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Overview The circuit was designed to provide more efficient output with a dynamic microphone by using a low noise preamplifier. Terminology - Dynamic Microphone – one variety of microphone that has the following features of being robust, rugged, relatively inexpensive and sturdy, versatile, ideal for general purpose use, uses simple design with few moving parts, better suited to handling high volume levels, resistant to moisture, no internal amplifier and do not require batteries or external power.
- Preamplifier – a device typically used along with sound equipment to help improve the overall quality of sound where it precedes another amplifier to prepare an electronic signal for further amplification or processing
Circuit Explanation As the dynamic microphone produces 200 to 600 ohm output impedance, it consists of discrete amplifier that runs on 3 stage of amplification with gain control. The gain is a measure of the ability of a circuit or an amplifier to increase the power or amplitude of a signal, usually defined as the mean ratio of the signal output of a system to the signal input of the same system. The transistors utilized was BC547 with 150ºC junction temperature and through hole package, but other types of transistors may also be used. With the first stage functioning on Q1 via the common base configuration, the transistor enhances the overall signal to noise ratio and conducts at low noise levels. While a direct coupled amplifier comprises of Q2 and Q3. The circuits gain is reduced to zero by the 47K ohm preset resistor.
The roll off frequency provided by the circuit is at 30 Hz while the frequency response is at 100 KHz. Roll off is defined as the Gradually increasing loss or attenuation with increase or decrease of frequency beyond the substantially flat portion of the amplitude-frequency response characteristic of a system or transducer. The first stage on Q1 has a gain of around 4.7 from the ratio of Rc & Re but will be reduced when supplying into the impedance of Q2 and Q3 with a shunt resistor. The amplifier gain from Q2 and Q3 is based on the ratio of Rc & Re while Q3 conducts on as emitter follower. As 12 V is supplied to the base voltage, the collector current of Q1 is 200 uA during the first stage.
The presence of minimal gain by setting the collector voltage of Q1 to half the supply voltage is because the signal from a dynamic microphone is normally low in about 10 mV. When the supply voltage is biased by half it allows the highest overload margin and maximum voltage swing. As Q1 operates with a collector voltage of 2.4 V and low collector current, it raises the input impedance to around 400 and ensures low noise performance of the stage. This is better since the dynamic microphone operates at 200 – 600 ohms.
The composite amplifier is on the last stage where Q3, designed for maximum voltage swing, operates in emitter follower, buffering the output and has a low impedance output, suitable for driving long cables, if necessary, while Q2 operates in common emitter and provides the voltage gain. The changes in temperature and current gain variation is stabilizing the biasing to a certain degree.
Application
Most dynamic microphones have a limited frequency response, which makes them well-suited, along with their ability to withstand high sound pressure levels, for loud guitar amps, live vocals, and drums
The preamplifiers may be mounted within or near the signal source such as turntable, microphone or musical instrument like electric base or electric guitar and can associated into the chassis or housing of the amplifier they feed or in a separate housing. They are also utilized in a foil electret microphone, can be a switched capacitor circuit used to null the effects of mismatch offset in most CMOS comparator based flash analog to digital converters, a circuit inside of a hard drive connected to the magnetic heads or the circuit inside of CD/DVD drive which connects to the photodiodes, a masthead amplifier used with television receiver antenna or a satellite receiver dish, as part of a stand-alone channel strip or channel strip built into an audio mixing desk, a stand-alone unit for use in live music and recording studio applications, and used at the first stages of an instrument amplifier. Source:www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm
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