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 In practice you can live with Vs at 1 to 3.5V which will correspond to a Vd of 7.4 to 4.9. Ideally you would like Vs to be about 2.5V and Vd to be about 6V (assuming a 8.4V Nicad PP3) 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’d run the FET from a higher power supply voltage, 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 batteries just because the manufacturers couldn’t be bothered to grade by Vgs. 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. It’s easy enough to test, and this parameter is a given for a particular device – it doesn’t age or change greatly with temperature.ĭesign manuals get all sniffy about that sort of thing because selecting FETs obviously adds to the cost if you are mass producing something. You’ll have to get more FETs than you need and throw out the dogs. 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 Many of the things you want to record with a contact mic are whacking great big structures that have BASS so you don’t want to go throwing it out at the off.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. Less tinny, but the 200Hz rolloff still takes the guts out of the recording. The same thing simulated into a 7k mic input This is a nice strong magnet, and since many resonant structures are steel you get an instant easy good contact with the object. 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. 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. The FET can be the typical 2N3819 or a BF245A/B. Musical instruments and anything where you bash the sounding structure with an object fall into the leave C2 out territory You can leave C2 out if you need less gain in your application. You also raise the signal level on the interconnecting cable run, which also helps to improve the system signal to noise ratio.ĭiodes D1,D2 stop you destroying the FET with large signals from the piezo device if you drop it. 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. FET high impedance inputīy using a separate box for the battery and load resistor the circuit is automatically powered when you plug in the piezo device. A FET amplifier will sort out the mismatch, and give a useful amount of gain for a contact mic. Australian Made AS/NZS 3718 approved, Galvin Specialised ProLab ® laboratory tapware range is supplied standard with swivel outlet, Stirling DIN 13792 approved easy use handle and button, 15mm male BSP inlet and fine control needle valve all finished in white epoxy coat.This is the solution to the tinny contact mic sound problem – at least the electrical matching issue.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |