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Home grown tone control

Discussion in 'Pickups & Electronics [BG]' started by kb9wyz, Sep 17, 2008.

  1. kb9wyz


    Sep 8, 2008
    Ok. I am trying to design a passive tone control circuit to go into a kit bass I am planning to build. I really liked the idea of the Stellartone ToneStyler™ tone knob, but it is like $100, and I think that this is, like, stupid. I figured that I can make the same thing for about $10-20, depending on how much I do with it. They say their knob will bleed of the highs, but will leave the mids alone, preserving your tone better.

    Here is my preliminary design:

    Component values are:

    R1 & R2: Trim pots, set to 4K Ohms
    C1, C2, C3: 0.020 uF
    R3: 250 K Ohm Pot

    The signal path splits into a Crossover (1st Order), set at about 2KHz. (I came up with this number from Audere's website. That is how they set the treble controls for all their products) R3 & C3 form a regular tone control to bleed off the highs that are let through by the High Pass Filter formed by R2 & C2. I chose that setup because that would allow someone to build the rest of the circuit and just drop it into an existing passive axe. This setup allows you to completely roll off the highs (Better than 12dB @ 8KHz).

    You might have noticed the red marks. They are for an optional idea I had. That would put 16K Ohm resistors at each red mark, that could switched in or out of the circuit, using a push-pull pot or miniswitch. The extra resistance would lower the crossover freq from 2KHz to 400Hz. That would allow you to use the circuit more like a regular tone knob, but would leave your lows untouched.

    I would very much like to know any thoughts, ideas, or any corrections (hopefully none) you all might have about this.

  2. Jarno


    Jan 27, 2005
    Have you simulated this schematic? Do you know how much this circuit dampens the signal, you might need a preamp after this.

    Best regards,

  3. kb9wyz


    Sep 8, 2008
    No, I haven't done any kind of simulation. I don't have the software for that. As for the math, I feel good about that. I have an A.A.S. in Electronics. That doesn't make me an evil genius, but I certainly know my way around equations & calculators.

    For the loss, it should be minimal. The kind of crossover that this is, has the least amount of loss of any kind of circuit. The expected loss of this circuit won't affect the volume of the axe in any meaningful way. 1st Order crossovers are regarded as "transparent."

    As for the rolloff of the highs, that is why the actual "rolloff circuit" is the same as what you'd find in any bass. It will perform the same way in this circuit. The crossover just limits the lowest frequency that the "tone control" can affect.

    Besides, the option the switch the crossover from 2KHz to 400Hz will make the difference between this circuit and a standard tone control negligable, so I still have the option. The benefit to this setup is that it will still preserve the lows at full rolloff. The difference between this and a normal tone control will be subtle. This should do a much better job of keeping the sound full, though.

    I am trying to emulate the ToneStyler™ because I don't like how the tone knob on passive basses work, but I want to keep the system passive.
  4. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    This probably wont work, as you will have a lot of insertion loss. It really needs a recovery amp.

    The Stellartone ToneStyler is actually a 16-position rotary switch with a number of different value caps, not a pot.

    It's kind of a remake of the old Les Paul Recording guitar's "decade" control, and pretty much is a decade box inside the guitar.

    It wouldn't be hard to make something like that. I'm not sure you can find 16 steps between a 0.1M cap and something like a 0.02M, which is probably the smallest that works with bass well (that's what I have in my passive bass), so the rest of the steps might be in combination with resistors.
  5. The crossover is also going to give you a notch at the crossover frequency. The crossover frequency is where each part of the circuit is at half power, not where the rolloff starts.

    Edit - never mind. You are mixing them together again, so it flattens out.
  6. Frequency response with 4K:

  7. Frequency response with 20K:

  8. Notice that both of the frequency responses start at -6dB - which will probably require some kind of boost.
  9. kb9wyz


    Sep 8, 2008
    Thanks Slyjoe,

    I guess the voltage levels from the pickups are a lot lower than I thought. I didn't think that the circuit would attenuate the signal anywhere near that much.

    Do I have any other options, then? Or should I just say,"Nuts" to the whole thing and do without the fanciness?

  10. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Just build a copy of the Steller. That will work with pickups, and it's fancy.

    I'd just pick up a 4 position rotary and use 3 tone caps. Something like 0.02, 0.047, and 0.1.

