Replies to This Discussion

Permalink Reply by jabes on July 22, 2011 at 7:40am

it's been mentioned elsewhere that tone control doesn't do much, i use a danelectro FAB o/drive footswitch ,it has a gain pot and a tone pot ,i use these to get a bit of control of the sound  - the o/d pot just sounds horrible  ,,i don't use it ....happy building   ;)

Permalink Reply by Glenn Reither on October 25, 2011 at 9:31pm

Unless you include a 9v preamp circuit, you'll struggle to get meaningful tone control. If you include a preamp circuit like the Tillman one, then you can use 25k pots with a .047uf tone cap (.022 doesn't seem to do quite enough to tame the highs).

I got this from Charlie Hall (thanks Charlie!) but haven't attempted it because the advantages in building Tillman preamps and sticking them in my gats were too great to mess around with passive systems any more.

http://charliehall.proboards.com/index.cgi?board=general&action...

 

Permalink Reply by Glenn Reither on October 25, 2011 at 9:33pm

In case you can't find it in the link above:

 

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

Glenn Reither said:

Unless you include a 9v preamp circuit, you'll struggle to get meaningful tone control. If you include a preamp circuit like the Tillman one, then you can use 25k pots with a .047uf tone cap (.022 doesn't seem to do quite enough to tame the highs).

I got this from Charlie Hall (thanks Charlie!) but haven't attempted it because the advantages in building Tillman preamps and sticking them in my gats were too great to mess around with passive systems any more.

http://charliehall.proboards.com/index.cgi?board=general&action...

 

Permalink Reply by Wichita Sam on April 13, 2012 at 3:18pm

single disc piezos will not produce any tone shaping.... dual (or more) piezos wired in series will offer minimal tone shaping, but not really enough to be worth the effort.  If you want to go ahead, use a .049 cap....

 

the best,

 

Wichita Sam

Permalink Reply by Glenn Reither on June 20, 2012 at 10:01pm

An update on all this with details for a working passive tone control here: http://www.cigarboxnation.com/forum/topics/passive-tone-control-for...