Creating a crude low pas filter

[Signal:Noise]

There was a point made in a previous post about using capacitors to act as a low pass filter to remove hum and it got me thinking about making some sort of crude lpf module. Had a quick search via google and there are diy filter modules you can make, but they're all quite complex and expensive to do, All I'm wondering is if it's possible to build a very crude lpf filter and if anyones got any schematics for one.

[sponge]

you can make fairly simple filters using op-amps.  the circuits are really easy too...  i'll post circuit diagrams if you like.  the easiest ones are bass cut, bass boost, treble cut, treble boost.  you can also create (more specific) band pass filters, to enhance a particular range of frequencies

[Signal:Noise]

Yeah, that'd be wicked. I'm thinking it'd be interesing to add one into a bent keyboard or something of that ilke.

[Pehr]

I've read somewhere that you can make a LPF with only a resistor and a cap! 
Just use a resistor (or a pot) between the "signal in" and "signal out", then add a cap from the "signal out" to ground.
The COF becomes:

f=1/(2*pi*R*C)

For a HPF the resistor and cap need to switch places I think...

I haven't tried it yet, so I don't know how good (or bad ) it works. Has anybody else tried it?

[catweazle]

Thats right, but a simple RC-LPF (Resistor Capacitor Network) will work but falls off at high frequencies at the rate of only 6 dB per octave. You can connect two or more of them in a row but the signal will be lowered (you need to amplify it - simple OP-noninverting Amp)

http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/filcap2.html

i recomend a OP Filter (2nd Order) or LC-Filter instead...

http://www-users.cs.york.ac.uk/~fisher/lcfilter/

http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html

[Pehr]

Yes I thought so, but couldn't 6 dB/Oct be enough to reduce the digital noise of some sample-based toy-keyboards? I'm going to try it anyway, without the op amp

[Signal:Noise]

I'll have a go at the Rc one, I need to up my understanding of electronics before I try the more complex designs.

[catweazle]

i tried it out with a 10nF Cap and 10k Pot (~4k) and it works fine ...