Mica caps - what values to buy

[motterpaul]

I have seen some circuits where using "mica" caps is recommended, especially between certain pins of op-amps. Usually in the range of 82pf.

I want to get some of these - what values of these would you recommend keeping on hand - where they are preferred over ceramic? Between ___pf  and ___f ?

[Silver Blues]

Silver mica caps, especially compared to the ceramic ones of the same value, are expensive as fuck. You can still find some okay deals on eBay but retail they're very costly. I wouldn't consider anything in a pedal circuit to be critical enough to warrant one. I've only ever used/seen them used in RF applications, because their extremely high precision and low losses at high frequencies.

I can't imagine an application where a mica cap would make any considerable improvement in a stompbox.

I'm almost sure you're asking because of the Bacon Bits. You'll notice no such recommendation exists in the 2014 version's build notes. That cap is just part of a filter.

However to answer your question I'd get pF values. 27-470 should do fine. You can get them in small nF values as well, but at that point you're better off with film.

[motterpaul]

You are right. I built Bacon Bits. Thanks for the reply!

[Silver Blues]

No problem. I edited it a bit for clarity.

[IvIark]

I've got them from 3pf to 1nf.  I found someone selling a bulk lot a while ago which were 25 x about 15 values, a lot of which were very low, but got reasonably cheaply so wasn't too bothered.  The rest I bought mostly from KW Tubes on eBay who used to sell lots of values up to 1nF very cheaply.

http://stores.ebay.co.uk/KW-TUBES

These were the values I got, and I do use them sometimes and will use them in the future for "premium builds", but I really like multilayer ceramic and you can get them for pennies.  I've marked with a cross those I've used most of.

3pF
6pF
20pF
33pF
39pF
50pF x
68pF
82pF x
99pF
100pF x
105pF
140pF
150pF x
240pF x
240pF
253pF
270pF
330pF x
360pF
360pF
390pF x
430pF
470pF x
750pF
820pF
820pF
1nF

[motterpaul]

I saw a very similar variety pack on Ebay for not too much (considering how many pieces you get) - 200 capacitors for $39.

http://www.ebay.com/itm/Assorted-100V-Radial-Silver-Dipped-Mica-Capacitors-LOT-/121454114976?_trksid=p2054897.l4275

Pretty much the same values as you mentioned - but then I realized I probably don't need most of the values and would probably save if I just selected a few, like the ones you pointed out.

So, I also see bundles of 25 pieces of one value for just about $2.00 - that is reasonable.

I am still learning what caps do in this application (I know, I need to  look it up). If they might reduce noise or oscillation in certain circuits then I would go for it.  I also understand most guitar effects are not "audiophile" builds and the range of audible frequencies is certainly not in the RF area, but I also know that RF can affect noise levels with high impedance inputs, when using cheap cables, for example.

So - while they wouldn't improve the tone, it is possible they could reduce the noise levels. Is that the idea here?

[IvIark]

Ceramic caps can be microphonic, and the standard ones and even some of the multilayer dielectrics have poor tolerances and susceptibility to variation due to things like temperature.  Mica can be found with very tight tolerances, I have some which are 1 and 2% and have much better temperature stability.

You just have to remember that any cap type can be used if you can get it in the correct value.  Differences aren't likely to be significant assuming the measured value is close, but sometimes it's nice to have choice just so you can try various options yourself and see what is and isn't important for you.

[motterpaul]

yeah, very good point. There might be things I hear in circuits that other people wouldn't hear or maybe wouldn't care about that would make a big difference to me. Like anything else.

[Silver Blues]

That's absolutely true, of course. Perhaps I spoke without completely fleshing out my idea in my first post there. If absolute precision is what you're after, then by all means, go for it. As far as differences go:

In terms of tone by virtue of material, probably none.
In terms of tone by reducing compounded tolerances, it really depends how many caps you'd be "replacing" and how out of spec they were to begin with. If you replace ten caps that were all off by 8% then yes, they will make a difference, probably one you can hear, but if you replace one cap that was only off by 1% then it really won't.
In terms of temperature stability, that also depends on the environment in which you play, and whether or not you can hear discrepancies attributed to this fact in the first place. If you gig in a range of temperatures and find your pedal sounds different each time, mica caps will definitely help you. By the same token, you can get temperature-stabilized ceramic caps (ever seen the ones with little spots of color painted on top?) but that's your prerogative, for the prices eBay can offer you may as well get the micas.

Just my opinion, not to be taken as absolute truth

[JaviCAP]

In my opinion, with the exception of ceramics over 1nf (because of the high tolerance, and that as Mark tells, can become microphonic, there's no real difference in audio while using the same measured values.

Silver mica caps are great, but have found no difference in sound from ceramic discs from 10pf to 680pf. I'd even say that ceramics used for filetring purposes are my first choice due to their lower inductance.

Not to forget, that in some effects, like Big Muffs, ceramics are a must if you wanna catch the original sound, as on several nanofarads values, they're prone to be noisy and harsh...as the effect used to be.

Many people has asked me how I get the sound of my Big Muffs, and the answer is easy (even thought many ones refuse to believe it): Follow the schematic, forget about filtering caps, use the lowest quality caps that you can find, and don't use a wallmart PS.

On the other hand, would never use ceramics on clean or modulation effects, because what you try to get is just the opposite.

So, if in doubt, just socket & try.

J.