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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.
for a demo of the tone control in action.
That sounds pretty good. It seems to have quite an effect on the tone. Good job and thanks for sharing this.
Got to give this a try.
This is good stuff. Pretty substantial effect on the tone. One question - in the last section with tone off, would that be the same or similar as having no tone control at all or is it the other way around? The first two sound nice and rich to me and the last with the tone off sound like a couple of guitars I have that are just piezo>jack.
Nice work on the elusive passive tone control for piezos. Maybe you should try cold fusion next :)
Thanks. Other way around. The last section is with tone wound all the way down. You can use a bigger cap like .0047uf (code 247) and get it a lot duller. You could also try a 500k log pot for the tone and that SHOULD give you more cut at 50% setting. Experiment with it, but at least this gives you a workable starting point.
The gats you have are probably just a bit dull-sounding anyway. How do you seal your piezo discs? I've got a method that tames feedback but keeps them pretty lively in the top end. The tone wound off sound is really only useful when overdriving - takes out the harsh buzz. If your gats sound like that direct to jack, I would wonder if your piezo is not picking up the full tone, possibly through being over-damped.
I glue my disc to the lid, centre and a little forward of the bridge (which reduces low end thump). Then I run a bead of hot glue around the edge, getting as little as possible on the disc, and sit a circle of 2 ply on the top. Then I seal all around the edge of the 2 ply with hot glue, and the hole in the middle from when I cut the 2 ply with a circle saw. Now the piezo is completely sealed off but still relatively free to vibrate. Works for me.
I've been using cold fusion and haven't had an elec bill or needed to put gas in the Delorean for years :)
Thanks for the info Glenn. I think you might be correct about the damping/dullness. I've tried several different ways of mounting and sealing piezos and have yet to get a sound I'm completely happy with. I like the idea of sealing off the piezo while leaving it to vibrate and I'm going to give your preamp method a try soon.
Thanks for posting this! I am just about to tear apart one of my builds (with a rod piezo) to put in a volume control. I thought I might as well add a tone pot, too, but thanks to your research, I won't do it the wrong way first and then have to reinvent the wheel.
This is what I did on the last one I built: It's a neck through 3 string on an Arturo Fuente box. I am using a disk piezo that I counter sunk into the neck ever so slightly so it would be flush with the top of the neck. The box lid sits right on top of the neck so it seals the piezo in place when all put together. The bridge is mounted directly over the piezo so everything is super tight when strung up. That sucker ain't going anywhere. Now, I did stick a regular volume and tone pot from a donor 'strat' and the tone pot seems to work as it should. I did find that it is more effective when using overdrive/distortion than when running clean. Clean doesn't have that much of an effect so I use the EQ on the amp to get the tone I need.
thanks for sharing this, i find this tone thing most puzzling...
Gremlins! Gremlins, I say! Always mucking about! Throwing monkey wrenches into otherwise perfectly good theories.