Film capacitor selection - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: November 05, 2011, 10:51:33 am
content: I'm designing a guitar pedal and googling around a bit, I've gotten the understanding that ceramic caps are unsuitable for use in the audio path because of non-linearities and microphonics. I still use ceramics for decoupling since I refuse to believe that this has an effect on the sound (I'm certainly no audiophile). For the audio path, I'm using film caps instead.But it seems that compared to ceramics, the selection of films is much more limited. For some sizes, it's fine. Anything above 1 nF and under 1 uF seems to be common but outside of this, the selection seems to be limited to weird sizes. When designing a filter, I needed a 560 pF capacitor but I couldn't find any in a reasonable size. Going to 680 pF, there was a few which could work from Wima but the were very big witih widths of 4.5 mm over the usual 5 mm with and ~ 7.5 mm length. This is looking at Farnell anyway.Am I missing something? Is it the scale of the rest of my feedback network which is off? Should I change the resistor values so that they allow for larger cap values? Is there some capacitor type I haven't heard of?

author: Unknown
author_rank: Unknown
timestamp: November 05, 2011, 11:54:16 am
content: Panasonic has some PPS caps in 560pF (Digikey carries them), no idea about the audio properties. Film caps tend to be much larger and more expensive than ceramic, especially the self-healing high(ish)-voltage ones.

author: david77
author_rank: Frequent Contributor
timestamp: November 05, 2011, 11:57:07 am
content: You could look at mica caps, they are suitable for audio stuff and are available in smaller sizes like ceramics. However they tend to be a little bit more expensive than film caps.WIMA used to make the blue and yellow FKC type film caps, I think they were available in values below 1nf. Not shure if those are still around, though.I've just look them up: Wima FKP are available starting from 100pf.https://www.buerklin.com/default.asp?event=ShowArtikel(42D2602)&l=e&jump=ArtNr_42D2602

author: shadewind
author_rank: Frequent Contributor
timestamp: November 05, 2011, 12:09:27 pm
content: I think I'm starting to get it... film caps are simply expensive and large? I just find it odd that I can find a 22 nF cap in a reasonable size but not a 680 pF one.

author: Unknown
author_rank: Unknown
timestamp: November 05, 2011, 12:17:58 pm
content: The PPS one is 1206. My guess is that the 680pF one you refer to is something like a self-healing 630V cap designed to withstand large transients, and the 22nF one is not. The FKP one appears to be a more reasonable size. Note that there are also supposed to be differences in frequency response/distortion between eg. polyester and polypropylene, although I'm not sure how much of this is electronics vs. audiophile fantasy.

author: david77
author_rank: Frequent Contributor
timestamp: November 05, 2011, 12:27:59 pm
content: What exactly are you looking for? SMD or through hole? Farnell do have 560 & 680p film in 1206 in stock, they also list 560 & 680p micas in through hole version.I wouldn't worry too much about what type of film cap or mica to use, I'd certainly not recommend ceramics though due to microphonics. I think that could have some nasty effects in a guitar pedal that people stomp on.

author: shadewind
author_rank: Frequent Contributor
timestamp: November 05, 2011, 12:52:27 pm
content: Quote from: david77 on November 05, 2011, 12:27:59 pmTo be honest, as small as possible. I have no problem with 0402 but obviously there aren't many film caps in that size.The reason I decided to use through-hole caps was because I was worried about breaking them when reflowing with hot air but it seems that it won't be a problem with the SMD types since they seem to a bit more durable. So I think I'm gonna redo the layout using SMD caps.Panasonic EQH series caps seem to be plenty in stock at Farnell so maybe those will work?http://se.farnell.com/jsp/search/browse.jsp?N=203072+727+502+110130329+100067&Ns=P_PRICE_FARNELL_SE%7C0&locale=sv_SE&appliedparametrics=true&getResults=true&suppressRedirect=true&No=0&divisionLocale=sv_SE&catalogId=&prevNValues=203072+727+502+110186063+110130329+100067&mm=100000||,1002849||,&filtersHidden=false&appliedHidden=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D203072%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Dsv_SE%26divisionLocale%3Dsv_SE%26catalogId%3D%26prevNValues%3D203072Anyway, I bet there's a lot of traditional wisdom when it comes to guitar pedals but I'm not sure I care a lot, really. I have no desire to replicate something classic, I know exactly what I want to do, build a simple filter with low noise to boost the high mids a bit.I have a bit of a fetish for making boards as tiny as possible so the SMD caps will help a bit with that

author: FenderBender
author_rank: Super Contributor
timestamp: November 05, 2011, 03:09:29 pm
content: Never used SMD film in audio, so I couldn't really tell you, but some cheap polyesters will likely be fine. If you want more reliability, you might try some polypropelene, but they are typically larger, and I doubt you could find too many good ones in SMD packages.

