{"id":1604,"date":"2014-10-03T09:54:44","date_gmt":"2014-10-03T09:54:44","guid":{"rendered":"http:\/\/www.richardmudhar.com\/?page_id=1604"},"modified":"2021-12-04T10:35:29","modified_gmt":"2021-12-04T10:35:29","slug":"piezo-contact-mic-amplifier-measurements","status":"publish","type":"page","link":"https:\/\/www.richardmudhar.com\/blog\/piezo-contact-mic-amplifier-measurements\/","title":{"rendered":"piezo contact mic amplifier measurements"},"content":{"rendered":"

A stereo version of this<\/a> was constructed using a NE5532AN dual version of the NE5534. The performance of the low noise amplifier was measured using Rightmark. Because this version was designed to explore the sensitivity at low levels the a higher gain was selected. R4 was made 820 and R5 15k to give a gain of about 19x (as opposed to 11). A 10mV p-p signal was injected into the input via an attenuator of 10k in series with a 12 ohm resistor to ground. From the junction of the 10k and 12ohm resistor the signal was taken via a 15nF capacitor in series with the input, to simulate the piezo source capacitance. This meant a signal of 12uVp-p (equiv to 4.2uVrms) was present at the input. You can see the effect of the source capacitance in the rising noise floor at low frequencies where the input noise current is developed across a rising impedance.<\/p>\n

\"noise
noise performance – unweighted<\/figcaption><\/figure>\n
\"noise
noise performance – A weighted<\/figcaption><\/figure>\n

The 4.2uV reference tone corresponds to about -105dBu, coming in at -18dBFS on the plot. The noise floor is about -66dBFS, 48dB lower, -153dBu measured in 1Hz. Scaling this up to the 22kHz bandwidth means 43dB has to be added (=20log sqrt(22000)), so en is about -110dBu A weighted for this amplifier when presented with the piezo impedance (modelled as 15nF in this case).<\/p>\n

\"sweep\"
frequency sweep, output about -4dBu. The HF rolloff is part of the sound card, the LF rolloff is due to the rising impedance of C1 and C4<\/figcaption><\/figure>\n

hum<\/h3>\n

These tests suffered high hum levels relative to the noise floor – this is because the amplifier was mounted in a plastic box. The capacitive source impedance of 15nF is about 200k at 50Hz, and as a result it is easy to couple hum into the amplifier capacitively. You really should build this into a metal box connected to signal ground at the input if you plan to use this indoors at low levels. For most sound art applications signal levels are so high this is not a major problem, and outdoors hum is also not generally an issue. Most outdoors interference is high-frequency RF which finds a low impedance into the 15nF capacitance across it.<\/p>\n

noise – terminate unused stereo inputs<\/h3>\n

Running in mono with a stereo version of this amplifier it is important to terminate the unused input. If the unused input is left open, the noise of the high source impedance unloaded by the piezo source impedance is amplified. This results in a noise floor some 30dB higher, which makes it hard to work with quiet signals if you are getting the unterminated noise in one channel of the headphones. A 0.1uF capacitor across the unused input terminals works fine – you shouldn\u2019t just short it as there is DC on the input since the piezo disk source capacitance is used as an input coupling capacitor. Wire a 0.1uF capacitor across a dummy plug if you use separate input jacks for L and R piezo disks, and plug that into the unused input to quieten the unwanted racket.<\/p>\nhttps:\/\/www.richardmudhar.com\/sounds\/060825_15k-820opa_unterm_termed.mp3<\/a><\/audio>\n

Noise comparison of unterminated and terminated channels (left and right channels respectively)<\/p>\n

At this level of sensitivity you have to be ready for some unusual second-order effects – this is the disk held by the signal cable at arms length. The low frequency noise is the noise of the muscles acting to hold the cable steady, and the slight residual vibration of the piezo disk held by the cable at one end. The second example is the disk also held by the cable sticking over the side of a chair but held steady by a weight on the cable. If you raise the gain you can hear that I should have turned my PC off for the lowest noise. Both these tracks were recorded on a HiMD NH700, Mic Lo-Sens 20, where 0dBFS corresponds to -29dBu.<\/p>\n

Piezo disk cable held at arms length, disk in air supported by cable.<\/p>\nhttps:\/\/www.richardmudhar.com\/sounds\/060825_15k-820opa_piezo_arms_length.mp3<\/a><\/audio>\n

Piezo disk cable weighted on a chair, disk in air supported by cable. Record level same as above.<\/p>\nhttps:\/\/www.richardmudhar.com\/sounds\/060825_15k-820opa_piezo_gnd.mp3<\/a><\/audio>\n

Comparing this version with the original bench model<\/a> using the NE5534, it is slightly more noisy, the signal to noise ratio of the 1k tone is 49dB compared with 62dB (in a 1Hz noise BW). You have to allow for the fact that the gain of this version is 5dB higher, and the wanted signal is 4dB weaker, so although the wanted signal looks about the same level the signal to noise ratio would be expected to be 4dB worse, so the comparison comes to about 62dB against 53dB, about 10dB worse. I should probably have tested the performance of the NE5532 before soldering it in, along with some other assumptions.<\/p>\n

Using Piezo contact mics right<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

A stereo version of this was constructed using a NE5532AN dual version of the NE5534. The performance of the low noise amplifier was measured using Rightmark. Because this version was designed to explore the sensitivity at low levels the a higher gain was selected. R4 was made 820 and R5 15k to give a gain … Continue reading “piezo contact mic amplifier measurements”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"footnotes":""},"class_list":["post-1604","page","type-page","status-publish","hentry"],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/P5aOO7-pS","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1604","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=1604"}],"version-history":[{"count":7,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1604\/revisions"}],"predecessor-version":[{"id":4524,"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/pages\/1604\/revisions\/4524"}],"wp:attachment":[{"href":"https:\/\/www.richardmudhar.com\/blog\/wp-json\/wp\/v2\/media?parent=1604"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}