Need help understanding an RC high-pass filter

[McCreed]

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.

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.

[Cliff Rogers]

Okay, I will see myself out.

[Simon Barden]

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.




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.

[Simon Barden]

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.

[Simon Barden]

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

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.

[McCreed]

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:



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.

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

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

[Simon Barden]

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,

[Simon Barden]

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

[McCreed]

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

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

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.

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

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

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

[Simon Barden]

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.