Power filter Capacitor

[RandomGlitch]

I'm about to build a TubeScreamer on vero. 

The layout I have has the 100uF capacitor for power filtering right in the middle of the board.

In my builds up until now, I've always put this capacitor on the DC jack terminals, with the idea that I'm filtering out any DC-borne noise and LED pops before the rest of my circuit "sees" it.

Am I correct in thinking this, or is it irrelevant just where it goes? Or should it go very close to the IC - I've seen that kind of thing mentioned before too.

In a way, I'd rather always mount it on the board than like I've been doing.

Thanks!

[chromesphere]

Pretty sure as long as its connected to plus and minus it doesnt make any difference if its on the board or on the dc jack.
Paul

[brett]

Hi
usually, you'll want the big filter cap 'upstream'. And yes, it does make a difference where it is. (Think of it like a dam that regulates water flow. The water flow is a lot more consistent downstream than upstream)
The idea mentioned of having a cap upstream of ICs (e.g. op-amps) is important because high-frequency 'junk' can get put into the power supply, which upsets both the IC and things nearby. For op-amps, a 0.1uF film cap within a couple of cm is good practice.
hope that helps
cheers

[chromesphere]

Actually yeah, ill go with bretts answer! i was just thinking about the lm386 i was tinkering with on the weekend.  It was basically useless without a cap close to the op amp (from plus to ground). 
It had to be within cm's of it or it didnt work.  This was also with a power filter (1mf of capactiance to ground), at the start of the signal chain.  The power filter made no difference.
I'd forgotten about that.

Paul

[RandomGlitch]

Hi
usually, you'll want the big filter cap 'upstream'. And yes, it does make a difference where it is. (Think of it like a dam that regulates water flow. The water flow is a lot more consistent downstream than upstream)
The idea mentioned of having a cap upstream of ICs (e.g. op-amps) is important because high-frequency 'junk' can get put into the power supply, which upsets both the IC and things nearby. For op-amps, a 0.1uF film cap within a couple of cm is good practice.
hope that helps
cheers

OK, that sounds like my practice of putting a big cap on the DC is just that. 

On the op-amp, do you mean connecting a 0.1uF from the +V (pin 8 on a dual op-amp) to ground? I saw that on a circuit the other day and was not sure if it was needed as there was already some big power filtering caps on the supply.  But you are saying it needs it's own, "local" cap too. 

Cool!  thanks for clarifying!

[Keppy]

The main problem with putting the filter cap on the DC jack is it's one more thing to blow up if you reverse the power supply.

[armdnrdy]

The main problem with putting the filter cap on the DC jack is it's one more thing to blow up if you reverse the power supply.

But then we all put diodes in place to keep that from happening...don't we? 

[Keppy]

Well, if the protection diode is on the board, which is grounded to the enclosure, and the DC jack is connected to the input jack ring, and you happen to not have the input cable plugged in yet... Boom.

It doesn't happen often, but it does happen to some people sometimes. Placing the cap on the jack is fine if you don't mix up your power supply. You might trust yourself that far, but I wouldn't do it on a pedal intended for another person. This being a DIY forum, that obviously won't matter to everyone here, but most of us have built a pedal for a friend a time or two, and it doesn't hurt to be careful.

[Kesh]

caps as close as possible to power pins of a chip is generally a good idea. helps kill oscillation. the resistance and inductance of a long run from chip to cap weakens the cap's effect, substantially at high frequencies.

my first tube screamer did this with no input, until i put a cap on the pins