Need help understanding an RC high-pass filter

Please be gentle and explain things as if you were talking to a six-year-old...

With my recent experience with a bass cut circuit with humbuckers, I've also been looking into simple high-pass filters.

I have used a passive RC filter before on neck humbuckers before but my understanding of how it works is rudimentary at best. My application is a parallel capacitor and resistor used in series with the pickup output.

I have experimented with the capacitor values and know on a very basic level that the lower the cap value, the more high frequencies will pass (or low frequencies filtered out).

What I would like help with is the role of the resistor in this application.
In very simple terms (remember I'm a six-year-old) what will increasing or decreasing the resistor value do to the operation of the circuit, and how will it effect the tone?

I've read something about "cut-off threshold" or something, but when I read that stuff, my vision gets blurry and my brain misfires...

Let me know if you need more info from me ie: pot/cap/resistor values etc.

http://www.learningaboutelectronics....calculator.php

If you play with it a bit you will see that the higher the value of the resistor, the lower the cutoff point and the lower the value of the resistor the higher the cutoff point.
If the resistor value is too high or infinite (not there) the cutoff point is so low you won’t hear any change at all.
If the resistor value is too low or zero, all the audible signal will be cut off.

Thanks Cliff. I did come across that same site and calculator during my quest. (also added it to my bookmarks)

I can see what you mean about the changing values and how they change the outcome, but I'm still trying to wrap my head around how those numbers translate to real world sound and what my ears will hear.

Right now I'm using a .0068uf cap in parallel with a 500k Ω resistor. I've settled on that for today. I'm braindead now...

Those values won’t make a lot of difference.
https://encrypted-tbn0.gstatic.com/i...WtNaQ&usqp=CAU

The best trick is to have a couple of leads with alligator clips on them hanging out of the guitar or rig up a special guitar lead with the clips on it and try several different values and pick something to your taste.

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Those values won’t make a lot of difference.

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Actually, they do. Subtle, yes, but noticeable and effective.

edit to add: The intent is just to brighten up the neck humbucker. It is often referred to as the "de-mud mod".

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Originally Posted by Cliff Rogers

The best trick is to have a couple of leads with alligator clips on them hanging out of the guitar...

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Yep. That's what I've been doing... and another reason why my brain hurts...

Why do you have a 500K resistor in parallel with the capacitor?
What is the value of your volume pot?
The high pass filter is formed by the capacitor in line and the resistor going to ground, in this case, the volume pot.
All the resistor in parallel is doing is bypassing some of the signal around the capacitor therefore bypassing the high pass filter.

Other ways to help ‘de-mud’ a humbucker are to reduce the height of the pickup, particularly the bass side, and you can just turn the mids up a bit on the EQ on the amp.

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Why do you have a 500K resistor in parallel with the capacitor?
What is the value of your volume pot?

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I’m using 500k pots, and with a 4 conductor humbucker, the parallel RC is in series with the north and south coils.
With my limited understanding of electronic principles, I can’t tell you why what goes where, but I have followed schematics that show this configuration and placement.

I’ll post the drawing later. I’m away from my computer atm.

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Other ways to help ‘de-mud’ a humbucker are to reduce the height of the pickup, particularly the bass side, and you can just turn the mids up a bit on the EQ on the amp.

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Yeah, I’m well aware of pickup heights and EQ, I’ve been playing and working on guitars long enough to know that.
It’s not always that simple, I’m not the first person to be dissatisfied with neck humbucker tone that has sought a solution.
Just google “muddy neck humbucker” and see how many threads there are on the subject.

Okay, I will see myself out.

If you've got a muddy neck humbucker then I'd suggest you really want to change the pickup for a clearer one.

I suppose it all hinges on what you call 'muddy'. My clear neck humbucker with a reasonable amount of bass might be Mr Muddy McMudface to you.

I can understand a plain RC and a plain CR circuit, but it's once it's in a guitar, it's hard to know what resistor is what in relation to the capacitor.

I'm assuming that the resistor that's part of the CR circuit in the G&L PTB circuit is the volume pot resistance.


Attachment 41054

The 1 meg pot is then set up as a bypass pot, to bleed in as much of the high pass filter as you want.
The numbers though for the filter frequency knee point (which is already 3dB down on unity) come out quite high when I do them, so I can't see you wanting anything near a no-bypass situation, so your pot travel will be quite limited. I'd be tempted to use a slighty bigger cap value, say 0.0033uf, which with 250k would give a 200Hz cut-off value, which is still cutting out some of the low mids as well as the bass end.

