This audio mixer circuit schematic we propose here is built around four amplifiers, which are current controlled, all of them incorporated in IC SSM2024, produced by Precision Monolithics Inc. (PMI). To obtain a small offset and a high rejection, the four mixer entries must have a 200 Ω impedance. Those impedances are obtained with resistors R5-R8, which belong to the voltage divider from every audio input.
With the values from the diagram, the input signal is 1V (0 dBV). At this level, the distorsions are about 1%, and at lower levels do not exceeds 0.3%.
The gain of the current controlled amplifiers (CCA) is determined by the current supply of the control inputs. At 10 kΩ for R1-R4, CCA’s are blocked if the control voltage is under 200 mV. Maximum gain is obtained for a control current of 500 uA, in this case the input voltage is increased with 0,5 V.
The current consumption is between 5…9 mA. Signal/noise ratio is 90 dB, and bandwidth is 130 kHz. SSM2024 wirks pretty well and power supply voltages between +9 V and +18 V, but best performances are obtained at +-15 V.