A New Tone Control Concept — or Is It?

Hey, smart people — let me get your take on this. I’ve been playing with a new tone control idea that’s so simple, I can’t believe no one’s done it before. (Chances are someone has.)

Here’s the idea: Conventional electric guitar tone controls employ a single pot and single capacitor connected to ground. As you turn the pot, more signal goes to ground for a darker sound. The capacitor value determines the cutoff frequency — the larger the cap, the lower the cutoff frequency and the darker the sound. In other words, the cutoff frequency is fixed, but the percentage of signal that gets cut off changes as you move the pot.

Meanwhile, the Gibson Vari-Tone circuit uses a rotary switch rather than a pot, and a set of capacitors of ascending size. The small caps have a brighter tone, and the large ones sound darker. But once a cap is engaged, it’s engaged all the way. In other words, the cutoff frequency varies as you move the switch, but not the percentage of affected signal—it’s always 100%.  (The Stellartone ToneStyler employs the same concept, with as many as 16 caps arranged around a rotary switch.)

But do you really need all those caps? Why not use the tone pot to fade between a small cap and a large one, like so:

double cap

Here, the brighter/lower-value cap is engaged when the pot’s all the way up. As you roll it back, the larger cap is introduced, producing greater capacitance and a deeper treble cut. When you arrange caps in parallel, their total capacitance is the sum of their values. For example, I tried a .0047µF cap and a .047µF, so the minimum value is .0047µF (a very modest cut) and the maximum is approximately .052µF (a very dark tone).

So far I’ve only tried this on breadboard, though I plan to deploy it in a new “parts” guitar I’m assembling. So far it sounds … really good. A lot like a ToneStyler, actually, but with fewer parts and handpicked values. The only tricky thing was finding a good pot value where all the action wasn’t bunched up at one end of the knob’s range. A reverse-log pot worked best for me—I got nice results with both a C500K and C1M.

I often use similar wiring to alter the value of the input cap on distortion pedals. (High values filter our more bass for a brighter/cleaner sound.) But I’m not aware of anyone having tried this on a guitar tone control.

Another issue is the fact that, in this circuit, the tone pot always has a cap engaged. You could use a really tiny value for the smaller cap so there’s little perceptible cut at the minimum setting, but that can make a substantial part of the pot’s range a little too subtle. So my plan is to combine this with a Ned Steinberger-designed JackPot as the volume control. This part has an “off” setting that bypasses the tone circuit entirely for a maximum-bright sound. That way, I’d choose for the smaller cap a value that provides the minimum treble cut I’m likely to want. (I suspect I’ll wind up with something between .0022µF and .0047µF.)

Have any of you seen or heard of such a guitar circuit? If so, any observations or advice?

 

52 comments to A New Tone Control Concept — or Is It?

  • Senny

    I recently did something like that with a bass. My reasoning is that I didn’t want excessive overtones or other trebly sounds even with the pot all the way up. I found that a log taper worked best for me.

  • Steve Newton

    Surely to fade between 2 caps you would have the signal enter on the wiper and have a cap to ground on each end of the pot?

    So that in the middle it would have, in the case of a linear pot, half the resistance between a cap and ground?

    As posted above there’s always a cap to ground. Cool if you want a slightly darker tone, ALWAYS, and even darker when the tone is cranked.

    But a different maximum roll off on each end would be more useful?

    • joe

      That would work, of course, but the result would be very different. You’d have minimum cut in the center position, with treble cuts of differing size in either direction, depending on the caps’ values. I’m not sure that would be particularly useful — the activity would be the same in either direction up to the point where the value of the greater cap kicks in. (I suppose one advantage would be that you could leap quickly to two “preset” values by turning the pot to minimum or maximum, as if you’d installed to caps on a DPDT switch, as seen here.) Or course, you’d have to worry about centering the pot, unless you could can find a part with a center detente. The resonant peaks, and therefore the overall tone, differ from standard. HOW exactly they differ is one of the things that I’m going to explore. 🙂

      In my diagram, though, the pot’s motion is the same as on a standard tone pot: Fully up is brightest, and fully down is darkest.Here you get varied capacitance, as opposed to varied percentages of signal to ground.

  • Steve Newton

    Ah right, I didn’t get the ‘same rotation action as a regular tone’ bit. You are right a totally different ergonomic consideration.

    Though surely you set the tone pot by ear? And knowing that one way is a deep treble cut and the other way ‘just enough to take the edge off’ you’d grab the pot an twist accordingly ?

    Though who am I to question it? These days I’m removing tone controls from single coils guitars and converting them to bass roll offs on humbucker axes.

