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Capacitor as a passive high pass filter

Discussion in 'Pickups & Electronics [BG]' started by Jeremy5000, Jan 15, 2013.

  1. Jeremy5000


    Feb 25, 2008
    Searched but couldn't exactly find the answer to this.

    So a capacitor can cut out the low frequencies. Can it just act as a high pass filter?

    Another question is (god I looked), is there a chart showing what microfarads equal what frequency?

    Last and most important question - how do you wire it? Just in-between last hot lead and jack?

    Thanks folks
  2. Rickenbacker and G&L do it. The common values are around the 0.0047uF range. Run the capacitor in series with the signal path, and then place a high-value pot parallel to the capacitor if you want a bass-cut control. 1M Ohms is common for high-impedance pickups.

    The frequency cutoff point is very difficult to figure out, as it is determined partly by the pickup impedance, which is difficult to calculate or measure, since it is frequency dependent. Most people find it easier to simply experiment with values.
  3. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

    Capacitors do not "cut low frequencies" what they do is to block DC and pass frequencies above a certain point. The result depends on how theyu are used.

    If they are in series with the audio - the lowest frequencies cannot get though, cutting the lows.

    If they are in parallel with the audio (like a bass tone control) the send the high frequencies to ground, cutting the highs.

    The cutoff or rolloff frequency depends on the size (value) of the cap. The bigger the value, the lower the frequency it will pass.

    This frequency also depends on the resistance and impedance of the circuit that it is used in. SO - in a guitar tone control with a pickup of impedance of perhaps 10K Ohms, it will have one result. If the impedance is different, the frequency will also be different, so it is not as simple as looking it up in a chart, although it is easily calculated by any competent tech or engineer. (That's why we go to school!)

    (edit) Good advice Line6man - just try different caps and see what happens!
  4. Well, it's not that the math is complicated, this can actually be done by high school students if you give them the formulas. It's just that it's hard to obtain accurate measures in some regards. The average person cannot measure pickup inductance, for example, because measuring inductance requires specialized equipment, and even then, accuracy is questionable, as it will pertain to specific reference frequencies. Change the reference frequencies in the real world, and the entire nature of the signal impedance will change, and that will throw off your formulas.

    Like I said, your ears will always be the better judge, anyway, so don't be afraid to just mess around with different capacitors until you figure out which value you like. :hyper:
  5. Jeremy5000


    Feb 25, 2008
    I'm using Fender custom shop 60's which I think have a DC Resistance of 7.1K
    Is this the impedance rating or is that something different?
    Thanks for the lesson btw ;)

    After test no noticeable difference, (recorded on separate tracks in pro tools, and even with a low pass EQ to try and hear difference in lows...)
  6. Jeremy5000


    Feb 25, 2008
    So - what does cut frequencies? Resistors? Or do I have to go active?
    Thanks again
  7. Hactar


    Sep 25, 2011
    Boulder, CO
    You might be after a high-pass filter, or perhaps a low-shelf filter for more control.
    A resistor will not do this.
    You can make passive filters, but they often attenuate the signal significantly.
    If all you want is a high-pass filter, a capacitor wired as line6man described will do it. If you want more control, then it's probably off to the land of active filters.

    Suggested reading for a start:
    en.wiki: HPF
    en.wiki:Filter Design
    Passive RC Bandpass

    If this is all rather too much fiddly bits, than you may want to just get a preamp that will do this for you.
  8. DC resistance is the measure of resistance to DC (0Hz) current.
    Impedance includes resistance, but it also accounts for Reactance, which is related to the inductive and capacitive components of the circuit. Reactance is frequency-dependent, while resistance is not.
  9. Jeremy5000


    Feb 25, 2008
    Sort of want to simplify things.
    Guess I'll have to just do some testing with a few different voltage and "uf" sizes and run them through a spectrum analyzer.

