Power filtering at the DC jack

[ACS]

Hi all

I've searched and searched, and while I've found a wealth of information on Power Supplies, loads of questions about filtering, and more than a few DIY PSU's, I've not been able to find a definitive answer to effective filtering at the DC jack.  About the closest I got was this link http://www.diystompboxes.com/pedals/ps.html from the FAQ.

So while I understand basically what's going on to achieve filtering, what I don't know is what values are best where!  Judging by the link in the FAQ and inductor and a cap are the best bet.  Would you guys agree?  If so, are the values of 1mH and 220uF that are quoted about right?  Anyone got any better solutions for DC filtering?!

Cheers in advance
Aidan

[zachomega]

Cheaper than an inductor would be a resistor.  Maybe 100 ohms and a 220 - 2200uf cap at twice the supply voltage or higher. 

-Zach

[ACS]

That was my first plan of attack, but was concerned about the associated voltage drop ...

[petemoore]

  nominal, easy to measure and jumper if necessary.

[earthtonesaudio]

You can calculate the voltage drop with V=IR, then go here: http://www.muzique.com/schem/filter.htm ...and plug in your resistor value and cap value to get a rolloff below 60Hz.  (You're filtering to get rid of hum, right?)  ...I think this is correct, people please correct me if I'm wrong.

If you're using a 100 ohm resistor, then the smallest cap you should use is 33uF.  Good engineering practice is to use the smallest capacitance you need.  Oh and keep your leads SHORT!

For best results, you shouldn't filter at the DC jack, but as close to your active component as possible.  If your active component is an op amp, for example, your filtering cap would ideally be on the power/ground pins.  If you wanted to be really anal about it, you'd use a big surface mount tantalum with zero lead length and tack it onto a big wide ground plane.  (Note: this applies mostly to high-speed signal stuff, guitar is much less picky)