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Thread: Capacitor mini-tutorial.

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  1. #1
    GAStronomist Simon Barden's Avatar
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    The tone control capacitor isn't in the direct audio path, so it's really not that important as to its type, but higher tolerance varieties will give more predictable results. Some capacitor types also vary their value more with temperature.

    The tone circuit in guitars is a form of an RC (resistor + capacitor) low-pass filter, where the cut-off frequency = 1/2*pi*C*R.

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    So both the capacitance and the resistance value have an effect on the cut-off frequency. The bigger the capacitance or the resistance, the lower the cut-off frequency.

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    But that resistance value isn't the value of the tone pot, oh no. It's the resistance of the pickup itself (or more correctly, the 'impedance' of the pickup, as its resistance will vary with frequency). Which is why humbuckers and high DC resistance single coils tend to use use lower value capacitance values than single coils with lower DC resistance values in order to achieve roughly the same cut-off frequencies.

    You may be surprised to learn that a single-coil pickup and a 0.047uF capacitor arrangement has a cut-off frequency of just 564Hz. An 8k ohm humbucker with a 0.022uF capacitor has a cut-off frequency of 904Hz. Both of these are quite low values. So why doesn't the sound always sound muffled? Because of the tone potentiometer. This acts as a signal attenuator, so the amount of signal sent to ground via the tone circuit is at maximum with the tone pot set to zero - i.e. no resistance. With the tone control at maximum, i.e. maximum resistance, you've got almost none of the signal above the cut-off frequency being sent to ground, and so the signal sent to the output is nice and bright.

    The above is slightly simplified, as in most instances, it’s not just the pickup’s resistance that makes up the ‘R’ value, but the resistance of the pickup and the volume pot in parallel. Because the volume pot has so high a resistance value compared to the pickup, it only has a relatively small effect on the overall resistance, but it isn’t negligible. For instance, a 6k ohm single coil with a 250k ohm volume pot has an equivalent resistance of 5.86k. An 8k humbucker with a 500k ohm volume pot has an equivalent resistance of 7.87k. So not that great a change, but the slightly lower value does push the cut-off frequency up slightly.

    There are other complications to add in to make a 100% realistic simulation of a guitar's complete tone response, as the pickup itself has a capacitance an inductance, and the guitar amp's input impedance also reacts slightly and the guitar lead's own resistance and capacitance also form an RC circuit and filter off a bit more high-end frequency (which can sometimes be quite a lot).

    But the basic principles are that the higher the value of the pickup resistance x the tone capacitor value, the lower the cut-off frequency. So for higher resistance/impedance pickups, you need lower value capacitors to achieve a reasonable filter cut-off point, and for lower resistance/impedance pickups, you need higher value capacitors to get a reasonable cut-off point. The higher the resistance value of your tone pot, then the less the higher-frequencies will get filtered off and the brighter the guitar tone will be with the tone pot set to maximum. So for brighter single coils, this is another reason why 250k tone pots are used in comparison to duller sounding humbuckers which typically use 500k tone pots.

  2. Liked by: corsair

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