Piezo contact mic amplifier
This is the solution to the tinny contact mic sound problem - at least the electrical matching issue. A FET amplifier will sort out the mismatch, and give a useful amount of gain for a contact mic.
By using a separate box for the battery and load resistor the circuit is automatically powered when you plug in the piezo device. By keeping the FET close to the piezo disk you maximize signal levels and reduce the extent of the high impedance part of the circuit, which has a tendency to pick up hum. You also raise the signal level on the interconnectng cable run, which also helps to improve the system signal to noise ratio.
Diodes D1,D2 stop you destroying the FET with large signals from the piezo device if you drop it. You can leave C2 out if you need less gain in your application. Musical instruments and anything where you bash the sounding structure with an object fall into the leave C2 out territory
The FET can be the typical 2N3819 or a BF245A/B. You may be able to get away with RF FETs like the J309 but you may find that the input noise starts to rise at audio frequencies.
My original version of this (no diodes, C2 and slightly different source resistor) I glue the contact mic using epoxy resin to the back of the actuator magnet of an old hard disk. This is a nice strong magnet, and since many resonant structures are steel you get an instant easy good contact with the object.
| original recording, from resonant bridge | |
| using a 7k mic input | |
| using a 50k line input |
The bad news, however, is that the manufacturers of FETs don't control their parameters well, and have
somehow conned us into living with the problem. The gate-source voltage needed to bias the
transistor into the linear region can vary between 0.25V and 8V, which leaves a good 7.75V down to a hopeless
0.4V for the transistor and load if used with a typical NiCad 8.4V PP3
You'll have to get more FETs than you need and throw out the dogs. It's easy enough to test, and this parameter is
a given for a particular device - it doesn't age of change greatly with temperature.
Design manuals get all sniffy about that sort of thing because selecting FETs obviously adds to the cost
if you are mass producing something. That's not the case here, and there's just no way to cope with a
manufacturing tolerance which can throw more than 90% of the battery voltage away in variations
in manufacture without screening the bad 'uns. Ideally you'd run the FET from a higher power supply
voltgae, like two batteries in series and perhaps double the values of R2 and R3, but it would be a shame to have to use two batteres just because
the manufacturers couldn't be bothered to grade by Vgs.
You can tell if you have a good 'un by measuring the voltage at the drain and source of the FET in circuit. Ideally you would like Vs to be about 2.5V and Vd to be about 6V (assuming a 8.4V Nicad PP3)
In practice you can live with Vs at 1 to 3.5V which will correspond to a Vd of 7.4 to 4.9. This will run the FET at 0.25mA to 0.9mA
I prototyped this and tested it out with all the spare FETs I had in my junkbox, on a supply voltage of 8.5V
| device | Vs | Vd | usable |
|---|---|---|---|
| 2N3819 #1 | 2.07 | 6.3 | OK |
| 2N3819 #2 | 2.03 | 6.2 | OK |
| 2N3819 #3 | 1.4 | 6.8 | OK |
| J309 #1 | 2.1 | 6.1 | OK |
| J309 #2 | 2.27 | 5.98 | OK |
| BF244B | 2.39 | 5.86 | OK |
| BF244 | 4 | 4.2 | BAD |
If you really can't live with the tastelessness of the select on test FET, don't want to use two batteries or are looking for the lowest noise performance, you can do better. The majority of piezo contact mic apps just don't need that performance, but here it is.
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