Another Noob Question - Film Vs Ceramic disc Caps

[Neta]

Hey guys,

Sorry for all the silly questions, I just really want to do everything perfectly since I'm usually really sloppy.
In DIY Layout Creator the symbol for film and ceramic cap is the same. How do I choose?
I've found that ceramic have non-linearity in their response but I saw in some places they specifically choose it. Is it desirable sometimes?

The layout in question: http://www.diystompboxes.com/smfforum/index.php?topic=97041.0

Thank you as always!

[R.G.]

There are a couple of principles at play here that will stand you in good stead, and help you understand things.

1. There can be no disagreement about matters of taste.
If someone says they like something, they are making a statement about something that is going on in their head, and there is no way to read their mind and refute that. You may disagree with their choices, even violently or viscerally, but there is no way to refute that they like it. The important thing to remember is that what someone else says about something being good (or especially "the best...") may be an opinion, not a measurable fact.
2. If you can measure it, you can call it a fact. If you can't measure it, it is an opinion.

It is a fact that film and ceramic capacitors can be measured to be very, very slightly different. Whether this is a good difference, a bad difference, or not particularly audible is an opinion.

[amptramp]

Here is a page with some answers and links to two of his other pages:

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/caps.html

One of the problems with ceramic capacitors is they tend to act like ceramic microphones and the behaviour of a capacitor for practice in your bedroom will not be the same as at a gig where the sound levels are high and may cause mechanical feedback.  Some of the electrostatic voltage across the capacitor tends to bend the capacitor plate dielectric and the energy going into that is deleted from the voltage at the output.  I prefer to use ceramic for radio frequencies where the lower values and lower dielectric constants make for less of the piezoelectric effect.

[GibsonGM]

Some say ceramics sound "grainy" or "Gritty", and some say poly types are "sterile".
One man's trash is another man's treasure.

Guess I'm old enough that where i use ceramics, in pF values....the freq's they're passing are so high I can't hear them anymore!  LOL

I "like" poly types for the lower audio range...because they are so commonly available in a nice variety of values and come in a few really cool colors.

[Gus]

There are different types of ceramic capacitors.  What type of ceramic?

[Neta]

I'm guessing "Ceramic disc" is not a good answer... 
If you're referring to the materials involved, which I'm guessing you are, I really don't know what are the differences etc.

I did see a graph of the curvature of freq response for several different ceramic caps, and they were very different, but I still don't know if the difference is audible or ptherwise important.

[bool]

In short... for the ignal pathe

Use poly types where possible (if space is concern Wima makes miniature R2,5mm leaded types)

C0G/NP0 types of ceramics (disk or MLCC) are very fine

X7R type is passable if space/size is of concern (usually the MLCC's)

[anotherjim]

There are usually only a few places for ceramics in circuit. The best for use in audio circuits are NPO or COG type.

I use a lot of 100nF NPO ceramics. I always put them across DC power supplies close to each integrated circuit power, or fit 1 at least if it's discrete transistor design. This is even if there isn't any shown on the schematic. They do a better job than large value polarized cap at keeping high (radio) frequency noise out of the power supply - but note that this is in addition to a higher value polarized cap, not instead of. On that point, older circuits relied on the battery to decouple power supply noise (a battery is a large value cap), so I find myself adding a polarised cap not on the scheme.

For really small values - below 100pF, they are used for radio interference rejection in the signal path. For this job I don't care what type of ceramic it is. Most early guitar circuits don't fit any caps specifically for this job, but these were designed before many of the causes of interference we have now were invented.

For signal coupling and tone networks, generally you won't find cap values below 100pF. I'll use ceramics for experiments/trial on a protoboard (longer wire legs make them easy to work with) - but always use film caps for finished circuits. I'm not fussy about type of film cap. Polybox style are easy to get in a wide range of values.

If you want to use ceramic in the audio path, and you're worried about the microphonic issue (tap the cap with a small screwdriver handle or pen and listen to the audio), you can always cushion the cap by pouring some melted petroleum wax over it - the same stuff used to "pot" pickup windings.

[pupil]

@anotherjim

I've heard, mainly from synth people, about putting a small ceramic cap across the power rails as close to the IC power pin as possible. Especially when the design uses more than one IC.  Are you saying you put a small value cap across the power supply for a transistor stage as well? This is the first I've heard of this, but it makes sense. I suppose you would put it above the collector resistor?

EDIT: I guess i don't really have a question. That all makes sense.  thanks

[davent]

Adding ceramic PS decoupling caps for IC's close to the pins.


dave

[thelonious]

Adding ceramic PS decoupling caps for IC's close to the pins.

Nice!!

[duck_arse]

Adding ceramic PS decoupling caps for IC's close to the pins.


dave

are they the tumeric-based caps, davent?

[davent]

Those are the curries so there's some tumeric as well as other flavourings in there as they are going onto an EQ board. Saving the cayennes for the high gain stuff.
dave

[PRR]

> I just really want to do everything perfectly

You will never get everything done "perfectly".

To some degree, caps is caps. When the dog-porch sags and I need a rock to shove-under, I might prefer granite, but schist will do. Traditionally, pedal cap choices have been even less fussy than my porch-rock choices.

For our purposes, "ceramic" comes in two flavors. Smaller values can be had in "NP0"/"C0G" which is more than perfect enough for any audio. Large values only come in X7R and worse, which has-a-flavor in some audio circuits. But hey, we play geetar Real Loud!! We make nasty noises! A little X7R grunge may be good; OTOH a "lack of X7R" will not be missed.

Film caps tend to have no major dirt; however there are people who will argue paper vs poly, prop vs ester, oil vs wax, orange red yellow brown or tropical-fish colors.

> In DIY Layout Creator the symbol for film and ceramic cap is the same. How do I choose?

As a general guide, for maximum usefulness, or to accomodate cap-sniffers, pick the "largest" cap that you might ever want to use. Then you can fit a 0.6" box-cap or a 0.1" micro-ceramic, as you wish. Ideally you will lay-out several holes to suit the lead-spacing of the various types; though not all layout programs do this trick.

For minimum size, you pick small caps and lay-out just that size.

[Neta]

Thanks everybody for all the great answers!!
Really appreciate it.

Can anyone please explain the concept of decoupling caps? I read in wiki that it's supposed to prevent transfer of something (was unclear as to what, can only guess DC) between two components.
But if that's the case, why purposely connect it across an IC socket (as the picture above illustrates)? I must be missing something.

Thanks again for all the help

[PRR]

> concept of decoupling caps?

Any useful audio system has multiple stages.

Each stage takes a big boring power supply and a weak precious signal to make a more powerful signal.

Usually more than one stage shares the same power supply.

Inside each stage the signal and power are worked together.

Coupling caps connect signal from one stage to the next.

Some signal in each stage sneaks out to the power supply, where it can then sneak back into other stages. This is usually a bad thing.

DE-coupling caps absorb signal on the power rail so there is less sneak-back.

[tubegeek]

are they the tumeric-based caps, davent?

Don't be silly.
Those are tiny corn dogs.

[davent]

In this sport i don't think i've ever seen the pads/spaces for the small ceramic decoupling caps as close to the op amp pins as possible (and i believe always recommended) on any pcb or layout. I soldered then to the sockets as an easy fix/addition for the op amps going on a purchased pcb.

(Curry covered grains of rice.)

[tubegeek]

(Curry covered grains of rice.)

Then why do they have sticks thrust through them then? Corn dogs.

[duck_arse]

corn dogs? what's that, something like food? we don't get mucha that kinda stuff down here. curry, rice, tumeric we got plenty of. also sticks.