Homemade dynamic noise suppressor

If audiophiles want to obtain high-quality sound playback effects, in addition to having high-fidelity audio equipment, they must also find ways to effectively suppress the noise of the sound source itself. Through many experiments, a dynamic noise suppressor was produced, which significantly reduced the background noise of sound sources such as tapes, records, and radio heads.

The circuit principle is shown in the figure below. The input signal is divided into two channels, and one channel passes through R3 and C6 to the output terminal. V2 is a junction field effect transistor, which is used as a voltage-controlled resistor in the circuit. Its source voltage is determined by the voltage dividing ratio of R5 and R6. C5 is a bypass capacitor, and fine-tuning W2 is used to adjust the gate voltage of V2 so that it is slightly reverse biased when there is no signal input. At this time, the resistance between the source and drain is about 100Ω, and the resistance is small, so C4 An AC path is provided through V2 drain-to-source and through C5 to ground. So C4 and R3 together form a simple RC high-frequency filter to attenuate the noise (mainly in the high-frequency band) in the output signal flowing to c6.

The other input signal is input to the base of V1 through C1, working in the common-emitter amplification state, and the static bias is determined by R1. R2 is the collector load resistor of V1, and W1 is the emitter resistor of V1, which is used to change the amount of negative feedback of the amplifier and control the gain. The output of V1 is sent to the rectifier circuit composed of D1 and D2 through C2. The rectifier output is connected to the gate of V2, and its output voltage controls the channel impedance of the high-frequency filter circuit.

When a weak signal is input, the output of the rectifier circuit is very small and has no impact on V2. The noise of the input signal is filtered out by the C4 and R3 networks. When the input signal level becomes larger, the rectified output increases, causing the V2 gate potential to decrease, blocking the C4 and R3 filter channels, reducing the attenuation of noise and the attenuation of high-frequency components. If the rectified output is large, V2 will be cut off, the filter channel will be completely cut off, and the output signal will not be affected by the filter circuit. In short, the noise suppression circuit has no effect on low and mid-range audio signals and higher-level high-frequency audio signals, but only attenuates the high-frequency components of weak signals to a certain extent while suppressing noise.

C3 is a filter capacitor, used to stabilize the output voltage of the rectifier circuit and prevent the V2 gate from suddenly dropping due to the influence of pulses, causing signal distortion. The capacity of C3 should not be too large, otherwise the voltage on it will not be able to keep up with changes in the input signal level. C1 and C2 are coupling capacitors. In order to improve the response speed of the amplifier to high audio signals, their capacities are used smaller. The action level of the noise control circuit is controlled by W1. When the boom is at the upper end, the action level reaches the minimum value.

This circuit has no special requirements for components. The V1 β value should be ≥70. In actual use, connect the input end of the noise suppressor to the input end of the sound source with a shielded wire. The best position of W1 can be repeatedly adjusted and determined. After determining the best position, fine-tune W2 to achieve the best effect.