Sometimes the phone does not suit the consumer: bad sound and audibility, noise, etc. To improve the characteristics of the device by collecting offer conversation node.
Diagram of the device shown in the figure.
(click to enlarge)
Quad operational amplifier DA1 eats bipolar voltage generated from one polar cascade on transistors VT5, VT6. At the operational amplifier DA1.1 and the transistor VT1 is assembled the microphone amplifier, and DA1.2 and VT2, VT3 - amplifier audio line. For protivoglistnoe system antiphase audio signal DA1.2 is not taken from the output of the transistor VT1, as is done in the most simplest speaking of knots, and is formed by a separate amplifier DA1.3. This is achieved good quality work protivoglistnoe system.
The cascade at the shelter DA1.4, the diodes VD2, VD3 and the transistor VT4 is designed to master gain control DA1.2. If the microphone signal is sufficient level, strengthening Oh DA1.2 is reduced, which provides an even greater suppression of his voice.
At a very high level of the signal at the noninverting input of op-amp DA1.1 linearity work DA1.1 and DA1.3 is deteriorating and depth of the suppression of his own voice reduced. To compensate for this effect the sensitivity of the amplifier at the shelter DD1.2 you need to downgrade. This makes the field effect transistor VT4. The channel resistance the source-drain is greatly increased, and strengthening Oh DA1.2 is determined only the ratio of the resistance values of the resistors R25 and R21.
The switch SA1 is designed to reduce the sensitivity of the microphone the amplifier. The switch SA2 is possible to gradually reduce the volume of the signal in the handset.
Electret microphone WM - FEM-389, FEM-332, FEM-377, M34LOR or NM similar. Diodes VD2, VD3, VD6 - any low-germanium or silicon, VD4 and VD5 - low-power silicon. The Zener diode VD1 - any low-power on voltage 15...30 V.
Transistor VT1 can be any of the series KT815, CT, CT, CT, with CT the current transfer ratio base 150...200. Field-effect transistor VT4 - any of series K, K, K, but it is necessary to test for voltage cutoff, which should be no more than 1 V.
To reduce the interference cost with conversational node is better positioned inside handset and use the screen of copper foil.
The establishment of the node is to select some resistors that affect certain parameters. Below lists the reference designators of these resistors and the parameters they affect, and the nominal values of the parameters:
- R3 - depth suppress their own voice;
- R7 - sensitivity microphone amplifier;
- R9 - sensitivity microphone amplifier;
- R12 - trigger system desensitisation phone amplifier;
- R14 - voltage supply conversational node (8... 11);
- R21 - volume with decreased sensitivity of the telephone amplifier;
- R27 is the volume of the input sound signal;
- R29 - symmetry of the shoulders of the bipolar source voltage;
- R31 - forced decrease the volume.
Author: A. Butov, S. CORBA Yaroslavl region