This is for a piezo contact mic that is powered off a field recorder’s plug-in-power supply. That is for small recorders like Olympus’s LS10, LS-5, LS14 series, which are typically unbalanced audio inputs, stereo on a 3.5mm jack, not for a P48 phantom power from a mixer or pro recorder1. You also get an opportunity to drink beer.
Batteries are a pain in field recording. One battery to rule ’em all is best in my book, the fewer things with a battery required to make a system work the better. The recorder generally has to have a battery, and Sony agreed with that many years ago, back in the days of Minidisc, and came up with plug in power (PiP). to eliminate the battery in the microphone. An electret mic doesn’t need much power so battery life is weeks not hours, but it has a switch you often forgot to switch off.
Plug-in-power
Plug in power is about 3-5v supplied through a series 6.8kΩ resistor, with the output developed across the 6.8kΩ resistor. It so happens that electret microphones were coming in at that time, and typically had a FET to buffer the high source impedance of the capacitive electret mic, an audio source through a few tens of pF. The 6k8 resistor is the drain resistor of the electret mic. You can read more about plug in power here.
Ideally a contact mic would work with this just like an electret mic, after all the problem and the solution are the same – a high source impedance 2 requiring a FET to buffer it.
there’s not much power available in plug-in-power
The trouble is that both the FET piezo amplifier and the opamp version need a 9V battery, which is a pain in the backside. An Olympus LS10 needs two AA cells and will run hours, and then the piezo contact mic wants a box with a 9V battery all to itself. That’s not good, and you tie yourself up in wiring.
The opamp version will never run on plug in power. The basic FET buffer is optimised for 9V, so it won’t run off PiP either.
The design
Initially I tried this with R2 omitted (replaced with a short) but there is not enough current to get the drain off the 0V rail. That’s fair enough – the IDSS is specified as 2-20mA and the most current you are going to get from a 3V supply through a 6k8 resistor is a little bit shy of 0.5mA. I bought a job lot of 2n3819s and experimenting with the value of R2 showed 470Ω put the drain voltage about 1.7V, with the source voltage about 0.5V, dropping about 1.2V across the transistor and 1.3 across the 6k8 drain resistor, which is good enough. Surprisingly, though they were from Ebay, nominally Fairchild parts, all five I tried biased about right with 470Ω up to 680Ω. Ordinarily you’d bypass R2 with a capacitor to maximise gain, but you are rarely short of signal level with a piezo mic, so I omitted that, it will slightly reduce gain, and also limit the current slightly if the piezo puts too much through the gate/source junction if I drop it. I have decided to take a chance with this omitting the diodes of the original FET version, because they’re hard to fit in. I expect peaks to be lower because I have mechanically damped the mic a lot. Time will tell if I get to regret that and the mic goes noisy or fails in service if I drop it once too often.
Your optimal value of R2 may be different due to the spread in FET Vgs- put a multimeter on the drain and aim for about half the open-circuit voltage3 ±10%. Mine is chosen purely for the supply through the left hand channel. If you parallel the inputs you may need a different value. I have hopes of using two to record stereo, else for mono paralleling the inputs makes for easier monitoring in the field.
Plug-in-power contact mic construction
First open a bottle of beer, trying to preserve the shape of the crown seal rather than bend it in the middle cavalierly. Obviously you can’t leave an open bottle of beer in the lab, so drink it. A 2cm piezo disc fits in a typical bottle cap, as shown
Now pick out any plastic seal in the bottle cap, and then clear the residue. I used a Dremel and the face of a small stone attachment to get this
You are going to solder to that surface, it will be the ground, and the cap provides shielding on that side, the brass disc shields the other side. This is a good time to drill a 3mm hole in the side of a cap, I used a small modelling pillar drill. If you don’t have a pillar drill you are probably going to give blood and turn the air blue with cursing. So better file a U shaped slot with a round needle file instead 😉 A hole locates the cable better in the later glue stage, but it’s not essential.
