Audio High/Low pass filter components.

[shred101]

Can someone tell me in plain english what components I would need for a combined variable high / low pass filter? Im a musician/producer and I use so many of these effects that it just turns into either a waste of desk power/effects units or bass eq bands.
Every time I ask this on audio forums I just get a crapload of math that I dont understand thrown at me and to be honest theres no way its that complicated because wikipedia implies that I just need a capacitor/resistor and a potentiometers for a simple circuit. I just need to know which ones. Theres no way im building and using this thing until im 100% sure of components.
So basically the idea is for a box with 1/4 input jack socket, a circuit that shears off all bass below the point dictated by what the bass potentiometer is set to and all treble above the point dictated by what the treble potentiometer is set to and then a 1/4 inch output jack socket. Can someone advise me on the values for the pots, capacitors, etc. for this please?
Thanks.

 

[Nigel Goodwin]

You fail to understand what it's all about - what you actually asked for is impossible - and I'm sure even a Wikipedia article doesn't say otherwise. To do 'nearly' what you actually asked would be quite complicated, not as simple as you're imagining.

 

[shred101]

Yeah that wouldnt surprise me. Im a n00b when it comes to this stuff. How is it impossible? I assume that you mean I cant actually select a cutoff point? If so would I be best constructing filters with simple on/bypass switches? I was talking about this on an audio forum today and I was told thusly:

470uF cap with a 1K pot will give you an effective high pass selectable from 200Hz to 20KHz.

 

[crutschow]

How effective is open to interpretation. A simple RC (1-pole) filter does indeed have a cutoff (the -3dB rolloff) point, but such a filter rolls off only at a 6dB/octave rate which is likely too slow for you to hear much effect (3dB is normally considered a barely detectable change is volume). If you want a sharp cutoff then you need multiple pole (active) filters. A 2-pole will give a 12db/octave rolloff, a 3-pole gives a 18dB/octave rolloff, etc.

A free program from TI that allows easy design of such filters is Active Filter Design Application - FILTERPRO - TI Software Folder (no crapload of math, the program takes care of that).

But, of course making them tuneable is another problem which is not simple to solve.

 

[shred101]

Well if its too complicated to make them tunable via a pot i'd happily comprimize on my design a bit as long as I can figure out how the simple filter circuits a created. I mean, I read in an old copy of sound-on-sound magazine the other day that capacitor-resistor series circuits create high pass filters and resistor-capacitor series circuits create low pass filters but the article doesnt give any indication of what value/type of capacitors/resistors you would need to to use.

 

[MrDEB]

Somewhere on the web there is a chart showing the capacitor resistor combos for different frequencies but I can't locate the printout as my wife and I are cleaning house/reorganizing so the print out is in limbo but it is on the web.
I recall searching for equalizer circuits

 

[crutschow]

A capacitor in series with a resistor to ground is a high-pass filter. A resistor in series with a capacitor to ground is a low-pass filter. The -3dB rolloff frequency for either is 1/(2*PI*R*C). (PI ≈ 3.14)

But don't expect to hear much rolloff from such a filter.