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Sunday, January 19, 2014

Tone capacitor myths

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Many people like to think that the type of capacitor used in their guitar's tone control makes some sort of tonal difference, so they will toss the ceramic or poly film caps that came stock and replace them with expensive audiophile caps like Orange Drops, Black Beauties, or any of several other types.

Obligatory Disclaimer: This is an attempt to debunk the "mojo" surrounding tone capacitor types *as used in guitars* (that is NOT to say that they don't have such properties in other circuits). If you are someone who fervently believes that your expensive new-old-stock audiophile tone caps sound better than your OEM tone caps, and that anyone who says differently speaks heresy, this is your opportunity to stop reading this and leave. Again, *I am not* implying that audiophile caps have no "mojo" properties, I am suggesting that these properties can't possibly be perceived in any shunt-to-ground application, such as a guitar's tone control circuit.

First we need to look at how a guitar's tone control works. Here is a schematic of a typical guitar (for simplicity, only one pickup and its associated volume and tone controls is shown). Also keep in mind that a guitar signal is not a single frequency, but is made up of many frequencies all superimposed on top of each other.

Notice that the tone control circuit is parallel to the signal path (it is strapped across the signal and ground). When the tone pot is rolled all the way up (the wiper rotated all the way to the unconnected terminal), there is a 500k resistor in series with a 47nF cap to ground present across the signal. A cap's reactance varies with the applied frequency (reactance is basically the AC equivalent of resistance) with it being the lowest at higher frequencies, causing them to pass through the cap very easily. The pot's resistance does not change with frequency. So the total impedance (resistance+reactance) of the tone circuit will be 500,000+Xc where Xc is the reactance of the cap *at a specified frequency*. Put simply, this puts a very light load on the signal (ignoring the parallel loads of the volume pot and whatever the guitar is plugged into) and only the highest frequencies in the signal are bled off because the load gets heavier as the frequencies get higher.

When the tone pot is rolled all the way down, it is out of the circuit (being unconnected at one end) and only the reactance of the capacitor is present across the signal. This loads the signal quite heavily--but again, the load varies with frequency. All the highs are dumped from the signal, along with a good chunk of the mids depending on the cap value. The cap's reactance is highest at lower frequencies so the lows stay in the signal. With a big enough cap the tone circuit could let all the frequencies in the signal pass and the tone control would then become a terrible volume control. Conversely, very small tone caps will subtly shave off very high frequencies to smooth out the sound without it sounding like a useless, muddled, treble-dump.

It should now be obvious why the tone cap type is insignificant in this circuit: the output signal doesn't even pass through it. Whatever bits of the signal *do* pass through the cap are shunted to ground and out of the signal path, never to be heard ("ground" can usually be thought of as "oblivion"). Since this is a passive tone control (it uses no amplifying devices) it can only cut frequencies out of the signal. The circuit is not boosting lows, but is just throwing away highs.

Having said that, there are most certainly tonal differences among the various capacitor types. Any application where the main signal, or at least part of it, passes through the capacitor will let the distinct tonal properties of that capacitor to become apparent. The best way, by far, to exploit the "mojo" of a capacitor is to use it as a coupling cap in an amp or effect.

So, in closing, any shunt-to-ground circuit, such as most guitar tone controls, will not display tonal differences between capacitor types. Some people will refuse to believe that, and that's just fine, though people should use their ears and experiment and not blindly buy into hype and parroted myths. Question everything! But no matter what your opinions and prejudices, remember that golden rule of guitar gear:

If it sounds good, it is good.
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