    All that said though I CAN see it being useful on bass though, pick the small cap to take the sdge off the worst of the string squeek on new strings and have the big cap as your max treble roll off to taste.

  • joe

    No, you’re right. The more I think about it, the more I see the attraction. Plus, I was wrong in my previous comment when I said the sides would sound the same up to the limit of the smaller cap. Because I think they’re be audible resonance differences between the two values. It’s the same reason why the scheme I’m discussing differs from a standard tone control. And I sound vague, it’s ’cause I am! One reason I’m eager to explore this …

    • Steve Newton

      Here’s another idea for you: one side has a cap to ground for treble cut, the other side a cap and inductor for mid cut.

      This really is the kind of stuff you could go nuts with!

      • Steve Newton

        Here’s another idea, stolen from small amps of yesteryear. A tone control that rolls off treble one way and enables treble bleed, and therefore bass cut, in the other direction.

        The treble cut will be totally interactive with the volume control’s position, with no effect at full volume.

        If the image attaches properly…

  • Bebah Palulah

    As someone else noted, it’s like a regular tone control except you’ve added some additional capacitance “always” and increased the total capacitance when they’re shorted together. The same thing would happen if you used a really long cable on a regular guitar.

    With the tone pot all the way up, the larger cap is essentially isolated from the rest of the circuit so it doesn’t have much effect. All the way down, yes it adds to the other cap.

    In the middle settings of the pot, you will not get as much of a peak as you would with a Tonestyler because the resistance reduces the “Q” or resonance of the circuit. That’s generally true whether resistance is in series or parallel. A variable cap is not the same thing as two caps with a variable resistor between them.

    But if it rocks your world, then who cares about all that?

  • Slava Azarov

    We can definetly see something similar in old fuzz boxes and amps (at least when 2 caps connected to lugs 1 and 3 of a pot, and lug 2 makes an output — for example 3 knob tonebenders). Personally, whenever I heard it, it sounded crappy for me, but I never heard it on a guitar. It is curious, are you going to make a demo?)

    By the way, I think, you’re wrong about “the cutoff frequency is fixed, but the percentage of signal that gets cut off changes as you move the pot”.
    Tone pot usually connected just as a variable resistor (one lug is not connected), so you got some “small” resistance (when compared to amp’s input) and a cap going to the ground. When you crank your tone there is no significant resistance, so all signal above cutoff freq is shunted to the ground (with graduate slope) — but still this is the brightest position of the tone pot.
    This happens because tone and resistance before it, make an RC circuit — low-pass filter. In such a circuit as resistance or capacitance increase — cutoff freq decreases (f=1/2pRC).
    So you are increasing resistance before cap and cutoff freq decreases.

    • Fuzz boxes is the very first thing I thought of. I remember seeing some single-transistor fuzz design where the “gain” was a pot in series with a big cap, with the pot-big cap branch in parallel with the input coupling cap, which was a small cap.

      I’m not sure if you’re right about Joe being wrong. My memory of exactly how tone controls are usually wired is kind of failing me, but I think I remember that what you’re saying would be true if the output was taken from the node connected to the capacitor, but it’s not–the output node is the node on the opposite end of the tone pot from the capacitor, unless I’m remembering wrong. I think that filter-characteristics of guitar tone circuits are easier to visualize if you imagine them as reacting to a current source. Meaning “a big resistor in series with a capacitor” reacts the same way that “a big resistor” does. Basically, current above the cutoff frequency is shunted to ground through the tone pot–so if the tone pot is high, very little percentage (compared to if it was just a capacitor) of this current gets shunted, whereas if the tone pot is low, a high percentage gets shunted.

      I do think you’re right about the cutoff frequency changing as the knob is turned, BUT I’ve been assuming that it only changes the cutoff frequency slightly, and the percentage of the current that is actually shunted is the dominant effect that we hear. We’d have to do more math to figure out if I’m assuming wrongly.

  • Well, it’s not actually achieving what the varitone is intending to achieve, so technically you do need all those caps.

    But if this sounds better to you than a varitone does, then that doesn’t matter, and then realistically you don’t need all those caps.

    I don’t think I would like having that small cap there on all the time on my fat strat’s neck tone, which I rarely put below 10, but I’d love it for my middle/bridge tone, which I rarely put above 7. Maybe a push-pull knob could switch out the smaller cap?

    Honestly I think that next time I decide to make any of my guitars more complicated, I’m going to make it really complicated: op-amp pickup mixer, baxandall tone stack, bjt boost, etc.