author: jimmc
author_rank: Frequent Contributor
timestamp: November 05, 2011, 07:35:39 pm
content: Do you really need a film capacitor?Your understanding is correct forsometypes of ceramic caps, however class one dielectrics (COG or NPO) are linear and non microphonic. These low-K dielectrics limit the maximum capacity available but 560pF is no problem at all.For a physically small SMD capacitor this would be my first choice especially if hand soldering.The higher values more commonly used in audio circuits require the use of medium or high-K dielectrics (X7R, Y5V, Z5U etc) which exhibit the non linearity and microphony you mention.For more information see...http://www.kemet.com/kemet/web/homepage/kfbk3.nsf/vaFeedbackFAQ/0EA15565AFC16E668525722500594468/$file/2006%2007%20ArrowAsiaTimes%20-%20MLC%20Noise.pdfandhttp://pdfserv.maxim-ic.com/en/an/AN4333.pdfJim

author: shadewind
author_rank: Frequent Contributor
timestamp: November 05, 2011, 08:55:08 pm
content: Quote from: jimmc on November 05, 2011, 07:35:39 pmI'm going to need larger capacitances as well. But I suppose I can mix and match as I like?

author: jimmc
author_rank: Frequent Contributor
timestamp: November 05, 2011, 10:35:45 pm
content: I don't see why not, unless you need matched capacitor ratios over a wide temperature range.(The tempco of NPO/COG is 0+30ppm, much less than film capacitors.)NPO/COG caps are available to >100nF but start to become expensive around 10nF and above.If you're interested in further information Cyril Bateman wrote a series of articles "Capacitor Sound" in Electronics World back in 2002. (Not sure about copyright so no direct link but easy enough to find.)Jim

author: BravoV
author_rank: Super Contributor
timestamp: November 06, 2011, 02:59:19 pm
content: Quote from: jimmc on November 05, 2011, 10:35:45 pmHey Jim, that reminds me the copy I have but its lurking somewhere around, maybe its time to find and give it another read.Sharing another reading about capacitor that is in my bookmark ->http://sound.westhost.com/articles/capacitors.htmIf you're bored, just skip it and jump to the conclusion at the end, below are quoted parts from this link above ..."One thing you can count on ... if anyone wants to sell you 'special' capacitors, designed to replace 'inferior' types (such as polyester, aka PET, Mylar®, etc.), then you know that there is a problem. These vendors are cashing in on the audio snake-oil bandwagon. Like cables, many of their offerings are likely to be of good quality, but at many times the genuine value of the part. Others will be perfectly ordinary parts that have been re-badged. For example, there are many capacitors sold as polypropylene that are actually PET or Mylar. It seems that no-one has ever heard the difference, simply believing that it is polypropylene, so therefore sounds 'better'.""DIY audio is supposed to be fun, not an endless search for the mystery component that will make everything sound wonderful. Sad news ... that component does not exist.".

author: shadewind
author_rank: Frequent Contributor
timestamp: November 06, 2011, 05:55:28 pm
content: Yeah, this why I'm asking here instead of on an audio forum because I don't trust the objectivity of audio people. I believe high fidelity comes from good design and sensible component selections, not magic capacitors.

author: ciccio
author_rank: Frequent Contributor
timestamp: November 06, 2011, 06:30:41 pm
content: It is very difficult to source film or mica capacitor smaller than 1 nF.If you are referring to the schematic you report in your other thread, you can reduce the values of the resistors so to increase the capacitor values and arrive to an available one.Yes, ceramics have a "bad sound reputation ", and with reasons (I've verified this many times, you can even measure their distortion), but it was years ago, the new dielectrics are better.In any case, they sounded, and measured, better than a polarized electrolytic without polarization....regards