Tuning AF power stages

B Vogel 75tuning AF power stages Simple, economically priced audio output stages, such as, for instance, those using the hybrid ICs in the STK series, may be improved in a simple manner as regards distortion, noise, and off-set voltage. To this end, the output amplifier is included in the feedback loop of an op-amp. Fig. 1 shows the set-up for inverting output amplifiers, and Fig. 2 that for non- inverting ones (the normal situation). In the calculations to arrive at the new gain of the output amplifier, determined by Ri and R2, it is assumed that the LF356 provides an undistorted signal of 5 Vrms; note also that this type of op-amp must work into a load of not less than 5 kilo-ohms to prevent distortion. For an output power of 50 W into 4 ohms, the output stage must pro- vide a voltage, U =PR = 14.2 Vrms. If the amplification of the stage is 3, the op-amp should deliver 4.73 V. For the set-up in Fig. 1, the value of R2 is then R2=3111, while for that in Fig. 2, R2= 2Ri. Note that...