![]() Example: piano, tuned metal drums or ‘hang’, cajon, xylophone (this is the ambitious one). I would like to experiment using multiple (identical) piezos on percussion instruments. Hello, I’ve built your NE5532 preamp and I love it. Transistorising audio gear killed off the crystal mic as they sounded tinny and harsh with the lower input impedances for the same reason. Crystal mics also used the piezoelectric principle, and were valued for their low cost and high output voltage which overcame the preamplifier noise better. The move to transistor circuitry meant consumer audio inputs tended to end up around 50k which is reasonably easy to do. Valves do high impedance really well, and inputs were often in the area of 1 megohm input impedance. These things are very much like the crystal mics people used to use with valve open reel recorders in the 1950s/60s. With the mic input you are looking at putting in a 25dB boost by the time you get to 50Hz which is just asking for hum problems, and even the 8dB boost on the line input version is more than doubling any hum you may have. Plus you will boost all the noise and hum at low frequencies. The trouble with that approach is you need the specs of the mic and amplifier input to know where to start your 6dB/octave LF boost, so you will probably have to tune that by ear if you don’t know these. You could try and equalise in post production. What you need is a high impedance input, which gets you the blue line in the plot above. This tends to be compounded with the problem that these things aren’t really designed as mics and resonate at 2-4kHz anyway unless you stick them down so you get a harsh ugly result. blue line is the high-impedance, green is a typical 50k input and red is a 7k mic input ![]() Which is does – from the frequency response you can see that the mic input starts losing low frequencies below 1kHz (red line) and the line starts to go below 200Hz (green line). So what to people do? They go and stick this into the line input of their recorder, typical impedance 50k, or the plug-in-power mic input of their recorder, typical impedance about 7k, and they start to grouch that this damn thing sounds tinny. I’ve terminated mine in 3.9M because that’s what I had to hand. If that is 20Hz, since the impedance of the capacitor is 1/2 pifreq*C then you want that to be > 530k. You need to put this into a load which is higher than the impedance of the series capacitor at the lowest frequency of interest. Hence the reputation for poor bass performance….”Ī piezo element can be thought of as a sound-dependent voltage source in series with a large capacitance of about 15nF. If wired to a consumer plug-in-power microphone input of about 7 kilohms impedance, the result is a 1kHz high-pass filter. ![]() When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass. The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. However, the main reason these have gotten a bad rap is that many people couple them into a standard audio load, which loses low frequencies. This will tend to lead to a peakiness at mid-frequencies, which is why I glue these to a magnet to try and spread this peak and make the coupling to the source better. ![]() The brass disc on which the element is glued is designed to resonate at the design frequency of 2-4kHz so that a large audio output is achieved with a small power input. The sound to voltage conversion isn’t noted for its high quality – most piezo contact mics are tuned speaker elements used in reverse. First of all though, why do these things sound rotten the way they are most commonly used? Two solutions are proposed – the cheap and cheerful FET buffer which probably meets all the requirements of 90% of users who are using the devices on outside structures, and the high performance version for those who want higher performance and lower noise. The pickup needs to work into a much higher impedance, typically 1 megohm or so. By their nature they can generate a lot of signal, but they cannot drive a 50 kilohm typical line input. The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs. They seem to promise a lot, but easily sound rough as commonly applied. They seem to cause grief when used as contact mics. Cheap, innocent looking piezo elements do okay as greetings cards sounders. ![]()
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