Passive high pass filter in stompbox form

[Mo Jiggity]

So, I recently experimented with putting a 32-band graphic EQ in between my guitar with EMGs and my amplifier, and I found that having a 12db cut on all the notch filters up to 200hz give me a tighter low end. I want to be able to have this as a switchable option in my rig, and I figured that the best and cheapest way to do this would be to build a simple passive high-pass filter to approximate such an EQ curve.

Due to my extremely limited electronics experience I consulted the internet to find out how to do this, and according to Wikipedia one simply puts a capacitor in the signal path and a resistor after it with the other leg to ground. Using the equation on High-pass filter - Wikipedia, the free encyclopedia , I calculated that I could use a 1 uF cap and a 1k resistor to achieve this effect.

My question here is twofold - one, will this idea work in that I'll be able to build this into a stompbox enclosure and use it as a switchable low-cut with no other appreciable tonal effects, and two, could I substitute a potentiometer in for the resistor putting the middle lug to signal and the other two to signal and ground to make the rolloff frequency adjustable?

Forgive me if I am vague or don't know what I'm talking about... I know how to solder and how to figure out basic switching concepts but that's about it.

[uvacom]

You've got the right idea, but that particular circuit won't work well. You'd do a lot better to choose something like 1nf for the cap and 1Meg for the resistor.

The issue is that your guitar is a passive device, so when you put the 1k resistor to ground, you completely change the impedance of the circuit and it will no longer output a sufficient signal. Using a smaller cap and a bigger resistor helps to prevent this. Also, this circuit will be a single pole filter, so it will only have a 6dB rolloff, you might adjust the cap to get the sound you want. You could use a pot for the resistor to change the cutoff frequency, but again this will interact with the impedance of the circuit and probably won't work as well as you'd like. If you do that, try using like a 500k pot in series with a 500k resistor to ground. You won't have a lot of range, but at least your circuit won't load down the signal too much.

In general, active filters work a whole lot better and you could build one very inexpensively. If you're putting this in a pedal, that's my suggestion. Being able to adjust the cutoff frequency in a second-order (or more) active filter is somewhat more challenging (although by no means impossible).

[tedmich]

You've got the right idea, but that particular circuit won't work well. You'd do a lot better to choose something like 1nf for the cap and 1Meg for the resistor.

The issue is that your guitar is a passive device, so when you put the 1k resistor to ground, you completely change the impedance of the circuit and it will no longer output a sufficient signal. Using a smaller cap and a bigger resistor helps to prevent this. Also, this circuit will be a single pole filter, so it will only have a 6dB rolloff, you might adjust the cap to get the sound you want. You could use a pot for the resistor to change the cutoff frequency, but again this will interact with the impedance of the circuit and probably won't work as well as you'd like. If you do that, try using like a 500k pot in series with a 500k resistor to ground. You won't have a lot of range, but at least your circuit won't load down the signal too much.

In general, active filters work a whole lot better and you could build one very inexpensively. If you're putting this in a pedal, that's my suggestion. Being able to adjust the cutoff frequency in a second-order (or more) active filter is somewhat more challenging (although by no means impossible).

A nice unity gain linkwitz-riley 24db/octave high pass would give you something similar, but you'd need some power for op amps...

you'd only need a single dual op amp for a high pass:
2-Way Electronic Crossover

or 1 more for a typical highpass/lowpass combo
XM1 Electronic Crossover Network

[pdf64]

Are you EMGs active or passive? Or do you currently plug into any stomp boxes? If so have they got a buffered output, ie if you switch the fx to bypass, and remove the fx battery/power supply, does the sound still come through? If so then they're not buffered.
Basically, 1k would be too low of a load for your guitar, 1M would be fine but 1nF might cause noise in series with your signal.
You need a high pass filter with the R between about 22k - 100k, in the signal chain after a permanently engaged active/buffered circuit.
Bear in mind that a simple filter will have a gentle -6dB/octave rolloff, whereas your graphic eq, as you describe the set up, will have a much steeper rolloff. If you need that steep rolloff, then tedmich's active circuit solution is needed.

[chipaudette]

Be aware that if you build your pedal using either a 1st order (ie passive) highpass, you will not get the same response as a graphic eq with all the low freq sliders pushed down. There are two key differences: (1) your graphic eq is actually making a "shelf" shaped filter and (2) the graphic eq is capable of sharper changes in tone versus frequency. Here's what I mean:

GEQ Can Shelf: Look at the shape that your sliders are making on your graphic EQ. The low frequencies are all flat at -12. This flat area is called the "shelf" and it is very different than what a 1st order highpass does. The 1st order highpass is flat in the high frequencies, but in the low frequencies it just keeps rolling off down forever. It doesn't shelf. The lowest frequencies will be cut more than just the kinda low frequencies.

GEQ Can Change Sharply: The other problem with a passive filter is that it doesn't roll off very quickly. On your graphic EQ, you might have 200Hz set to -12 dB and 400 Hz set to 0 dB. Compared to what a passive filter can do, that's a pretty big change over a very small change in frequency. By comparison, a 1st order passive filter with its cutoff at 200 Hz will have this kind of response:

200 Hz: -3 dB
100 Hz: -9 dB
50 Hz: -15 dB
25 Hz: -21 dB

So, you can see that your 1st order low pass is going to have a hard time achieving 12 dB of cut between 400Hz and 200Hz. It just can roll-off the signal that sharply.

With all that said, you still might like the sound of the passive high-pass filter very much. It is nice and simple and has much smoother phase response than comes from most graphic EQ. So, I say, despite the differences, give it a go. Just be aware that it may sound quite different.

Have fun!

Chip

[Enzo]

Why does it have to be passive?

Here is a nice little Danelectro graphic EQ pedal for $30. SO you have to throw a battery in it once in a while. It has multiple bands for you, with the bonus of a level control for a little no tone added boost.

Buy Danelectro DJ14 Fish and Chips 7-Band EQ Pedal | Equalization & Tone Shaping | Musician's Friend

And if your amp has an FX loop, consider doing this in there instead of out front of the amp.


But if you want to plat with little passive tone filters, consider that the freq is RxC related. SO instead of running the guitar into a low resistance with a large cap, maybe something more like a high resistance with a smaller cap.