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Use bleeder resistors in audio opamp design?

Discussion in 'Electronic Design' started by Grant, Mar 23, 2011.

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  1. Grant

    Grant Guest

    I'm making some audio filter circuits, using 1% resistors and capacitors.

    Capacitors only available in 22n, 33n or 47n cheaply, so I use them in
    combinations to get a reasonable impedance points. One filter would
    use three caps in series -- what happens to the DC isolated nodes on
    the series connected capacitors here?

    Would you put a high value resistor, 10M or something, across caps in
    series? I recognise the need to balance DC midpoint with electros in a
    power supply, but what of signal nodes around an opamp?

    Around one opamp I have three in series caps to ground, two in series
    on the feedback (8.25n and 16.5n from 22n and 33n caps).

    Anyone have a horror story of what happens if one leaves these nodes
    'floating' DC wise. Or do nothing at all, board leakage will fix it?

  2. Phil Allison

    Phil Allison Guest

    ** Where do yo get your 1% tolerance 47nF caps ??

    ** Nothing much - it will acquire a voltage near to zero DC.
    ** Many designers do not like to see electros with more than rated DC volts
    across them - so they add " ballast " resistors to swamp electro leakage
    in series strings. But long as the electros are the same type and age and
    have a reasonable margin of voltage over that needed - such resistors
    hardly matter.

    I regularly put 350 volt electros in series and run them across a 500 volt
    rail with no ballast.

    ..... Phil
  3. Rich Grise

    Rich Grise Guest

    If the total DC voltage end-to-end of the series string is less than
    the voltage rating of the individual caps, then it shouldn't be necessary
    to do any "balancing: - let each cap develop as much voltage as it needs,
    and don't worry about artificially "balancing" them.

    If it's such a critical circuit that ESR becomes an issue, then that's
    beyond my purview, and you'd need a "real" engineer.

    Hope This Helps!
  4. Grant

    Grant Guest

    Usually present in PC power supplies where they'll put a couple 200VDC in
    series for the rectified mains, 150k is common. Wide variation.

  5. Phil Allison

    Phil Allison Guest

    "Rich Gripe"

    ** How ambiguous.

    ** Where a series string of poly film caps must withstand a DC voltage that
    is comparable with the SUM of the individual cap voltage ratings - the use
    of balancing resistors is almost essential.

    The alternative is a kind of Russian roulette to decide which cap will be
    overvoltaged enough to make it suffer an internal, insulation failure
    discharge next.

    Snap, crackle, pop.

    ** Instead of a know nothing jerk off like you.

    .... Phil
  6. Phil Allison

    Phil Allison Guest


    ** Do you ever take any notice of what folk say to you ??

    No reply required....

    .... Phil
  7. Rich Grise

    Rich Grise Guest

    Do you?

  8. Rich Grise

    Rich Grise Guest

    What part don't you understand?
    If you've got three capacitors, each with a breakdown voltage of, say, 100V,
    in a circuit where the maximum total voltage across the string is, say, 50V,
    which one gets the overvoltage?

    Where do the extra volts come from?

  9. Phil Allison

    Phil Allison Guest

    "Rich Gripe"

    ** How ambiguous.....

    ** Then you can tell a funny capacitor joke.


    No fuckwit hypotheticals .


    You stinking TROLL.

    .... Phil
  10. Rich Grise

    Rich Grise Guest

    What kind of insane voltages are you using in your opamp circuits where
    you could punch through a series string of 100V caps? For punch-through,
    from where are all those volts coming?

  11. Grant

    Grant Guest

    I'm not doing that around an opamp circuit ;)
    But at low voltage/power levels? These are 160VDC, 100VAC caps in a
    nominal 1V level audio signal chain. Perhaps I should have mentioned
    the caps I have are polypropylene film type 1837 MKP and although
    there's not much power involved around an opamp, I simply wonder if
    there's some effect that would be heard? Like a hissing or crackling
    as the nodes balanced themselves over board leakage, or whatever.
    Not used polymer electros. Replaced a lot of low ESR electros in PC
    related gear (mobos and monitors) that seem top be selected at too low
    a rating for the job. There I try to fit higher voltage caps, but that's
    a different topic.

    Another OT: I've replaced failed mains 400V electro with a pair of series
    electros and ballast/balance resistors, to fit the space of the blown
    original. That monitor still running too after couple or more years.

