piezo disk transducer volume control? passive?

[Quackzed]

I'd like to install one of those undersaddle pickups along with a preamp-eq thing in my acoustic guitar.
but i'd also like to put  some piezo disks inside the guitar with some way to blend  the 2 (undersaddle and disks)
i was planning on wiring 3-4 disks in parallel then wire them also in parallel with the undersaddle transducer.
i'm wondering if there is a way to adjust the volume of the disks 'before' they get to the preamp.
normally i'd just use a volume pot between them and the pre, but because of the transducers very high impedence signal i don't think it will work well.
so is there a way to do this?
i've read that you may be able to use a variable cap to ground to attenuate the signal capcitively, would a cap blend work like this or is the resistance of a cap blend pot a problem?
i don't really need to be able to blend on the fly, a set and forget setup would work fine,  i'd just like to be able to get a good balance btween the 2 then leave it alone...

any ideas would be appretiated.

[Gus]

Try something like this.


Note C4 it works with the piezo as a capacitive voltage divider.  Adjust C4 for volume/level.   I used guessed values for C5 the piezo and C4 the "pad" You will need to try different values for C4.

Build a preamp for each section you want to match(two sections if I understand your post) and blend them after the the two preamps

[Quackzed]

aha. so its like a resistive voltage divider, only with caps!
thanks man! that makes sense.
but i dont think i'll be able to affect just one signal if theyre parallel...
i'm guessing i will build the above circuit for one or the other(undersaddle or disks)
and use the onboard preamp for the other...
maybee include an input for the preamp out(say the undersaddle)on the pcb and put a blend pot between that signal and your circuits output. (disks)
i think that would work pretty good.
i was thinking i could get aaway with a passive cap to ground from just the disks but it would also affect the undersaddle...

[GFR]

Gus,

What's improved by adding a bipolar in this circuit? Why not just the JFET as a follower?

[Gus]

The follower works at about 1.2ma about 4.6VDC at the emitter(R2).  Sim shows about .172ma JFET Id, most of the current in the NPN
I like to use a JFET at 1/10 or less IDSS.

So for a Source Follower(SF) circuit(not the circuit above) with 1.2ma source current I would like to use a JFET with an IDSS of 12ma at voltage D to S in the circuit

Also with the JFET BJT circuit R1 and Q1 Vbe is something like a constant current circuit