A pickup is an essential device used to capture sound from a specific environment, functioning as an electroacoustic transducer that converts sound vibrations into electrical signals. It serves as the first stage in a sound monitoring system and plays a crucial role in determining the overall audio quality. Typically, a pickup has three wires or a binding post, which includes the positive terminal of the power supply, the audio signal output, and the common ground. The rear-end equipment usually consists of a digital video recorder (DVR) or a recording device. The pickup is generally powered separately, with the remaining two wires providing the audio output connected to the input of the recording device.
To ensure optimal performance, it's recommended to use RVVP audio shielding cables rather than network cables, as the latter can introduce interference. Additionally, a regulated power supply, such as a transformer with a copper coil, is preferred over a switching power supply. A switching power supply may significantly degrade the sound quality due to noise and instability.
The following circuit is designed for monitoring pickups and is widely used in practical engineering applications. Unlike standard amplifying circuits, this design minimizes the impact of cable length, distributed capacitance, and input impedance on the pickup’s performance. Through careful engineering, the circuit offers low noise, excellent sound quality, low power consumption, and high stability.
**Circuit Structure and Function**
The entire circuit consists of a power supply section and a main amplification circuit. The main circuit includes a microphone input stage, a voltage amplifier, a current amplifier, and a filtering stage. The power supply circuit uses a three-terminal voltage regulator, 78L09, to provide a stable and clean power source. Capacitors C1 and C2 are used for filtering, while LED1 and resistor R1 serve as a power indicator. A diode, VDI, is included for reverse polarity protection.
The microphone input stage uses an electret microphone (MI CI) along with biasing components like R2, C3, C9, Rpl, RIO, R9, and C8. This stage is followed by a voltage amplifier built around operational amplifier IC2A, with resistors Rp3, R4, C10, and R11. A current drive stage is formed using IC2B and its surrounding components, including R12, R13, R14, R5, R6, and C5. Finally, a filter circuit composed of R6, C6, R7, C7, and R8 ensures a clean output signal.
One key aspect of the design is the microphone bias circuit, which is critical for reducing noise. Instead of directly connecting the microphone to the power supply, this circuit uses an RC filter (R2, C3, C9) to minimize noise coupling. The microphone is placed close to the amplifier input to further reduce interference. Adjusting Rpl allows the microphone to receive a bias voltage between 2 and 3 volts, which also controls the circuit’s sensitivity.
This circuit is similar to S-type amplifier designs but with fine-tuned parameters. It enables the voltage-controlled amplifier to operate in a near-no-load state, allowing it to handle various loads without compromising sound quality. The voltage and current amplifiers are coupled via a Wheatstone bridge (R12, R13, R14, R5), ensuring minimal distortion even under challenging conditions.
In the voltage amplifier stage, IC2A acts as a non-inverting amplifier, presenting a lighter load to the microphone and reducing its impact. The current amplifier (IC2B) provides additional gain, and the bridge configuration helps maintain balance and stability. The choice of component values ensures that the circuit remains functional even when unbalanced.
Simulation results show that increasing R14 improves frequency response but reduces current gain. However, since the monitor circuit doesn’t require high power, this trade-off is acceptable. The output of the voltage amplifier remains stable, maintaining the circuit’s performance characteristics.
A two-stage RC high-pass filter (C6, R7, C7, R8) is added to remove unwanted high-frequency components, improving the signal-to-noise ratio. This filtering ensures cleaner and more accurate sound reproduction.
**Component Selection and Operation**
When building the circuit, attention should be paid to the following:
1. Use metal film resistors to minimize noise, especially in the bridge section.
2. Choose authentic NE5532 chips from the US SIG company for better performance.
3. Select high-quality electret microphones with good sensitivity and directional characteristics. Ensure correct polarity during installation to avoid poor performance, such as reduced range or excessive noise.
4. Use a double-sided PCB with large-area grounding for improved stability.
5. After assembling all components, test the circuit and adjust Rpl and Rp3 to achieve optimal signal quality. Avoid setting sensitivity too high, as this can lead to noise and interference.
This circuit is a reliable and effective solution for monitoring pickups, offering excellent sound quality and ease of use in real-world applications.
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