    For testing purposes, (and maybe permanently), would you suggest bypassing the whole tone knob for now and going straight from hot lead > cap > input jack? or output jack as I would call it :eek:
  10. Depending on the signal impedance and pot value, there is generally only a slight difference in tone between removing the tone control and just leaving it on "10." If you are removing volume pots, however, things will generally get brighter.

    There is no reason to experiment with voltage ratings. You are dealing with a maximum of a few Volts, so just about any capacitor works. No need to pay extra money and waste control cavity space for a higher voltage handling capability.

    Also, indeed, it is an output jack. Anyone that calls it an "input jack" does not understand the function of the jack.:hyper:
  11. eddododo


    Apr 7, 2010
  12. bassbenj


    Aug 11, 2009
    This will give you some decent information as to a choice of values.

    Note that for a low pass filter (tone control) the R is determined by the resistance (impedance) of the pickup.

    But if you want a high pass filter (like a G&L) the "corner frequency" (which is where the circuit starts rolling off) is determined by the volume pot. (assuming wiring that is NOT of the shorting type of volume control) The load on the cap that makes the filter is the value of the volume pot PLUS (if the volume is up full) the amp input impedance in parallel. Amps can run from maybe 100k up to a meg of input reistance so it may or may not make a difference.

    If the volume control is of the shorting type and is BEFORE the filter then it will be the amp impedance that is the R you use for the filter.

    Go look at a G&L L2X00 circuit and see how Leo did it.
  13. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    let's clear away the underbrush for a second; what are you tying to accomplish here? what's the problem you're trying to solve?
  14. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    I think the other questions have been well answered and the suggestion to take a step back and tell us exactly what you are trying to accomplish rather than asking a lot of questions about details is an excellent one.

    In the mean time, no there is no chart. A capacitor is not in itself a frequency selective device. It can be used to build frequency selective circuits but the frequencies selected or rejected depend on the values of the other resistors, inductors, and capacitors in the circuit as well as on the configuration of all those components. So you could not publish a single chart that would give the information you seek. You could publish a book of charts that would but it would have the considerable disadvantage of needing an infinite number of pages since there are an infinite number of variations of all of the above.

    One reason that it is so hard to calculate numbers even when given component values is that as far as the pickup inductance is concerned it has no value. Or rather it has a different value at every frequency and above a certain frequency it actually looks like a capacitor. The reason for this is that there is a tiny amount of capacitance between each of those thousands of windings in the pickup inductor and a tiny amount of resistance in series with each of them and so the inductor is not an inductor alone but an inductor in series with a resistor and both together in parallel with a capacitor. Sort of, even that description is a simplification though generally a useful one. The result is that even if you have an inductance meter and even if it is a good one you cannot get a single useable inductance reading. You instead have to characterize the pickup's impedance across the entire frequency range you are interested in. Only then can you sit down with the other component values in the bass and make accurate calculations about the frequency response.

    As you can imagine this can get to be quite tedious even for engineers with the right equipment and good circuit simulators. It is possible to break down the complexities of the pickup impedance into a small RLC circuit model as implied above and then use that model to make predictions but you need to make those accurate impedance measurements mentioned above to derive the model's component values and of course every pickup will need to have a different model made. You can find a handful of these models on line if you look carefully but the chances of finding a good model for the pickup you have are nil.

    When all is said and done sitting down with a collection of capacitor values and trying them one by one to see which ones you like is generally the easiest and quickest way to proceed. There is one saving grace in this particular instance though. If you truly just want a high pass filter you could build that by simply putting a capacitor in series with the output lead to the output jack and letting it "work against" the 1 MegOhm input impedance of your amp. In that case the 3 dB corner frequency is the frequency at which the capacitor impedance equals the amp input resistance, 1 MOhm. I'm not going to build a chart but the equation is very simple:

    R = 1/(2*pi*f*C)

    which can be rearranged to:

    C = 1/(2*pi*f*R)

    and then simplified to:

    C = 1/(6,280,000 *f)

    since we know that pi is 3.14.... and R is 1,000,000. But of course you want C to be microFarads rather than Farads so that simplifies further to:

    C = 1/(6.28 *f)

    and in every case f is the frequency in Hz that you want for the corner frequency. Now this assumes that the driving impedance of the rest of the circuit in the bass is low enough to be negligible which may be roughly true at full volume and less so at low volumes. But it should at least tell you roughly what capacitor values you want to start experimenting with.

