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Knowles FG-3329 microphone buffer amp/preamp

Elena Zucchini
Prodigy 10 points

Dear all,

Hello, I am currently involved with some hearing research and need to amplify a signal from the tiny Knowles FG-3329 microphone capsule (placed in the ear canal).

We have the capsule, a (very good and clean) studio preamp and a AD converter.  However, the output impedance of the mic is way too high for a pro-audio preamp, with a nominal value of 4.4kOhm (2.8 - 6.8 kOhm).

As such, I am now into Day 3 of my electronics career (!), having blitz-read Tom Duncan’s electronics text and scoured forums and datasheets for hours.  I really need some advice on selecting the appropriate TI opamp and designing this schematic.

So here’s the summary of where things stand.

Design goals: 

  1. low distortion and low noise: the recorded signals will be used to for both impulse responses and audio recordings for playback
  2. consistent output: early recordings will be convolved with latter recorded impulse responses after many months
  3. sufficient dynamic range for live recording

Options for solution:

  1. a buffer amp to interface with a studio preamp
  2. an IC preamp to interface directly with AD convertor

Since I’m new to all this, I feel might be safer to take route 1 with a battery to keep my scope for error as small as possible.  However, route 2 is attractive as the mic would be self contained, small with provide invariant amplification.

I’ve managed to find the circuit diagram of the capsule IC which is just a MOSFET.  The IC designer recommends having Vdd = 3V and adding another 22kOhm load resistor to -3V to get the required dynamic range.

My first instinct was to build a circuit with 3V coin cell, add the extra resistor to the microphone negative and use a TI opamp to build a simple non-inverting opamp buffer.  If I was capable, I would love to have built a (fixed gain?) phantom powered amp using a high quality audio IC amp but I fear this may be beyond me and/or give inferior sonic quality.

Could I ask the community for any recommendations of opamps/ICs, circuits and how they might need adjusting to suit my needs?

Mic data: http://www.knowles.com/search/prods_pdf/FG-23329-C05.pdf 

Mic IC schematic: http://www.draftonline.co.uk/downloads/FG-internal-1.PDF

Many thanks for reading!

Elena