In this post we discuss a powerful preamplifier circuit for microphones, which can be used for amplifying an electret MIC signal, using just a couple of BJTs.
Microphone signals will often be too weak to get carried to units for example mixing consoles and recording devices with sufficient strength. Preamplifiers boost a microphone signal to line level (i.e. the amount of signal power necessary by this kind of equipment) by giving steady gain and also protecting against induced disturbance that could in any other case distort the signal.
The output voltage for a dynamic microphone is quite low, usually in the 1 to 100 microvolt range. A microphone preamplifier raises this amount by up to 70 dB, to just about anywhere up to 10 volts. This more powerful signal can be used to operate equalization circuitry inside an audio mixer, to push external sound results, and also to sum for some other signals to produce an audio mix for audio recording or for live audio.
Circuit Description
By using just a couple of BC547 NPNs or Equivalent, and through a directly coupled circuit we can make a MIC preamplifier circuit for amplifying minute mic signals into very strong signal.
Figure 2: Electrical diagram (the voltages indicated have
Were measured by supplying the circuit with a voltage of 12 V).
TECHNICAL CHARACTERISTICS of this MIC preamp :
Required Supply voltage 9 to 18 V
Current consumption at 12 V 1.5 mA
Minimum input signal 30 mVpp
Maximum Output Signal at 12 V 7.5 Vpp
Bandwidth 10 Hz to 40 KHz
Average gain 250
Bill of Materials
R1 =100 k preset
R2 =47 k
R3 =82 k
R4 =560
R5 =4.7 k
R6 =680Ω
R7 =680Ω
R8 =100Ω
R9 =47 k
R10 =100 k
R11 ... 100 k
C1 =390 nF polyester
C2 =10 μF electrolytic
C3 =1 μF polyester
C4 =1 μF polyester
TR1 =BC547
TR2 =BC547
PCB Design and Component layout
