Should there be a cap between a piezo pickup and the gate of FET preamp?

[Carlos]

Hi everybody,

maybe my last post was not exactly to the point.
Should there be an input cap at the gate of a FET (2N5457, Stratoblaster) when hooked to a piezo pickup (Schaller piezo tunamatoc bridge)?
Should I block the DC coming from gate?

Regards

Carlos

[Paul Perry (Frostwave)]

I wouldn't think so. Just so long as the gate has some bias resistor to somewhere so it can't float free.

This looks like an interestig Japanese collection of piezo preamp circuits: (click where you would expect)
http://www.geocities.jp/dgb_studio/G_circuit.htm

[Carlos]

Thanks for the answer and the link, Paul.
My Stratoblaster has no bias resistor at the gate, I even left out the "grid stopper" resistor.



Shall I add a "grid stopper"?

You see, I would like to hear your comments before I take out the preamp through the f-hole which is a pain in the neck.
Regards

Carlos

[Processaurus]

Great link Paul, thanks.  The dummy pickup coil in the feedback loop of an opamp in the first diagram is super interesting.

[Carlos]

When I put the side of my right hand on the bridge muting the strings, the sound is completely off: not even a muted sound. It sounds as if the FET switches off.

Does the pickup create a voltage affecting the biasing of the FET (remember no resistor at the gate)?

I guess I will have to take the preamp out through the f-hole again ....

[stm]

I wouldn't think so. Just so long as the gate has some bias resistor to somewhere so it can't float free.

This looks like an interestig Japanese collection of piezo preamp circuits: (click where you would expect)
http://www.geocities.jp/dgb_studio/G_circuit.htm

I'll take it one step further by linking the same site but in plain english: http://www.geocities.jp/dgb_studio/index_e.htm

Next, choose "Electronics Circuit" and then "Inside of Guitar" in the navigation bar on the right and you are there.

Now take a look at the "Phase Shift Pickup" circuit (second project on he page).  It's a phase shifter built around a humbucker pickup that uses an LDR to vary phase!

My browser (IE) didn't show the schem at first, so I right-clicked on the link and saved the schem to disk, then opened in an image viewer program and voila!

[Carlos]

My 335 copy had another f-hole operation and I got the preamp out. I added a 2M2 resistor at the gate. The PCB I got from a forum member years ago did not provide space for such a biasing resistor. So the gate had indeed been floating.
The FET is biased correctly now and the palm mutes work, too.
The sound does not die at all. So I guess I was right:
Applying pressure on the bridge piezo when you do palm mutes puts DC at the gate of the FET. The FET is not biased properly anymore and cuts off the sound.

You should try a piezo bridge for modern sounds. I like the blend of acoustic (via mixing console) and electric tone (via amp) a lot.
John Wesley of Porcupine Tree uses these sounds quite often.

Regards

Carlos

[Paul Perry (Frostwave)]

That Alembic circuit has the gate conected (from a DC perspective) to ground via the pickup, if a magnetic pickup is used..... I can imagine it being a bit sensitive to interference with a piezo. But I havn't had any hands-on experience.

[Dave_B]

My Stratoblaster has no bias resistor at the gate, I even left out the "grid stopper" resistor.
Carlos

Carlos, I know you meant no harm, but you should change that image to a link.  I don't think they'd want you hotlinking to it.