In contrast to line-level signals, microphone signals require amplification of the signal levels. Also, the impedances do not allow for a purely passive design as with the Passive ADC. That is why there is now an alternative ADC with an active input stage. Principally, this is initially the same design as the passive ADC, followed by an active amplifier stage, which either only delivers the same level or optionally amplifies up to 14dB in switchable 2dB steps. For microphone signals, the input transformers are virtually switched at the push of a button – of course only electrically, not physically – and thus take on an up-conversion (amplification) instead of a down-conversion (attenuation). This results in a total gain for microphone signals of between 24dB and 30dB with 0dB (no) active gain (compared to the line ADC). This can then be increased in 6dB steps.
With the adda191 reference or the system19x-adc2mic-line-reference module, this “passive gain” can be varied by 6dB by switching the gear ratio, from 24dB to 30dB. The active stage can even be bypassed, thus obtaining the purely passive signal path. With the system19x-adc2mic-line module, this “passive amplification” is fixed at 28dB.