Is this a filtering cap?

[gutsofgold]

The 120pF in parallel with the clipping diodes?

And would I notice a difference between a high quality film cap and a ceramic cap in there?

[kurtlives]

No its not. Power filtering caps go between the input voltage and ground.

Its common to put small value caps across clipping diodes. Its just cutting some highs.

[earthtonesaudio]

To be more specific, it is passing some more highs from the output back into the inverting input.  When those highs go into the inverting input, the opamp does its magic and you hear less of the high end coming out of the output.

So you could say it is filtering (because it is), but as kurtlives said, the common term for a "filtering cap" is one that is filtering noise on the supply voltage.


And the only way to tell if different caps sound different in there is to listen yourself.  Some people swear they hear a difference, others not so much.  If you replaced the ceramic with an aluminum electrolytic, on the other hand... big difference.

[Zben3129]

I can't remember if there is information in "technology of the tubescreamer" at www.geofex.com that pertains to that specific cap...

...but those 2 diodes and cap in paralell with a resistor/cap to ground is nearly the exact same as the clipping loop of the ts, with the exception of the gain pot. I'd try messing around with that cap once you build it to find what value best suits your gear/style/ear.

Zach

[gutsofgold]

so it's a frequency filter cap...  

I built this pedal for myself and it sounds amazing. I modified the etching a bit to allow for a larger film cap there but I etched it a bit smaller for a friend's build. I built his exactly the same way except for this small cap which I used a ceramic for his. His sounds much fizzier at higher gains, very raspy and "ear piercing".

I used these ceramics and they are TINY...very very very small in size.
Murata RPE 120pF


Are these low quality or something?

[foxfire]

so it's a frequency filter cap... 

I built this pedal for myself and it sounds amazing. I modified the etching a bit to allow for a larger film cap there but I etched it a bit smaller for a friend's build. I built his exactly the same way except for this small cap which I used a ceramic for his. His sounds much fizzier at higher gains, very raspy and "ear piercing".

I used these ceramics and they are TINY...very very very small in size.
Murata RPE 120pF


Are these low quality or something?

by larger do you mean a larger size or value? a larger value would cut more highs making it less fizzy. also in general a film cap will have less noise or a cleaner sound than a ceramic cap which could explain the differences in sound you are getting.

[gutsofgold]

I meant larger in size, not in value.

It's a very prominent degradation in sound, not as much as I would have expected.

[earthtonesaudio]

so it's a frequency filter cap... 

I built this pedal for myself and it sounds amazing. I modified the etching a bit to allow for a larger film cap there but I etched it a bit smaller for a friend's build. I built his exactly the same way except for this small cap which I used a ceramic for his. His sounds much fizzier at higher gains, very raspy and "ear piercing".

I used these ceramics and they are TINY...very very very small in size.
Murata RPE 120pF


Are these low quality or something?

Here's a page that has graphs of cap linearity:
http://members.aol.com/sbench102/caps.html
BUT, just because something is more linear doesn't necessarily mean it is better sounding, especially for a distortion. 
You sure the values are the same?  And that both pedals are otherwise 100% equal?  Even going from ceramic to polystyrene shouldn't make a huge difference like you describe, I don't think.

By the way, that's a monolithic/multilayer ceramic cap.  A bit different from "vintage" ceramic disk types.
Also, there are different types of "film" caps.  Polystyrene (for hifi crazies), polycarbonate, mica, polypropylene, polyester, etc.

[gutsofgold]

Hmm, yes the pedals are exactly the same. The same right down to the model of TL072 used along with wiring and brand of resistors.

Would trying a ceramic cap be more appropriate here? Or worth the effort of ordering some?

I'm really hoping that these caps aren't bad because I ordered enough to do quite a few different pedals, mainly a BMP I want to use on.
Perhaps they only sound bad in this specific application 

[R.G.]

It's almost impossible to get film caps other than polystyrene (and those are hard to find) that are as small as 120pF. Most people use NPO/COG ceramic for pF caps.

I suspect you may have made a mistake in the value of the film cap you put in your pedal rather than having the wrong kind of cap in your friend's pedal. If you accidentally got  a 10x or 100x larger cap in yours, it would make it much smoother.

[R.G.]

ohhh.... I'm feeling Zen-y.

Every cap is a filtering cap, when you get right down to it.

[gutsofgold]

Wow so he dropped the pedal off and I socketed the cap. The film cap completely gets rid of this hiss noise on top of the gain. Both caps are 120 pF thats for sure, small bear carries film caps that small.

I don't trust these murata ceramics at all, would it be wise to stock up on ceramic *disc* caps and not he multilayer kind?

[R.G.]

Could be the murata device has an internal crack or something.

Multilayer ceramic is a good and useful technology; no reason not to trust them.

I don't know what's happening in your case, but Mother Nature is whispering something in your ear. You just need to figure it out.

[earthtonesaudio]

Since you socketed the cap, did you try more than one Murata, to see if it's just a problem with that one cap?  When caps fail they short-circuit more often than they open-circuit.  A partial short might explain the harshness... maybe.

Could be the murata device has an internal crack or something.

Multilayer ceramic is a good and useful technology; no reason not to trust them.

I don't know what's happening in your case, but Mother Nature is whispering something in your ear. You just need to figure it out.

Today's moment of Zen brought to you by R.G.