Designing and building an active contact microphone for field recording and sound design

I have now written an updated version of this blog entry over on Oaka Insturments, check it out here.

As I have just released my Tellus contact microphones for sale over on Oaka Instruments, I thought it would be a good idea to talk about the design and build of the active contact microphone. Its no secret that I have used a modified version of the Alex Rice Piezo Preamplifier and following his notice in the original PCB files he asked to be credited according to the Creative Commons Share-Alike 3.0 license.

I’m not going to go into pre amplifier design much here as Zac Poff has done a fantastic job gathering Alex’s notes on the design on the link I shared above. I encourage anyone who has basic electronics knowledge to give it a go. I learnt loads from it. As I said before, after lots of testing and not actually being 100% happy with the original design I changed it considerably to my own taste and adapted it to be used for sensitive recording, useful for field recording and sound design. I completely re-designed the board and made it tiny with SMT components to fit inside an XLR plug.

The image above shows the board design and also the inside of the piezo element. There are a few things to note here. I have 3d printed a small horseshoe shape to fit around the piezo element. This slightly increases the mass of the element which helps reduce that classic piezo ‘tinny’ sound. You can also see the larger resistor in the circuit board, I put that there to enable me to change the voltage at which the circuit operates. I thought this could be handy if ever I wanted the microphones to operate at say, 12v, for art installations or workshops. It gives me a degree of flexibility without having to print loads of circuits in the future.

The image above is an early prototype using Zac Poff’s through hole version of Alex’s circuit design. In this version the the circuit is inside the element along side the piezo. This was an experiment and it worked, but I was not happy with the size and chunkiness of the capsule. It was good that it happened though and demonstrated the creative, practical journey which happens when designing things. It made me think how important it is to have a really lightweight and low profile capsule and cable. It makes attaching it to stuff way easier and allows it to be a lot more versatile and dynamic with placement.

The final stage of making the Tellus microphones was coating the elements in a durable plastic coating. I soon released after creating the first prototype that it was really important to mark the back of the microphone with a small bump, so you can tell which side to stick it too! In theory this coating should waterproof the microphone, but I have not tested it yet!

The finished product is shown in the picture above. I am actually incredibly pleased about how it turned out. It has been quite a journey designing this microphone and I almost gave up on many occasions. Its ended up sounding really good and I use it constantly in my own work. If you are intested and would like to buy a microphone, please follow the link here. Also, pease do get in touch if you have any questions about giving it a go yourself, I definitely encourage you to do so!

Thanks for the support.

Ezra