Dynamic Microphone Preamp

Description: This is a three-stage discrete amplifier with gain control. Alternative transistors such as BC109C, BC548, BC549, and BC549C may be used with minimal impact on performance. The first stage, built around Q1, operates in a common base configuration. This configuration is generally not used in audio stages, but in this instance, it allows Q1 to function at low noise levels, enhancing the signal-to-noise ratio. Q2 and Q3 form a direct-coupled amplifier, similar to a previously designed microphone preamp. Since the signal from a dynamic microphone is typically low, often less than 10mV, setting the collector voltage of Q1 to half the supply voltage is not beneficial. In power amplifiers, biasing to half the supply voltage maximizes voltage swing and overload margin; however, with low input levels, any value within the linear operating characteristics is adequate. Q1 operates with a collector voltage of 2.4V and a low collector current of approximately 200µA. This low collector current ensures low noise performance and increases the input impedance of the stage to around 400 ohms, which is suitable for dynamic microphones with impedances between 200 and 600 ohms. The output impedance at Q3 is low, and the graph of input and output impedance versus frequency is provided. The overall gain of this preamplifier is approximately +39dB or about 90 times. The gain of the first stage Q1 is roughly Rc/Re, which is 4.7. This gain is reduced by the input impedance of Q2 and Q3, as well as the shunt formed by the 47k preset. The amplifier formed by Q2 and Q3 has a gain of approximately Rc/Re since Q3 is an emitter follower and has unity gain. The gain of Q2 and Q3 is roughly 10/0.47, resulting in an overall gain of 4.7 x 21 = 98. The circuit's gain can be reduced to zero using the 47k preset. The frequency response is flat up to 100kHz, with a low-frequency roll-off at 30Hz. Any amplifier will introduce its own noise into the signal, which can degrade overall performance. The noise of this preamp, measured with a 10k load resistor, is also documented. The first stage, Q1, is designed to operate with a collector current of 200µA. With 15k and 47k bias resistors and a 12V supply voltage, the base voltage is calculated as 12 x (15/(15+47)) = 2.9 Volts. The emitter voltage will be the base voltage minus 0.7V, resulting in 2.2V. For a collector current of 200µA, the emitter resistor is calculated to be 2.2/0.2mA = 11k, and a 10k resistor is used. Since Ic and Ie are approximately equal, the collector voltage is 12(0.2 * 47) = 2.6V. The last stage is a composite amplifier similar to a previously designed ECM preamp. Q2 operates in common emitter mode, providing voltage gain, while Q3 operates as an emitter follower, buffering the output and offering a low impedance output suitable for driving long cables if necessary. The last stage, Q3, is designed for maximum voltage swing, with an emitter voltage targeted around 6V. Variations in transistor parameters may result in measured voltages differing from calculated values. The collector voltage of Q2 is a base-emitter voltage drop higher, at 6.7V, and the collector current is set at (12 - 6.7)/10k = 530µA. This current is also the emitter current for Q2, leading to an emitter voltage of (0.53 * 0.47) = 0.24V. The base voltage will be higher by 0.7V. Since the base of Q2 is connected to the emitter of Q3, biasing is stabilized to some extent against variations in temperature and current gain. If a meter is available for testing transistor beta, the transistor with the highest current gain should be used for Q2.

This three-stage discrete amplifier is designed for optimal performance in audio applications, specifically for use with dynamic microphones. The use of alternative transistors allows flexibility in component selection while maintaining performance integrity. The common base configuration of the first stage minimizes noise, which is crucial for high-fidelity audio amplification. The direct coupling between stages Q2 and Q3 ensures that the signal remains intact and that the amplifier can handle low-level microphone signals effectively.

The gain structure of the amplifier is carefully calculated to provide a substantial increase in signal level while allowing for adjustment via the 47k preset resistor. This feature enables the user to tailor the gain to specific applications, accommodating a variety of input levels from different microphones. The overall frequency response of the amplifier is designed to be flat up to 100kHz, ensuring that audio signals are reproduced accurately without coloration.

The careful selection of biasing resistors and supply voltage ensures that the amplifier operates within its optimal range, providing low noise and high input impedance. The choice of an emitter follower configuration for the output stage further enhances the amplifier's capability to drive long cables without significant signal degradation. This design approach ensures that the amplifier remains stable and reliable under varying conditions, making it suitable for professional audio applications.

In summary, this discrete amplifier circuit effectively combines low noise performance, adjustable gain, and robust output characteristics, making it a valuable tool for audio engineers and professionals seeking high-quality microphone preamplification.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

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 47k preset. The response is flat to 100kHz, low frequency rolloff at 30Hz, a simulated plot is shown below: Any amplifier will add its own noise to the signal, degrading the overall performance. The noise of this preamp measured with a 10k load resistor is shown below. The first stage, Q1 was designed to operate with a collector current of 200 uA. With 15k and 47k bias resistors and a 12 V supply voltage the base voltage will be 12 x ( 15 / (15+47) = 2.

9 Volts. The emitter voltage will be the base voltage -0. 7 V or 2. 2 V. For a 200 uA collector current the emitter resistor will be 2. 2 / 0. 2 mA =11k. A 10k resistor was used. As Ic and Ie are approximately equal then the collector voltage is 12 (0. 2 * 47) = 2. 6 V. The last stage is a composite amplifier similar to my ECM preamp. Q2 operates in common emitter and provides the voltage gain while Q3 operates in emitter follower, buffering the output and has a low impedance output, suitable for driving long cables, if required. The last stage, Q3 was designed for maximum voltage swing, hence Q3 emitter voltage should be around 6 V.

Variation in transistor parameters however, means that measured voltages will be different to calculated voltages. Q2 collector voltage is a base-emitter voltage drop higher or 6. 7 V and collector current is set at (12 6. 7) / 10k = 530 uA. This is also the emitter current for Q2 abd hence emitter voltage will be ( 0. 53 * 0. 47) = 0. 24 V. The base voltage will be higher by 0. 7 V. As the base of Q2 is connected to the emitter of Q3, then the biasing is stabilised to a certain degree against changes in tmperature and current gain variation.

However if a meter is available for testing transistor beta, then the transistor with the highest current gain should be used for Q2.