    If I get some spare time today I might come up with some spice models of the tone controls you and some of the commentors mentioned. (Not that spice models of guitar pickups are ever particularly accurate, but whatever.)

    • I feel silly for somehow not reading the paragraph about using JackPot. I do think I’d rather switch out just the small pot instead of switching out the entire tone circuit, though.

  • CroftyTTL

    What could be interesting is dropping a Fender style no load tone pot into this circuit as well. The cap values might need more tweaking to find a minimum that doesn’t immediately cut the treble as soon as it’s engaged, but it’d be cool to have a max cut to min cut to off with the need for a push/pull or toggle.

  • Mil

    Sorry Joe but I don’t really any value of this arrangement. It could work only in super bright guitars that would always need some treble bleed. It’s much better to change pot values for guitars like that, go from 500K to 250K etc. When changing pots doesn’t cut it it’s better to get rid of the pickups/guitar than to have a bleed circuit on all the time, some of the magic is always lost.
    Something different would be to put the small cap between legs 2 and 3 instead of grounding it. Than at certain resistance levels treble would be boosted and than when the bigger cap overpowers, the treble would end up in the ground.
    The trouble with this would be to find musical values, it’d be probably hard to find something universally acceptable and for each guitar you’d need to try different caps and pots. You’d need to start with a big value pot for this, at least 1M. Keep soldering…

  • The exact effect of the smaller cap depends on the other components in the circuit, including the guitar cable but for a typical humbucker with the tone control at max treble, volume at max and 470pF of guitar cable you get a 6dB boost peak at 3K rolling off at 12dB thereafter without the small cap.With the small cap, the peak shifts down to 1KHz and you get 10dB boost. This is because the capacitance of the guitar cable forms a resonant circuit with the inductance of the pickup then you add a further cap in parallel which shifts the resonance down in frequency. if you then roll off the volume to about 7 the peak drops to about 9dB of boost and shifts up a little in frequency because the resistance of the volume control decouples the cable capacitance from the pickup inductance.
    With the volume rolled up again and the tone control at max treble cut the resonant peak (and following 12dB cut) moves down to around 300Hz and boost is about 3.7dB.
    I'd guess that is what happens anyway *-)

  • Sebastian Enriquez

    What about an RC Filter, with the pot being the R. With a twist of the pot you change the resistance therefore the cutoff frequency.

    • Bebah Palulah

      You are driving this RC combination with a pickup, whose inductance likes to get involved. The relative effect of each part depends on the details of the values.

      If you were talking about an active pickup then you’d be isolated from the inductor, and it would behave as you suggest.

  • Joe – you asked the question – Have any of you seen or heard of such a guitar circuit?

    The answer is yes, it’s is already inside almost every electric guitar.

    Your circuit shows a conventional RC tone control with a further small capacitor wired directly across the pickup. If you regard that extra cap, not as an additional component, but instead as the self capacitance of the pickup, which is normally a few hundred pF, then your circuit is already present in almost all electric guitars. OK I realise I’m being a bit smartarse here, it did strike me as another way of looking at this.

  • In all my years I have never seen filter cct's like this but as tleco tech the filters have never been variable, When I put my guitar together I had a 0.022uF and a 0.047uF and for reasons that I have long forgotten I put in a switching matrix that allows me to get 0.047, 0.022 and 0.015uF. After many revisions to the cct (it had coil taping and variable taping) I almost put in a 0.033uF and taking out the switching, well I ended up getting some single ended 9 Watt amp and all of a sudden this flexibility made scene I have one tone control that I can control the cut frequency a coil control pot and a volume control. Now the funny thing If I put in a single cap 0.015uF (as close as you can get) It doesn’t sound like the two 0.022 and 0.047uF in parallel, Its in the harmonics that get let through from what I can hear. But when all said and done could be something to give it a go.

  • I like the pseudo PTB scheme where the tone cap (I prefer .033 or .022mF) is attached to lug 3 of the pot and to ground. Lug 2 is connected to the Volume pot’s lug 1 and there’s an additional small cap (0.001mF or smaller) connected between tone pot lug 1 and Vol pot lug 2. Assume both pots are Log types: with tone at 7.5 (ie halfway along the pot’s sweep) there is no appreciable treble cut, and the volume pot gives a pretty even tone throughout its range. With tone pot higher than 7.5 bass is cut as you roll back the volume, with tone pot lower than 7.5 treble is cut. Minimum bass setting – vol approx 3, tone full up. Minimum treble setting vol full up, tone full down. I have this in most of my guitars and love it.

