The (wee) Fettle Boost (5) – The Fettle Boost

written Jan 8th 2018, backdated to 30th May

So some time around May I went back to the Wee Fettle Boost schematic, and thought about how to improve it. I didn’t want to move far from the original design, but I though a couple of minor tweaks could make it a far more useable pedal.

I’d like to point out that I realise this is nothing ground breaking, and the world probably doesn’t need another boost/ fuzz pedal, but it’s all good learning! And the Fettle Boost will likely be pretty short lived as a result. It might feature as one half of a double boost/ fuzz in future.

Anyhows, first thing was swapping that volume trim pot for a proper pot. Simple enough, I just needed to change the footprint in KiCAD.

One thing I noticed with the Wee Fettle Boost was that is could be a bit muddy and indistinct with some setups. I have my guitar tuned to B, and my testing amp is the Orange Micro Terror which can be bass heavy. I didn’t want to lose this sound, I’m just aware that it might not be for everyone. The simple solution was to put a high pass filter on the input. I decided on a 3 way switch to allow for 3 options. The basic idea was to use a DPDT (on-on-on) switch to swap between the original input cap and two others. It was time to do some maths!

Just to clarify, a high pass filter is a frequency dependant voltage divider made from a capacitor and a resistor (in its simplest, passive form). The combination of values of these two components set the cut off frequency of the filter. A high pass filter leaves the frequencies above the cut off frequencies untouched, and rolls off the frequencies below, hence the name ‘high pass’. A low pass filter does the opposite (rolls of higher frequencies, passes the lower ones), and it does this by swapping the position of capacitor and resistor. See below.


Very simple. There are plenty of free online high pass/ low pass filter calculators out there, this is my usual go to. Anyway the equation to work all the cut off frequency is

Fc =          1         

               2π RC

Fc means cut off frequency – this is actually the frequency where the cut is 3dB

R is resistance in ohms (Ω)

C is capacitance in Farads

The roll off from the cutoff frequency is 6dB per octave or 20dB per decade

So next, I just needed to work out what frequencies I wanted to cut. On a standard electric, the lowest fundamental (low E) is 82.41Hz and dropped D is 73.415Hz, so theoretically anything below that would be worth cleaning up for most guitarists.

In audio recording and mixing many people recommend rolling off everything from 100Hz to help the guitar cut through the mix more. Below 100Hz is where the guitar and bass most heavily overlap. The low E on a bass is 41.204Hz, which is also roughly where the bottom end of the bass drum sits (40 – 50Hz)

Just a quick note on frequencies and your sound…

How your guitar or bass sounds when you play by itself, and how it sounds in a band situation are not the same thing. A prime example of this is scooped mids. It may sound great in the bedroom, but it will always sound terrible with a full band. Each instrument needs to have its own band of frequencies where it can sits unchallenged by other instruments, otherwise you end up with mush. Don’t get me wrong, loud mush can sound great, but most musicians aren’t chasing that wall of indistinguishable frequencies tone.

So the idea for this high pass switch would be;

setting 1 – everything passes

setting 2 – cut out subharmonic frequencies for all guitars/ tighten bottom end for basses

setting 3 – guitar only – some bass cut.

Why not have a pot instead of a switch!? I hear you whimper. Well if you check the schematic, you’ll see that the high pass filter is on the input, and that resistor to ground  can’t be swapped for a pot without causing other issues. I decided that I’d aim for a cut at around 100Hz and around 50Hz. Bearing in mind that the resistor in the filter is 1MΩ, I realised that the cap would have to be very small, and finding the right value when you don’t have much leeway might be a problem. Using the calculator I mentioned above I inputted the resistor value and the cutoff frequency I was aiming for. The closest values came out at:

150pF plus 10M resistor = roughly 100Hz

330pF plus 10M resistor = roughly 48Hz

which wasn’t too bad at all.

Here’s the maths…

R = 10000000 (1MΩ)

C = 0.00000000015 (150pF)

RC = 0.0015

2π = 6.284

2πRC = 0.009426

1 ÷ 0.009426 = 106.1(Hz)

So, a couple of simple tweaks later the Fettle Boost was ready to go on sale!

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