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Old 9th November 2011, 09:23 PM   #1
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Default Cap bypass for dummies

I was wondering if someone could explain to me the applications/benefits of a bypass cap.

Correct me if I'm wrong but from what I understand instead of replacing an existing cap with a better more expensive one you can use a lesser value cap (10% for example) that is of better quality to bypass it . Thus imparting some of its sound characteristics.

Also from what I understand this is done by soldering the leads of both caps together in parallel.

Is this generally done to bypass film caps or could it be done with electrolytics as well?

Ex 1 : Bypass 2200uF lytics in power supply
Ex 2: Bypass 3.3uF film caps in tube output
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Old 12th November 2011, 03:28 AM   #2
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You can't change the important characteristics - ripple current, esr, leakage current, tan theta, inductance, etc, but you can alter some aspects to improve things a little in some aspects.

Caps are very much horses for courses. So if you have a course in mind, then it's better to as specific as you can be and ask about the specific application you have in mind.
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Old 12th November 2011, 04:22 AM   #3
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Good question.

I would love to learn some more about this subject also.

Dean
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Old 12th November 2011, 05:11 AM   #4
qusp is offline qusp  Australia
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its all about the frequency of interest. if you are wanting for example to get rid of high frequency noise on power lines at a chips power supply pins, then you may want to bypass the larger electrolytic caps with a small value film or quality ceramic cap as they have a much lower impedance at high frequency, so will provide a more attractive path to ground for the high frequency noise than the pins, where the electrolytic cap may have high impedance at high frequency and not provide a path to ground that the unwanted currents would rather take.
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Old 12th November 2011, 05:32 AM   #5
benb is offline benb  United States
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While not directly answering the OP's question, interesting reading is the two-part article "Picking Capacitors" linked to in Walt Jung's list of articles:
Services
I googled capacitor frequency response to look for those "V" graphs of capacitor impedance (magnitude) with frequency, and stumbled across yet another Rod Elliot page:
Capacitor Characteristics
The "V" graph is fig. 14 on that page. The idea of bypassing is that smaller capacitors have their resonant frequencies (the bottom of the V) at a substantially higher frequency, and thus help the larger capacitor maintain low impedance out to this higher frequency. Whether this is actually necessary in an audio coupling circuit is hotly debated, and Mr. Elliot clearly argues that bypassing is not necessary due to the inductive reactance portion of the graph still offering a very low impedance up into MHz frequencies.

I feel confident other posters will be along with an opinion different from Mr. Elliot's, and I can't wait to read them. <insert popcorn-eatimg emoticon>
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Old 12th November 2011, 05:47 AM   #6
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Good question.

I would love to learn some more about this subject also.

Dean
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Old 12th November 2011, 10:39 AM   #7
marce is offline marce  United Kingdom
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Benb,
great article from Mr Elliot, nice to see some sensible veiws on MLCC caps for decoupling.

A couple of links that may help Thunk, the AVX site has a lot of info...
PCBDESIGN007 Inductance of Bypass Capacitors, Part I
- Technical Articles AVX Online
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Old 12th November 2011, 11:15 AM   #8
DF96 is offline DF96  England
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If inductance of the big cap is a problem then adding a small cap in parallel can make things worse by creating a parallel tuned circuit. The size of the peak you have created depends on circuit losses, including ESR: the lower the ESR the higher the peak. Adding a bypass may sometimes change the sound of a circuit, but not necessarily for the better. You need to measure (or calculate) what you have, then decide whether a bypass will improve it or make it worse.
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