Of course that volume pot resistance plays its part as well, and with a typical +/- 20% on a lot of pots, then you could be anywhere between 200k to 300k. Which with a 0.0022uF cap could give 361Hz to 241Hz. And the cap value has its tolerance as well, so the final possible cut-off frequency range is even bigger. So best to measure the volume pot resistance and choose a cap value that suits that, but check the value of the cap you use as well.

With a 500k pot, a 0.0068uF cap should give a 46Hz filter point. Which is far to low to do anything worthwhile to the mud. A 1.5nF cap would give a 200Hz cut-off and a 1nF cap about 300Hz, so somewhere in between those would replicate the 250k pot G&L circuit response.

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Originally Posted by McCreed

I’m using 500k pots, and with a 4 conductor humbucker, the parallel RC is in series with the north and south coils.

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I think you mean CR (capacitor series with the signal, followed by a resistor to ground), as an RC (resistor in series with the signal, followed by a capacitor to ground) is a low pass filter. CR and RC are standard filter descriptions in electronics, so it's important not to get them confused.

Thanks guys.

Sorry Cliff, just re-read my last post from last night, and I didn't mean to sound so cranky, but it did. It was late when I typed it...

Here's what my wiring is based on:

Attachment 41057

This has become known around the internet as the "Artie Too De-mud Mod" originally designed for use with a SD '59n.
I had exchanged messages with him on Seymour Duncan Forum (amongst many many others) and he informed me that if I wanted to cut the low frequencies more than the standard .01uf, to try a lower value, hence my using the .0068uf.

I know that the frequency crossover is only 46hz (I ran it through the online calculator) but it absolutely does make an audible difference. I will reiterate that it is subtle, but audible when taken in and out of the circuit via switch or clips.

My original question came from some people using a 1M resistor in place of the 500k, and I was trying to understand the what and how the resistor value effected the operation of the circuit.

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I think you mean CR (capacitor series with the signal, followed by a resistor to ground), as an RC (resistor in series with the signal, followed by a capacitor to ground) is a low pass filter. CR and RC are standard filter descriptions in electronics, so it's important not to get them confused

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Maybe I should have just referred to it as a high-pass filter, I thought RC was the correct term based on other things I've read, but I guess not. Thanks

That’s certainly not a standard CR high-pass filter design. It’s more akin to a treble bleed circuit, but it’s not that either.

That capacitor in that position will give a phase shift to the output of the first coil with respect to the second coil, so you will definitely get some frequency cancellation happening and a thinning of the sound. The resistor is again a bypass resistor (as far as I know) but 500k won’t bypass a lot, if any. I’d think you need to look more at the phase shift value of the capacitor rather than seeing it as a conventional high pass filter.

But then you’ve also got the coil inductance to consider, so there’s probably a lot more going on than just a phase shift. Beyond a very simple explanation, methinks,

What’s the rest of the control arrangement like, standard LP type wiring? Had you tried a 1meg volume pot fir the neck pickup?

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The resistor is again a bypass resistor (as far as I know) but 500k won’t bypass a lot, if any.

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I'm sure you're correct, and I did read that somewhere.

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I’d think you need to look more at the phase shift value of the capacitor rather than seeing it as a conventional high pass filter.

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FWIW, all the live testing I did, I only used the 500k resistor, but changed cap values.

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What’s the rest of the control arrangement like, standard LP type wiring? Had you tried a 1meg volume pot fir the neck pickup?

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2 HB /1 v /1 t/ 3-way toggle

You could still try a 1 meg vol pot (or add a 470k in series as a test) What’s the actual value of the pot? >500k is always consideted better with humbuckers.

Using 500k pots.

I also have seen a Seymour Duncan diagram with a single .047uf cap in series with the hot lead of the HB going to the switch(or volume pot), but not tried it.

SD w/ .047uf cap

That .047 uF looks wrong to me, as you’d get a 7 Hz filter point as it must take the pot resistance into account. 0.0047uF/4.7nF should give a 67Hz filter point, which would be more useful, but maybe the pickup inductance also comes into play here. 7Hz is the typical DC blocking cap filter frequency on an FX pedal input stage.

With passive electronics, everything affects everything else. Add in some semiconductors or valves to separate the stages and you’ve got far more control over what’s going on.