  • um, read this: http://www.beavisaudio.com/techpages/BigMuffToneControl/
    and as far as "always having one cap engaged" just use a switching pot.

  • um, about that "jackpot" volume control: are they really charging $10 for a simple switching pot? that's insane! is the thing made of gold? go search Mouser.com, Digikey.com, or http://smallbearelec.com/

  • Shizmab Abaye

    Sorry for the off topic, but I met this guy today who has written an iPhone app to identify resistor color codes: http://resistorvision.com/

  • Wee-Lamm (a.k.a. Steven)

    An interesting idea would be to connect the smaller cap to pins 2 and 3 of the pot.

    I would expect it then to act more like a band filter such that not only would the pot determine whether the larger cap bled off fewer or more highs, it would also change the reference to what would be considered low and high.

    That is, if the tone caps in parallel filtered from within th 1k-2k and 3k-5k frequency band with the pot determining how much of those ranges are filtered off, then moving the 2nd cap parallel to the pot should shift the 2k window of the bigger cap so it would be filtering less of the 3-5k at one end of it’s sweep, and more of the 1k-3k filtered at the other end of it’s sweep. (my frequency selection and math here are conceptual only, and not to be taken as accurate measurements).

  • Roland

    Joe, Did this project come to a resolution? I’m interested in applying such a circuit when only one tone pot is available. ta

  • Roland

    thanks!

    I’ll certainly consider implementing this on a single tone pot single coil guitar I have.

    If I don’t want to have a push-pull pot to engage/disengage it, I need to use a tiny value small cap, is that correct?

  • joe

    Hi Roland! Check out http://tonefiend.com/pickups/ultimate-lipstick-tube/, where I deploy the idea in an actual guitar.

  • Craig

    What about using a variable capacitor, would that work? It seems like that was the gizmo that controlled the tuning frequency on old transistor radios…

    • joe

      Wow, I don’t know much about such parts, other than what I read on Wikipedia: https://en.wikipedia.org/wiki/Variable_capacitor

      Theoretically it should work, assuming you can find a part compact enough to fit inside the control cavity and with the desired range of values. But since fading between two caps works great, costs next to nothing, and takes up little space, I’m not sure what the advantage of a variable cap would be.

      • Shizmab Abaye

        Variable caps used for radio tuning have about 1000 times smaller capacitance than needed for guitar tone controls. The advantage of a variable cap, could one be made in the right range for guitar, or those many-caps-on-a-rotary-switch, is that the resonant peak is maintained regardless of setting. With Joe’s “cap fader”, that peak gets flattened out in the middle of the pot rotation. Doesn’t mean it’s a bad sound, but certainly different.

  • MMh, for years I had a tone control in my Framus ES copy that followed a similar combo idea. I got it from the Rockinger.de site and it might have had a Bill Lawrence conncetion. (Have you tried his q-filter?) It replaced the cap with an inductor AND had the other end of the sweep connected to a regular, but smaller, cap. So you could go either thick tone or thin, but you could not bypass it all. That would have required an extra switch.

    • joe

      Wow, I’d never heard of that models! Thanks for telling me about it. Adding a tone-circuit bypass is theoretically an easy mod — except that NOTHING is easy when you’re trying to work with electronics inside an ES-style guitar! 😉

      The Bill Lawrence notebook is a treasure. He was a fascinating dude with fascinating ideas.

  • Some clarifications: The guitar had a volume, a treble and a bass control (cap in series) when I got it. And I got the schematic – which referred to Lawrence – from the Rockinger paper catalogue. This was way before the internet. And yes, the guitar is drenched with my sweat from MANY operations on its intestines.
    And for the last number of years I have strung it With flatwounds for that Nashville 50s sound. I do not find a pic of it right now. There are similiar models on the Framus museum site, but they kept chaninge PUs and other Things.

    • joe

      Sounds cool! You’re right that flats are a key to the ’50s Nashville sound. But a lot of guitarists forget that almost EVERYONE used flats until the latter part of the ’60s. Early Beatles, Stones, Who, Kinks, Motown and other R&B, surf, and of course anything jazz-related — it’s all flatwound guitar work till ’66, ’67 or so. Also, the main reason we migrated away from nickel is because the material became markedly more expensive at the end of the decade. (Though yes, some did prefer the brighter tones of replacement materials.)

  • Hehe. the funny thing is that fuzzboxes appeared exactly when flatwounds disappeared. It makes total sense for all country, pop and early rock music, but I have to listen to all my blues records again how the flats sound there!

