.

    Oh yes, you'll have fun fitting 22uF and 100uF plastic films.

    Let me guess. You're not doing this because you have a problem with the sound
    but because you read somewhre that al elco are 'bad' for the sound ?

    Graham
     
  5. me

    me Guest

    I had an amp with a sub-sonic filter switch, think about it. things that
    make you go what?!?!
     
  6. Wimpie

    Wimpie Guest

    Hello Ectoplasm.

    Whether better or not, depends on where they are in the circuit. When
    the AC voltage across the capacitor is very small with respect to the
    signal voltage, there is no problem with electrolytic capacitors. This
    is the case for most decoupling applications where the capacitor is
    only used for DC separation. This means that the cross over frequency
    of the capacitor in the circuit must be far below the lowest working
    frequency.

    When they are part of a filter, foil capacitors are a better choice.
    In a filter, a large part of the driving AC voltage can be across the
    capacitor. You can use polypropylene, but why not using polyester?
    They are smaller, cheaper and also very linear.

    Your questions.
    Small AC voltages on high voltage foil capacitors is OK. The voltage
    on the capacitor mentions the maximum DC or AC voltage that may be
    applied for a long time.

    I think it is not a good idea to just replace all electrolytic caps by
    foil type capacitors. As mentioned before it depends on the function
    of the capacitor. You may run into a space problem and probably it
    isn't worth the money.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
     
  7. ectoplasm

    ectoplasm Guest

    No, only 10uF, 22uF, 10uF. In this circuit:
    http://sound.westhost.com/project24.htm

    But I bought those yellow caps already, 100V ones. They are bigger
    than alu ones, yes. But they're not expensive ones.
    Yes, right. But 40 cents for an alu elco, 1.50 for these polypropylene
    caps who have a better audio reputation....
     
  8. ectoplasm

    ectoplasm Guest

    Thanks a lot for explaining. I get what you mean. I will take a closer
    look on the functions of my capacitors.

    But polyester, you say. Those are the plastic block ones, right?
     
  9. Eeyore

    Eeyore Guest

    Wimpie wrote:>
    Absolutely correct. The actual voltage *across* an electrolytic capacitor in
    zero bias mode (as opposed to the signal voltage magnitude itself) that gives
    trouble is when it exceeds ~ 100mV.

    Since using a large value capacitance reduces Z and hence that voltage it's hard
    to wrong with values of around 10uF for input coupling caps and 100uF for output
    coupling caps in most real world circuits.
    This the critical difference.

    Polyester is indeed the least expensive of the plastic film dielectrics and
    works just fine. The obsession with polypropylene and indeed Teflon as
    dielectrics is no doubt due to the audiophool idea that more expensisve = better
    in some rather undefined way.

    Graham
     
  10. Eeyore

    Eeyore Guest

    1.50 for a 22uF, 100V polypropylene ? Somone's giving them away it seems.

    You should ignore those ppl who just talk about a capacitor's 'reputation' without
    applying any science btw. There are lots of idiots in audio whose life's goal seems
    to be to use the most expensive components (including wire!) possible for no good
    scientific reason. There are certain caps you *should* avoid btw and I bet they've
    never mentioned them ! Notably so-called medium and high-K ceramic types. The
    dielectric behaviour in these is known to be non-linear. Low-K ceramics are fine
    though.

    Graham
     
  11. Eeyore

    Eeyore Guest

    They often come in 'box' construction but not exclusively by any means. Mylar is
    the same stuff btw.

    Graham
     
  12. YD

    YD Guest

    I much prefer the blue variety, clearer highs and less graininess in
    the lows.

    - YD.
     
  13. Better, how?
    I see only one capacitor in that circuit that will have any
    significant effect on the sound. It is the 1 nF. The rest
    have essentially no changing voltage across them in the
    audio frequency range, so will have almost no effect on
    audio. That said, I think some of the capacitor values are
    a bit small, like the two 22 uF caps in the output stage.
     
  14. Subsonic energy being produced by your headphones or
    speakers is a waste of amplifier power that can cause them
    to distort the stuff you can hear. It is usually a good
    thing to get rid of.
     
  15. MooseFET

    MooseFET Guest

    The only capacitor that it is worth considering changing is the 10uF
    at the input.

    The 470pF on th efirst op-amp is shown hooked up wrong.

    [.....]
    What kind of resistors are those? They may be making more distortion
    than the capacitors.

    The 22uF feeding the pot serves no purpose.
     
  16. Eeyore

    Eeyore Guest

    Really ?

    It looks like an active filter configuration to me.

    Yes it does.

    Graham
     
  17. I have a bigger problem with the choice of output
    transistors. Can anyone see why 300 volt transistors with
    gain that falls pretty sharply above 200 mA collector
    current (and at 10 volts collector to emitter drop for the
    PNP) were chosen for this design? With the output running
    80 mA class A there is not much current capability left to
    drive the headphones.
    http://komponenten.es.aau.dk/fileadmin/komponenten/Data_Sheet/Transistor/MJE340.pdf
    http://www.uib.es/depart/dfs/GTE/education/industrial/tec_analogiques/MJE350.pdf

    I would like to see the amp tested, side by side with one
    using a TIP29C, TIP30C output pair.
    http://www.fairchildsemi.com/ds/TI/TIP29C.pdf
    http://www.bourns.com/pdfs/tip30.pdf

    Of course, if you change from class A to class AB (say, 5 to
    10 mA idle current), you could probably use a pair of TO-92
    output transistors, like ZTX692B and ZTX792A and cut the
    power supply size in half.
    http://www.zetex.com/3.0/pdf/ZTX692B.pdf
    http://www.zetex.com/3.0/pdf/ZTX792A.pdf
     
  18. Eeyore

    Eeyore Guest

    It's probably what he had on hand.

    No worse than using 2N2222s in that respect.

    Defeats the audiophile purpose of the Class A operation.

    Graham
     
  19. I think it and the 15 pF are an overcompensation scheme for
    the NE5534.
     
  20. D from BC

    D from BC Guest

    I made my own amplifier electronics and put it in an old (1970's I
    think) amplifier box. The original front label has "solid state
    amplifier". It still has the original rumble filter switch which I
    think is the same thing..
    D from BC
     
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