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Switched tone caps instead of tone control

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  • Switched tone caps instead of tone control

    This is inspired by the recent tone cap thread.

    Instead of a tone knob on my main guitar, I installed a three way tone switch. In one position, there is no capacitance. In the two other positions, a capacitance is placed across the output: 1nF and 2nF, respectivelly. There is no series resistance at all. Though I do not have a continuously variable tone control this way, the use of a pure capacitance means the resonant peak in the response is retained. The pickup and the capacitor form an LC resonant tank. Without resistance in the circuit, the resonant tank is undamped. A regular tone control kills the resonant peak as you turn it down. A peak then reappears when the control is turned all the way down.

    I just recorded a quick little MP3 with my smartphone to demonstrate the switched cap setup. This guitar has only one pickup, a DiMarzio DP-156 "Humbucker from Hell" in the bridge. "Tone One" in the clip is with the 2nF capacitor across the pickup, the most rolled off tone. From there it progresses to "Tone Two", with the 1nF capacitor across the pickup, and then to "Tone Three" which is just the straight tone of the DP-156 without any shunt capacitance, other than that of the instrument cable and preamp's FET input.

    The effect is like having three pickups in one.

     

     

     

    Music DIY mailing list: http://www.kylheku.com/diy
    ADA MP-1 mailing list: http://www.kylheku.com/mp1

  • #2

    Gibson used switched caps in a circuit called the varitone since 1959. The circuit also contained a 1 henry coil to make the circuit an actual passive EQ filter which you cant get with caps alone. Caps by themselves only with only cut or pass highs. With a coil added you can create notches with the midrange which is what most guitar players desire.   

     

    The development of the solid body electric guitar in the 1950s initiated

    radical innovation in pickups, switches, amplifiers, and accessories that

    continues with ever greater technological sophistication. First there was a

    single volume control, then tone control then three-way switches and on

    and on. The designer's goal was to enhance the players' control of their

    sound. As each manufacturer rolled out a new feature, their competitors

    responded with their own.

    Gibson introduced the Varitone six-way tone

    control in 1959 on the ES-345. Then in 1982 B.B.

    King and Gibson joined forces to create the one

    and only Lucille featuring the Varitone with a

    wide range of expressive sounds from a blazing

    humbucker to a cutting single coil pickup. And

    yes, there is a Varitone on the new Little Lucille.

    VARITONE CIRCUIT

    The Varitone circuit is a notch filter that utilizes a variety of capacitors to

    achieve a reduction in frequencies at set points within the overall

    bandwidth. As the varitone is rotated from Position 1 to Position 6 the

    center frequency of the filter shifts from higher to lower frequencies.

    Figure 1 demonstrates in general the type of frequency response that is

    created by the varitone.

    The specific characteristics are:

    Position 1: Bypass (no effect)

    Position 2: -8.5dB at 1875 Hz

    Position 3: -12dB at 1090 Hz

    Position 4: -15dB at 650 Hz

    Position 5: -16dB at 350 Hz

    Position 6: -20dB at 130 Hz

    Mono/Stereo Output Jacks

    When using "Mono" Output Jack-1, both pickups are operational. If you

    want "Stereo", you MUST use both "Mono" Output Jack-1 and "Stereo"

    Jack-2 together.

    "Stereo" Output Jack-2

    When using output Jack-2 alone only the treble pickup is functional. When

    using both jacks together Jack-2 carries the treble pickup signal.

    "Mono" Output Jack-1

    When using output Jack-1 alone both pickups are functional. When using

    both jacks together Jack-1 carries the rhythm pickup signal.

    Gibson's former R&D Guru, J.T. Riboloff, who won an award from

    the Music Trades magazine in 1993 for his design of the Nighthawk, went

    a step further by implementing a Varitone circuit in the Blueshawk. The

    Varitone filters out specific, fundamental frequencies. Combined with two

    volume, two push-pull tone, three-way switch frequencies in it's five

    settings, the Varitone gives the Blueshawk enough sounds to satisfy a

    session player. Click here to hear the Blueshawk sing.

    "The Varitone opened up the spectrum of the tone paths of that instrument

    quite a bit,"

    Riboloff said.

    "The other nice thing with that Varitone is the push/pull tone pot which

    bypasses the circuit. The Varitone selector is a rotary switch, and

    somebody might want to change his tone in the middle of the song, but

    he's also thinking about keeping his timing. He doesn't want to count how

    many times he's clicked his Varitone knob. What this allows him to do is,

    for example, leave the circuit in bypass and preset his Varitone at the

    beginning of the song. So if you want to locate position 5 on the Varitone,

    for example, preset the knob so that all you have to do is push that pot

    down."

    VARITONE CIRCUIT

    The Varitone circuit is a notch filter that utilizes a variety of

    capacitors to achieve a reduction in frequencies at set points within the

    overall bandwidth. As the varitone is rotated from Position 1 to Position 6

    the center frequency(f0)of the filter shifts from higher to lower

    frequencies. Figure 1 demonstrates in general the type of frequency

    response that is created by the varitone.

    *The specific characteristics are:

    Position 1: Bypass (no effect)

    Position 2: -5dB at 1875 Hz

    Position 3: -6dB at 1090 Hz

    Position 4: -7dB at 650 Hz

    Position 5: -10dB at 350 Hz

    Position 6: -14dB at 130 Hz

    Push Pull Tone Control:

    - With the push pull tone control in the down position the varitone is

    operational.

    - With the push pull tone control in the up position the varitone is removed

    from from the circuit.

    *Decibel levels based on 1000Hz : 1 Volt sinusoidal test signal

    Comment


    • Kazinator
      Kazinator commented
      Editing a comment
      WRGKMC wrote:

      Gibson used switched caps in a circuit called the varitone since 1959.

      The Varitone is interesting, and really, the question is, why don't guitars ship with useful circuits like that.

      The circuit also contained a 1 henry coil to make the circuit an actual passive EQ filter which you cant get with caps alone. Caps by themselves only with only cut or pass highs. With a coil added you can create notches with the midrange which is what most guitar players desire.   

      Capacitors and resistors can form a band-pass or band-stop filter, just not one with a very high Q (narrow frequency band).

      Any circuit built around a passive pickup does contain a coil: namely, the pickup!

      Also, there is already capacitance even if no capacitor component is present: the capacitance of the instrument cable. Together with the pickup, it forms a LC resonator: it has a peak at a particular frequency, and a -12db per octave rolloff (second order).

      Added capacitance (cap across the pickup) will lower the frequency of the resonant peak, and where the rolloff begins. The capacitor in parallel with the pickup does not simply cut high frequencies.

      If you simply want several tones which resemble the natural responses of various other guitar pickups, this is a nice approach which is also very easy to implement, with a minimal part count. The most basic implementation is a single cap in series with a switch, bypassing a pickup (or all of them: basically in parallel with the volume pot).  I have the second least complicated thing which is to switch two capacitances or nothing.

      The Varitone circuit needs a coil because although there are pickup coils in the circuit, they are not there in a topology that allows notches to be created in the frequency response.

      It's hard to comment on what most guitarists want. If I were to guess, most guitarists want is to sound like So and So player, on such and such recording, and don't really have a clue how to get there. Probably the most common experimentation with guitar electronics is: replacement pickups. And that mostly shifts around the resonant peak.



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