Wiring up a capacitor and resistor in series

[LH]

When wiring a capacitor and a resistor in series, does the order make any difference?

More specifically, I'm talking about how a guitar's Tone control is wired up, either with the capacitor "between" the Volume and Tone pots, or the capacitor "between" the Tone pot and ground. Since guitars are wired up using both methods, I'm thinking there might be some difference... :?

Any comments? Thanks for any info.

[Ansil]

to make a low pass filter you need to go from first lug volume to the pot         and from center lug into cap to ground

so it cuts highs as you turn down
]
if you go cap first then into pot and then to ground it cuts bass


http://www.muzique.com/schem/filter.htm

[Mark Hammer]

In the case of a guitar tone control, the pot works a little differently than a conventional RC lowpass filter.  If you have a resistor in series with the signal, and a cap going to ground from the "output" side of the resistor., there will be a fixed slope rolloff of frequency content according to the formula F = 1/(2*pi*R*C), where F is the point where it is 3db down, R is in megohms, and C is in microfarads (uf).  If you make R smaller, the point where the rolloff begins gets higher infrequency.

Remember that capacitors act like frequency sensitive resistors, where they provide a higher impedance path for lower frequency content, and a lower-impedance path for high frequency content.  In the case of a guitar tone control, the tone pot is simply a variable resistance in series with the cap to determine how much bleed there is at a fixed designated frequency.  Think of it as being like this: the impedance of the path to ground for frequency F is the sum of whatever the frequency-dependent impedance is for the cap, PLUS whatever the tone pot adds to that (since the pot affects all frequencies without prejudice).  Changing the tone pot setting does not alter where the rolloff begins, merely how much rolloff there is.

In a wiring scheme as simple as the typical guitar, it is difficult to insert any sort of variable-rolloff-point lowpass filter that does not interact too much with the volume pot.  On the other hand, "normal" tone pots do interact a little bit with the volume pot, depending on where they are tied to the volume pot.  You can either tied the tone control to the input lug of the volume pot or the wiper.  Input lug is preferred since there will be less interaction between volume setting and what the tone pot does.

On my guitars, now, I use a bidirectional tone pot where the wiper of the tone pot (1 meg linear) is tied to the input of the volume pot, and each outside lug of the tone pot has a different value cap tied to ground.  Rotate it in one direction from the midpoint and it rolls off at one frequency.  Rotate it in the other direction and it rolls off at another frequency.  I use a .022uf cap for one side, and a 4700-6800pf cap for the other.  One direction gives muting while the other gives "rounding" of the edges.  If you have one tone pot for two or even 3 pickups, this provides a very nice degree of flexibility so that the treble-cut action is appropriate to the pickup/s being used (e.g., for bridge pickup you really only want to round; if you wanted bassier/mute you'd switch to the front pickup anyways).  This arrangement also has the perk that the entire range of tone sweep is squished into half the arc of rotation, which makes it easy to use your pinky for "wah" effects with the tone pot....assuming you can reach it.

[LH]

Very interesting stuff! Thanks

So, the "placement" (before/after Tone pot) of the Tone capacitor makes no difference?

Thanks.

[matt239]

You are correct. The order of series elements make no difference.

[hymenoptera]

Phew.. I bet LH was anxious for an answer to that one!