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Coupling capacitor voltage rating?

Discussion in 'Electronic Design' started by Terry Pinnell, Feb 14, 2004.

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  1. When you use a coupling capacitor to isolate the input and output of a
    simple audio amp from DC, how do you decide what voltage rating is
    appropriate please?

    Checking a couple of old projects, I found virtually no DC present
    *across* the output capacitor, regardless of amplitude, frequency or
    load. So could the rating be arbitrarily low, allowing me to use some
    cheap surplus electrolytics I still have around?
  2. I read in that Terry Pinnell <[email protected]
    For best long-term reliability, if there is no DC present, you should
    use a reversible electrolytic, or two ordinary ones back-to-back, maybe
    with some DC bias applied to their junction. But in many cases, just a
    single ordinary electrolytic is quite OK. I don't like to use anything
    less than 6 V caps, though, in such applications.

    If your caps are elderly, re-form them by connecting to a DC supply
    equal to the marked working voltage and monitor the leakage current.
  3. I would think that with a simple amplifier with a single supply,
    a capacitor is needed, and that you would measure 1/2 the supply
    voltage across the cap.

    With a dual supply, no DC is measured, but you would not need
    a cap then, a protection relay perhaps...
  4. Fred Bloggs

    Fred Bloggs Guest

    Well if there is "virtually no" DC present then the simplest course is
    to not use a capacitor at all.
  5. Jim Thompson

    Jim Thompson Guest

    What is the definition of "virtually no DC"? 1mV, 10mV, 100mV ?:)

    ...Jim Thompson
  6. Jamie

    Jamie Guest

    if your using 2 Elctrolytics with back to back connections then the
    voltage rating is the lowest value of the 2.
  7. Thanks John.
  8. Not sure I follow. I'm using a capacitor (as usual with coupling
    capacitors I thought) to *ensure* there's no DC...

    See the example in my reply to Frank.
  9. Not sure I follow that, Frank. Maybe a specific example would help, so
    here's one of the circuits I had in mind:

    I applied 1V rms as input. Regardless of supply voltage, DC across C3
    is a mere 270 mV.
  10. Your op amp +input dc level is 0V, thanks to R4 and R3. Why is the
    output at 270mV ? I expect that to be closer to 0V.

    I would not expect an output capacitor in this situation, where
    you have dual supplies. The same applies to power amplifiers, or
    bridge amplifiers, like the ones in car-radios. No output cap.

    With an amplifier that uses a single supply, the DC level from the
    output transistors is 'placed' at about Vsupply/2. In that case
    you need a cap between output stage and speaker.
  11. Active8

    Active8 Guest

    Well if there is "virtually no" DC present then the simplest course is
    to not use a capacitor at all.[/QUOTE]

    What is the definition of "virtually no DC"? 1mV, 10mV, 100mV ?:)

    ...Jim Thompson
    Yeah, and how would that affect the next stage is the real
    question. The next stage might run ok, but it might put a DC level
    on the speaker at the quiescent point.
  12. I read in that Frank Bemelman>) about 'Coupling capacitor voltage rating?', on Sat, 14 Feb
    No only that, but eliminating R3, C4 and C5, and making R4 10 kohms to
    earth would give more output swing. A 741 driving 1 kohm is not a very
    happy bunny.
  13. I was a bit puzzled about that C4 & C5 positive feedback. What
    is it supposed to do? And isn't the 741 'verboten' these days?

  14. I read in that Frank Bemelman>) about 'Coupling capacitor voltage rating?', on Sat, 14 Feb
    It bootstraps the 100 k resistor, making it look like maybe 10 Mohm at
    low frequencies. I'd rather not go into what it does at 20 kHz!
    Only by Bill Sloman, and he doesn't count. (;-)
  15. N. Thornton

    N. Thornton Guest


    Firstly caps are rated by voltage, not dc component of voltage, so
    what the dc component is isnt what matters. If you have a dual supply
    amp on +/- 12v you need a 12v output cap, since the cap will see
    nearly 12v at times.

    Secondly output caps are used on dual supply amps when either
    a) the V offset can not be guaranteed to stay satisfactorily low at
    all times.
    or b) as protection to the speaker in case of amp failure, which could
    apply full supply V to the speaker.

    Regards, NT
  16. for audio, NEVER have electrolytics in the signal path if you can
    avoid it. about the only thing worse to use is tantalum.
  17. Ban

    Ban Guest

    That circuit is crap. The guy doesn't know anything about opamps. Here is a
    more appropriate one:

    | | 9-30V
    .-. |
    220k| | | 100n
    | | | ||
    '-' +--||--GND
    | | ||
    |# | |\|
    o-+---|#---+----|+\ #|
    | |# | | >--+----#|---+--o
    | 10u | +-|-/ | #| |
    | | | |/| | 100u |
    | | | | | |
    .-. .-. +---)---+ .-.
    | | 220k| | | | |
    1M| | | | | 1/2 TL072 | |1M
    '-' '-' | '-'
    | | | |
    === === === ===
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    ciao Ban
  18. I read in that Philip A. Marshall>) about 'Coupling capacitor voltage rating?', on Sun, 15 Feb
    Indeed. All professional audio recording and broadcasting equipment uses
    ONLY Leyden jars in the signal path.
  19. Guest

    Most caps rated at dc.
    Many have only 1/4 (or less) rating at 1kHz.
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