I use this "tone" control sometimes;
(http://www.diystompboxes.com/pedals/lowpass.GIF)
Would someone explain how/why it works? A regular low pass filter would take the output from between the pot and the cap, but this one takes the output from before them both. Also, the affected frequencies don't seem to be anywhere near the 1/(2.pi.R.C) that applies to low pass filters. Can any experts out there help me please?
This is just shunting higher frequencies to ground. That's how a tone control works. So the larger the cap, the more "highs" get removed. The series resistance, simply allows you to adjust the tone control.
This simple little control works surprisingly well.
This is very similar to some of the newer circuits where they have .001, .002 etc.. caps to ground on the output section of circuits.
Think of it as connected in series with the output impedance of the previous stage of the circuit. This makes a frequency-dependent voltage divider. As the resistance of the pot is reduced, you get just the output impedance and the capacitor as a simple RC filter.
In standard RC cap-to-ground lowpass filters, the RC relationship determines the *point* at which frequency-selective attenuation begins, but NOT THE AMOUNT OF ATTENUATION. In the case of the traditional guitar-type tone control, the resistance determines the AMOUNT of attenuation at the given turnover frequency. Change the cap value and keep the pot value the same and you will still note very little "tonal" difference when the pot is only rolled back a bit. Decrease the resistance of that pot even more and you start to here more treble loss, but at a different starting point. With the "Rat-style" tone filter, resistance changes comparable to turning down a guitar tone control halfway tend to result in more pronounced treble loss, largely because the steepness of the rolloff is the same, just moved down the spectrum more.
The important thing to remember is that just because they are both *called "tone", and just because they both use one resistance and one cap, doesn't mean they do the same thing or work the same way.
Thanks Mark. That's a very helpful description. So it puts an adjustable slope on a single pole filter. That would explain why such large caps are used...a low cutoff frequency with a low slope still has a very mild effect. It also explains why adding extra capacitance didn't do much unless the pot was turned down (for a substantial effect) (I'd been playing with filtering white noise and watching what happens on my o'scope, but I'd been getting frustrated working out what was going on). Many thanks. :D