It is designed for a dynamic microphone which output impedance is between 50 and 200 ohms. It is possible to connect a microphone which the output impedance is higher, but at the cost of a slight decrease in gain. Use with an electret microphone will be discussed in other article. The -3 dB bandwidth is 30 Hz to 100 kHz. With the above  scheme, the gain is about 35 dB to 40 dB (40 dB = amplification by a factor of 100).

This transistor microphone preamp circuit is a classic, implementing an NPN common emitter (common emitter simply means that the emitter of the transistor is common to input and output of the preamplifier stage, the base forming the entry and collector delivers the output signal). The gain is mainly determined by the value of the resistors R3 and R4. R3 contributes to the temperature stability of the assembly, and its value fairly high given the location of the component, reduces the gain significantly. Therefore there branch parallel capacitor C2, which “bypasses” the resistance from the moment or is this a sound input (if no audio signal input, the capacitor sees only one component continuous and performs as an insulator of high value – it acts here in the presence of AC signals). By removing this capacitor C2, the preamp still works, but with a gain of about 15 dB below. The capacitor C4 mounted between collector and base of the transistor allows to limit the bandwidth up to a reasonable value (around 100 KHz), and limits the risk of developing an unwanted parasitic oscillation.

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No major difficulties for the realization, but may be difficult to find the SSM2017 and SSM2142. Note that you can easily replace the SSM2019 and SSM2017 directly by replacement it INA217, INA163 or by THAT 1510 or  THAT 1512. Similarly, the SSM2142 can be replaced by DRV134. In all cases, the replacement is made pin to pin and does not require modification of the circuit.

The SSM2017 (or replacement SSM2019 or INA217) allows to make a good preamp with few components. In the schema on the internet is seen where the resistance that determines the gain is the only component in the Annex SSM. No phantom power supply provided (and therefore no protection against power surges), no RF input filter, in short, the ideal scheme for the beginner. The Mono preamp circuit diagram above includes all the “functions” basis, you will be completely free to simplify assembly by excluding one or all of these functions.

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