  15. kevteop


    Feb 12, 2008
    York, UK
    Out of interest / related question: If you're wiring a cap to a tone pot to act as a low-pass tone control, is it possible to use too big a cap? I mean, if you replaced a .047 with a .1, I guess you could still get all the range that the .047 had, plus some more, right?
  16. bassbenj


    Aug 11, 2009
    No this isn't quite the way it works. Cap filters consist of two parts: a cap and a resistor. (or many resistors) Those values determine what is called a "corner frequency". That frequency will be the dividing point where on one side all frequencies go through and on the other side where frequencies begin to be turned down. You can see how this works on a standard tone control. It's a low pass filter, so that means that low frequencies always go through but highs get turned down. Note that in addition to the frequency determining resistance one also has a pot that isn't supposed to shift frequency points but just turn the filter on and off. On a tone filter the pot is in series with the cap. On a High Pass filter it would be in parallel with the cap. The controls typically do NOT shift the corner frequency (very much :).

    So what happens if you change the value of the cap? Say on a standard tone circuit you went from .047 to .1? Remember the filter is turning down high frequencies. So say previously the corner frequency was 100 Hz. That means with the tone full max, all the lows up to 100 Hz would get through normal but above that all higher frequencies would start to get turned down. The higher you go, the less would get through. And that effect we are all familiar with. Eliminates all the "bright" tones.

    But going to .1 mfd in effect doubles the capacitance and that has the effect of cutting the corner frequency in half! So now only things up to 50 Hz will get through normally but above that everything else will start to turn down dramatically. The bass will sound VERY tubby and dark. Not usually what you want.

    I put a switch on my basses with a .015 cap for the tone. If I flip to that it moves the corner HIGHER and lets more mids through which is a tone I sometimes like.

    The same ideas hold for a high pass "bass control" where all frequencies get through except for the lows below the corner frequency.

    See what I'm saying? Does your head hurt yet?
  17. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    TL;DR version:

    a bigger cap "crosses over" lower, taking out your midrange along with your treble when you turn it down.
  18. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    It will depend on the pickup's impedance and other things. Since there are set values for caps, just try some to get the tone you want.

    G&Ls do this for the passive bass control.
  19. Jazz Ad

    Jazz Ad Mi la ré sol Supporting Member

    Yes you have to make several tries to know what works the best.
    It isn't difficult or expensive, you can get 10 caps per dollar from electronic shops.
    One of the best feature of high pass filters on pass is that you can mimic a J bridge pickup with a P.
  20. Jeremy5000


    Feb 25, 2008
    Ahh yes, great question of course.

    So when recording, (and in rare instances where I get behind a live console), I usually like to exclude some low frequency from the bass guitar to "tighten up" the kick drum.
    So why not use the amp, preamp, EQ? Why not let the sound engineer take care of this?Well in the ladder case you can't always count on having a sound person who totally knows what they're doing or is on the same page as you and every now and then you wind up with your amp in a spot in a room you just cant control (in terms of resonance), or a poorly designed room, bad plywood stage etc.

    So I just want to try simplify the all around control of bass frequencies by having a bass that cuts the lows. This is where the experimentation comes in, where to roll off the frequencies, to even do it internally in the instrument, or to do it later in the signal chain.

    I would like to start off with a bass that I could walk into any situation with and regardless of amplification have the low frequencies controllable. I'm thinking of disabling the tone pot all together, or - you could look at it hypothetically or not - imagine the tone pot working in reverse where it (the capacitor attached to it), sent the low frequencies to ground instead of the highs.

    Sorry if I muddied the waters with the way I posed the questions.