4-Channel Mixer

014 AUDIO & HI-FI The proposed mixer is designed around four current-driven transconductance amplifiers contained in a Type SSM2024 from Precision Monolithics Inc-PM!. To obtain a low off-set and high control rejection, the four inputs should have an impedance to earth of about 200 O. These impedances are obtained from resist- ors R5-R8 that also form part of a potential divider at each input. With values as shown in the diagram, the nominal input signal is 1 V (0 dBV). Distortion at that level is about 1% and at lower levels not more than 0.3%. The amplification of the current-driven amplifiers- ccAs-is determined by the current fed into the control inputs. These inputs form a virtual earth so that calculating the values of the bias resistors (to transform the inputs into voltage-driven inputs) is fairly simple. With a value for R1-R4 of 10 kg2, the CCAS are switched off if the potential at the control inputs is lower than 200 my. Maximum amplification is obtained at a dr...