Dolby noise suppressor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Dolby noise suppressor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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Dolby noise suppressor is one of the most effective noise reduction devices, however, it requires a hard binding of signal levels both in recording and playback modes.

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The device consists of an input emitter follower (V1), an active notch filter (V2) that suppresses the subcarrier frequency (31,25 kHz) when recording stereo broadcasts; adder (A1), regulation channel, control signal generation channel and electronic switch (V3), which, if necessary, can be replaced by a conventional mechanical switch.

In turn, the control channel consists of a high-pass filter R12C7 with a time constant t = 106 μs, a controlled attenuator R13V4 and a limiting amplifier based on the op-amp A2 and diodes V5, V6, and the control signal generation channel consists of an amplifier based on the op-amp A3 and a rectifier based on diodes V9, V10.

Instead of the KT342A transistors indicated in the diagram, other transistors of this series, as well as transistors of the KT3102 series, can be used in the noise suppressor. KP103K transistors can be replaced by transistors of this series with letter indices L, M, N; at the same time, it is necessary that the cutoff voltage of the transistor used in the controlled attenuator (V4) be at least 0,9 V. Instead of diodes KD512, diodes KD503, KD521, etc. can be used. Op-amps A1-A3 can be of any type, but with appropriate correction circuits.

Establishing a noise suppressor is reduced to setting the trimmer resistor R27 voltage at the gate of the transistor V4, equal to its cutoff voltage. To do this, set the resistor slider to the upper (according to the diagram) position, switch S1 to the Record position, and switch S2 to the "On" position, and an alternating voltage of 7,9 mV (-30 dB from the nominal, equal to 250 mV) with a frequency of 3 kHz is applied to the input. In this case, the signal voltage at output 1 should be equal to 22 mV.

By moving the slider of the resistor R27 down (according to the diagram), they find its position at which the voltage at output 1 barely noticeably begins to decrease. In this position, the engine is fixed.

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