Help understanding bypass caps

[DC2100]

Hi, I am sourcing parts to build a few power supplies but I am confused with the choice of bypass caps due to my lack of knowledge. In the past I have used audio grade polyprops for all my bypassing but I would like to see if I can get the same results from 'normal' types. (ok I cant afford nice caps anymore :-( ) I am only looking for caps to bypass my lytics so 10-100nf, I have searched the forum and checked the suggestions spit out when I created my thread but havnt really found what I am looking for. I was wondering if someone could clear up my confusement?

I have been using the usual sources for my selection, I narrow my choice down to mkp or polyprop and then check the data sheets for the avail caps. This is when I loose the plot, every polyprop is either, 'supression', (x1/x2etc) or snubber/pulse mode. I know 1837's are popular but they only go upto about 100v and they mention 'high frequency' decoupling. Ive found some higher voltage mkp's, vishay 1839, these are very cheap but they look the part, unfortunately the data sheet says: 'AC Pulse,
Pulse operations, SMPS and thyristor circuits, storage, filter,
timing and sample and hold circuits'.
The only capacitors, apart from the 'audio grade' types, that state 'bypass' 'coupling/decoupling' in the data sheets are mkt's?
I may have been led down the preverbial garden but I have always took it as read that polyprops were better than polyester when it came to bypassing electrolytics or anything else for that matter regarding audio?
I would be very gratefull if someone could fill in the gaps for me, I dont really want to know that 'v-caps are the best' etc, I just want to try and understand why there are so many different types of polyprop with different uses and if i can just ignore these dam data sheets without affecting the performance of my psu?

Alan

[h_a]

Uhoh, difficult matter ahead.

Subjective fun apart, you can use any film cap for decoupling. The idea is simply to use a cap with better high frequency impedance (means lower) in parallel to the electrolytic (as that hasn't a great high frequency impedance).

So, basically you can use Teflon, Polyester, Polypropylene, even polymere electrolytics, with or without metallization.

If you look closer at the properties of the film, you find simply that also these films are not perfect caps and closest to the perfect cap come Teflon, Polypropylene, Polyester (in that order roughly, differences are pretty small). Metallization gives smaller caps, but worsens its properties slightly.

So feel free to chose whatever pleases you. Or whatever you can or want afford, better a cheap decoupling cap than none.

Last, don't worry what the datasheet says about its intended purpose. What's written there is just some idea of the guy that wrote the datasheet. You can use in principle all films for decoupling, regardless from what the datasheet says. Just choose a sufficient voltage rating.

Have fun, Hannes

EDIT: oh and don't forget that placing the film cap as close as possible to the power consumer is certainly more important than a few % difference in technical properties.

[DC2100]

Thanks for that Hannes, I was begining to think I had opened the holy grail or something!

[cbdb]

These caps are only there to minimize the RF on the rail. No audio passes thru them so you dont need audio grade caps.

[analog_sa]

Originally Posted by h_a

better a cheap decoupling cap than none.

Not sure what kind of factor price is but IME random bypassing often results in worse sound than no bypassing. Actually i find the process of selecting size and type for the bypass so arduous that usually altogether forego bypassing in analogue circuiits. Maybe just a little local decoupling. Digital circuits are a completely different matter and bypassing and lots of local decoupling is an engineering requirement.

[DC2100]

Thats the kinda thing I was hoping to hear aswell cdbd, I was just overwhelmed with the amount of applications for these things, cheers.

analog_sa:
This is indeed a supply for an analogue output stage, of a cdp using valves. The designs I have looked at have all used bypass for the electrolytics without specifying anything more than, '10 or 100nf film&foil polyprop' one of these designs is by Thorsten Loesch so I am guessing it is probably very accurate. Its a bit large on the parts count for this particular cdp so i was just getting a feel for the basic concept and wanted to order some 'off the shelf' bits. I will see if there are anymore opinions on the subject before I do though as you have given a cheap alternative, none! I dont have a scope so I rely on tried an tested circuits that have been freely published. Cheers.

