resistors on passive piezo tone control

Hi Blind Gee,
Welcome to the site. Very unusual and interesting instruments you have there.
Piezo pickups need a very high impedance. Usually something like 3 to 5 megohms. Unfortunately typical tone and volume pots do not come in this range so ideally you do need a preamp before any controls.
A standard electric guitar pickup is affected by input resistance and capacitance. Lowering the resistance will reduce the amount of the pickup's resonant peak. Increasing capacitance will change the resonant frequency to a lower frequency.
A piezo pickup reacts completely differently. Lowering the input resistance will reduce bass. Increasing capacitance will reduce volume. This is why when adding the usual type of guitar tone control to a piezo pickup it acts as a volume control instead of a tone control. Adding a suitable resistor in series with the piezo pickup will help to convert the piezo signal so that a guitar tone control will work more like expected. I would start with a resistor of maybe 100K between the pickup and the tone control. Wattage rating doesn't matter so you can use a resistor type that is physically very small. The tone control should be a fairly high value, maybe 1 meg log. The value of the pot won't affect the bass because the capacitor will stop bass from being lost. The tone capacitor can probably be one of the standard values used in electric guitars such as 0.022uF, 0.047uF or 0.1uF. If the 100K resistor value works then it should do. If it works but not effectively, try a larger value resistor like 220K or 470K until it does. I would then connect that point to a volume pot of 1 meg log or even 2 meg log if you can get one. The pickup will see a total resistance that is the value of the resistor and the volume pot added together. This total will be reduced because of the input impedance of whatever the instrument is connected to. The larger the pot values, the less signal you will lose. The downside is that with a larger value volume pot, you will lose more highs from the signal when the volume pot setting is reduced but you will not lose quite so much bass. Adding a bypass capacitor to the volume pot might help to retain lost highs when the volume setting is reduced. The larger value that your added resistor is, the more highs you could lose with the volume control backed off. I would try something like 100pf as a bypass capacitor if you think you will need one. The value chosen will depend on the capacitance (length and type) of cable you use from the instrument. I would choose the shortest cable that you can comfortably work with. For an external preamp, you can still use a preamp that has a high impedance designed for piezo pickups and you may find one that has extra tone controls to fine tune the sound. Or you can use an FET design with an input resistor to earth or ground of 3.3 or 4.7 megohms. It won't work as well as having the preamp in the instrument and the controls in the usual place within the preamp design itself, but I think it is the only way to do what you want. By juggling with values, you will be able to figure out compromise settings that work as best possible for your instruments.
Regards,
Charlie

Hi Blind Gee,
I like all the various designs based on the cigar box theme. The sounds are great.
Regards,
Charlie

Hi Blind Gee,

Looks like interesting instruments you are building and fascinating sounds I might add.

I have seen folks do this kind of thing with passive circuits, but the result it is so-so.... A FET preamp is what you need, something along the lines of the Tillman design. I described such a tiny FET preamp in the "Projects in Progress" section that is small enough to fit in a regular jack plug. It will easily fir inside a cigar-box guitar, and will run hundreds of hours off a few button cells. After the preamp you can install a 25K volume pot and regular treble-bleed tone control. The ouput impedance of the FET preamp is nice and low and there will be more than enough drive for any guitar amp.

Good luck with your project.

Regards.

Johan.

Hi Blind Gee,
Not sure is this is useful or not. They seem to have an Australian outlet and prices would seem to make it worthy of further investigation. search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2163624&k=battery%20box
Regards,
Charlie

I happened upon this old thread from way back and thought I'd give you an update.
I ended up building Tillman pre-amps for my gats, and still do. At first I built them on perfboard but luckily I have an elec engineer buddy who gets prototype circuits printed. He laid out the Tillman with all the options and he squeezes them onto unused parts of the big slabs of board. SO I've got lots of lovely labelled boards now and I just drop the components in and I'm all good. He also has a fighter jet and we do aerobatics in it. Yeah, he's a good guy to know

I did occasionally return to the challenge of passive tone control and did get a working solution. Obviously a preamp is MUCH better but at least I showed that passive can work. Some people have reported good results with it.

I put it in a gat that was going to a hobbyist who will probably never plug in to an amp.
www.youtube.com/watch?v=paGWTAotNUU

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. The only advantage of doing this over simply going without a tone control is that you have control at your fingertips instead of messing about with pedal settings.


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).

Hello everybody
I need your help: i've just bought an hybrid guitar : the cort m custom. It has two magnetic pickups and a fishman piezo bridge. The acoustic sound is controled by a volume knob and tone knob. Actually it's a stacked concentric volume/ tone pot. The issue is the following : the tone knob doesn't work, whenever i turn the tone knob, the sound is the same whereas the volume knob works perfectly.

I've read in this forum lot's of things i don't understand about the fact that piezo tone pots doesn't work well or stuff like that.

Here are some other information : there's a 9v battery for the piezo pickup and when opening the wiring cavity, i notice some kind of circuit stacked with the tone/volume pot.

I removed the stacked knobs to see if the problem came from the knob instead of the pot but whent turning directly the pot, no effect neither.

As this guitar is brand new, i'm thinking of returning it but if there is a genuine problem with tone controles of piezo pickups, i'd like to know.

Thanks for the help.