Narrow microphone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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To record the voices of birds, animals, the sound of the sea, etc., you need a microphone that has a narrow radiation pattern and effectively cuts off extraneous noise. The problem can be solved using the device described in this article.

The microphone contains a pre-amplifier, the output level of which is sufficient to connect it to a tape recorder. The directivity of the microphone significantly increases the ratio of the useful signal to acoustic noise at the amplifier input and allows you to amplify and record the sounds of distant sources with high quality.

A highly directional microphone consists of a dynamic microphone itself (MD-38, MD-45, MD-200) and a low-noise amplifier placed in a special cylindrical case. The characteristic of these microphones in the frequency range of 50...15000 Hz has a non-uniformity of 8...12 dB. With an output signal level of 0,2 ... 0,5 V, the required voltage gain of the microphone amplifier is 50 ... 55 dB, and the signal-to-noise ratio is no worse than 60 ... 65 dB. The coefficient of non-linear distortion - no more than 0,2%. The amplifier must have good temperature stability and consume a small amount of current from the power source, which is used as a battery or a battery of galvanic cells.

These requirements are met by the amplifier, the circuit of which is shown in Fig. 1. It is based on the playback amplifier from the "Mayak-001 - stereo" tape recorder. The first stage is assembled on a silicon low-noise transistor VT2, the second - on an operational amplifier (op-amp) DA1. The BM1 microphone is connected directly to the base circuit of the transistor VT2, which operates in microcurrent mode, which makes it possible to obtain the required signal-to-noise ratio.

A feature of the amplifier is the use of two independent OOS circuits. The first of them, consisting of R5, VT1, C2, R1, C1, provides temperature stabilization of the DC input stage operation mode, and the second (C3, R4) forms the required frequency response of the amplifier. The voltage transfer coefficient of the amplifier (50 dB) is approximately equal to the ratio of the resistances of the resistors R4 and R2, it can be changed by selecting one of them (for example, R4) with little or no change in the DC mode of the device. Capacitor C3 determines the upper frequency of the amplified signal, which is 15 kHz.

The divider R6 - R9 is used to create an artificial midpoint and supply the required bias voltage to the non-inverting input of the op-amp DA1 (pin 5). The R2C1 circuit determines the lower limit of the amplified frequencies, which is chosen around 20 Hz. From the output of the amplifier (pin 10 of the op-amp), the amplified signal is fed through the capacitor C7 to the level controller - a variable resistor R10, and from its engine - to the X2 connector. Pins 2 and 4 of the connector are the power switch. When a tape recorder is plugged into the jack, these pins supply power to the amplifier from a GB1 battery. The amplifier consumes a current of about 2,5 mA from the source, its performance is maintained when the supply voltage is reduced to 5 V.

The details of the preamplifier, except for the resistor R10, are placed on a printed circuit board (Fig. 2) made of one-sided foil fiberglass.

Resistors R1 - R9 - MLT, C1-4, C2-33, variable R10 - SPZ-4. Capacitors C1, C2, C4 - C8 - foreign, similar to K50-35; C3, C6 - CT1, CD. In place of VT1, you can use transistors KT3102 with letter indices A - B, D, KT342B, KT358D, and in place of VT2 - KT3107 with indices L, F, with slightly worse results - D, I, K. Op-amp DA1, except for that indicated on the diagram , K153UD2 is suitable, as well as KR140UD608, K140UD6, KR140UD708, K140UD7, taking into account the difference in the pinout. In addition, microcircuits, with the exception of K153UD2, have internal correction circuits, so you do not need to install capacitor C6.

The amplifier practically does not require adjustment, you just need to check the compliance of the operating modes shown in the diagram. Depending on the sensitivity of the microphone used, it may be necessary to adjust the gain by selecting the resistor R4.

The design of the microphone described in [1] is shown in fig. 3. Its base is a cylindrical case 1 with a diameter of 60 ... 65 and a length of 450 ... 600 mm, which is easy to glue from drawing paper.

To reduce the reflection of sound from the walls, the case is pasted over from the inside with a layer of foam rubber 2. The microphone capsule 3 is attached to the case with wire rings 4 and rubber extensions 5. An amplifier 6 is placed near the microphone, enclosed in a screen, for example, made of tinplate from a can of condensed milk. Under the amplifier there is a battery 10. The back side of the case is closed with a cover 7, on which the connector 9 and the variable resistor 8 (R10) are fixed. For ease of use, a handle-bracket 11 made of polystyrene 5 mm thick is attached to the case. A nut 12 is attached to it, with which the microphone can be mounted on a photo tripod.

A highly directional microphone allows you to record sounds from a distance of more than 100 m. Even better results can be obtained by changing the design of the microphone - placing it in the center of a parabolic reflector or additionally equipping it with a set of resonant tubes [2, 3].

In any design, increasing the range of the microphone will allow narrowing the bandwidth of the amplifier. On fig. 4 shows a diagram of an amplifier operating in the "telephone" frequency band - 280 ... 3400 Hz. It is assembled on two op-amps, which are part of the K157UD2 low-noise amplifier.

The cascades are identical and are inverting amplifiers connected in series. The lower limit of the passband of each of the stages of the amplifier is determined by the elements R1, C1 and R2, R3, C2, and the upper - by R4, C3 and R5, C4. Capacitors C5, C6 serve for the frequency correction of the op-amp, the divider R6R7 forms an artificial midpoint. Capacitors C7, C8 shunt the power supply circuits of the op-amp. The variable resistor R2 is a signal level regulator, with its help the gain of the device can be changed within 50 ... 64 dB.

Headphones with a resistance of 9 ... 1 Ohms can be connected to the output of the amplifier (pin 16 of the DA100 chip). With a supply voltage of 6 ... 9 V, the amplifier operates stably and the power released at the load is quite enough for listening. If another type of op-amp is used, a current-limiting resistor with a resistance of 5 ...

A drawing of a printed circuit board and a layout of elements on it are shown in fig. 5.

Capacitors C1 - C4 can be of the K10-17, K10-47, K73-5, K73-9, K73-17 series; C5, C6 - KT1, KD. As an op-amp, you can use KR1434UD1, which is an analogue of K157UD2, as well as K140UD20. In the latter version, the drawing of the printed circuit board will have to be corrected, not forgetting the current-limiting resistor at the output of the second op-amp (pin 10 of the K140UD20 chip). Resistor R2 - SP4-1, the remaining elements are the same as in the previous design.

This amplifier also does not require adjustment, you just need to make sure that there is zero voltage between pins 2 and 3, 5 of connector X2.

Literature

1. Vdovikin AI Entertaining electronic devices. - M.: Radio and communication, 1981.
2. Nikolaev Yu. Supersensitive microphone. - Radio, 1992, No. 10,
3. Makarov D. Spy passions. - Radio, 1995, No. 3, p. 40, 41; No. 4, p. 44-46.

Author: V. Mosyagin, Veliky Novgorod

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