I'm building my own (rugged) headphone. I am planning to use a MM20-33639-000 microphone with CSR8675 DSP. My question here relates to impedance matching.

Based on what I have read, most modern audio equipment use voltage matching as opposed to impedance matching (if there are multiple devices powered) to preserve the voltage across each input of the destination block. Anyway, let me start with some facts.

The output impedance of the MM20 microphone is 5100 ohm (max = 6350 ohm).

The CSR8675 datasheet states the following:

"When the input pre-amplifier is enabled, the input impedance at MIC_LN, MIC_LP, MIC_RN and MIC_RP varies between 6k Ohm (pre-amplifier gain >0dB) and 12k Ohm (pre-amplifier gain = 0dB)."

I am planning to use 2 microphones. The CSR8675 accepts 2 input lines individually. Therefore each microphone gets its own pre-amplifier (no sharing needed.)

As can be seen above, the output impedance of the mic and the input impedance of the preamp on the CSR8675 are pretty similar (6350 Ohm and 6000~12000 ohm). Since I'm not sharing the mic output across multiple pre-amps, I think I can stick to the concept of impedance matching (not voltage matching) without having to worry about if the signal voltage will drop across the pre-amp input and hence causes noise issues (SNR degradation). From what I have read, if I do voltage matching, the preamp will need to have an input impedance that is 10 times larger than the mic's output impedance.

Is my selection of devices based on impedance matching correct and use these two components, or do I have to use a preamp that has a higher input impedance?


1 Answer 1


The advantage of impedance matching is that you have maximum power transfer. The advantage of voltage matching is that you have no signal amplitude loss under different load conditions (e.g. cable length or multiple loads).

I think with the setup you described, it does not matter what you do.

  • \$\begingroup\$ i just wanted a second opinion to confirm my thoughts. Thank you. \$\endgroup\$ Commented Dec 14, 2018 at 5:38

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