Taming A Bright Pickup

[guitar2005]

That's actually backwards Guitar2005.

The cutoff frequency formula is 1/2*pi*r*c. If we assume a 10k resistance for the PUP, using the 4.7nf I suggested as a start point, gives a cut off frequency of 3386hz. If you go to 3.3nf, it goes UP to 4823hz. Not down. Remember, the cap is essentially a short to ground for very high frequencies, but won't pass DC. To the cap, lower frequencies look more like DC. Bigger caps will move the cut off freqency down.

http://en.wikipedia.org/wiki/Lowpass_filter

However, we don't hear sound in a linear way, the formula is probably not very accurate, and since you're ear can use harmonics to create a perceived fundamental tone, the math is only a starting point. So 4.7pf or 3.3pf, or 10pf . . . it doesn't matter. All that matters is what sounds good to you.

Regards,

Todd

With the 68pf, it cut off bass frequencies and as I moved up in values (i.e. 10nf), more bass was present... therefore increasing the cutoff point. That's what I noticed.

Using the formula is 1 / (2pi)rc, I agree that I should have see the opposite.

I know what I heard... There must be something about how I wired it. Basically, I put the cap in series with the hot lead, not in parallel with the pickup. I 'll experiement some more with the cap in parallel and report back.

[ToddW]

Even in series, the larger the cap, the more bass it'll pass. You've sort of wired it like a coupling cap.

That said, it may be that you want to cut the lower frequencies more, because it's the harmonics that you'd lose moving the PUP closer to the bridge. Without those harmonics, the tone gets very thin and sharp. Play with this: http://www.till.com/articles/PickupResponseDemo/index.html

Our hearing isn't linear, and, because the strings are fairly short, the fundamental bass tone is not that strong. But your ear/brain fills in the fundamental. If you ever play the lowest bass notes on a baby grand piano, you hear the notes, but if you analyze it on a computer, the tone is barely audible. We fill in the note by analyzing the harmonics. It's also the harmonics that give an acoustic guitar its sound, and why the neck PUP may sound more acoustic than the bridge. You get more fundamental, but also more harmonics.

From what you're describing, my guess is that a notch filter might be what you want, if you decide you need to go there. You could play with a graphic equalizer to find out the center frequency and then choose your cap and inductor from there. It's a cheap and easy filter, just an inductor and cap in series to ground from the hot lead, but if you can get to where you're happy with a simple cap . . .

[Paul Marossy]

0.0047 is 4700mF. That's pretty big.

It's really a pretty small value. I would start with that and tweak it from there. You might even find that it's not quite big enough for your taste.

Hmmmm, you must be building some JUMBO guitars there Actually, you were wearing guitarspeak goggles when you saw .0047, and turned it into mf. Guitar2005 actually read what I wrote like an engineer would. I have no idea how you'd fit a 4700uf (4700mf) cap inside a guitar, but I hope Guitar2005 reads this before he tries. The 12w ones I have upstairs are 3" tall and 1.5" across!

Todd

Oops, I thought he meant 0.0047uF=4700pF - I looked at that again this morning and slapped myself on the forehead.

Man, I need to quit trying to do too many things at once while I'm on the computer.

[guitar2005]

Even in series, the larger the cap, the more bass it'll pass. You've sort of wired it like a coupling cap.

When I put the cap in parallel with the pickup, I got the desired effect. Lower cap values (i.e. 68pf) remove less high end than higher cap values (i.e 10nf); meaning that the higher the cap value, the higher the frequency cutoff value.

The series wiring probaly had some weird interaction with the pickup own capacitance, inductance and resistance.

I ended up going with three 3.3nf in series, which is about 1.1nf. That tamed down the high end enough, while keeping the pickup's character. I think that having a slightly lower value and a second order filter would help even more. I'll have to research how to make the second order filter though. My electronics days are so far away...

[ToddW]

You're going beyond me if you want a second order filter. In fact I thought it would require an op amp, which shows how little I know.

I think you could put a small inductor in series with the PUP as well as the cap to ground. The inductor would drop the resonance frequency which is sort of like adding a second filter, wouldn't it? And since an inductor passes low frequencies more easily than high frequencies, intuitively it seems like it'd increase the slope of your curve low pass filter's curve. But like I said, you're going beyond my circuit design skills.

Anyway,these are pretty cool if you need an inductor: http://www.mouser.com/catalog/specsheets/164201.pdf

Regards,

Todd

[thirdstone]

Tune your amp tone knob down...........

[Prostheta]

I wouldn't think a passive filter would require an op-amp unless you're wanting gain.

[guitar2005]

I wouldn't think a passive filter would require an op-amp unless you're wanting gain.

+1

http://en.wikipedia.org/wiki/Low-pass_filter

A second order filter simply has a more defined cutoff where the frequency response drops faster. I was thinking of something along the lines of the Butterworth filter or something similar.

ToddW was right in that I'll need an inductor for that though.

[ToddW]

Third Rock, the goal is to only alter the tone of the bridge PUP without altering the tone of the neck PUP.

Prostheta,

I thought a second order filter meant you'd basically be passing the signal through two filters or setting up a feedback loop.

To use two "passive filters, you'd have to isolate the filters from each other somehow. You could use a big resistor or such between them, but that alters the signal again, right? And if it's in series, wouldn't it drop the voltage swing you'd get at the jack making the bridge PUP lower output than the neck PUP? Going by that logic, I thought a second order filter would be be something like a low pass -> op amp -> low pass -> jack. That or setting up a feedback loop after an op amp with a cap in series with it to cancel some of the higher frequencies. Remember I already said we were out of my leaque here.

Throwing a coil in series with the PUP seems easy and might accomplish what Guitar2005 wants, but I don't think it turns this into a second order filter.

This is the point where I find an electrical engineer at Guitar Nuts or AX84.

[guitar2005]

Tune your amp tone knob down...........

The treble controls on my amps don't remove the right frequencies for this guitar/pickup and that wouldn't be very practical at shows.

[Paul Marossy]

Cool, sounds like you accomplished what you wanted. I'm putting this one in the memory bank for future reference.

[ToddW]

ToddW was right in that I'll need an inductor for that though.

That must be my one for the week. Anyway, those mini transformers from Mouser really are cool as inductors. I'm going to use two in my current build to offer different notch filters. Might be a closer source thought in Canada, or even on ebay?

[Prostheta]

A parametric EQ might be a good thing to consider if it's more tone sculpting than "repairing" an overly bright sound.

[guitar2005]

I played the guitar a little more today just to see if I still liked the results and I have to say that the caps really did the trick. The guitar sounds nice and meaty and still as enough highs to sound nice and clear. The Paradise City intro is sounding real nice and it no longer has the ear piercing ice pick treble. Pinch harmonics sound reall good too.

I highly recommend this to anyone trying to control highs in a guitar.

[Paul Marossy]

Then, there's this unknown value I tried (I think I prefer the 3.3nf though) which sounded pretty good. Its marked 100v, 68cp, 8821, 682k. Not sure what value it is but I'm curious.

I believe that is a 0.0068uF cap.

[Bygde]

Happy to hear that it work out for you! However if the axe was mine, I'd put in a tone knob. I have read that you don't like tone knobs, and I used to be the same. But then I bought my Ibanez, and it really opened up my ears to the tone knob. It can sweep the tone really nice between modern metal (generally thin sounding), and a Fear Factory/Dream Theater sound (generally thick). I'm just hooked on it, and I really think it puts that little extra on my tone.

Edited June 22, 2008 by Bygde