{"id":1590,"date":"2014-10-03T08:55:57","date_gmt":"2014-10-03T08:55:57","guid":{"rendered":"http:\/\/www.richardmudhar.com\/?page_id=1590"},"modified":"2022-07-27T12:35:07","modified_gmt":"2022-07-27T12:35:07","slug":"piezo-contact-microphone-hi-z-amplifier-using-a-fet","status":"publish","type":"page","link":"https:\/\/www.richardmudhar.com\/blog\/piezo-contact-microphone-hi-z-amplifier-using-a-fet\/","title":{"rendered":"Piezo contact microphone hi-Z amplifier using a FET"},"content":{"rendered":"
\n

This is the solution to the tinny contact mic sound<\/a> 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.<\/p>\n

\"FET
FET high impedance input<\/figcaption><\/figure>\n

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 interconnecting cable run, which also helps to improve the system signal to noise ratio.<\/p>\n

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<\/p>\n

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.<\/p>\n

\"FET
FET buffer<\/figcaption><\/figure>\n

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. I tested it out on a footbridge<\/a><\/p>\n

\"060817_bridge_p1000662\"<\/p>\n

 <\/p>\nhttp:\/\/www.richardmudhar.com\/sounds\/060817_bridge_piezo_mass_2.mp3<\/a><\/audio>\n

The original recording<\/p>\n

The same thing simulated into a 7k mic input<\/p>\nhttp:\/\/www.richardmudhar.com\/sounds\/060817_bridge_piezo_mass_2_1khpf.mp3<\/a><\/audio>\n

Ugh. Somewhat tinny…<\/p>\n

Now try using a 50k line input<\/p>\n<\/div>\n

\nhttp:\/\/www.richardmudhar.com\/sounds\/060817_bridge_piezo_mass_2_200hpf.mp3<\/a><\/audio>\n<\/div>\n
\n

less tinny, but the 200Hz rolloff still takes the guts out of the recording. 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\u2019t 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<\/p>\n

You\u2019ll have to get more FETs than you need and throw out the dogs. It\u2019s easy enough to test, and this parameter is a given for a particular device – it doesn\u2019t age or change greatly with temperature.<\/p>\n

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\u2019s not the case here, and there\u2019s 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 \u2018uns. Ideally you\u2019d 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\u2019t be bothered to grade by Vgs.<\/p>\n

You can tell if you have a good \u2018un 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)<\/p>\n

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<\/p>\n

I prototyped this and tested it out with all the spare FETs I had in my junkbox, on a supply voltage of 8.5V<\/p>\n\n\n\n\n\n\n\n\n\n\n
device<\/th>\nVs<\/th>\nVd<\/th>\nusable<\/th>\n<\/tr>\n
2N3819 #1<\/td>\n2.07<\/td>\n6.3<\/td>\nOK<\/th>\n<\/tr>\n
2N3819 #2<\/td>\n2.03<\/td>\n6.2<\/td>\nOK<\/th>\n<\/tr>\n
2N3819 #3<\/td>\n1.4<\/td>\n6.8<\/td>\nOK<\/th>\n<\/tr>\n
J309 #1<\/td>\n2.1<\/td>\n6.1<\/td>\nOK<\/th>\n<\/tr>\n
J309 #2<\/td>\n2.27<\/td>\n5.98<\/td>\nOK<\/th>\n<\/tr>\n
BF244B<\/td>\n2.39<\/td>\n5.86<\/td>\nOK<\/th>\n<\/tr>\n
BF244<\/td>\n4<\/td>\n4.2<\/td>\nBAD<\/th>\n<\/tr>\n<\/tbody>\n<\/table>\n

If you really can\u2019t live with the tastelessness of the select on test FET, don\u2019t 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\u2019t need that performance, but here it is<\/a>.<\/p>\n

introduction<\/a><\/p>\n

a version of this to work from plug-in-power on field recorders<\/a><\/p>\n

high performance version<\/a><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

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 … Continue reading “Piezo contact microphone hi-Z amplifier using a FET”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"jetpack_post_was_ever_published":false,"footnotes":""},"class_list":["post-1590","page","type-page","status-publish","hentry"],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/P5aOO7-pE","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1590","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/comments?post=1590"}],"version-history":[{"count":10,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1590\/revisions"}],"predecessor-version":[{"id":4665,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1590\/revisions\/4665"}],"wp:attachment":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/media?parent=1590"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}