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OK, so a day of frigging around with different stuff has lead me to a passive tone control for piezo.

Some background: If you ever tried a normal guitar volume and tone setup on a passive piezo, you'll know the tone control doesn't work as it should. If anything, it acts more as a high pass filter than a low pass. This is because piezos have massive impedance compared to magnetic pickups.

I usually use an active preamp circuit based on the Tillman design and that means I can put normal volume and tone controls after the preamp and they work as they should. But I've had a couple of commissions recently from people who will rarely plug into an amp anyway, so that brought me back to the search for a passive tone control that will work for a piezo. It's seems a bit silly to put a really good preamp in a gat that will rarely get plugged in.

I tried a bunch of stuff I had read about with RCL filters but none of it worked, really, despite what some dude claimed. I suspect that, as usual, the very high impedance makes the RCL filters unworkable.

That left a more traditional approach with some modifications to make it piezo-friendly. Thanks to Charlie Hall for pointing me in the right direction yonks ago. I finally got around to trying out his suggestions. Apart from the tone cap value, he was pretty much right.

Piezo red lead > 100k resistor > usual volume and tone pot wiring as per this basic setup BUT

1) vol pot is 1M log (I tried a 2M pot but it didn't make any difference)

2) tone pot is 1M log

3) the tone capacitor is .0022uf (code 222), i.e. 1/10th of what you'd normally use on a tone cap.

4) you'll need a treble bypass cap across the input and output of your  volume pot or you'll lose heaps of treble as you turn the volume down. Use a .0022uf (same as what you used as a tone cap).

All grounds (including the piezo black lead) go to the back of the volume pot and you run a ground from there to the output jack ground.

You will notice a loss of about 3dB output level, but you can plug into an external preamp or booster or eq pedal or whatever to up your level and get your impedance matched for an amp if you want.

This is still inferior to an active setup but if you want proper tone control and don't want to build a preamp circuit and put a 9v in your gat, you now have a way to do it.

NB: I tested this on a disc piezo. I think rod piezos have a different impedance again, though still massive, so I can't vouch for how it will work on a rod. You might have to use a larger resistor (220k or more).

Let me know how it worked out for you.

cheers

Blind Gee