Single-supply piezo-preamp circuit for e-guitar

I think your way too high there on your assumption, I've fed Piezo guitar pickups, both buffered and unbuffered into a mixer board, my guess is between 1 volt and 100 mv max rms output.

Wait, how is that possible? And in this case it is just a simple ac coupled single supply op amp circuit with a gain of about 5, sheesh! What the hell are they teaching you these days?

Rocky

My thought was to make sure I get enough headroom to avoid clipping. But I sure will mesure the actual output as soon as I get the piezos delivered.

Hmmm, I also feel a little bad about this. I sure learned about the theoretical side of it. But I never got some hands-on experience. That's why I'm asking for help from people who have this experience.

But hey! As I mentioned, I'm focused on the digital side (computer vision, coding and signal processing, speech recognition and synthesis, VHDL, ...). O.k. the name electrical engineering is probably just misleading, as I'm using it in a generous way.

Simon

You don't need gain, you don't need an op amp. To impedance-buffer piezo pu use a J201 FET with 2k21 drain to ground, 3M3 gate to ground and 47µ drain to output. At Vss=9V, Id Å500µA, linear headroom is Å2Vpp. If overdriven It will saturate gracefully but if you are digitizing the signal for the DSP purposes the TDH is irrelevant anyway.

j.

Ok, for starters your circuit will be noisy as hell since you're using an inverting configuration and you have to add the thermal noise of the 10M input R to any analysis.

You should use a non-inverting voltage follower. I doubt that a typical piezo output will exceed 3v pk-pk. If it does - attenuate the signal after the follower.

Do you have to run it off a single supply of 5V ?

The PCM1802 has a single ended input. You can get far better results using differential input converters.

Lol ! It's internally differential anyway !

If you're worried about 'overdriving' the input - I would suggest adding some clamp diodes to gnd and 5V on the analogue input ( pins 1 and 2 ).

The best way to be sure of 'safe' input levels is to measure the pk-pk maximum signal swing available from the op-amp stage and scale it actually.

Graham

It's a mystery to me too.

Should mean I'll be handy for anyone needing competent analogue design. It just doesn't seem to be taught any more. And certainly not well.

Graham

You might *want* clipping. I used piezo wafers as percussion triggers, and when glued on 1/8" plywood under a rubber pad, a medium whack with the handle of a screwdriver yielded pulses of several hundred volts (with no load of course, just a high-impedance oscilloscope probe). Make sure you have sufficiently low input impedance!

Clifford Heath.

Most would be in that range. To a large extent, the mechanics determine the output from a piezo, but open circuit piezos can occasionally reach as much as 10Vp-p. As a result, many designers opt for 18V or even 36V batteries. But all that's really needed is a regular hi-Z buffer with low voltage supply; because the piezo is a capacitive source, one simply shunts it with a larger capacitor to get the voltage swings under control (and at the same time, make it much more tolerant of lower input impedances). This is generally a far better solution all 'round than trying to handle very high voltages at very high impedances. My new piezo buffer runs fine on 3 AA cells for decent battery life (along with a DSP to do the "Variax" thing).

Tony Tony (remove the "_" to reply by email)

I would definitely NOT apply clamping or amplification in this case.

These pickups work as differential pressure sensors, voltage output is proportional to a velocity difference between a vibrating string and a vibrating top. These excursions are minimal. You would have to whack the strings really hard to get more than 2Vpp into a ³1M. I'm assuming we are using a bridge piezo pu which is the only 'correct' way of utilizing piezos for further signal processing.

In practice, instrument piezos are buffered by a JFET voltage follower which, among other positive features _saturate_ rather than clip when overdriven. Hard (diode) clipping at any stage is a big no-no as it causes realy nasty transients which will fool DSP into 'hearing' what's not there.

Further, you should 'phantom' power the buffer and place it as close to the pu as possible and run low Z signal to the DSP.

Maybe you should take a look here:

j.