The amplifier has 2 stages: voltage amplifier and current amplifier with each stage having a separate feedback loop. The motivation for removing the overall feedback loop is to reduce the overload of the input stage due to crossover distortion on the voltage amplifier, implemented using IRFP240/IRFP9240 vertical power mosfets biased at relatively low current (100mA). The crossover distortion occurs each time a power transistor turns on. and pushes the gain of the power stage down to cero (close to). When cascaded with the voltage gain stage, the gain is still cero. Independent of the amplifier bandwidth, feedback cannot correct the crossover distortion if the gain is cero. It is difficult to predict the output waveform because the circuit is time-varying. But single inspection indicates that the error signal produced at the input of the amplifier is very large. The resulting transient response at the crossover point shows a large overshoot and ringing that last a longer time than the charge time of the gate capacitance, that means, the time required to have the gain reestablished. By splitting the feedback into two separate loops simulations indicate that transient distortion at the crossover point is reduced both in amplitude and duration. Overall distortion is higher, as circuit theory predicts, but tests show to be below 0.1% at any frequency for this particular circuit. The voltage amplifier is composed of a cascade of a common base input stage and a folded cascode. The power stage, composed of a complementary pair in compound (quasi Darlington?) configuration with single loop feedback (common base - common source), achieves the required unity voltage gain.
The two monoblocs are build using chassis and power transformer from old PA amplifiers, includes additional capacitors (4 x 4700uF each) on the supply rail, and semi-translucent acrylic front panel for esthetics. Would like to discuss described feedback topology.
The two monoblocs are build using chassis and power transformer from old PA amplifiers, includes additional capacitors (4 x 4700uF each) on the supply rail, and semi-translucent acrylic front panel for esthetics. Would like to discuss described feedback topology.
