Mixer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Musician

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A simple stereo mixer with several inputs and different sensitivity is useful for creating soundtracks for videos. The preference for transistors over microcircuits is given mainly in order to minimize the block's own noise.

Fig. 1

The mixer is made on transistors (see figure). It has a 5 mV sensitivity input with tone controls on each channel; two inputs with adjustable sensitivity, variable in the range of 5 mV ... 500 mV, also with tone control; three stereo inputs with a sensitivity of 500 mV without frequency correction.

When developing the unit, the main goal was to minimize the noise level; managed to achieve a level of about -75 dB. As far as I know, it is very difficult to achieve such a result from microphone inputs to an op-amp. The mixer described in [1] is taken as a basis. Mic amplifiers have added mitter repeaters to match subsequent tone controls.

The diagram shows only the left channel, the diagram of the right one is identical. Common to both channels is a stabilized 12 V power supply (based on the LM7812 chip) made according to a typical circuit. To introduce the "Mono" mode, sources of two-channel phonograms can be connected through inputs 4 and 5 or 5 and 6. If necessary, two stereo channels of the mixer can be combined (mixed at the connection point of capacitors C8, C9.

About the details. In order to reduce noise, oxide-semiconductor capacitors K53-1 or K53-14 are used. Transistors - KT3102B or KT3102G or imported low-noise transistors, for example, VS547 Variable resistors in the tone control with group B characteristics. The rest of the variable resistors are imported, with group A characteristics. SHURE 48 microphones are used with the mixer, you can MD-85. Connectors: input mono - sockets for plugs with a diameter of 6,3 mm (6 pcs.); stereo and output - RCA ("tulips").

During installation, everything is screened if possible, especially cases of variable resistors. For a mixer, it is better to take an all-metal case. It is also desirable to use the power supply in a separate case; its output voltage is 12 V with a low ripple level at a load current of at least 20 mA.

Setting up the device is not required, but it is desirable to control the shape and amplitude of the sinusoidal signal at the output, as well as the identity of the parameters of the right and left channels, by applying a signal with a frequency of 1 kHz to the inputs. When applying a signal with a voltage of 5 mV to the microphone and 500 mV to the line inputs, there should be no clipping of the output signal; the voltage at the line output should be about 500 mV.

The mixer can be supplemented with an amplifier for headphones (not necessarily a low-noise one and a level indicator, made according to the diagrams of the corresponding nodes published in the magazine [2-5] The number of inputs can be increased.

The output impedance of the mixer allows you to connect any amplifiers with the appropriate sensitivity to it (even several amplifiers in parallel in the absence of their mutual influence).

This mixer has been used many times to mix signals from a synthesizer and several microphones - it turned out very well.

Literature

1. Nechaev I. Multi-channel mixer. - Radio, 1994, No. 1, p. 31,32.
2. Sapozhnikov M. Headphone amplifiers. - Radio, 2004, No. 1, p. 15,16.
3. Kuznetsov E. Amateur mixing console. - Radio, 2003, No. 3, p. 10-12.
4. Kuznetsov E. Sound signal level meter with a measurement range from -40 to +3 dB. - Radio, 2007, No. 7, p. 17-19.
5. Logarithmic level indicator. - Radio, 1983, No. 9, p. 61.

Author: A. Kasatkin, Moscow; Publication: radioradar.net

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