This blog post explains the deisgn and creation of Tellus, an active contact microphone. You can purchase the microphone here.
I’ve been experimenting and building active contact microphones for a while now and have playing with a variety of different approaches. Recently, after using the microphones substantially in the field, I have settled on a design which I am happy with.
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.
An early Tellus prototype
The first versions of the microphone I was using a 3d printed case and a plastic dipped protective layer. I found this worked well at keeping the microphones durable, however, after experimenting with other materials I was finding that increased sensitivity was possible by not covering up the brass element.
Version one of Tellus, with a plastic coated element
I was also finding that the process of dipping the microphones was very time consuming and a lot of the time, the results were visually quite bad. So I went back to the drawing board……
A prototype of Tellus V2, using old bicycle inner tube to attach
In the latest design, I have created a version using a brass element, I have found that using a metal element is a massive improvement than using plastic. Subtle high frequency response is greatly increased and its possible to hear insects and bugs moving in the soil. I have also now put the circuitry inside the microphone body, decreasing interference. I have also made the cable thicker and more durable. The case is now larger and made out of a high quality and highly durable plastic.
Tellus Version Two - Active Contact Microphone
Tellus Version Two - Active Contact Microphone Brass Element
The final design I think is a huge improvement. Its taken a while to get there but the result is a really durable and solid tool. I have been using the microphone for my own work and the results are amazing. If you would like to hear some sound clips check them out below:
