Passive vs. Active Tone Controls

[Hendrixisgod]

So I'm modifying a circuit for the first time, and I'm experimenting with where to replace my resistors with pots to control gain and that is all well and good.  BUT I don't know the first thing about tone stacks! What are the pros and cons of passive vs. active? (if I place this before the final cap it is active with 4.5v, but afterwards is passive). Also, does anyone have diagrams for any sort of low-pass/high-pass combo or any sort of tone control? Show me your favorite tone controls! If it's active I'd also appreciate help on changing the circuit for a voltage of 4.5v.

[Incubusguy]

The Distortion+ is a good place to start with modifying since it's a pretty simple circuit and you can really see/hear the effect of most changes.

Passive tone controls have the advantage of simplicity (the simplest being just a resistor and capacitor) but you can expect some signal loss in the passband as well as the stopband. Whether the losses are too great is an issue you'll need to decide yourself but most distortion circuits still have enough output post tone control.
Active tone controls are more complex and require a power source but yield negligible losses in the passband and can provide greater than unity gain. They are also less susceptible to changes in the cut-off frequency by a load.

You need to understand that placing a filter/tone control before the final cap does not automatically make it active. The 4.5V DC measured is the bias voltage for the opamp and balances the signal halfway between 9V and ground for maximum voltage swing. The final cap blocks this DC voltage to balance the output signal back around ground (swings positive and negative with respect to ground). An AC signal (your guitar signal) swinging around a non-zero DC voltage can damage equipment after the pedal, hence the reason for the capacitor.

Now, the Distortion+ actually has a low-pass filter after the final capacitor acting as a fixed tone control. I don't know the exact version you have but the GGG layout has a 10k series resistor and 0.001uF parallel capacitor. These provide a cut-off frequency of 15.9kHz, so any frequencies above this will be attenuated at a rate of 6dB/octave.

Increasing the value of the resistor will attenuate more highs. You can use this to fix the maximum amount of treble you want, then break the connection between the resistor and the parallel diodes and capacitor. Between these, put a potentiometer wired as a variable resistor (http://interactiondesign.sva.edu/classes/physicalcomputing/files/potentiometer_diagram.jpg, signal goes in A, comes out W). If the pot works the opposite way to intended, move the signal to the other outer lug. Using a 100k pot will allow you to vary the cut-off frequency between 1.5kHz and 15.9kHz which should be more than enough for your needs.
You can choose to just use a pot instead of the fixed resistor but at the pot's maximum you may get overly-piercing treble and possible oscillation.

For more comprehensive reading about filters, try reading each page of this article: http://archive.chipcenter.com/circuitcellar/october01/c1001ts2.htm