Identifying High/low pass filter

[craggsy]

Hi!

I've built one pedal (MXR Distortion +) and have 2 more to build. Im going to need to modify them to allow for more bass but im new to this low/high pass filter stuff. I've seen a page explaining how to add more bass to a Tubescreamer clone using a 1.0uf cap.

Can anyone explain to me what I need to look out for when changing caps to a bigger value to allow more bass.

These are the schematics for the two pedals ill be modifying.

http://www.muzique.com/schem/pcrat1.gif
http://www.generalguitargadgets.com/diagrams/dist_plus_sc.gif

Thanks!

[niftydog]

Capacitors can be thought of in general terms as an AC short circuit, and a DC open circuit. That is to say, they pass AC signals but block DC signals. This isn't a hard and fast rule, but it helps you understand what's going on. Firstly, all caps block pure DC - that is a given.

Now, lets loosen up the rule a bit. Low frequency signals are 'closer' to DC than high frequency signals and high frequency signals are 'closer' to AC than low frequency signals. You follow?

So, a cap in series with a signal line will, depending on it's capacitance value, pass high frequency signals while blocking low frequency signals. Hence, simple high pass filters use caps in series with the signal.

A cap from a signal line to ground will, again depending on it's capacitance value, direct high frequencies to ground, while allowing low frequency signals to pass unaffected. Hence, simple low pass filters use caps from signal to ground.

So, to allow more bass to pass through a circuit, you need to find the high pass filter that is most responsible for blocking the low frequencies and alter the capacitance value. This will affect the filters characteristics and either allow more low frequencies, or block more low frequencies.

Get your head around this concept first, then have a search around the forums for some other threads on this subject. There are formulas that we use to calculate the filter characteristics, but often people just try a few values of cap to find the one they like. Suffice to say, when you find the high pass filter cap and you want more bass to pass, you'll need a larger capacitance value. This can be achieved temporarily by just holding another capacitor in parallel with the one that is in circuit. The total capacitance will be equal to the two added together.

[craggsy]

Thanks for that niftydog! One last question, ive noticed some pedals have variable high/low pass filter such as Jacques (http://www.ts808.com/) copy of the Tubescreamer. How could you vary the capacitors value or is it the resistance before it your changing?

Thanks again for you help!

EDIT: ill go check out the simple mods page!  

[niftydog]

usually you would vary the resistance or some other parameter. Variable caps are an oft discussed topic around here, but as yet there are no designs of variable caps that are useful in audio applications. They are sometimes useful for high frequency applications such as comms and video etc.