I47 Discrete Mic Amp
Very low noise, transformerless, discrete Microphone Amplifier filters for 4U frames
The discrete Mic Amp uses a transistor low noise input stage instead of an input transformer. A stepped switch controls input gain of the discrete mic amp and ranges from 6dB to 72dB. As the electronically balanced input stageof the discrete mic amp accepts level up to more than + 30 dBu no input pad is required at low gain settings. An infinitely variable fine fader with a range of ± 6 dB with a calibrated center detent sets the intermediate values. The maximum gain amounts to 78dB. The input has an input impedance of > 10 kOhm and an input CMRR of at least 70 dB to 15 kHz.
Two switches can be used to add a resistive and capacitive load to the input of the discrete mic amp. This will of course not change the performance of the amplifier, but modify the load that is applied to the microphone. Therefore the microphone itself determines the effects resulting in this load. The standard values are 600 Ohms and 200 Ohm for resistive load and 1.5 nF or 4.7 nF for capacitive load. These resistor and capacitors can be exchanged easily.
Special filter circuits of the discrete mic amp that have no influence on the frequency and phase response of the amplifier are used to provide effective oppression of RF disturbances. The signal to noise ratio of this module puts it in the absolute top bracket of microphone amplifiers. Even with a very low gain setting of 40 dB the noise figure of the amplifiers remains below 2 dB.
The switch P48 turns on the 48volt phantom supply. When activated the phase switch reverses the phase of the output.
The switch FILT turns on the high-pass and low-pass filters. The high-pass filter has a slope of 24dB per octave and a range of <20Hz to 600Hz. The low-pass filter has a slope of 12dB per octave and ranges from >25kHz to 2kHz.
A LED chain with 4 LED's indicates the output level. The output of the discrete mic amp is electronically balanced or is implemented balanced and floating.
Balanced and floating output