Now assemble the amplifier Manhattan style over the ground plane of the bottle cap, which also takes the shield of the cable. Use relatively thin cable for the output signal, because else handling noise will drive you bonkers. Some cable protection against the sharp metal would be wise, heatshrink in my case. Solder the piezo to the relevant place. You don’t want too much of the red and black cable going to the piezo element because you will have to lose this in a load of glue. I had too much.
Now would seem a good time to test this with your recorder, handling the piezo should be easily audible.
I wanted to keep a continuous solid interface to both sides of the piezo. Air is quite high impedance mechanically, and a contact mic detects signal in low-impedance solids. The sharp resonance of piezo discs is because they are a solid in air on both sides, and I wanted to damp this and match the disc to the solid medium. I used glue for that.This takes out a lot of the dreadful 2kHz honk in the sound, which is the natural resonance of the disc in air. In an ideal world you would put the contact mic in a solid medium with the same impedance as the ceramic either side to pick up the longitudinal vibrations, which would get rid of that resonance, but getting rid of the piezo-air interface on both sides helps a lot, even if epoxy and hot-melt glue aren’t a great match for the ceramic – they’re much better than air! The cap and glue mass-load the rear of the disc somewhat, which usually improves the sound.
In the final device, you want the disc to sit slightly proud of the bottle cap so the brass disc is in contact with the sound source, but the bottle cap isn’t. For that you can use hot melt glue all the way, though probably in two stages.
I wanted to glue a neodymium magnet to the other side of the piezo disc. Many things that are interesting with a contact mic are steel surfaces, and a magnet saves you trying to clamp or stick things to such a surface.
I discovered the hard way that hot melt glue is above the Curie point for neodymium magnets, and the loss of magnetism is permanent, turning it into an expensive metal disc. So I used epoxy resin to stick the magnet to the back of the piezo disc. I also got to resolder the red lead to the friable silvering on the back – best done quickly.
It pays to manipulate the magnet with non-ferrous tools, the best way to hold it in place is put it on the disc over a steel surface covered by masking tape. The next stage is glue this to the cold hot-melt glue encapsulated amplifier in the cap – the depth of the glue plus the magnet should set the brass disc proud of the cap. I also had to lose all the red and black wire in the gap and fill this with epoxy. I did this glue operation is two stages, because otherwise it was going to be a very messy job.
This stage is used to prove the brass disc is clear of the wires and the bottle cap. Only just in the case of the wires.
The second stage infills the gap between the disc and the cap, by mixing the glue on a piece of paper and using a fold in the paper to pour the epoxy into the gap.
You will get to scrape some epoxy off the disc. Aim to make nothing stand proud of the brass disc, which will then be held on steel surfaces with minimal gap.
Field testing – what does it sound like?
This is the mic attached to galvanized farm gate, the excitation is entirely the wind. I could not hear this sound standing next to the gate, it was a strong breeze, not a hurricane. I found it was very easy to overload, because you can’t really hear those low frequencies and metering isn’t fast enough for the transients. I record the main output on the LH channel, and put a series 15k resistor to the RH channel, which gives me about 10dB attenuation, but also more noise. I had to take this recording from the RH channel because I made a mess of the recording level.
Experimenting with stereo
I made another one of these, to see what stereo sounds like in a contact mic. Using a Y cable that connected the tips of two TRS jacks to tip and ring on another, I can combine these. This was on a farm gate, tapping the gate to test the sound
this was recorded on the opposite sides of the middle vertical strut of the fence below
this was recorded on adjacent struts
and this final one was recorded as shown below
You can hear a bit of the wind impacting the cables, perhaps I will have to clamp them to the gate to get rid of that effect. The speed of sound in steel at nearly 6km/s is much higher than the speed of sound in air at 330m/s o although the spacing of the third sample (pictured) would be outrageously wide for a spaced pair of microphones in air, the stereo effect is clearer for me on that than on the first two.