Remote sound probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When designing and assembling sound probes, both in the traditional minimum configuration and in the extended one, very often little attention is paid to its parameters such as input capacitance and input impedance. You can improve these parameters for previously assembled probes by supplementing it with a simple transistor device, which is proposed in this article.

For the repair and preliminary adjustment of various sound-reproducing equipment, for example, tape recorders, radios, telephones, intercoms, simple audio frequency amplifiers are often used, usually having an output power of 0,1 ... 4 W and a maximum sensitivity of 30 ... 100 mV. This simple device, usually assembled on widely used microcircuits, for example, K174UN7, K174UN22, TDA2822M, KA2209 or several transistors, is in the arsenal of every radio amateur almost from the very first steps in electronics. Some time after the assembly of such a probe, its modest properties no longer satisfy the radio amateur, and he gradually supplements it with new functional devices - amplitude and several frequency detectors (10,7, 5,5, 6,6 MHz), an arrow logarithmic signal level indicator and etc.

In cases where the probe needs to control the presence and passage of a low-amplitude audio signal in high-impedance or especially specific circuits - in microphone and corrective amplifiers, active and passive filters - the parasitic capacitance of the connecting cable and the first stage of the ultrasonic frequency converter is even just a few tens or hundreds of picofarads, relatively low input impedance can not only distort the test results, but also lead to the complete inoperability of the tested node. To solve these problems, I propose to assemble a simple prefix for a sound probe, made in the form of an active remote probe.

The diagram of the device is shown in the figure. It has an input impedance of more than 5 MΩ and an input capacitance of less than 3 pF (for an input signal amplitude of less than 7 V).

(click to enlarge)

The device is a single-stage inverting amplifier, made on a diffusion-planar field-effect transistor VT3 with an insulated gate and a depleted n-type channel. To improve the load and frequency characteristics of the node, the field-effect transistor is paired with an emitter follower on an n-p-n bipolar transistor VT4. The audio frequency input signal, passing through the separating protective circuit C1R2R1R3C2, enters the gate of the field effect transistor VT3. The voltage gain depends on the ratio of the resistance values ​​of the resistors R5 / R6 and the parameters of the field-effect transistor and can reach 6 ... 9 with open contacts of the switch SA1. Bipolar transistors VT7, VT12 connected in anti-series and operating as a micropower low-capacity two-anode zener diode with an opening voltage of 1 ... 2 V protect the field-effect transistor from breakdown by high input voltage. The R8C4 power filter prevents hum and self-excitation.

On the resistor R9 and the LED HL1, a node for indicating the supply voltage is made. If necessary, with a tuned resistor R7, you can set an integer input voltage transfer coefficient, for example, equal to 5, which can be convenient if any calculations are involved or an oscilloscope is connected to the probe output.

You can use any small-sized fixed resistors, for example, S2-23, MLT, S1-4 or similar imported ones with a dissipation power of at least 50 mW. Trimmer - SPZ-28, RP1-63. If the adjustment of the input signal gain is not needed, then it is desirable to replace it with a constant of the same resistance. Non-polar capacitors - any small-sized ceramic or film, for example, K10-17, KM-5, K73-9, K73-24. If the probe is used to check the performance of lamp stages, then capacitor C1 must be at least 250 V. Oxide - K53-19, K50-35.

The field-effect transistor is suitable for any of the KP305, 2P305 series, including in a frameless version. It should be noted that 2P305 transistors at room temperature withstand the gate-source voltage of ±30 V, which is twice as much as that of KP305. When mounting this transistor, be sure to take measures to protect it from breakdown by static electricity. Transistors VT1, VT2 - any of the KT315, KT312, KT368 series. The KT3102G transistor can be replaced with KT3102, KT6113, KT3130, SS9014, 2SC1222, 2SC1845 with any letter index. A green LED with a glow brightness of up to 15 mcd of a rectangular shape (5,0x2,5x9,6 mm) can be replaced with any similar one, for example, L-383SRWT, L-362GD, L-1513SURS, L-1543SGS, L-934SRD / J , AL307PM, AL307NM. If desired, the indication of the supply voltage can be omitted.

Switch SA1 with position fixing, it is desirable to use a miniature design, for example, from the mode switch of an imported car radio.

The remote probe is connected to the sound probe with a two-core shielded wire, a stabilized supply voltage of 12 ... 15 V is supplied through one of the cores, a signal through the other, and the braid is used as a common wire. The adjustment consists in selecting the resistor R6 until half the supply voltage is obtained on the resistor R7.

All design details can be placed in the case from the used stationery marker AD8004, AD6002. Mounting is carefully shielded with brass or several layers of aluminum food foil. The length of the unshielded part of the brass probe tip must not exceed 15 mm. Solder a piece of flexible stranded insulated wire 25 ... 35 cm long with a crocodile clip to the common wire of the probe. If desired, and the capabilities of the radio amateur, the design can be supplemented with another switch (not shown in the diagram), with which it will be possible to switch the input signal bypassing the nodes of the active probe, which can be useful when the input signal amplitude is more than 1 V.

Author: A.Butov, p. Kurba, Yaroslavl region

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