Originally Posted by HCEarwicker
No one? Another of the Universe's unfathomable mysteries?
It's a problem with bad shielding. Perhaps a cheap guitar cable or amp shielding.
It's like this. The amount of hum pickup depends upon the impedance of the circuit. This is why microphones use low impedance to keep hum pickup low since their output is low. The higher the impedance the more hum gets picked up if shielding isn't 100% effective.
It depends on the wiring, but imagine this: A pickup hooked to a volume pot. Now the pickup has a general impedance (consider the resistance of the coil) of a few K Ohms. But the volume pot is usually 250K or 500K.
OK. When the volume is at full, the volume slider is all the way at one end of the pot shorted to the pickup output. Hence the amp is looking at about a 4 or 5K circuit which doesn't pick up much hum. Now move the volume control to the center. Suddenly the impedance seen by the amp may be serveral hundred thousand Ohms which greatly increases hum pickup if say your cord is cheap with ineffective shielding (or less than optimum shielding anywhere in that circuit).
Not all bases do this since some wiring is such that the volume controls short the pickups and for those the impedance to the amp never gets high. Active basses can do this too, but then it's a problem of shielding in the bass rather than the output as they usually have low impedance cable driver circuit that prevents high impedance to the amp.