[Gordy]

Sounds like you previously fell for the audiophile-guru nonsense. Good to see you heading in the right direction now.

One approach is to not bypass anything and simply assess it that way. Then decide.

For general background info on supply bypassing have a little read...

http://www.analog.com/static/importe...8865AN-202.pdf

http://www.designers-guide.org/Design/bypassing.pdf

http://www.tentlabs.com/InfoSupport/...decoupling.pdf


: )

[DC2100]

Thanks Gordy, Its going to take me a while to get through that lot but its very helpfull, seems the 'thank' icon has gone?

[DSP_Geek]

Originally Posted by DC2100

Thanks Gordy, Its going to take me a while to get through that lot but its very helpfull, seems the 'thank' icon has gone?

One more note: I wouldn't worry about using polypro for bypassing, as their dissipative characteristics are more important for series applications like crossovers or coupling. As others have mentioned, getting the bypass closer with a slightly "worse" capacitor is more useful than a "better" cap you have to place farther away because the thing's so big. I bypass my radial electrolytics with little ceramics wired right into the legs, which seems to work well.

[wintermute]

Hi DC2100 If you are still worried about finding something that mentions audio then you could consider these evox rifa SMR caps

ALso might want to read Walt Jungs Picking capacitors article part1 & part2

The above is an excellent article, quite technical but demystifies the humble capacitor with real tests which reveal distortions and what to look for when choosing a capacitor for audio:

here is a quote of the summary of the article:

If we have done a good job on this article, a glance at Table III and considerations of the distortion discussions should allow the reader to easily select a good capacitor. For reasons of practicality and other rationalizations, there are the inevitable trade-offs. However, here is the way we see it.

Up to values of about 10,000 pF, polystyrene is the best all around choice, as it has reasonable size and is readily available in many sizes, with tight tolerances available. Above 10,000 pF, and up to 0.1 u F, it still can be used but is much harder to obtain.
Above, 0.1- u F polypropylene (or metalized polypropylene) is the dielectric of choice, as it has nearly the same relative qualities of DF and DA as polystyrene. Tight tolerances are available (but will be special order), and you can get capacitors up to 10 u F or more.
Teflon may well be the best dielectric of all for audio, but is produced in limited volume and is generally not practical. Parylene is an excellent dielectric also, but limited in electrical size ( 1 uF or less) and not widely available. Polycarbonate is perhaps the next best all-around choice behind these and is generally available in a wide range of values.
Polyester types are the most widely available for all the films and are already widely used in many audio circuits. There is no doubt that this is due to the generally low cost of these capacitors, but convenience and low cost should not be primary selection criteria to a critical audiophile. Polyester capacitors can be readily heard in good systems, with defects similar to those described for tantalum but, of course, reduced in magnitude.
In our opinion, polyester capacitors should be very carefully applied in an audiophile's system, and any system using them in the signal path may potentially benefit by the substitution of (equal value, voltage and tolerance) polypropylenes or polycarbonates. We have done this ourselves on different items of equipment, tube and transistor, with always the same result�a stunning upgrade in sound quality. Further, we have observed others do similar things, either completely independently or at our direction, with the same type of results.
It is not surprising to us that this type of reaction occurs, since one single polyester or electrolytic (or other polar type) can be heard, and a typical update to an old preamp or amp might replace a dozen or more! If you did nothing more than take an old (stock) Dynaco PA5 preamp and change the capacitors to polypropylenes, you can be literally astounded at the results. All of this is available at moderate cost to anyone who can solder, and you need not send your amp off to the specialty audio shop either!

I decided on PPS caps after reading this, based on their comment that after polystyrene & polypropylene, polycarbonate is the next best all round choice... Polycarbonate are no longer available, but PPS are generally considered to be the replacement for Polycarbonate and tend to measure better in almost all regards.

Tony.