STRENGTHEN THE VOICE OF THE HANDSWhen I needed a megaphone, I became acquainted with the prices of Chinese industrial products: as it turned out, they cost from 1500 to 8000 rubles., and found that such a device can collect a couple of hours with her hands. A megaphone (or loud speaker), an electrical circuit which recommend and bring forth, be useful for sports, in advertising, in conditions of constant excessive noise, playing games with the child and even in the minibus for broadcasting information to passengers in the rear seats.

The traffic rules in force today in Russia, not allowed to be used by drivers and passengers of loud-speaking devices (except the cars of operational services), so the use of the proposed device for any other purpose not prohibited by law, that is not outside but in the cabin of the car. I confess that specially planned the device as a megaphone for car travel in a big company in my minivan.
The schematic of the device is available for repetition not only radio Amateurs, but even the car owner with little experience in radio. Configure the diagram is not necessary, and with proper items and proper installation of the device begins to work immediately. Undistorted volume and a homemade is not inferior, this parameter is inexpensive industrial Chinese counterpart.
Distinctive features of the proposed development are: high gain, simplicity and reliability, its focus on Autonomous operation (from batteries with a total voltage of 12V), no bipolar power supply.
So, the megaphone is a graph repeatedly reinforcing (hundreds of times) level of the input signal. Let us consider the principle of work itself and the electric circuit device (Fig. 1).
DA1 chip is a power amplifier with an output power of 2 W at 4 Ohms to work in the sound paths of the instrument and included according to the classical scheme.
However, since IC output amplified signal is supplied to “rock” transistor cascade amplifier has higher output power, reaching, according to my data, a 4.5…4.8 W at a supply voltage of 12 V. This power is enough to, not straining voice, to lead tours on the street or voice to inform the attendees, located in the rear seats of a minivan or minibus.
Fig. 1. Electric diagram of the device
Fig. 1. Electric circuit device
On Vysokomol input circuits is connected a highly sensitive microphone, taken from an old cell phone Nokia3310 (however, in appearance and fit microphones from other cell phones, for example, microphones LG KP500, KP501, KC910, KF510, etc.). Can fit electret type microphones MKE – in this series a few models, but the most popular FEM-3).
As can be seen from the diagram, the device is implemented on a single chip, the amplifier К174УН5, resulting in signal amplification (lossless quality).
The IC output is connected to the output stage to the complementary pair of silicon transistors, which, in this design, provides a ten-fold signal amplification AF (possible replacement transistors shown in Fig. 1). The oxide capacitor C3 (at the operating voltage 25 – 35) is necessary to avoid component and a DC voltage on the dynamic head (protects it during peak signal values).
The capacitance of the capacitor is not advisable to change lower than 200 UF, since this depends on the maximum amplitude of the AF signal at the output of the amplifier. The role of C3 works well with the oxide capacitor Tesla.
Chain R2С2 – compensation; it prevents distortion of sound.
However, when you first turn a radio Amateur himself may decide, based on the characteristics of the use of loudspeakers and transistor pairs are there in this circuit the need. Thus, specified the RC-chain can be without consequences be deleted from the scheme.
In practice it turned out that it is possible to provide smooth adjustment of the volume level (power) of the device, using instead of resistor R3, variable resistor 22 to 33 ohms In series (with linear characteristic). Pin 3 of the chip К174УН5 in this case is connected to the middle output of the variable resistor.
Changing the capacitance of the capacitor C1 in the range 0,05 – 1,5 µf, within a small range to adjust the sound volume and tone of the signal. Its tolerance of a possible 10-15 %.
DA1 chip must be mounted on the heatsink.
I do not recommend this device to Orient on a network power supply, since the scheme was originally designed for Autonomous battery power for a portable device voice amplification. Moreover, the chip К174УН5 not protected from accidental increase of the supply voltage operates at a voltage of 12 V ± 10% (max voltage 13.2 V; it is a long time, this chip also will not stand). But it has a great economy, why he was chosen for such an unusual scheme with independent power supply from batteries or rechargeable batteries; current consumption К174УН5 only 30 mA.
Fig. 2. The electrical circuit microphone amplifier-adapter for dynamic earpiece with impedance of 70...200 Ω and above
Fig. 2. The electrical circuit microphone amplifier-adapter for dynamic earpiece with impedance of 70…200 Ohm and above

Is defined in the diagram the dynamic head will fit modern: YDP5090-11, speaker ВС25ЅС55-04 or who have, the old “Soviet” 6ГДВ-5D.
The same circuit can be used not only high-sensitivity condenser or electret microphone, but also dynamic capsule, for example, the type of DAMS. The electrical circuit adapter for dynamic capsule shown in Fig. 2.
The output of this microphone amplifier on the VT1 transistor should be connected in the gap of the conductor extending from the regulating resistor R3 and the capacitor C1.
Elements (R1 and C1 in this case, from the initial scheme (Fig. 1) are removed. Transistor VT1 is selected with a large amplification factor and transmits the signal about 40 dB when used in conjunction with primer type DAMS.
Is specified in the scheme of the transistor can be applied to the transistors КТ373А, КТ342В, КТ3102А. Oxide capacitors C1 – C3 in this scheme, apply to any working voltage.
During the soldering/mounting chip use caution; do not overheat insights. Manufacturer К174УН5 advises to maintain the duration of the soldering output no more than 3 seconds, the interval between neighboring rations conclusions – 10.
The length of conductors from the pins of the chip should aim for the minimum to reduce the effect of spurious relationships.
A. KASHKAROV, St. Petersburg

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