Automatic amplifier input selector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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This device can work as part of a sound amplifying complex or as a separate attachment to any amplifier. It automatically connects one of the four inputs to the amplifier when an audio signal with a level of more than 60 mV appears at this input. This makes the radio more convenient to use and eliminates the need for input switches.
The circuit of the device does not distort the audio signal, since its switching is carried out by polarized relays K1 ... KZ type RPS32 or similar, with two groups of switching contacts. This allows the circuit to be used in high-quality radio equipment. The second distinctive feature of the above circuit is unipolar power supply, as well as low current consumption. Polarized relays do not require a constant power supply for latching contacts and in this circuit consume energy only at the moment of switching a group of contacts to select an input with a signal source.
The selector circuit (see Fig. 1.10) is assembled on three microcircuits, one DD3 transistor matrix and three relays (K1 ... KZ). It consists of four comparators on the elements of operational amplifiers DA1, from the outputs of which a signal with a logic level is fed to one of the triggers on DD1. At the moment of switching the corresponding trigger on the positive edge of the voltage drop, the differentiating circuit from the capacitor C9 ... C12 and the resistor R14 ... R17 generates a pulse, which is amplified by one of the transistors of the matrix DD3. This pulse switches the relay contacts to the desired position.
At the initial moment of powering up the circuit, even if there are no input signals, input X1 will always be connected - this provides the pulse generated by the circuit R13 and C13 at the moment of switching on. It is better to connect the signal source that is most often used to this input.
When setting up the circuit, it is necessary to set the response threshold of the comparators DA12 with resistor R1 so that in the absence of input signals at the outputs of the comparators there is a logical "0".
When using polarized relays with a low voltage supply, for example, from the RPS45, RPS43 series, the supply voltage of the circuit can be significantly reduced. The circuit uses a relay type RPS32B RS4.520.224.
The second selector circuit shown in fig. 1.11, is designed to work with two inputs and contains two microcircuits and one polarized relay RPS45 RS4.520.755-08 (or RS4.520.755-18) with a nominal winding voltage of 6,3 V (in fact, it operates at a lower voltage).
The circuit can be powered by any source with a voltage of 4,5 ... 9 V and can be used even in portable radio equipment. The current consumption by the circuit does not exceed 3 mA, and without the use of LEDs for indicating the working input HL1 and HL2, it will be even less. The use of a connected input indication is not necessary, and they can be excluded from the circuit if it is powered by a self-contained source.
The principle of operation of the circuit and its settings are similar to those shown in Fig. 1.10. Since the DA1 chip, due to internal resistance, cannot provide zero voltage at the output, the circuit is made on diodes VD2 and VD3 to offset the supply voltage of the logic chip DD1.
The operational amplifiers used in both circuits can be replaced by 140UD6 (with the appropriate number), but this will increase the current consumption and the dimensions of the device.
Resistors and capacitors are suitable for any type, small-sized.
Rice. 1.10 (click to enlarge)
Rice. 1.11 (click to enlarge)
Publication: cxem.net
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