ADAU1701 as Dynamic Mic Preamp

Hello,

The -1701's ADCs are current-sensing -- 100 uA RMS for a full-scale input. The inputs include a 2K ohm internal resistor. Typically an 18K external resistor is added, for a total of 20K ohms. This provides a sensitivity of 2V RMS for a full-scale input. This picture from the ADAU1701 data sheet page 20 shows the resistors and their effect. You may wish to download the data sheet which includes more details.

As you suggested, a 60 dB gain would provide a 2 mV RMS full-scale, suitable for most dynamic mics. Yet, 60 dB of gain within the DSP would cut directly into the available signa-to-noise ratio, yielding a rather noisy sound perhaps even worse than a cassette tape. Shorting the ADC input resistor reduces the full-scale input to 200 mV -- 20 dB better than before, so you only need 40 dB gain inside the DSP. This should work marginally better. If you go this route, place a DC Block Filter before the gain blocks, as the ADCs have a small DC offset which can get huge with added gain.

However, the best solution would be to add an external mic preamp ahead of your ADAU1701 board. Such a preamp may support a differential input, reducing hum and buzz. Ready-made preamp boards are available from Amazon, etc.

Best regards,

Bob