Piezo Pickup Tone And Volume

[octafish]

I've used piezo-electric pickups in a few applications (ukelele, cigar box, a percussion bicycle) but I have always used an off board buffer and EQ. I am finishing off some half-done and half-baked projects before I get started on a proper guitar project. One of these is a cigar box guitar with a piezo and on board tone and volume controls. I realize I'd be pushing it uphill trying to keep it passive so I'll be using an onboard buffer (jfet or opamp, I haven't decided yet).

The question is: After the buffer what sort of tone control should I use? I was hoping to use a simple cap to ground tone and voltage divider volume arangement as in an electric guitar. Any recomendations for pot and cap values? After a buffer I should be able to use a low value for volume like 100K yes?

Thanks in advance for your advice.

edited for spelling

Edited September 5, 2005 by octafish

[lovekraft]

Yes - any decent buffer should drive 100K with no problems, and you can probably go as low as 25K wihtout worrying too much, but it'll ultimately depend on the circuit. Keep in mind that a lower load impedance will lower the noise floor, but it will also require more power supply current, so don't get crazy! Just curious, opamp buffer, or something discrete? I've got a simple little two transistor buffer (JFET input/bipolar follower) that should be cheap and well-behaved, if you're interested.

[octafish]

Just curious, opamp buffer, or something discrete? I've got a simple little two transistor buffer (JFET input/bipolar follower) that should be cheap and well-behaved, if you're interested. 

I've built a couple of those "mint box buffer" single JFET devices , but I am thinking with a dual op-amp I could have an inverting buffer and then an inverting amplifier to give me a modest amount of gain.

[octafish]

Just found this http://www.west.asu.edu/rlerman/PDF%20File...Schematics2.pdf

Just what I was looking at trying to design myself but someone has done the work for me. All I have to do is add a Vr voltage divider and where and how to add the tone control. Any Ideas?

[lovekraft]

What kind of piezo are you using? What's the output level?

[octafish]

Its a 30mm disc cut down to about 20mm

The only specs I can get on it are:

- Max. input voltage: 30V pp

- Resonant Frequency: 4,200 +/- 500 Hz

- Resonant Resistance: 300 ohm max

- Electrostatic Capacitance: 20,000 pf +/- 30%

Plus If I set my DMM at 200V AC I get a 1/2 volt when I tap it with my finger (obviously not that accurate).

[lovekraft]

It doesn't sound to me like you need a preamp - 500mV (1/2V) is pretty stout, even for a tap transient. A simple voltage follower will probably give you all you need, not to mention avoiding the problems with headroom that can result from running a hot signal into a preamp with a lot of gain. That mint-box buffer is almost ideal for this application - this one might be a little better (but only marginally so - it will drive a load as low as 1k though):

BTW, this buffer will work great for adding passive pickups to an EMG setup too, you'll just need one for each passive pickup. I've got a vero layout that's fairly small, if anybody's interested.

If you simply must have a preamp, the one you linked to will work, but it's got a lot of gain (26dB!), so you'll have to keep the input level fairly low (especially if you're going to use a single 9V battery), or you'll overdrive the opamp, and that clipping isn't very pretty, even using a JFET input chip like the TL072. Just my two cents - YMMV!

[octafish]

Edited because I am a bonehead. Thanks for the circuit, looks better than the opamp, more compact as well.

Edited September 8, 2005 by octafish