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Ultra low noise mic preamp

   Noise of the amplifier is extremely low. And it has to be like this because electret capsule noise is very low and has only 1.4 uV RMS (A - weighted). The preamp noise should be lower than 1.4 uV RMS in order to avoid any additional noise to capsule noise. The preamp noise should be at least three times lower than the capsule noise to consider it negligible. It's because in the way random uncorrelated noises are added. For example, three times less is 1.4 uV/3 = 0.47 uV.  The total noise of the capsule and the preamp will be sqrt (0.47^2+1.4^2) = 1.477 uV. So we see that the noise increased only 1.05 times and it can be said that the capsule noise is the dominant noise source. 

   Preamp gain should definitely be such that after the gain the capsule noise dominates over ADC analog digital converter noise. In my case 6.12 uV RMS (A). This noise is produced by Focusrite scarlett 2i2 2 gen sound card which is perfect for its price. So 6.12 uV x 3 = 18.36 uV RMS (A) and preamp's gain has be at least 18.36/1.477 = 12.4 times. These are the requirements for the amplifier in order to avoid any additional noise on top of the capsule. It is exactly what Baffless microphone amplifier does. If the sound card is 106 dBA of dynamic range in reference to max input level +4 dBu, it will work with this preamp without any degrading sound. In this example by using a very affordable Focusrite scarlet 2i2 sound card, we get maximum what it is possible with the microphone capsule. Surely usable dynamic range is reduced from 106 dBA to just about 96 dBA. But this is the nature of all sound cards. Up to 10 dB reduction of dynamic range is necessary in order to call for noiseless dynamic range of sound card. This sensitive setting is perfect for low sounds. There is a switch at the back side of head for attenuation of louder sounds. When recording nature sounds or other low sounds you have to work with max sensitivity to get the best quality possible for binaural head records. This sensitivity switch enables users to get better performance with relatively cheap sound card compared to the best sound card available but without the switch. The electret capsule mounted in the ear is of 121 dB dynamic range and even can go higher, therefore dynamic range should cover more than 130 dB. The sensitivity switch solves this problem.

ultra low noise electret capsule mic preamplifier

Pic 1. Ultra low noise mic preamp for electret capsule

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