Conversation node for a telephone set. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Conversation node for a telephone set. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Sometimes the operation of the telephone set does not suit the consumer: poor sound and audibility, noise, etc. You can improve the characteristics of the device by assembling the proposed conversation node.

The diagram of the device is shown in the figure.

(click to enlarge)

The quad operational amplifier DA1 is powered by a bipolar voltage generated from a unipolar cascade on transistors VT5, VT6. On the operational amplifier DA1.1 and transistor VT1, a microphone amplifier is assembled, and on DA1.2 and VT2, VT3 - an audio signal amplifier from the line. To operate the anti-local system, the anti-phase audio signal for DA1.2 is not taken from the output transistor VT1, as is done in most of the simplest conversational nodes, but is generated by a separate amplifier on DA1.3. This achieves a good quality of the anti-local system.

The cascade on the op-amp DA1.4, diodes VD2, VD3 and transistor VT4 is designed to control the gain DA1.2. If a sufficient level of signal is received from the microphone, the gain of the DA1.2 op-amp is reduced, which provides even more suppression of one's voice.

With a very high signal level at the non-inverting input of the op amp DA1.1, the linearity of the DA1.1 and DA1.3 deteriorates and the depth of suppression of one's own voice decreases. To compensate for this effect, the sensitivity of the amplifier on the op-amp DD1.2 must be lowered. This makes the field effect transistor VT4. The resistance of the source-drain channel increases greatly, and the gain of the op-amp DA1.2 is determined only by the ratio of the resistance values of the resistors R25 and R21.

Switch SA1 is designed to reduce the sensitivity of the microphone amplifier. Switch SA2 can step down the volume of the signal in the handset.

Electret microphone VM1 - MKE-389, MKE-332, MKE-377, M34LOR HM1003 or similar. Diodes VD2, VD3, VD6 - any low-power germanium or silicon, VD4 and VD5 - low-power silicon. Zener diode VD1 - any low-power voltage 15 ... 30 V.

Transistor VT1 can be any of the KT815, KT817, KT961, KT608, KT630 series with a base current transfer coefficient of 150 ... 200. Field-effect transistor VT4 - any of the KP302, KP303, KP307 series, but it must be checked for cut-off voltage, which should be no more than 1 V.

To reduce interference, it is better to place the board with the speaking node inside the handset and use a copper foil shield.

Establishing a node comes down to the selection of some resistors that affect certain parameters. The tags for these resistors and the parameters they affect, as well as the ratings of the parameters, are listed below:

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R3 - depth of suppression of one's own voice;

R7 - sensitivity of the microphone amplifier;

R9 - lowering the sensitivity of the microphone amplifier;

R12 - response threshold of the telephone amplifier sensitivity reduction system;

R14 - supply voltage of the conversational node (8 ... 11 V);

R21 - volume with reduced sensitivity of the telephone amplifier;

R27 - the volume of the incoming sound signal;

R29 - symmetry of the arms of the bipolar voltage source;

R31 - forced volume down.

Author: A.Butov, village of Kurba, Yaroslavl region

See other articles Section Telephony.

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