Neck pickup high pass filter cap and pedal input cap interaction

[Halkbi]

I recently acquired a guitar whose neck pickup has that classic mud going on. I'm a big fan of using unorthodox pickup combinations to combat this, but for this specimen I'm keen on keeping the cost down. Google led me to this wonderfully simple solution by Seymour Duncan:



Various comments on different boards suggest that 47nF is too big and is unlikely to change the tone much, with values between 4,7nF and 22nF seeming more common. One drawback of this solution is that the shift in phase induced by the cap will alter the in between setting where both pickups are wired in parallel. If I ever wanted to use the middle position (which I rarely do) I could just make the mod switchable via a push-pull pot and all would be fine. That could also be a nice way of giving the neck pickup a switchable boost; bypassing the high pass filter and letting the mud through is likely to push an amp set to edge of breakup into overdrive. Happy days!

However, I realized that there's another issue with this approach. The capacitor on the neck pickup output will end up in series with the input cap of a pedal when the volume is set on full. This could potentially make things very unpredictable (imagine accidentally cutting the input cap value of a treble booster in half when turning up the guitar volume), so my first thought was to isolate the cap by putting a small resistor after it (pickup > cap > resistor > pot). How small of a resistor can you get away with in order to keep caps from seeing each other, and would it's addition alter the tone and impedance in any significant way? I realize that this is a typical trial and error situation, but I would like to keep the trial on a minimum as the guitar is a semi-hollowbody. Pickup is an Alnico V P90 that measures about 8.4k and pots are 500k if that's something to consider.

[anotherjim]

Well, no. If the series cap is always loaded by the volume pot and that is wired wiper to the output (Fender style it will be), then the HP filter is the cap and the full resistance of the pot. Treat that as one filter pole.  If you have the pickup feeding the vol pot wiper then the filter becomes variable with the volume.
It's more complicated than that because the pickup is an inductor, so you have an LCR filter. 47nF might be a good value. The wrong RC only filter calculation makes it full bass, but the inductance of the pickup raises that. It could roll off bass under 350Hz. That's guessed for 4.5H inductance but I don't know what you have.

[Halkbi]

Well, no. If the series cap is always loaded by the volume pot and that is wired wiper to the output (Fender style it will be), then the HP filter is the cap and the full resistance of the pot. Treat that as one filter pole.  If you have the pickup feeding the vol pot wiper then the filter becomes variable with the volume.
It's more complicated than that because the pickup is an inductor, so you have an LCR filter. 47nF might be a good value. The wrong RC only filter calculation makes it full bass, but the inductance of the pickup raises that. It could roll off bass under 350Hz. That's guessed for 4.5H inductance but I don't know what you have.

Thanks for the explanation! And yes, I'll be wiring input to lug 3 (not a fan of decoupled volume pot wiring). So are you saying that the fact that we got a whole filter circuit going means that the filter cap wouldn't be in series with the input cap of whatever pedal I run my guitar into? That the load of the pot alone isolates the filter cap from any cap put after it even if it's on full blast, with all the resistance going to ground? Sorry if I didn't get you right or if my question didn't come across properly, my english is sloppy at best (as is obviously my understanding of basic electronics, but I'm working on both  ).

[PRR]

> unlikely to change.... One drawback .. shift in phase ... in between setting...overdrive. Happy days!..... in series with the input cap of a pedal ... unpredictable .... isolate the cap .......

IMHO, you are over-thinking this. It costs four bits to just try a cap. It may not be all that you fear.

[Rob Strand]

Another option is the G&L passive bass-treble tone control method.  Instead of a series cap you use a parallel cap and resistor in series. Only focus on the bass control part (C1 & VR1) which is before the volume control like your set-up.  Tone control can be moved to wherever you want.  The VR1 resistor tunes the amount of bass cut.   It's OK to use  fixed value but you could use a trim pot.  The capacitor tunes the frequency.   You might need a bigger cap.

Keep in mind a small cap will also give the effect of cutting the bass because the frequency kicks in is above the bass frequencies.

I suspect the extra resistor of the G&L method will let the response be tuned more evenly.



Pickups characteristics aren't consistent and the guitar's volume control pot value can vary.   So in the end you need to tune the values instead of using a recipe.


Here's a few simulations comparing the single cap method with the variable resistor + cap method (G&L).
I've set the single cap to 33n.



Here the cap in the R+C method needs to be increased a bit to match the single cap method.  As expected
the lower R values raise the level at low frequencies and give control of the bass.


In this case the cap in the R+C method is set to small value (10n) and the resistor is set to a low value in order to reduce
the amount of bass cut.    While the lows are cut it attenuates the mid zone. 


The sims highlight the need to trim the cap value.  The resistor can be used for fine tuning but it's not like a variable cap.

The G&L cap value is much lower than the 10n I've used here so the G&L modifies the response quite a bit.

I didn't put much effort into the pickup model.  It didn't seem to change the results much.

Note the guitar loading waters down interaction effects between the bass-cut filter and the up-stream filtering in the amp or pedals.   It's not the series cap values as such it's more the two RC time constants of bass cut cap + guitar load resistance and amp/pedal input cap + amp load resistance.

[Elektrojänis]

How about an alternative solution... If the pickup has adjustable pole pieces, screw them out a bit. If needed lower the pickup itself to compensate for volume and possible too much magnetic pull. This should affect the inductance of the pickup slightly and it should make it a bit brighter. I have not tried this with P90's though, but there is no modification needed to try and you can easily revert the adjustment if it doesn't work for you.