In order to keep the FireBox in compliance with IEEE 1394 powering standards, the voltage rails were reduced, which resulted in a slightly lower preamp gain than the preamps found in other PreSonus preamps. The FireBox has approximately 45 dB of analogue gain with an additional 12 dB via the boost for a total of 57 dB total gain. This is sufficient for the majority of the FireBox’s users’ needs. However, in situations where a low output microphone is used, quiet sound source is recorded, or very hot digital signal is required, it is recommended to pick up an external preamp (like the BlueTube DP, Tube Pre, Eureka, ADL 600, etc) and use it through line inputs 3 & 4.
Take, for example, the Shure SM58. Based on its specifications, a 94 dB signal into the microphone will result in an output signal of -54 dB. (90 dB is roughly equivalent to the volume of a blender.) So if you record something as loud as a blender with an SM58, the result would be a maximum attainable signal level of +3 dBV (or -15 dBFS). An SM57 has an even lower output (-56dBV/Pa) and would result in a maximum signal level of approximately +1dBV. Conversely, the Shure KSM27 condenser microphone has a sensitivity of -37 dB/Pa. This gives a preamplified signal of +20 dBV (or +3 dBFS) -- a signal roughly 30 times louder than that of the SM58.