  12. Grant

    Grant Guest

    Thank you, I did discover that in the early 80s when designing and
    prototyping a new portable LVDT readout unit. The LM324s were great on
    12V single supply, apart from needing that resistor to ground to kill
    the easily seen crossover distortion.

    For these filters I plan to use AD847, it's all high level audio signal
    and the opamps in follower mode (S-K filters). I bought a fair few at a
    decent price and individual opamps easier to force into class A than a
    quaddie on +/- 15V, for dissipating the extra heat?

    Probably use current mirrors for constant current sink to -15V. There I
    wonder at signal cross-coupling via the current sinks, maybe I should
    plan on individual sinks? This is not a production design where I have
    to minimise component count.

    Years ago (1981/2) I designed a LVDT portable readout unit, used the
    'proper' synchronous rectifier and related stuff to get measurement
    performance, and the LM324 on 12V single supply.

    Discovered the crossover distortion for myself and the obvious cure of
    loading the output with resistor to 0V, since they have that large source,
    lower sink current output design, just begging for more current to ground
    when run on single supply. Turned out to be the most accurate instrument
    the boss used, and first that showed an LVDT's 'S' curve, quite nice :)

    (LVDT: Linear Variable Displacement Transducer, not the modern signaling
    technique ;)

  13. Grant

    Grant Guest

    Phil is himself, and doesn't stand nonsense, a bit overboard in his
    responses, but I worked on building sites as a teenager, the language
    doesn't fuss me. Some people have short fuses (fuzes?), I don't have
    to buy into that.

    In this particular case I dunno if he was impatient for my reply to
    his query (where to buy 1% 47n, which I answered) or what?

    I don't see what else he's fussed about unless it's because I don't
    see the point in replying to posts I agree with, nor do I see a point
    responding to Rich G. when he was talking about a different point to
    what I'm interested in. I find myself quite busy lately (surprisingly
    for someone left the workforce in '93), so I don't spend a lot of
    time here.

    It's nice to pop in the odd questions and see responses, but I'm not
    here to play flame games. Unless I am ;)

  14. Grant

    Grant Guest

    Interesting thought. Worth a look at.
    Except the next available higher voltage is from power amplifiers, very
    ripply, so there's some interesting stuff for me to think about in there.
    That too.

  15. Grant

    Grant Guest

    What's wrong with using bipolar caps, the leakage? I've never liked
    back to back as they sort of expect diode action to keep the electros
    nice, so what does that non-linearity do to a signal? Or the cap itself
    after a while.

    Then there's all those circuits out there with little, none or slight
    reverse bias on tantalums in audio signal paths, how do they sound
    after a while?

  16. Phil Allison

    Phil Allison Guest


    ** Thanks - they were formerly very hard to get items in Australia.

    Farnell had some ( expensive) 1% polystyrenes up to 10nF and that was it.

    ..... Phil
  17. Phil Allison

    Phil Allison Guest

    ** My oversight, now corrected, in not replying.

    ** You have a reading comprehension problem.

    The concept of "context' seems beyond you.

    ...... Phil
  18. Phil Allison

    Phil Allison Guest

    "Bill Bowden"

    So, what's your opinion of using electros back to back to form non-
    polarized caps?

    ** Ask the NG, not me - shithead.
  19. Grant

    Grant Guest

    Too right :)

    There's more practical factors in my mind, like building a well known
    topology that will work first time.

  20. Grant

    Grant Guest

    That's why I'm using a couple windoze apps to do it for me ;)

    Anyway, as I was uncomfortable with series caps I ordered some 10n today
    for where I was going to series up to get lower value. I might even use
    the remainder of the 50 caps one day on ADC/DAC stuff.

    I did say up front I'm processing audio, though I do like the DC unity
    gain because I'll possibly use the low pass on data, dunno yet. Direct
    coupled is good too. Better than back to back electros.

    If I was a tad brighter, younger and better at maths I'd look further
    into other topologies, but you need to try harder to scare me into
    doing more than taking a brief glance at your leapfrog whatsits :)
    I never heard of them before. I used to work in industrial electronics,
    was called on to make a filter card once, multistage low pass thing
    with lots of trimpots, yuck. Stayed out there so it must've worked.

    That was DC accurate stuff for signal conditioners too. So there's my
    bias showing. If it don't work I'll chalk it up as lesson learned, be
    here back with a sad story to tell.

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