This assembly is a double preamp, perfectly suited for a stereo microphone. It is based on the use of an integrated circuit type LM381, which includes two preamps in the same case (14 pin) and which has good performance alone, mostly in terms of background noise. Datasheet (technical document) the manufacturer does not provide sample application for the micro preamp, too, here’s one.
As you can see, it is not very complicated, and apart from the LM381 now obsolete, uses only very common components.
The LM381 is an integrated circuit that includes two low noise preamplifiers and has mainly been used for the realization of preamps for K7 or Phono preamps readers (with RIAA correction). The circuit has an excellent separation between the two channels, as a decoupling and a power control are operated internally to each channel (separation of 60dB channels for a feed rejection of 120 dB). The circuit can operate with a supply value between 9V and 40V.
The legs of the LM381 which are not used in this assembly (legs 3, 5, 6, 10, 11 and 12) remain in the air.
The input impedance is determined by the value of the resistors R1 (left channel) and R2 (right path). It is approximately 100K for both channels.
Gain control is provided by resistors R3, R5 and R8 for the left channel (upper assembly), and resistors R4, R6 and R7 to the right channel (inféreiure part of the assembly). If the gain of about 30dB in this case, was not enough, you can change the values of these resistors, knowing that to increase the gain, you must increase the value of R7 and R8, or decrease the value of R3 , R4, R5 and R6. Values in parentheses are another alternative component values.
Note: If high gain you observe a parasitic oscillation, you can add a capacitor of 10 pF between the legs 5 and 6 and another of the same value between the legs 10 and 11 (these are tabs for defining external compensation frequency, which is found on many other operational amplifiers).
A simple power of 9V to 24V is required. In fact, any source of voltage between 9V and 33V is perfect. Note that the manufacturer of the LM381 Announces max voltage 40V, but it’s not really useful to push too far for a mic preamp like this. The main power is provided by decoupling the two capacitors C7 and C8.