Low-pass filters aren't too tough, but you may not get the results you're looking for no matter what circuit design you choose. Unfortunately there's no such thing as a "brick wall" filter that mutes everything above a certain frequency. Not in the real world, anyway. The best you can hope for in an analog implementation is "close enough"...but hey, it might just do the trick!
The first thing is to split the signal. The easiest way is to have one path go from your guitar straight to the amp, and another path go from your guitar to a buffer. The buffer will isolate your guitar from the filtering circuit and take care of any impedance issues downstream. You should also place a 100nF at the front of the buffer to block any DC from leaking back towards your guitar.
For the filtering section itself there are a few options, but I think the Sallen-Key low pass filter is a good balance of performance and simplicity. You can read all about it here:https://en.m.wikipedia.org/wiki/Sallen
Each stage is second order and provides 12dB/octave filtering. You can cascade multiple filter to improve the overall performance. If you use a quad opamp like a TL074, you can use one opamp for the buffer and three for the filters, giving you 36dB/octave total filtering. It's not a brick wall by any means, but at an octave above the 187 Hz F you noted the perceived volume will be reduced to less than 10% of normal.
Try these values as a starting point for your filter:
R1 - 33k
R2 - 33k
C1 - 56nF
C2 - 15nF
These values give a frequency of 166 Hz and a Q of about 1, which should get you in the ballpark. Don't forget to add another 100nF to the end of your filter chain to block any DC from leaking into your amp signal.
After that, you just need to worry about power, biasing the filter and construction. I suggest keeping things simple if this is your first build, so no switches or LED.
For power, just use a 9V battery. Use two 10k resistors in series from the positive terminal to the negative terminal. Put a 10uF capacitor to ground at the junction of the 10k resistors, and you have yourself a power supply with filtering and a reference voltage junction.
To bias your circuit, run a 1M resistor from the 10k junction to the input of the buffer (after the 100nF capacitor). Biasing this way provides a reference voltage for your signal and gives you maximum headroom for a clean signal.
Construction is fairly straightforward but you may have questions and you go along. Don't be afraid to ask!
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