1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
     
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Question about passive tone controls

Discussion in 'Pickups & Electronics [BG]' started by kraigo, Jan 18, 2012.


  1. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    I'm not really much with electronics. I can solder and do basic passive bass or speaker cabinet wiring, figure out coil switching, etc., but I've never really studied electronics.

    I'm wiring up a bass which will have a passive treble roll off (low pass filter) and bass roll off (high pass filter). I'm basing the work off of the G&L L-1000.

    The G&L used a 1 MOhm pot with a .002 uF cap for the bass roll off. Plugging those numbers into an RC filter calculator (Guitar Pedals: R-C Filter Calculator) suggests that the filter will roll off 6 dB/octave with around an 80 Hz. Super - that makes perfect sense to me.

    Then I put the standard Fender Jazz Bass and P-Bass cap and pot values and my understanding fails me.

    250 KOhm with a .047 uF suggests a 13.6 Hz filter. That can't be right. Help a guy understand, please.

    KO
     
  2. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    A quick hand calculation says that the online calculator is correct, not that this is surprising. The traditional high frequency roll off circuits in bass guitars are not arranged as an RC lowpass with the pot resistor appearing in series with the signal path as that calculator assumes, however. Instead both the pot and the capacitor are in parallel with the signal path and together they "work against" the internal impedance of the pickup. A pickup's impedance is a somewhat complex combination of resistances, capacitances, and inductance but for this purpose it could be very roughly considered to be just the DC resistance of the pickup. That is typically around 10k and when the pot is set to zero Ohms the calculator predicts a corner frequency of 339Hz. More like what you were expecting perhaps?

    The function of the pot in the typical tone control is not to provide a resistance for the cap to work against but rather to control how strong the cap's effect is and to effectively remove the capacitor from the circuit entirely when the pot is maxed out. When the pot is set to 250k Ohms it completely overwhelms the effect of the capacitor which is 10k at 339 Hz.

    Does that help any?

    Ken
     
  3. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    That's not taking into consideration the pickup impedance. The hotter a pickup is wound, the more affect a treble cut tone control has. This is why the passive tone controls on EMG pickups never did much, even though they use a .1µF cap. The newer EMGs have a lower output impedance, so they had to make an active tone control.

    So, any of these numbers you plug into one of these calculators might be right for a given impedance, but not right with a bass pickup.
     
  4. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    That helps a lot. Thanks.

    KO
     
  5. DavePlaysBass

    DavePlaysBass

    Mar 31, 2004
    CO
    The pot in a tone control is not creating an RC low pass filter element based on the 250K and 47nF values. The pot is acting more like a variable switch that isolates the pickup from the capacitor. When set at the full treble cut setting, the value of pot resistance between the pickup and the tone capacitor is nearly zero. This allows the capacitor to pass most of the highes to ground and hence you get the full treble cut . The exact frequecny of the treble cut is a strong function of the pickup inductance and resistance along with the tone capacitor value. As you turn the pot towards less treble cut, you are placing pot resistance between the pup and the tone cap. The increasing pot resistance provides some isolatation between the pup from the tone capacitor which in turn reduces the high frequency draining effect of the cap. At the full treble on position, you now have the full 250K of resistance between the pup and the tone cap, making the influence of the tone cap on the pup signal much less. There is still some loading which reduce highs but the equations for determining the high fequency loading at this position are a stronger function of the pup impedance and the pot value. When you consider all of the passive component (pup, cable, pots, caps, etc) values it is more accurate to simulate the roll off effects than to calculate them by hand. The challenge is getting accurate parameters from the pickups and in many cases, these parameters are not constant across the frequency range making them very difficult to simulate or calculate their responses on paper. The next best approach is to hook it up and play it. If you are a real geek, get out the spectrum analzyer and measure the response for a given arrangement.
     
  6. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    I can tell you guys that using 500 KOhm pots and a .02 uF cap in my P-Bass was not a terribly good idea. It's a pretty subtle effect. I'll keep the 500 KOhm pots and try a .047 uF cap in there.

    Thanks again for the illuminating answers.

    KO
     
  7. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    I like using .02µF caps in some of my basses. It doesn't get that deep thick tone, but gives more of an upper midrange bump and a cool tone. But you need something like a .047µF for a darker tone.

    If you use a push/pull tone pot, you can have both.
     
  8. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    Now _THAT'S_ an interesting thought. One for both the Skyline 55-01 and Tokai P-Bass.

    KO
     
  9. DavePlaysBass

    DavePlaysBass

    Mar 31, 2004
    CO
    I love the 250K 0.1uF on my MIM 50's P. One of the most useful tone controls I have used. It's thick. And when combined with a nice set of DR Lo Rider Stainless, it gets a nice balance of vintage with definition. When Fender first released the split pickup P, this was the configuration. Sometime later (60's, 70's), they changed to now common 250K 0.047uF.
     
  10. What do you guys consider "thick sounding"? My tone control doesn't have nearly as much rumble as I'd like. I have a passive split-p pickup, resistance at about 4k, 500k tone with 0.2u capacitance. That gives me about 199Hz, which doesn't sound very deep. What frequency should I aim for? Thing is, I don't like typical bass boost circuits, as they just muddy up the signal and hurt my ears, but I still get too much treble.

    Or am I asking for too much from my bass? I'm using dr hibeams, and want the high end for some stuff, but not all.
     
  11. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    I guess I'd look to a different pickup. The Seymour Duncan Quarter Pounder SPB-3 was too bassy for me (I prefer the simple SPB-1 with GHS Precision Flats). Or try the flats first and see if they give you what you want.

    KO
     
  12. Guess I should try flats. Plus I do want new pickups. Problem is, I got no money right now, so I guess I was wondering what I could do for now. I have a lot of electronics parts, so that won't be an issue. Now, I found out that p basses have a hump at about 200Hz, but j basses don't, and I love the deeper tone of j basses. Is there anything I can do to remove that bump? Also I have just found out that ~200Hz causes a boomier tone, but at the expense of clarity. How can I remove the high end without decreasing clarity and without booming, just deep chest-rumbling?
     
  13. Hey, I figured out my tone! Just stopped adding caps in parallel and went straight to one 0.47u. It sounds so awesome, and a 1u electrolytic is even more awesome, but by then the signal becomes really quite. 0.47u is the most attenuation I can stand, and is just on the edge of what my amp can handle... Oh, and i also breadboarded a greasebucket circuit, and was rather pleased. Certainly not my main tone, but useful enough to maybe add in with a switch sometime in the future. It has a nice growly warm tone when rolled off, and (to me anyway) is reminiscent of a j bass bridge pu. Whereas a regular tone control has a boomy but smooth tone. But enough of me, what is other people's experience?

    Edit:
    Oh, and I think my pots are 250k. I'd have to measure my bass again at some point.
     
  14. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    I think my pots are 500K and my cap is .22. It's a bit too subtle, but I never use the tone knob anyway.

    KO
     

Share This Page