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Thread: Musings on the De-mud mod.

  1. #1
    Toneologist
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    Default Musings on the De-mud mod.

    I've applied the De-mud mod to one of my guitars and was thinking about the how/why of it recently. Reading Wikipedia, I'm getting the impression it's a simple first order low-cut/high-pass filter, though I don't understand what the "first order" bit means.

    Here's the diagram of a such a filter from Wikipedia, and ArtieToo's diagram for comparison, noting that the volume pot acts at the resistor to ground, even though it's not shown in Artie's diagram. (edit, found a new image of a high pass filter. Ignore the cap and resistor values.)



    First thing is that since the volume pot acts as the resistor to ground, the amount of resistance is going to change as the volume is turned up and down, thus effecting the cuttoff frequency. Per Wikpedia, the formula cutoff frequencey of a High pass filter is f = 1/2*Pi*t, or f = 1/2*Pi*R*C, where f is the cutoff frequency in hertz, R is the resistance in Ohms and C is the capacitance in Farads. If my math is right (and if it isn't, please point out what I did wrong), the cutoff frequency of a De-mud mod with a 500K volume pot all the way up should be a 318.31hz. Now if the pot is turned down halfway, assuming it's a pot with a perfect linear taper, that gives us 250k resistance to ground, giving us a cutoff frequency of 636.62hz. Now, I realize pots aren't going going to measure perfectly their rated value in Ohms, nor is it going to have a perfect taper, liner or audio(logarithmic). But the general idea remains the same: the De-mud mod is going to cut out more and more bass as the volume pot is turned down. I don't know if that's an issue or not, but I guess it could cause the bass to get trimmed off too much making the lower volume setting sound thin if you used the wrong capacitor value.

    Looking at Artie's diagram, what I'm trying to figure out is what the resistor parallel to the capacitor, but in series to the signal is supposed to do, if anything. Does that some how change the resistance to ground, even though it's wired in series with the pickup? If so, does the pickup's own resistance count for anything as far as the filter is concerned, since the other end of the pickup is connected to ground, and if so, that messes up my math above, doesn't it?
    Last edited by Koreth; 12-16-2008 at 12:12 PM.

  2. #2
    Tone Member lyongraulty's Avatar
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    Default Re: Musings on the De-mud mod.

    If I'm remembering the demud mod correctly, the 500k resistor is separate from the volume pot. You can use a pot and use it control the amount of bass rolloff, but as you said, using a volume pot for the resistor might be a problem. He said the cap/resistor combo can be wired in series with eachother anywhere in the circuit. I tried with the bridge pup in my LP (right before the volume pot) but I didn't like the sound as much as the original way.

  3. #3
    Toneologist
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    Default Re: Musings on the De-mud mod.

    Looking at the diagram ArtieToo posted, the cap and resistor are parallel to each other, but are both in series with the pickup. Looking at the diagram for a low cut filter circuit from wikipedia, the resistor in Artie's diagram is in the wrong spot to be the resistor to ground needed to make a low cut filter circuit. As far as I can tell, the resistor in ArtieToo's diagram doesn't connect to ground and thus doesn't act as part of the low cut filter circuit. However, the volume pot works by sending all, some, or none of the signal to ground, so there's our resistor to ground, giving us a low cut filter circuit.

    So what does the 500k resistor in the diagram above do, if anything?

  4. #4
    Ultimate Tone Member skynyrd's Avatar
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    Default Re: Musings on the De-mud mod.

    I tried the demud mod,,well half of it since I didn`t have a 500K res. I put a .001 cap on the red and white wires and it sounded to me like it made it almost in paralell. The volume dropped on the PGn a good bit, so I am thinking the 500K is there to keep a load balance on the coils ? Going to hit the shack today and pick up some resistors and various caps to try.

  5. #5
    Ultimate Tone Member
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    Default Re: Musings on the De-mud mod.

    Yes, it is usually there to keep the resonant peak from shifting. It is in an odd spot in the above diagram. The way I've done it is one side of the resistor to ground.

  6. #6
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    Default Re: Musings on the De-mud mod.

    Here's a simple way to think about how the de-mud mod works. At low frequencies a capacitor acts like an open circuit (no connection), while at high frequencies it acts like a short circuit (a wire).

    So at low frequencies we ignore the capacitor and have just the resistor. If the volume pot has the same value as the resistor (500k in the picture) then we have a voltage divider where about 1/2 of the signal is across the de-mud resistor and 1/2 the signal is across the volume pot, so we have reduced the bass by half (3 dB).

    For high frequencies the capacitor acts like a short across the resistor, so it's like there are no added components in the circuit, the full signal shows up across the volume pot, and the treble/midrange are unchanged.

    Changing the value of the capacitor changes the frequency where the roll-off from full signal to half signal starts. For a single RC circuit like this the roll-off is fairly gradual.

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