High Impedance Preamp Project

schematic on a piece of scrap paper...click for larger version
schematic on a piece of scrap paper…click for larger version

For a while now, I’ve been wanting to build my own contact mic preamp and impedance buffer. The problem has been that I’m still basically an electronics neophyte. I’ve got a reasonable understanding of many of the underlying concepts…I’m not going to get completely lost if someone starts talking about “imaginary numbers” and phase interactions. I get all of that on a conceptual level, but when it comes to understanding how circuits are actually designed…? Forget it. There are some things that are finally starting to click into place, but I’m still way out of my element.

There’s a big disconnect for me in how I want to use the finished circuit…and how I even get there to begin with. The general idea is this…I want to build a quality preamp that I can use with home-made piezo disc and film contact mics. I want the pre-amp to be decent, but I want to be able to use transducers that I can put in precarious situations without worrying about the cost of replacement. I also want to use it with a professional (+4dBu) input recorder (i.e. my 744T). You can find a plethora of schematics online to build cheap preamps for use with high-impedance inputs (guitar amps, etc.), but very few for the lower level of impedance we have on professional gear. I had a light bulb go off not too long ago when I was browsing through the Jensen Transformers website…a direct input box is exactly the kind of impedance buffer I’m looking for!

You see, Jensen has a transformer designed specifically for use in a passive DI circuit…they even have several schematics for building your own DI using this particular transformer. This seemed like the perfect solution to me. Take a decent contact mic preamp schematic and feed that to the input of the DI schematic. While not cheap, the impedance matching section of this combined circuit is completely passive. That means I don’t really have to deal with all of the calculations that would be necessary to combine two active circuits…calculations I’m sure I’ll screw up.

So, that schematic at the top of this post is what I’ve come up with. I’m using the preamp schematic found on Colin Cunningham’s blog over at Make magazine with the basic DI schematic laid out in the JT-DB-EPC data sheet. Take a look at the frequency response data for that transformer. Very nice, and the 3dB roll off point for the high end is at 100kHz…hellooo high res. recording. The MPF102 transistor indicated in Colin’s blog is a little difficult to find (unless you’re ordering it directly from China, Thailand, etc.). So after a little research around the internet, I’ve decided to replace it with an equivalent model that people seem to be having luck with in basic audio applications (the NTE451). The whole circuit will be powered with one 9-volt battery.

Ultimately, I’d like to build 2 complete channels into a single case. I’m going to start out with a test of just one for now, and I’ve ordered all of the components necessary to build the prototype circuit on a breadboard. I’ve also wanted to build something similar for use with disposable electret microphone capsules. So I may test out a circuit for that purpose while I’m at it. I’ll post progress, and any results, as I move through this project.

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One Response to High Impedance Preamp Project

  1. Pingback: Stage 1: Preamp Circuit Test » Dynamic Interference Blog

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