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Hello!

I've been trying to understand some basics of modding guitar pedal circuits, specifically for bass.

After reading this handy "How it works" document over at beavis audio (http://www.beavisaudio.com/techpages/HIW/HIW.png)

and reading some of the threads in this forum I've come across at least two modifications that need to be made in most guitar pedals for suitable use with a bass.

1) Changing the capacitor and resistor that form the high pass filter to form a low-pass filter.

2) Increasing the value of the output capacitor to allow for more low frequencies.


Some questions!

1) Are these the only mods that need to be made? I currently have the BYOC Scrambled Octave kit waiting to be built, which I may or may not mod. (Schematic here: http://www.buildyourownclone.com/scramb ... ematic.pdf)

2) How do you identify the filter on a schematic? In the scrambled octave I'm guessing the filter is formed by C1 and R2. That's my guess because that's similar to the schematic in the beavis link above.

3) Do I need to lower or increase the values of the capacitor and resistor that form the filter to create a low-pass filter? And by how much do the values need to change?

4) I assume the output capacitor in the scrambled octave is C7. If that's correct does it have a large enough value at 1u to be suitable for bass?



Any and all help is welcome!

Thanks! Hope to hear from someone soon!

I am waiting for some responses because I want to mod my pedals for bass as well

Your conclusions are a bit wonky.

The deal is that the bass is one octave lower than the guitar. Effects circuits almost always use coupling capacitors - at the input, between stages in the circuit and at the output. Now since the bottom E on a guitar is at around 82Hz you could perhaps get away with calculating the values of these various capacitors based on the idea that they don't need to pass frequencies below the bottom E on the guitar. However what happens in practice is that most of the time the actual coupling capacitor values used are plenty big enough to pass 40Hz - the lowest note on a 4 string bass, so these capacitors do not need to be changed. Very occasionally you will find a coupling cap that is a bit on the small side for bass.

All this means that you have to look at effect circuits on a case by case basis, identify the coupling caps and calculate their roll off points relative to the circuit impedances they are driving and then make them bigger only if you need to. The other approach is to not bother with the calculations and simply double the value of all the coupling caps to allow for the extra octave of the bass. This still means you have to be able to identify which caps are coupling caps in order to be able to change them.

The next thing is - does the effect have any tone controls / filtering? If it does you probably want to move the centre of the tone control / filtering down one octave. To do this you need to identify any tone control / filter caps and double their values.

You say >> 1) Changing the capacitor and resistor that form the high pass filter to form a low-pass filter.

No you don't want to do that. Most filters in effects pedals are already low pass. What you want to do as mentioned above is move the operating point of the filter down by 1 octave.

I suppose the real simple approach is to just double the value of all the capacitors in an effect, but for the reasons given above this is often overkill and sometimes for things like stabilisation caps it might be the wrong thing to do.

Lastly, guitarists seem to cope with effects that lose their bottom end but bass player aren't very comfortable when their low end disappears, so for bass effects you often need to arrange to split the bass signal into an effects path and a clean path, the effects signal path needs to go to the effects and the output of the effects is then mixed with the clean path signal.

Let's look at the Scrambled Octave - it has no filtering as such, none at all. All of the capacitors in that circuit except for C4, which is a DE-COUPLING cap, are either input, output or inter-stage coupling caps. The input cap C1 0.01uF is on the small side for bass, I'd make it a 0.1uF. The two 1uF C and C3 should be big enough as they are. And so should the rest of the caps.

Bearing in mind that the Scrambled Octave is a fuzz / OCTAVE UP pedal I would say it is a very good candidate for a parallel mix path so you hear some of the effect, but you don't lose the clean bottom end from the bass. I think there is an effect pedal style mixer available form someone - the Paramix I think it is called, you need something like that and then you can run all your effects in the Paramix side chain, run your clean signal through the other side and then mix them at its output. Ah .... here it is the Paramix from http://www.guitarpcb.com/apps/webstore/products/show/3456364 As far as I can tell from a quick look that will give you the parallel mixing you need for bass and effects.