A Bloody Mess

Update Feb 4, 2011: I wrote up a number of ideas for solutions in this blog post.

Before plunging into the details, here’s the abbreviated version: the Epiphone Riviera P93 guitar has 3 pickups and 3-way selector switch.  There’s no way to turn off the middle pot other than turning it all the way down. Because of this, the volume pots must be wired with independent (aka decoupled) wiring, which results in bad treble loss when turning down. And because of this wiring, there’s no way to use a treble bleed cap since turning down one will result in loss of high frequencies in the other. This problem can be avoided on a typical two pickup 3-way switch (traditional Les Paul style) guitar since it can use non-indendent (coupled) volume wiring.  Read on for details…

A few days ago, I realized that something was not happy with the new treble bleed wiring on my Epiphone Riviera P93, which has three pickups and three independent volume pots tied together at a three way switch.  With treble bleed caps on all three volume pots, I was hearing strange interactions between the pots.   For example, with the switch in the up position (neck and middle together), when I turned down the neck volume to zero, the middle pickup sounded way duller than it should have.    Intuitively, I wouldn’t expect the treble bleed cap on the neck pot with the neck volume all the way down to affect the sound of the middle pickup!

Treble Bleed Circuit Signal Path

After drawing out the circuit (pictured here), it became clear what was going on.  Since the middle and neck outputs are tied together at the switch,  there is a path for the middle signal back to the neck pot and the neck cap to ground.  We added the treble bleed cap to allow high frequencies to bleed back through to the signal wire when turning the pot down, but in doing so, we also added a path to ground through that same capacitor, for the other pickup!

Note: this problem can be avoided on a typical two pickup 3-way switch guitar (Les Paul, ES-335, Sheraton, etc) because on those, with only two pickups, the 3-way switch has two non-blending positions which completely isolate a pickup.  So instead of using the independent-volume (decoupled) wiring (pickup signal wired to center volume pot lug), you can use non-independent (coupled) volume-wiring (pickup signal wiring to side pot lug).  In that configuration, there is no treble loss when turning down the volume, but if you turn down the volume all the way, it also kills the volume for the other pickup. (Hence the “non-independent” in the name). But this isn’t a problem because with only two pickups, if you want to turn off one of the pickups, you can just use the 3-way switch – you’ll never need to turn a volume pot all the way down.

I’m stumped on how to resolve this, without resorting to active electronics, so I’ll be skipping treble bleed in this guitar after all.  After googling about treble bleed ad infinitum, I don’t see anyone else discussing this at all.  Have I missed something obvious?  If you have a solution, please get in touch!