    • joe

      That is so true, though I never quite thought of it that way. (Though of course, the VERY early fuzz recordings would have flatwounds. (“Satisfaction” is almost certainly flatwounds.) But yes, by the time fuzzes became common, flatwounds were relegated to the realm of clean-toned jazz. Which is pity, because fuzz sounds AMAZING on flatwounds.:)

  • Cesare

    Give a look at the g&l l2000 bass wiring diagram. It has bass cut, treble cut, a small cap always to ground and treble bleed on volume.
    http://www.bassesbyleo.com/paul/g&l_bbe_wiring_diagrams/basses/l2000_asat_schem_old.jpg

  • Emil Carr

    I dont really like your explanation of how the cutoff works.. Let me try and explain.

    The cutoff frequency (f) in hz is inversely proportional to the resistance in megaOhms (R) x capacitance in microFarads (C) (R x C). Lets say our cap is 22nF and resistor 500k.

    f = 1 / (2pi * 0.5 * 0.022) f = 15hz

    When the tone knob is at it’s lowest value (0megaOhms)

    f = 1 / (2pi * 0 * 0.022) f=infinity. No frequencies are cut off.

    As the resistance decreases, the cutoff frequency rises

    f = 1 / (2pi * 0.2 * 0.022) f = 36hz

    (These frequencies would be for a low-pass filter. The calculation is the same for a high-pass filter, the only difference is the positioning of the resistor and capacitor)

    • Shizmab Abaye

      Emil your explanation would be ok but for the fact that the guitar pickup has its own set of inductance, capacitance, and resistance characteristics which interact with the tone cap and pot.

      The frustrating thing about this is that these parameters, while measurable, are generally not published. So, a guy hears his friend’s awesome tone control setup and decides that’s for him. Puts in on his own guitar and it sounds different. So then he buys some cloth covered hookup wire. Goes downhill from there.

    • I’m sorry Emil but your calculations are wrong and as Shizmab says, are based on incorrect assumptions.

      It is easy to make the mistake that the tone control set-up in an electric guitar is a simple single stage Resistor / Capacitor filter, where the two components are in series, the other side of the capacitor goes to ground, the signal is applied to the other end of the resistor and the output is measured across the capacitor. If that were so then your first calculation is roughly correct, while in a practical situation in the second, the capacitor would be fed from the impedance of the signal source. Lets say this is a test generator with an impedance of 600 ohms – the -3dB cut off would be around 12kHz. This is not the case for the typical electric guitar.

      If you think about it for moment you will realize that a guitar tone control operates the other way around – when the knob is fully clockwise the control is at maximum resistance and the cut-of frequency is high. When it is fully anti-clockwise resistance is minimum and tone is rolled off.

      This is because although the tone control resistor is in series with the tone capacitor, both components are wired in parallel with the pickup and the volume control takes the output from across the pickup. The tone control components are in effect fed from the pickup impedance.

      When the tone control is fully clockwise the large resistance isolates the capacitor from the impedance of the pickup and it has little effect. With the tone control in this position and the volume full up the inductance of the pickup resonates with its own self capacitance in parallel with the capacitance of the instrument cable.

      When the tone control is at fully anti clockwise the capacitor is effectively wired directly across the pickup. Combined with the inductance of the pickup it then forms a lowish frequency resonant circuit. Since the capacitance of the tone capacitor is usually much larger than the capacitance of the instrument cable, the tone capacitor dominates.

      With a typical Strat single coil pickup and assuming an instrument cable capacitance of 500pF, 250K tone pot, 0.022uF cap, guitar plugged in to a 1Meg input, the range of the tone control is roughly from 6.0kHz -3dB (tone fully clockwise – with a resonant peak of +7.4dB at 3.9kHz) to 950Hz (tone fully anti-clockwise – with a resonant peak of +4.5dB at 594Hz). These figures vary from pickup to pickup and depending on the instrument cable capacitance.

  • Andy

    Hey! I just opened up my 1984 g&l for the first time today and I found a design very similar to this. I had always loved the tone control on it but was too nervous to open it up for whatever goofy reason. I may try to draw it out and share it but maybe include a picture at some point as well. I’m glad someone else has looked at this because it definitely threw me for a loop when I first saw the two caps both headed to ground.

    • joe

      Cool! Yes, this idea is definitely stolen from G&L!It would be cool to see a picture or schematic, but it’s probably going to be really close to this, if not identical. Meanwhile: Don’t be scared to open up your guitar — they’re build for it! Just be gentle till you learn what you’re doing. I always stand by the Maker’s Faire credo: If you don’t open it, you don’t own it. 🙂

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