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Do I need shunt capacitors when using low ESR electrolytics?
Do I need shunt capacitors when using low ESR electrolytics?
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Old 11th October 2020, 07:56 PM   #1
quietcarlota is offline quietcarlota
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Default Do I need shunt capacitors when using low ESR electrolytics?

I am upgrading the filter caps on the linear power supply of an old tube pre-amp, and I was wondering how important it was to use small value shunt capacitors in parallel with the electrolytics if my replacements are already low esr (in the 100s of milliohms for caps in the 10s to 100s of microfarads)?
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Old 11th October 2020, 08:25 PM   #2
wiseoldtech is offline wiseoldtech  United States
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I've often added some 0.1uf' mylars across each cap in a CLCRC supply with no worries.
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Old 11th October 2020, 10:55 PM   #3
klasdiy is offline klasdiy  Sweden
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It depends on the capacitors frequency response. Take a look i the data sheet for 100 kHz. Modern eletrolytics are far better than old, but I do not know if it is so for HV caps as well. And polypropylene is better due to lower losses, if you care of all details.
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Old 11th October 2020, 11:46 PM   #4
benb is online now benb  United States
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Tube devices operate at a much higher impedance than solid state, so there's much less of a need to bypass electrolytics. Then again, extra caps are cheap enough for one-off DIY designs, so it can't hurt.
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Old 12th October 2020, 12:51 AM   #5
bucks bunny is offline bucks bunny  Germany
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Wherever low inductance rails are required - specially with class d-amps - these low inductance caps are located as close as possible to the sensitive part of the circuitry.

A shunting cap placed directly at the bulk caps is without effect simply because the wiring inductance is in between.

Certainly these do no harm.
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Old 12th October 2020, 04:07 AM   #6
PRR is offline PRR  United States
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Do I need shunt capacitors when using low ESR electrolytics?
Just close your eyes and pop a cap on?

The great thing about electronics is you can DESIGN with confidence.

Who here thinks about resonances? Every part has R L and C. Every part has at least one resonance. Any two parts together will have *multiple* resonances.

There was a time when electrolytic caps had large ESR and also large inductance. Back in 1959. Today the inductance is similar to a piece of wire between the cap terminals.

Say you put 0.1uFd across 40uFd. The 40u cap Z drops from 4r @ 1kHz toward 0.7r @ 10kHz. It may typically have 2uH of inductance which drives Z higher.

With a 0.1uFd across that, we have a parallel tuned circuit a lot like the input to an AM radio. (Who here built crystal radios?) Like the radio the impedance goes HIGH at some point. For the estimated values here, around 330kHz. And 40 Ohms, far worse than the 40uFd alone.

This don't help. Does it hurt? Most tube audio amps have little gain at 330kHz. Not just the nominal gain between jacks, but any internal gain may make trouble.

If you "need" to do this, you may actually want "high ESR" which would spoil the Q of the resonance, lower and flatten the 330kHz resonant tuned circuit.
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Old 12th October 2020, 05:51 PM   #7
klasdiy is offline klasdiy  Sweden
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Oh yes... Combinations with electrolytics and ceramics should be avoided of resonance reasons. But if you add 10u/400V to the big (100u or so) low ESR electrolytic? The resonance frequency will be lowered and may interfere with the signal. Many years ago I constructed a signal filter for telephone lines (to damp frequencies from 4kHz to some MHz) and struggled with a lot of resonances which needed to be damped out with resistors. Yeck. So, OK, I'll take back the most of what I wrote before and calim the opposite.
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Old 12th October 2020, 08:16 PM   #8
Elvee is offline Elvee  Belgium
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Some real, measured data can be found here:
paralleling film caps with electrolytic caps

Draw your own conclusions....
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Old 12th October 2020, 10:39 PM   #9
bucks bunny is offline bucks bunny  Germany
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I value your real measurements with different cap combinations. My conclusion is that these impedance curves are irrelevant to real audio amps as they where probed directly at the bulk caps terminal - certainly the best way to do it. But at the point where this matters you will find the local caps that dominate impedance no matter of any far distant shunt caps. Otherwise this was a poor design by itself.
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Old 13th October 2020, 07:22 AM   #10
Elvee is offline Elvee  Belgium
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Local bypass is OK, but many people simply add a 100nF cap directly across the E-caps (and I think it is the subject of this thread)
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