This FET audio mixer is an example of FET’s versatility. FETs are originally designed for high frequency applications but they can be used for audio frequencies and in fact they perform excellently in this area.
This mixer needs only four FETs and performs very well (actually you cannot expect a high quality mixer).
The mixer’s input impedance is determined by the resistance of the input potentiometers used, since the impedance of the FETs is very high.
The number of inputs that can be connected to the circuit is unlimited as long as the R1 value is chosen according to this formula: R1 = 22K / n, where n = the number of inputs.
The frequency response of the audio mixer is from 20 Hz to 80 kHz and linear within 3 dB
Audio Mixer Components
C1 = 0.1 uF/50V ceramic
T1 … T4 = 2N5397, 2N5398, 2N5486, MPF102, MPF106