#1 resource for Cigar Box Guitars, Free Plans, How-To, Parts & More!

Passive tone control for piezo - solved.

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

3 members like this

▶ Reply to This

Replies to This Discussion

Permalink Reply by Glenn Reither on May 8, 2014 at 11:47am

Probably start a new thread for this. I build my own on the Tillman design with a couple of extra bits to increase the tolerance for FET variation.

▶ Reply

Permalink Reply by Glenn Reither on May 9, 2014 at 5:12pm

Getting a proper soldering iron with variable temperature solved a lot of my soldering problems.

▶ Reply

Permalink Reply by Glenn Reither on May 10, 2014 at 1:54am

Bakon 936

▶ Reply

Permalink Reply by MATAN on May 10, 2016 at 4:55am

so, any news about all of this witchcraft you made here?

I a complete ignorant and does not understand half of wht you wrote here, but a DIAGRAM (hint) could really help.

If anyone of you is still here or you all went to work in high-tech electronics and left the cbg....

▶ Reply

Permalink Reply by Glenn Reither on May 29, 2016 at 7:59pm

Hi Matan,

The wiring diagram is linked in the original post. Just the stock standard vol pot and tone pot.

http://www.guitarelectronics.com/product/WD1H11_00/Guitar-Wiring-Di...

A few peeps have reported success with it.

▶ Reply

Permalink Reply by JL on May 31, 2016 at 4:05am

the changes described in the post are not in the linked diagram...please diagram the wiring???

▶ Reply

Permalink Reply by Glenn Reither on June 6, 2016 at 6:05pm

▶ Reply

Permalink Reply by MATAN on June 7, 2016 at 2:37am

Glenn the MAN!

now noobs and ignorants like me can cast your blues spells!

▶ Reply

Permalink Reply by Timothy Hunter on October 6, 2016 at 1:22am

is the third tag on the tone pot supposed to be grounded? i dont see how it is supposed to work as this picture shows.

▶ Reply

Permalink Reply by JL on October 6, 2016 at 1:46am

For a tone pot you only use the center and one of the outside lugs. used like this its a variable resister.

on a volume pot, you run the signal from one outside lug to the other connected to ground, and the middle one selectively siphons off some of the signal on its way to ground.

▶ Reply

Permalink Reply by Timothy Hunter on October 6, 2016 at 2:17am

my bad. after looking at the pick for the 12th time i finally realised that the second lead of the cap goes to the case of the tone pot and therfore ground. for some reason i saw it as across thefirst two legs of the pot hence my confusion as to how it would work...