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.
Hi Richard,
Your resources are really incredible for someone just getting into this side of field recording. Unfortunately I’ve noticed that a lot of your boards feature JFET’s that just seem impossible to source in the uk nowadays, including their crossreferences (or where there is something new, it’s stats are much different). Do you have any suggestions on how I approach actually constructing these? Even in this simple example, I can’t easily find a 2N3819.
While nearly all leaded FETs have gone the way of the dodo you can still buy them either NOS on ebay or you can take your chances with Chinese. Although the Chinese relabel all sorts of parts, I bought 10 2N3819s last year from China via ebay for this very project, and they work fine
For example https://www.ebay.co.uk/itm/171907152091
(That’s not the vendor I used, just one from a random search)
Having just said that, it seems some leaded parts are still made by onsemi. So if you want to buy them from an ISO9001 supplier where you reasonably know it’s what it says in the label then CPC will sell you some.
JFETS at CPC and the J113 looks a nice choice, since the spread in VGS (off)is more controlled to cut off at -.5 to -3 V, so most will be right for this, though still S.O.T. on a breadboard first, that is the price you pay for FET simplicity 😉 CPC won’t sell you fewer than 5 anyway.
Ah that’s really good to know, I was pretty nervous about ordering random fets for audio equipment, I like to try and use higher quality stuff to maintain quality. You mentioned that even the name brand edones QA sucks in a previous post for meeting their own specs. Have you noticed an equivalent variation in the knockoffs?
Since you actually read these comments I have two other things I’d like to ask. Where possible when making these fantastic posts, I don’t suppose you could post an actual parts list instead of just the schematic? Obviously I can just go through and list them myself, but sometimes the part is obscured in the graphic or the graphic is so complex that it can be really hard to figure out what something is/takes ages. It would also spare all of us who follow your stuff from each having to copy it down individually.
Secondly, I don’t suppose you have done any work/investigation into non-diaphragm piezos? I would be very interested to see the different responses and sounds that a spherical/hemispherical/toroidal..etc. piezo would make in a hydrophone for instance.
> but sometimes the part is obscured in the graphic or the graphic is so complex that it can be really hard to figure out what something is/takes ages.
I’ll think about it, Kicad should be able to do that. The nightmare is if i have to edit the schematic afterwards and the parts list/BOM gets out of sync, sowing confusion 😉
> spherical/hemispherical/toroidal..etc. piezo would make in a hydrophone for instance.
Tell me more about your suppliers?
You do get encapsulated piezo designed for sonar, these naturally go higher in frequency. And are often $$$
I did get one of the ones designed to replace ultrasound misters, they are designed to generate fog from a water surface. Haven’t got round to doing anything with it.
The normal gonzo way of doing hydrophones is to dip the piezo disk and attached cable in plasti-dip a few times. Worth a go, because you can’t really argue with the price 😉 Somewhere I saw one fellow glue a piezo disk into a tin can and then somehow get a seal on the cable and lid, trouble with that is it will want to float, but other than that an interesting approach with less damping.
Hello! I just wanted to ask – is the value of Vds critical in this circuit? I bought a few 2n3819s from mouser, they all gave me usable values as per the table on the 9v FET page; but in this circuit I had to change R2 to 2k to get 1/2 supply at Vd (testing with Zoom H5 at 2.4v and LS-10 at 3v). But for all R2 values from 470ohm up, Vds remains between around 0.1-0.18v, which is a lot less than the 1.2v you posted!
I had a couple 2n5457s around, I tried those and I got a larger Vds – closer to 1v with the 3v supply and around 0.4-0.6 with the H5 but also a significant increase in gain. Please let me know if you might have any thoughts about this?
Thank you!
The problem is that shocking order of magnitude spread in the IDSS, 2 to 20mA according to the datasheet. PiP usually is via a 6k8 resistor, so if the supply is 3V and you’d like 1.5V at the drain then you’ve only got 0.2mA to play with. If you’re unlucky enough to get a batch at the 20mA IDSS end you’ll have to raise the source resistor high enough to back-bias the GS junction. That will degenerate the gain against the 6k8 drain res – I didn’t figure that’s a problem with a piezo. But it may get so high as to starve the FET of VDS. As a sort of general rule you want about as much VDS as there is voltage across the 6k8 resistor. You’re SOL if you end up having to raise VGS much above 1V if you only have 3V to play with 🙁
That’s why it’s worth S.O.T. the FETs from a job lot. The AoE bitched about this in both of my editions, and sadly we’re stuck with picking the good ‘uns from a batch. At least IDSS doesn’t seem to drift too much with temp or time 😉
I would agree that the 2N5457 is a much more suited part, with IDSS of 1-5mA from the datasheet. I had a really hard time sourcing leaded FETs retail, and Mouser UK has these as a factory special order so they probably won’t sell you 5, but if you can get them, go for these
Hello Richard. Thank you for the wonderful circuit. 2N5457 can be purchased in Akihabara, Japan. When using 2N5457, are the values of each resistor as shown in the circuit diagram on this page? Once completed, I plan to use it for live performances where I swing around the bell. thank you.
The lower IDSS of 2N5457 should mean you can get away with a lower R2 or possibly eliminate it altogether. You’re probably best off selecting on test using a breadboard and your recorder input – observe the voltage on the drain. In an ideal world that would be half of the open circuit voltage with nothing connected. It will be too low iwth a 2N3819 and R2=0, but the 5457 may be OK with R2=0. Don’t get hung up on the half o.c. – anyhing from 1/4 to 3/4 is probably fine!
R2 is the only one to change – R1 is OK as it is and R3 is inside your recorder.
Thank you for your quick reply! I understand, I will check it on a breadboard and derive the value.