Low pass filter with variable control?

[Ben Lyman]

Is it possible to make? Anyone have a schematic? The simpler the better and passive is preferred if possible. It's for keyboards

[Groovenut]

Here are a couple of passive LPFs.


The one on the left varies the amount of signal allowed through the cap to ground, leaving the knee point static but varying the slope. The one on the right varies the knee point but leaves the slope static.

The values are arbitrary and only to show function.

[Ben Lyman]

Thanks, that's kinda what I thought, like a tone knob on a guitar.
As for the diagram on the left, is one of the potentiometer lugs left unconnected to anything?

[antonis]

You can leave it unconnected but it should be a good practice to connect it to the middle lug (pots wiper)..

[nguitar12]

exactly what I am going to ask also. Is there any equation for calculating the cut off frequency/slope?

[Groovenut]

exactly what I am going to ask also. Is there any equation for calculating the cut off frequency/slope?

f=1/(2piRC), R=1/(2piCf), C=1/(2piRf). R in ohms, F in Hertz and C in Farads

edit: apparently fat fingered the Z this morning

[antonis]

F in Herts

F hurts...

(better is Hertz - sounds less fizzy..)

[Groovenut]

F in Herts

F hurts...

(better is Hertz - sounds less fizzy..)

Some mornings it all F in hurts.....

[robdean]

LPF calculator: http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php

In addition to using a pot to vary the circuit response, you can add a switch between different value capacitors. I have done that in a guitar. I get the best results by ear once I know the approx. cap values. Cheap test leads with croc clips on the end are a great way to try different values, or just a bit of domestic wiring block. Ideally get a bunch of values either side of your best guess. Recently I did OK with a dozen identical caps by combining them in series/parallel to get other values.

[Ben Lyman]

Thanks Rob, very cool indeed