Microphone without screens and noise. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Microphone without screens and noise. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Microphones, radio microphones

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A difficult problem is the sounding of mass sports, entertainment or school events. You either have to supply 220 V directly to the speakers, or put up with long shielded cables, which are expensive, fragile and, in the end, do not completely save you from vodkas and interference.

The proposed scheme is suitable precisely for cases when it is necessary to quickly organize sound, using any available wires for the microphone (even single insulated conductors lying separately under the feet of the children!). You can "on the go" twist broken wires, even the wires going to the microphone - nothing will happen! When the wires break, a powerful hum does not pass to the ULF input. A low supply voltage cannot harm anyone.

The device circuit (see figure) contains two blocks - a remote microphone signal amplifier and a power supply, signal extraction and protection of the ULF input from emergency situations. The remote amplifier is built without the use of unreliable parts - capacitors. to amplify the signal, a ULF microassembly on field-effect transistors is used, operating with zero bias voltage at the input. Due to the specific properties of the assembly transistors, the output voltage is equal to half the supply voltage, it is this property of the microassembly that makes it possible to power and transmit an amplified sound signal over one pair of wires with an extremely simple circuit.

Microphone without screens and noise

Microphone B1 is connected to input DA1 through resistor R1. This allows you to connect and even solder a microphone cable (short) to the circuit without damaging the microassembly. Diodes VD1,2 serve as protection of the input from overvoltages. The amplified DA1 signal is fed to the base VT1, which is turned on by an emitter follower. the bodies of both reinforcing elements are connected to the "body" of the device, which is convenient for installation. From the emitter VT1, an amplified signal is fed into the line. Resistor R2 and the zener diode provide power to the microassembly, and the zener diode VD4 is necessary to match the potentials. Recall that the resistance of zener diodes in the open state for an alternating signal (the so-called differential resistance) is small, much less than the resistance to direct current.

The VD6 diode serves to protect against reversed polarity of the wires (there will simply be no sound). At the end of the line near the ULF (maybe even built into the ULF) there is a second block. Power is supplied to the line through R3. The variable (sound) part of the signal passes through C1 to the volume control R4. Diodes VD6, VD9 limit the signal at the moments of switching, as well as during a short circuit or line breaks. The zener diodes VD6, VD7 open only in the event of a line break and shunt the variable component of the pickup on the line (only a short click passes in the ULF).

The flattest box is obtained by mounting parts on a board made of foil fiberglass without drilling holes. The tracks are cut with a cutter from a hacksaw blade and tinned. The parts are pressed against the board by the housings. The terminals connected to the body are not insulated, they also hold the part. Conclusions 3, 4, 7 of the microassembly and B, E of the transistor are isolated. A soundly assembled board can withstand the weight of a person without damage! A ready-made case or a self-made one made of tinned sheet can serve as a screen - conductors are well attached to it and soldered. If someone wants to increase the linearity of the gain (signal quality), this can be done by adding to the OOS circuit for alternating current - a chain of series-connected 100 kΩ resistor and a 0,1 μF capacitor, which must be connected between the common point VD4, VD3 and pin 7 DA1.

By selecting the resistance of an additional resistor, a suitable output signal level is set. Reducing the gain will reduce distortion, but remember that an overly signaled signal will be more prone to pickup, so you should choose a "golden mean" so that hum or other interference is not heard in the pause. You should not connect the body of the remote unit with any metal structures - an alternating current background and distinct crosstalk may appear.

See other articles Section Microphones, radio microphones.

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