freestompboxes.org

Hello;
I am currently in the process of building my own pedal for my bass, based on the Rose Screamer, which itself derived somewhere from the Ibanez Tube Screamer.
But I have some questions about the capacitors that the Tube Screamer uses for filtering;

First question is about the capacitors in the signal path; Why do they have such different values? Isn't the capacitance supposed to be adjusted based on how low you want to go? So why have that 10µF capacitor (C9) at the output when the lows aren't going to get through the 20nF C1 in the input? Also, isn't C3 supposed to boost anything above a given frequency? Then why is it higher than C1?
I know that putting the diodes in the op-amp feedback path gives softer clipping than putting them in a path to ground. I figure that this would be the same for capacitors. So why have such a tiny C4 to reduce the high frequencies at that stage but such a high-value C5?
I believe that C3 is meant to give a boost to the mid-frequencies. So why is it a lower value than C5?

It's because the Z (Impedance) at each part you mention is very different.
Let's say a 100nF cap in front of a tube screamer might roll off the Bass at Frequency "X"
Now a capacitor in a hifi crossover might roll off at the same "X" Frequency BUT will use a 10uF Cap.
A crossover is working at low Z ,, only 8 Ohms. The Z at TS front end is ~500k Ohms.
So the values are quite different.

Remember when you look at a schematic you are looking at 2 circuits not one.
These circuits have two parts,, there is the DC part and there is the AC part.

The AC wiggle (your guitar signal) is floating on a DC voltage,, you set the DC parameters and then you tweak the AC so it passes the desired sound.
Working out the AC involves a fairly good knowledge of RC maths.
If don't like school you can cheat a bit,,,
The fastest way to get your head around that is simulation software ,, many come as free demos or limited use.
Which is all you need to understand 90% of guitar stomp boxes.
HTH,,
Phil.

What phatt says.

You have to take the capacitor value and the resistance value that shunts it's output to ground or a decoupled voltage like vref.

A quick and dirty look at the capacitors and resistors in the signal chain reveals the following 3db points (cutoff frequency of a high pass CR filter). The actual resistor values may differ slightly due to the interaction of other components but not enough to make an audible difference.

0.1uF 510k = 3.12Hz
1uF 10k = 15.9Hz
1uF 101k = 1.58Hz
0.1uF 510k = 3.12Hz
10uF 10k = 1.59Hz