Use bleeder resistors in audio opamp design?

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?

Thanks, Grant.

Reply to
Grant
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It doesn't matter.

Why add resistors to series electrolytics? Let the voltages balance themselves. Equal leakage currents sounds like a good deal to me.

It's not a problem.

John

Reply to
John Larkin

"Grant"

** 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

Reply to
Phil Allison

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! Rich

Reply to
Rich Grise

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

Grant.

Reply to
Grant

"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

Reply to
Phil Allison

"Grant"

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

No reply required....

... Phil

Reply to
Phil Allison

Do you?

Thanks, Rich

Reply to
Rich Grise

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?

Thanks, Rich

Reply to
Rich Grise

"Rich Gripe"

** How ambiguous.....

** Then you can tell a funny capacitor joke.

Hey:

No f****it hypotheticals .

Ever.

You stinking TROLL.

... Phil

Reply to
Phil Allison

Why is Phil such an asshole anyway? The few posts I've looked at recently rapidly turn into an argument with Phil being, by far, the least civil and throwing around the most four letter words.

Statistically speaking, it seems highly unlikely he's the rational one here, while everyone else has behavior and anger issues.

---this space reserved for Phil's almost certain nasty response---

---end space---

Reply to
hondgm

Film caps can go from near zero leakage to punch-through shorted in microseconds, so it's not safe to let them balance themselves. Electrolytics and ceramics generally have greatly increasing leakage at or above rated voltage, so strings self-balance. Startup transients must be considered, of course.

I've experimented a little with polymer aluminum caps. They have very low leakage up to roughly 2x rated voltage, then die suddenly. So they probably don't equalize and aren't very well suited for series-string use. Pity.

John

Reply to
John Larkin

He usually makes sense technically, and is occasionally even neutral and helpful.

The real looney here is Nymbecile, aka AlwaysWrong. He is universally profane, abusive, innumerate, and of course wrong. Scat fetish, too.

John

Reply to
John Larkin

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?

Thanks, Rich

Reply to
Rich Grise

I used to shocked by Phil's outbursts, but then I noticed his technical knowledge is very broad and deep. And sometimes he's damned funny, especially compared the usual off-topic nonsense that dominates this once-great n.g. Rich Grise is a case in point - rarely helpful and usually inane. Years ago he chucked it all for a life on the beach

- I cannot figure why he came back.

-- Joe

Reply to
J.A. Legris

You can have broad and deep technical knowledge, and share it even, without descending to swearing and petty insults.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Shouldn't be a problem... just let them float. Though, for simulation, you'll need paths to ground :-)

If your OpAmps "feature" class-B outputs you may want to bleed enough load current to force the stages into class-A. I've experienced problems with some brands of LM324 having enough dead-band to produce wild spurs in active filters. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Up to 20 KHz, maybe.

John

Reply to
John Larkin

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.

Grant.

Reply to
Grant

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 ;)

Grant.

Reply to
Grant

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