The circuit described here is based on the superior characteristics of dual-gate MOSFET (metal-oxide semiconductor field-effect transistor). It exhibits a very high input impedance that lends for good sensitivity and very less loading of the input signal source. Low cross-modulation characteristic leads to minimal distortion of the output with respect to the input signals. Also, the MOSFET offers low feedback capacitance and high transconductance. All these advantages make the MOSFET the most effective for high-quality mixer and converter applications. This dual-input audio frequency mixer circuit employs a single dual-gate MOSFET 3N200. One may, however, substitute it with any other dual-gate. MOSFET such as 3N187 and BF966. (It is to be noted that BF966 is not gateprotected and hence calls for suitable precaution in handling it.) The audio frequency (AF) input from the first channel (CH1) is applied on gate 1 (G1) of the MOSFET through 500- kilo-ohm potentiometer VR1. The AF input from the second channel (CH2) is applied on gate 2 (G2) of the MOSFET through another 500-kilo-ohm potentiometer VR2. Potentiometers VR1 and VR2 serve as gain controls for the mixer inputs.
Circuit diagram :
Dual-Input High-Fidelity Audio Mixer Circuit Diagram
Gate 1 receives the negative bias resulting from the voltage developed by the current passing through resistor R1 that is in series with the source. Gate 2 receives the positive bias produced across resistor R3 by the voltage divider formed by resistors R3 and R4. The mixed common output signal developed across drain load resistor R2 is coupled to the output through capacitor C5. This output can be, in turn, fed to any audio amplifier system for further amplification. The input impedance at each signal input is approximately 500 kilo-ohm, which is determined largely by the resistance of potentiometers VR1 and VR2. Higher input impedance may be obtained by substituting higher-resistance potentiometers, but this will lead to the pickup of stray signals.
The current drain of this circuit at 6V DC is less than 3 mA. The open-circuit voltage gain is 10 for each channel. The maximum amplitude of input signals at gates G1 and G2 is 0.1V RMS. Signals of higher amplitudes are reduced by the adjustment of potentiometers VR1 and VR2, hence evading the output signal peak-clipping. The corresponding output signal amplitude is 1V RMS. The entire circuit can be built on a general-purpose PCB or veroboard. The complete assembly is shielded using a metal container. The two input jacks should be fixed on the opposite sides of the container against the output jack. This simple circuit can be utilised for various combinations of devices at the input end. A few examples are two microphones, two audio players, or one audio player and one microphone, etc.
Note. Adequate precautions should be taken to prevent the destruction of MOSFET due to static electricity. The use of a grounded tip for the soldering iron is recommended.
Author : Prasad J. - Copyright : EFY
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