![]() Noise comparison of 2n3819 single fet with opamp version using a NE5534 (left and right channels respectively) įreshwater hydrophone recording using this with an air cell piezo disk hydrophone. However, substituting the TL071 for the usual audio workhorse opamp, the bipolar NE5534 brought the lowest noise of all. a 7mV rms 1kHz signal was fed into a 10k:12ohm attenuator, and then coupled via a 15nF capacitor to approximate the piezo source, generating 8uV. You can leave C1 and R4 out if you need less gain, though there is much to be said for the cheap version if you have lots of signal. FET Mic amplifier – opamp versionĭiodes D1,D2 stop you destroying the opamp with large signals from the piezo device if you drop it. The first thing that sprang to mind was a TL071 – the overall schematic is shown below. The calculation in the introduction showed a load of 330k was enough, so 1 meg is plenty for a contact mic. Although they are high impedance, they don’t need an outrageously high load impedance – not like an electret mic, where you need gigaohms to avoid losing bass. Piezo mics aren’t usually considered hi-fi, so distortion isn’t usually a problem of the amplifier – at high levels the mic will not be particularly linear. This is the purist approach solution to the tinny contact mic sound problem if you want the lowest electrical noise, or the lowest distortion at higher levels. ![]()
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