Pi Section Filters?

[finkfloyd]

Hi was browsing rapid, and came across these : http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Filters+%26+Suppression&tier3=PI+Filters&tier4=Pi-section+capacitive+PCB+filters&moduleno=60315

and smt version http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Filters+%26+Suppression&tier3=PI+Filters&tier4=SMD+Pi-section+capacitive+filters&moduleno=60319

Do you just drop these in like this? :



These apparently are made for rejecting rf/emi noise.

Im trying to find a magic noise cure for my effects, especially the tubedriver and bigmuff, both together produce a lot of unwanted noise.


I was also looking at this thread, in which RG mentions a Pi section filter by using an inductor and some film/ceramic caps as seen here : http://www.diystompboxes.com/smfforum/index.php?topic=45698.0

Question is:  substituting the 100ohm resistor for for the 1a inductor, what value would you use? 100uh? 220uh..etc..


Cheers
Chris

[R.G.]

Pi filters are called that because the two caps to ground and series inductors mimic the two-legged character for "pi".

They are a cascaded second order filter. The action is not hard to appreciate - the first cap shunts high frequencies to ground, the inductor is a large impedance at high frequencies, and the second cap makes a "voltage divider" for any highs that get through the first cap and inductor.

The inductor and cap values matter a lot on what frequencies they reject. The integrated ones that you show are intended to block RF, well above audio frequencies, so unless you have an RF oscillation problem in your pedals, they won't do a lot. But that is one possible source for noise, so maybe.

If your problem is plain old hiss, you may have a tough time getting rid of it. With enough gain, any front end hiss is amplified up hugely. This is a common problem with cascaded distortion devices.

But to answer your question, use the biggest value inductor you can afford and physically fit.

[Ardric]

They are three terminal devices.  Power in, gnd, power out.  They try to shunt noise to gnd.  They can be much more effective than a simple resistor and capacitor, which is what we usually do in pedals... but as R.G. points out, these integrated ones in a single package are much too small for our purposes.  We'd want electrolytic caps and an iron-cored choke for an inductor, and it ends up being pretty large.

Look at the schematic of an old Fender amp that has a choke in the power supply.  It's cap, choke, cap.  Duncan's Power Supply Designer is a fun windows tool for visually experimenting with this.

The thread you've referenced has a lot of good info in it on adding power filtering to pedals.

Re your particular pedals, the Big Muff sounds like a normal pedal, but the Tube Driver is probably powered by it's own A/C power supply from the wall... right?  So there could be grounding issues between it and another A/C powered device with a safety ground, like the guitar amp.

[finkfloyd]

Ahh, thank you very much for the clarification and explanations, and sorry for a late reply, (had a long hangover!)

The russian black big muff I have has had a power adapter added to it by myself, and there is a slight noticible hum now, i wanted to add an extra circuit to tame it a bit

but the tubedriver runs from its own 12AC and the hum gets apparant and loud when the drive is turned up past 50%, both of these units together get noisy until i play.