Piezo contact mics on low voltage power supplies

A few years ago I did a couple of piezo contact mic amplifier designs, because people often moan that these things sound tinny and crap. There’s a wrong way and a right way to use these – they want to work into a high impedance. Using Piezo Contact Mics Right sets you right. Trouble is these use a 9V battery, and it seems world + dog want to use 5V, because that’s what they had. Time was when power supplies were +/- 15V for analogue and 5V for digital, but that’s a different story for later.

So what can you do with your piezo contact mic at 5V then?

Not much. If you are looking for low signal level performance an emitter follower biased at an output of 2V would work well, but if you only have 5V available it’s likely you are trying to digitise this signal and bung it in an Arduino or something. In that case, think laterally. Toss the power supply. I developed those amps because as a field recordist I wanted to hear faint signals from the contact mic. You know, like the whispering in the rails as a distant train approaches, though you need to avoid the Fredzania Thompson ending.

These days people would look at you funny if you attach a box with wires to the underneath of the rails. Don’t try this at home and all that.

Turns out many people want to use their contact mics on an instrument, or drum pad, or generally something they bash seven bells out of. Life is a lot easier for you. As established in Using Contact mics right, you want an input resistance of about 330kΩ so the bass doesn’t roll off with the typical series tens of nF capacitance of the sensor. 330kΩ is a damn sight more than your typical plug-in-power audio recording doohickey, which usually feeds the electret mic power from 3V via about 6.8kΩ. I measured my Olympus LS-14 and even the line input is 10k.

So stick the 330kΩ resistor in series with the input. Even writing that makes me cringe, because it will lose a hell of a lot of signal level, making a potential divider with the input resistance – for a 6k8 input you’ll take a loss of 33dB. That translates into a direct worsening of your noise figure by that much, that’s a lot of performance to throw away¹. OTOH it works perfectly well down to 1.8V, it’ll be OK down to 0V as it doesn’t use power 😉

how much signal do you get from a piezo contact mic?

Let’s take a look at the sort of signal level you get from a piezo disc sensor. I got one on the bench and fed it into a DrDAQ signal capture device and Picoscope

Hmm, 60V eh? maybe I can afford to suck up a 30dB loss of signal. Also shows the rationale for the diodes on the input of my amplifier designs, to shunt some of this heat out into the power supply rather than wrecking the active devices. Sure, you’ll get a bit less into 330k, but it goes along with my general experience as a field recordist. You aren’t normally short of signal if you are using a contact microphone on something you’re going to hit with a stick 😉

How does this work in practice? I got myself a couple of fresh from ebay piezo elements and stuck them to a radiator with neodymium magnets, the magnets are 1cm diameter

I then wired these to a jack plug, one via a 330kΩ resistor to the centre pin (L) , the other straight to the other contact. Having seen the 60V pulse, I was careful to do my magnet wrangling before plugging the jack into the recorder…

Let’s cut to the chase. Setting the input to mic, with the 330k  resistor, tapping on the radiator with the back of a Sharpie pen, and lining up levels right I get

where if you compare that with the input without the resistor

you can hear the usual moan – this damn thing sounds tinny.

I did make a simultaneous recording on the LH channel to the above tinny recording, using the other piezo disc, but the problem is you can’t adjust record level for right and left separately. I experienced a 35dB difference between channels, so I had to boost the 330k series resistance channel by 35dB to get it to a similar level

It’s therefore noisier than the first recording because I couldn’t optimise the Olympus recording level. However, it’s exactly the same percussive event as the second recording, so it shows the tonal difference.

Don’t use the line input as shown in the pic. The series 330k into the 10k input resistance pads down the signal way too much, you can never get enough record level. You have to boost the recording in post, result: way too much hiss.

The first recording shows the 330k series resistor trick working at its best. Sure, you can hear some noise, whereas typical contact mic signals have zero perceptible hiss, but it’s very reasonable. To check that I filtered at 450Hz to get rid of the mains hum in the initial part, and the SNR is about 60dB unweighted. If you’re using contact mics to interface to some digital gizmo (these are often used in drum pads and the like) 60dB is far more SNR than you need. It’s a pretty good match for your Arduino 10-bit ADC, but remember that’s tapping a radiator with a Sharpie, not some demented drummer giving it his all. You have enough signal. If you are a musician using the audio out to listen to, then it’s more marginal. But if you’re a musician, you are used to the problem – try a DI box.

I wondered about all the good people saying but I want it to work on 5V. I want the moon on a stick and world peace would be a great thing too, but what you wants is not always what you gets.

The art of analogue design is thin on the ground

Surely everyone knows that your digital stuff uses 5V (or 3.3V) but your analogue stuff wants +/-15 if you’re lucky, though you can usually get away with a single 12V rail. Drop below that and you start to find your active devices perform worse, they get slower, you have to pay loadsamoney for rail to rail opamps, noise performance gets worse, it’s all down. Performance suffers as you starve analogue systems of voltage. If you want a low-ish noise microphone amplifier to run off a single 6V rail, you’re better off with a discrete transistor or two than some expensive low-voltage rail-to-rail opamp that has a compromised noise figure.

If you aren’t that fussed about noise, and most consumer items aren’t²,  then it’s easy – a MAX9810 will go down to 2.3V. And power the electret mic. But it’s not suitable for piezo discs, and the performance is average. Sound Devices aren’t going to be putting one of those in their field recorder inputs any time soon, and you’d be laughed off gearslutz if you demand your preamps run off 5V. In short, if you want your input amplifiers to run off 5V, you don’t care about audio quality, and this should be obvious. Then it struck me – people fall into this trap because

The internet happened as analogue electronics died

into niche signal conditioning islands. Many electronics engineers come across analogue as history, not practice. Analogue signal conditioning is now typically done by the OEM as part of the sensor. Which is why people don’t get just how unreasonable it is to ask for decent analogue signal conditioning from a single 5V rail. Designers used to use a switchmode to get +/- 15V rails if they had to go that way in a computer, or nasty things like the ICL7660 ³ to at least get a -5V rail to give their signal conditioning amps a chance with 10V across ’em.

Long story short, you can’t make a high-performance piezo contact mic front-end to run off 3.3V. You might just about get away with a single transistor emitter follower biased to about 1.5V on the output, but you’re pushing your luck.

But as this post shows, you can get a very good result with the series resistor that will be good enough for the vast majority of people’s applications, even if you are limited to a basic mic input designed for electret mics and impedance levels. Sure, you’ll be over 30dB noisier than you have to be, but a piezo element is not a hi-fidelity item in terms of frequency response. If it’s good enough, you can’t beat the resistor! 330k is also enough to probably save your mic input if you drop the contact mic onto a hard surface, at 100V it will keep the current into your input protection diodes to less that 1mA.

Win all round. The solution to getting a high impedance input on a low-voltage power supply is to ignore it and pad the resistance instead. It only works because piezo contact mics give a LOT of signal.