You can add a pickup and control without causing any grounding problems. A single coil pickup of any type will be susceptible to hum though. This can be minimal with something like Fender's low noise pickup. Or rail types are available in side by side humbucker versions that are as narrow as single coils. These also sound more like a single coil compared to traditional humbuckers (like you already have).

A piezo requires a very high impedance circuit. The existing circuitry would load it down too much.

Volume pots can be wired forward or reverse. If yours are wired forward, or you ever played a Gibson, you may have noticed that turning down one volume to zero, with two pickups selected, will kill the entire signal. So you don't want to wire the new added volume pot in the forward configuration or it will also kill the entire signal when rolled all the way back.

Forward volume pot wiring takes the pot output from the wiper (center terminal). Reverse wiring feeds the pickup into the wiper, and takes the output from the high side of the pot (an outer terminal). That's how you will want to wire the new volume pot. The Fender Jazz bass is an example that uses this configuration.

Wiring is simple. With pot viewed from the back with the three terminals on the left:

Pickup ground to lower pot terminal.
Pickup signal to middle pot terminal.
Pot lower terminal to output jack ground.
Pot upper terminal to output jack signal.

Anything you add to a passive circuit is going to affect things a bit. A 250K ohm audio taper pot might be a good starting choice. If there is a noticeable loss of highs with the 250K, a 500K ohm would be better. You may want a linear taper with the 500K ohm also.

The pot value and taper will affect the response (the feel) of the pot, how the pickup volume varies with the pot setting. In an ideal circuit, an audio taper volume pot will respond as one might expect; the volume will closely correspond to the pot setting. For the reverse wired volume pot in a passive circuit, the pot resistance and taper will both affect the response of the volume pot control.

Comparing audio and linear tapers, the audio taper will be more responsive at the higher volume settings. The linear taper will be less responsive at the higher settings, but more responsive at the lower volume settings. It can be a matter of preference.

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Last edited by RJ 1911; 01/20/23 09:09 PM.