    Or do a 6 position and add some in between values.
  11. kb9wyz


    Sep 8, 2008
    I may just do that. Mouser has all sorts of odd values I can play with. I have to do something with my soldering iron.:D

  12. put in a on/off/on toggle...wire a 0.022 to the pot...

    one direction throws a 0.033 in parallel (0.055)

    one direction throw in a 0.082 in parallel (0.104)

    I'm doing this on a P-bass very soon ;)
  13. Actually, I had a connection on the schematic wrong (output connected to end of R3 instead of wiper). Here's the real response with R1 & R2 at 4K:


    Notice that the large attenuation only happens when R3 is at 100% (in this orientation, 0 ohms). If you want to use this circuit, you need a series resistor with R3 to prevent the resistance from going to 0.

    That said, you don't really have a crossover in this circuit, since the R1 and R2 "branches" are actually in parallel. So what does the R1 and R2 part of the circuit do? Not much. :)

    If you want variability in your tone circuit the easiest thing to do with a passive circuit is to switch caps and/or resistors in and out of the standard tone circuit like SGD and PilbaraBass suggested.

    Let me know how it goes.
  14. kb9wyz


    Sep 8, 2008

    The parallel circuit was intended to function as a crossover. The thing is that I haven't done much with my electronics degree other than Ham radio for the last few years, AND I have never worked with audio stuff. Today, I realized that I seriously overcomplicated the circuit when I was rereading some info on crossovers. The thing was that I wanted more variability of tone. You proved that the circuit has the rolloff that I am looking for, I just need to be picky about the resistor and cap values so I don't over or under attenuate the highs.

    I thank you very much for helping me through this. I need to learn to think more simply, I guess.

    Oh, if I could ask a favor:

    In the pictures I have seen of wiring diagrams, I think that they always have the resistor (pot) and cap in series to ground, with the output coming off the top of the resistor. Or, in other words, they aren't set up to use the voltage across the cap only, like a basic low pass filter.

    Could you model that for me so I can see if there is a difference between that and the normal LP formed by R1 and C1 in my (wildly overcomplicated) circuit?

    Not trying to impose. This will be the last thing I ask concerning this subject.

  15. Hey Beast:
    I can do it, but I'm not sure what you are asking for. Are you just asking for a normal tone circuit at R1/C1? With the rest of the circuit unchanged? Maybe you could post a schematic - words tend to result in confusion, at least on my part :)
  16. kb9wyz


    Sep 8, 2008
    Ok. Thanks Slyjoe.

    The way tone knobs seeem to be typically wired differs from what a normal RC LPF is. So This is what I am seeing:
    Assume a 250K Ohm pot for R1 and 0.050uF for C1

    I'm curious about whether or not they actually perform differently. I'm thinking that they wouldn't because in the top one you'll only get the voltage across C1 & then in the bottom one you'll always get the full voltage drop from the volume because the "filter" is in parallel with the volume, so then how much attenuation does this circuit produce? Or is it the same?

    This is what I get for letting my education sit almost totally dormant for a couple years.:spit:
  17. Cernael


    Jun 28, 2008
    Huh? I don't see how the "textbook" one could work at all, really. Looks like the cap is wired directly between the tip and ring or the jack (well, functionally, at least), meaning that any treble the pickups produce would be shunted through the cap rather than make their way through the amp...the pot looks like it just indiscriminately changes overall volume.

    Then again, I was never educated on these matters, so if you were to explain why I'm wrong, I'd be happy to agree...
  18. Cernael:
    "Any" treble is a misnomer. It depends on the cap value. The textbook example is in fact a low pass filter. That said, the pickups are being ignored and other parts of the circuit are being ignored. You can't analyze a circuit without considering the entire circuit (we'll ignore superposition for now).

    For a jazz bass with SCN pickups, the entire circuit looks like this; the AC source, cap, resistor and inductor simulate a pickup.

  19. kb9wyz: The bass wiring diagrams are NOT a textbook LPF. However, when added to everything else in the circuit, the response is similar. The response of the above circuit is:


    A few things to note: The resistor is only swept over about 30% of its range to simulate an audio taper pot - farther out than that the response isn't really enlightening.

    The thing to do to explore this further is download a freeware/shareware SPICE program (like 5Spice), put your components in, and vary the values. I used SCN pickups because they published some of the pickup parameters (DC resistance and Henries). I assumed an internal cap of ~250 pf; it's not published. The model of the pickup, with different values, shows why pickups with different parameters have a VASTLY different sound to them.

    Hope this helps.
  20. RyreInc


    May 11, 2006
    Kalamazoo, MI
    In the first diagram the cap is coupled to the output, meaning it will always affect the tone. The pot adds series resistance to the output (without grounding it), lowering it's output in the process and also moving the knee of the lowpass up in frequency, as demonstrated by Slyjoe's graph.

    The second diagram is the one you want to use; this one adds resistance between the cap and the output, allowing it to be faded in and out.

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