The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs. By their nature they can generate a lot of signal, but cannot drive a 50 kilohm typical line input. The pickup needs to work into a much higher impedance, typically 1 megohm or so.
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. When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass line.
If wired to a consumer plug-in-power microphone input of about 7 kilohms impedance, the result is a 1kHz high-pass filter. Hence the reputation for poor bass performance.
Added to this loss of low frequencies, the piezo pickup can suffer from resonances above a few kHz. These are not removed by using a higher load impedance, but because more of the wanted sound gets through their prominence in the mix is reduced.
A way to solve this is to use a field effect transistor (FET) buffer. Ideally this is placed close to the contact microphone/pickup to reduce hum pickup, though using properly screened cables means this is not absolutely necessary.
Construct a FET High Impedance Buffer
Fig.1 shows an example schematic. All the parts are standard and the total cost should be less than US$10 to build.
- Q1 is used as a common-source amplifier.
- D1 and D2 prevent the FET being destroyed if a contact mic is dropped – the piezo element can deliver very high peak voltages, and these diodes conduct excessive voltages so they can be dissipated safely in the low-impedance parts of the rest of the circuit.
- C2 gives the highest gain – for musical instruments leave this out to reduce gain to about unity. The schematic has been drawn in two parts to show that left-hand part including the the FET can be mounted close to the contact mic, current will only be drawn from the battery when the jack is plugged into the battery box, which is the section on the right.
Unfortunately the gate-source voltage of FETs is poorly controlled in the manufacturing process, so it is possible to get a 2N3819 FET which will not work with this design.
With everything connected up the voltage across R2 should lie between 1V and 3.5V, assuming a 8.4V NiMH PP3. The simplicity of this circuit means that it is worth getting a few FETs and selecting on test. In the event that the first one does not fall in the right voltage range swap it out for another – there is considerable variation in Vgs even in one batch of FETs.
This is small enough to build onto the contact mic. The photo shows a simplified prototype with slightly different values build onto a contact mic on a magnet. The disc is about 1 inch diameter.
Conclusion: Increase the Bass of Pizeo Guitar Pickups and Contact Mics
Piezo guitar pickups and contact mics can give perfectly adequate bass response if used correctly into a high load impedance. Vacuum-tube equipment typically presented high input impedances, which were ideal, but typical modern audio inputs need the help of a buffer amplifier like the one shown here to get the best out of these.
Piezo devices deliver high signal levels and very low noise, and are not sensitive to stray AC magnetic fields as coil pickups can be.
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Richard Mudhar - Originally from London, now enjoying a less hectic pace of life in Suffolk, Richard Mudhar is a keen wildlife sound recordist and ...
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