Dynamic Microphone Preamp

Posted on Feb 6, 2014

This is a 3 stage discrete amplifier with gain control. Alternative transistors such as BC109C, BC548, BC549, BC549C may be used with little change in performance. The first stage built around Q1 operates in common base configuration. This is unusuable in audio stages, but in this case, it allows Q1 to operate at low noise levels and improves over

Dynamic Microphone Preamp
Click here to download the full size of the above Circuit.

all signal to noise ratio. Q2 and Q3 form a direct coupled amplifier, similar to my earlier mic preamp. As the signal from a dynamic microphone is low typically much less than 10mV, then there is little to be gained by setting the collector voltage voltage of Q1 to half the supply voltage. In power amplifiers, biasing to half the supply voltage allows for maximum voltage swing, and highest overload margin, but where input levels are low, any value in the linear part of the operating characteristics will suffice. Here Q1 operates with a collector voltage of 2. 4V and a low collector current of around 200uA. This low collector current ensures low noise performance and also raises the input impedance of the stage to around 400 ohms. This is a good match for any dynamic microphone having an impedances between 200 and 600 ohms. The output impedance at Q3 is low, the graph of input and output impedance versus frequency is shown below: The overall gain of this pre-amplifier is around +39dB or about 90 times. The first stage Q1 has a gain of roughly Rc / Re or 4. 7. This is however reduced by feeding into the input impedance of Q2 and Q3, and the shunt formed by the 47k preset. The amplifier formed with Q2 and Q3 has a gain of roughly Rc / Re as Q3 is an emitter follower and has unity gain. The gain of Q2 and Q3 is roughly 10 / 0. 47 = 21 and overall gain therefore 4. 7 x 21 = 98. The gain ofthe circuit may be reduced to 0 by the...

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    flyback with a pulsed DC signal
    Cash Register circuit
    Light Sensor Switch
    Stand alone CDROM audio driver
    Microprocessor power supply watchdog
    Single chip checks resistance
    Flashing Led Controller
    Auto gain Control op-ampCircuit
    1-Wire Two-way level converter unit (from 1.8V to 5V) Reference design
    Cellphone-Based Device Control With Voice Acknowledgement
    make this simple delay on circuit
    Chager circuit for SMF batteries
    Electromagnetic Field Probe with Meter
    Motorola TPN1154 and TPN1136 Desktop Power Supplies
    Controlling high-current circuits
    LED Dance Pad
    Power frequency noise filter PC822 schematic
    5G14 3 ~ 6V adjustable power supply