Simple low pass pedal?

[Lameller]

Hi

I would love some help figuring out how to build a simple low pass filter that passes all frequencies below 187Hz (the high F note on a guitar A string) and cuts all frequencies above 187Hz.

The box could be as simple as:

Input - split signal into two signals -
Signal 1 - Low pass filter - Output 1
Signal 2 - Output 2

I will be using the box to send only the E and A string on a guitar to an octaver pedal.

I have never built a pedal before - so any help would be greatly appreciated.

Best Lameller

[PBE6]

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–Key_topology

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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[Mark Hammer]

I have one of these puppies: http://www.hifiengine.com/manual_library/dbx/100-boom-box.shtml whose purpose was to provide an octave down, but only for what was low end in the first place.  I don't have a schematic for it, but there's a lot of lowpass filtering in there and a 4013 flip-flop.

[Transmogrifox]

This could be as simple as a resistor and a capacitor with an audio jack at the capacitor for the low-passed signal and a jack on the other side for input and "through" signal.

If you have a breadboard, start there and see if it is enough.  The nominal cut-off frequency from a simple RC circuit is:
F = 1/(2*π*R*C).  Use a large resistor like > 100k for minimal effect on the dry signal.

It would help to know what octaver you're using and what low pass filtering it already has.  Does the octaver work well enough already?  Maybe you don't need more low pass filtering.

Does the octaver sound ok when you roll the tone knob all the way back on your guitar?  The a simple passive RC filter will work.

If the first set of "simple" answers don't satisfy the requirement then you will probably need to start cascading sallen-key filter stages as PBE6 suggested.

Here is an online calculator for these that can help you get the correct resistor and cap values for each filter stage:
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

If you cascade several in series then the effective -3dB point goes lower in frequency, so you may want to set Fc higher if you combine more than 2 of these.

Use a Q factor of 0.707 unless you would like to have some peaking.  In that case, Q = fc/BW (center frequency divided by bandwidth of desired peaking).  Higher Q is narrow and tall peak, lower Q is wide and short peak.

[Ben N]

A filter won't segregate strings. To do that you need a hex pickup. Having said that, as noted above, you may be able to get to "good enough" with a second or third order filter.