polymer piezo headaches, SRPP or mu-follower questions

[murrayatuptown]

Hi:

Anyone have any luck making a JFET or MOSFET adjustable overdrive for a piezo sensor?

I am looking at the enhanced modified version of JFET mu-amp or SRPP variations to solve two frustrating problems with a piezo project (uh, for a violin...I have a penchant for not following normal paths).

I started with the Runoffgroove Peppermill and a couple types of Measurement Specialties polymer piezo sensors. (Flat ribbon and coaxial).

It's been a frustrating project compounded by errors and horrendous hum and it's just now getting to the point where I have something approaching usable/musical. I got rid of most of the hum, but...

...I don't have enough gain to overdrive the two small practice amps at home or even get any out of the Peppermill and way too much hiss...probably partly thermal noise from insisting on 20M gate bias resistors for a 100 pF piezo to avoid a high pass filter well into instruments midrange.

I read that the tweaked JFET mu amp options provide 'noiseless biasing' and like most similar circuits, a lot more gain than the MOSFET driving a JFET.

1) I'm wondering how well one can reduce the gain to a much lower value if needed...like with an unbypassed source resistor on the lower JFET.

2) I kind of gave up for a little while on the 100 pF piezo (total capacitance including cable)...it's too doggone high impedance and squirrelly... Now I'm using a larger ribbon polymer piezo about 1" x 3", and good 3000 pF with shielded cable and insulation tape then copper shielding tape. (Raw, the ribbon was 2300 pF with it's twisted pair wiring. The 3000 pF came about after the layers of film tape and copper tape and braid over the twisted pair

That large a capacitance will tolerate a ~ 500k AC input impedance resulting from a 1M-1M bias splitter...
...but a drive or range control at the input looks like a problem to me reducing the input impedance far enough to move the lower corner freq. back into the violin's midrange again... that's a fundamental quirk of a capacitor sensor feeding a resistance to ground
is that a high pass filter is formed.

So what I think I need to do is try varying the gain with a source resistor and not mess with the input stage.

I thought a buffer might help, but I think I would need to do the noiseless bias thing there too, and it seems unnecessarily complex to do this for two stages...surely a simpler mess can be made.

I thought the Tillman circuit should have sufficed, but if I don't have enough output with the Peppermill, perhaps a single stage won't cut it...

BTW, the sensor is held on with an elastic band with Velcro stitched to it wrapped around the 'hips' of the lower violin bout (crude guess as to how Headway's 'The Band' might mount.

Anyone been there, done that, any suggestions, or if nothing else, maybe you'll get a laugh out of someone else's insane ideas...

Oh, there is no bypass switch. It's in an Altoid's tin with 1/8" input and output jacks so the sensor can be cabled or run directly into an amp...the large piezo was a late concession, it seemed like it would be harder to control hum with vs. the shielded coax, but the lower impedance came to the rescue and it's passable direct into the amp but for hiss and hum..

Since it worked directly, I'm also going to try the only functional commercial pedal (Bad Monkey) I have with the larger piezo.

Thanks

Murray

[petemoore]

  IUC Peizo to be a chrystal structure that happens to transduce some AC when stimulated, mostly high frequencies.
  The AC voltage should be a decent output swing [many pzo's are just bare PU elements with wires], which can use some boost [straight peizo isn't strong enough for some tastes.
  And can definitely benefit from current boosting [buffer or booster] because they give up very quickly when driving current say through a cable or other load, often times this is done almost at the pickup point, in the guitar or other.

[sean k]

Well, your original question was... is anybody having luck with piezos? and to that I'd say I've never had any bad luck or hassles excepting the fragility of two little wires on a piezo blank coming off easily... or fairly easily.

I just use the srpp type circuits with just about any jfet in common use with 10M then follow with a100k pot as volume on the output. Never any problems getting good bass response.

But then again I just buy the discs for a dollar each, solder on my own wires and use double sided sellotape to attach them to the stuff.

[murrayatuptown]

OK, thanks...

I'll make a couple more mods to the Peppermill then do an SRPP variant next.

FYI, the piezo polymers are unusual looking. The ribbon type is just a thin layer of the polymer (PVDF=polyvinylidene fluoride I think)with silver paint on both sides and riveted wires attached. Some come with a layer of protective overlay, evidently mine isn't, I pulled some silver off with masking tape.

The coaxial cable made with PVDF dielectric between the center conductor and braid is just that...a piece of cable...one splices a conventional cable to it. Some commercial pickups use these...I'm pretty sure Headway's The Band is polymer. I think LR Baggs makes some and I don't remember the other name but the guy has patents. Some of the coax ones fit under a bridge with a milled-out slot to fit snugly over the polymer. I think the string instrument bridges for violins and larger have 1-5 tiny polymer sensors. I've never found super small ceramic ones.

I will eventually try an op amp, but there are enough that insist on discrete transistors for their overdrive behavior, I skipped op amp circuits...and I really liked the 'transparent OD' concept for it's subtlety...a matter of personal preference I guess.

I'll keep tryin'.

The polymer stuff is weird...it's essentially free of resonances the ceramic structures can have, the coax can be used hundreds or thousands of feet long for chain link fence, airport runway, and traffic light approach vibration sensors, and it's used in (P?)IR motion detectors...at very low frequencies, near DC, it has strong temperature sensitivity (hence the IR uses)...

This has been a bigger hassle than I expected, but it's a lot closer to usable than when I started now...