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The acoustic mixer is a device that allows us to record on a tape a mixed literary-musical program - for example, a message with a musical background, the transition from one melody to another by overflowing and changing the volume, etc. It is therefore a miniature DJ's device, similar to those used in radio stations and discos. The mixer can be used not only for recording on a tape recorder, but also for live transmission.
The device consists of three single-stage transistor amplifiers with a common load. Therefore, it can be used for mixing by summing three audio signals, the signal from input 1 (resistor 220 ohms), designed to attach a dynamic microphone, is amplified, and the other two identical inputs 2 and 3, through which can be include as sources a tape recorder, gramophone, tuner or crystal microphone, partially suppressed. As a result, signals of approximately equal amplitude are output.
The electrical circuit of this home mix console is charged with rechargeable batteries from R6 type with 1,5V each, for which i have not assumed a place in the circuit board. With this solution it is practically removed the opportunity for transferring parasitic voltages between the attached devices to the mixer (like the audio power amplifier, which can be found in m profile) and also blocks the incoming low-frequency noises from the main power supply.
As the current consumption does not exceed 0,5 mA, one set of battery works for a very long time. The use of transistors instead of operational amplifiers provides low noise coefficient.
Due to the small working space I have, I have simulated here the first transistor amplifier stage 1, which is designed for a dynamic microphone.
If you want to simulate the other two same stages (2 and 3), you can change the following things:
1. 220 Ohm Resistor to 430 kOhm;
2. Base capacitor from 1 uF to 680 nF;
3. For the base voltage devider, you can change 220 kOhm resistor to 200 kOhm;
4. Emitter resistors are 1,2 and 3,9 kOhms;
This project can be combined with the Universal Low Frequency Amplifier in my projects!
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