alternative for a 30pf capacitaror...27pf vs 33pf

[zwound]

Ok, i can't find a 30pf capacitator in my country...so i need to choose between 27pf or 33pf...will this make a big difference? which is better to use?

btw, im buildign a proco rat

[StephenGiles]

No noticeable difference at all!

[zwound]

Thanks for the reply, however this worries me slightly.

i've burnt way too many lm386's because of slightly altered circuits

http://www.acsu.buffalo.edu/~gsgordon/Rat1.html

this one is directly connected to the chip...which costs like 8 bucks...i dont really wanna fry that, since i'm making this for a friend, and he's paying for the parts...

will it be fine?

[Andre]

Since the tolerance of capacitors is 10% or more, even if you could find a 30pF cap, in worst case this 30pF cap could actually be 33pF or 27pF.

It is absolutely safe to use 27 or 33pF.

André

[zwound]

*sighs from relief*


phew. that's great news!

Thanks again for the help, all

[Mark Hammer]

In this instance, the cap provides a limit on the gain-bandwidth product for the op-amp.  That is, how much maximum gain can be produced at each frequency across the spectrum.  The "magic" to the Rat is that the circuit asks for more gain at higher frequencies than the chip is capable of providing.  The little drone tone you get is from asking it to do something it can't do; kind of like an electronic complaint.  Although I don't see it mattering all THAT much, my instinct is to go with 33pf instead of 27pf, since that provides a tighter grip on GB product at the high end.  I think I have a 33pf cap on one of mine and it works just fine.  Do keep in mind that caps of that type/range can easily vary by +/-10% so the one that says 27pf on it can be 30pf just as easily as one that says 33pf can be 30pf, and just as easily as one that says 30pf can be 27pf or 33pf.

It's good that you check, and the integrity you show in looking out for a friend is to be commended.  In this instance, though, it's more worry than was truly needed.  What you really want to worry about is the ears getting fried once your buddy starts using it!

[zwound]

  thanks, Mark, i've now ordered the 33pf

[dosmun]

33pF is what was used on the first run of Rats.

[jonathan perez]

27pF will fry anything it comes in contact with

[zwound]

27pF will fry anything it comes in contact with

sarcasm?

[jonathan perez]

very 

[petemoore]

27pF will fry anything it comes in contact with
 yupp, I had a good circuit fry toward the beginning of the DIY career.
 Enough to put you off about what's what exactly.
 I had some magic ideas about electronics...it IS powerful stuff, a few good shocks, a couple smoked items of interest missed sorely, coupled with rumor'e misnomers about 'you'll fry it if you try it', and a general 1/2 unwarranted paranoia about 'you could get shocked, the amp's plugged in' can easily set in and manifest itself..however, most of the time @9V or small voltage circuits, amps safety caps prevent HV danger when working with small voltage devices, I've never heard of electrocution by stompbox [it Is of course possible, say if your amp isn't grounded], mostly you can prevent fried components by observing polarity, component voltage ratings, and no shorting conditions.
 Smoked OA's, GE's, broken caps mostly, maybe a diode.
 Most of the time making a 10x value swap to a component will just alter circuit frequency response, bias or noise performance.
 As long as you have a cap of sufficient voltage rating [following polarity or polarities] in the position in question, the circuit will 'work', although it is possible to shunt or block all relevant frequencies, making it sound dead.