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Since Moamps put up that section of schematic (I used to have a PR99 and unfortunately gave it away) there are any number of aspects to that design that will affect the overall performance. The Op amps originally used were not particularly 'fast' (I forget which they were now LM748?) and C11 is I think feedforward compensation. The output transformer isolates the amplifier stage from a possible strongly capacitive cable loading so in itself probably assisting stability of the amplifier stage.The original design brief used to be a compromise between sounding good (with whatever it might be connected to), have good measurable specifications (to brag about) and be cheap enough to make a profit. There are of course many different ways to achieve these goals.Playing a 'devils advocate' it would be possible, but not advisable to omit R4 and feed in some HF oscillation from the bias/erase oscillator so that the 'switching' of the output transistors gets blurred. This would then require output filtering ) see also 'class D' power amplifiers. Yes I know that class D is different altogether but having just written this I am reminded of my STC 'amplifier designs' booklet from the early 1970's which shows a design for a power amp (where they had bravely shunned a driver transformer to have a multivibrator (2 transistor) oscillator feeding the output transistors. Their 200 watt design (2 Ohm load) recommeneded the use of 3 phase mains because continuous 12Amps DC from rectifier/capacitor design was deemed impractical. Thus 3, smaller power transformers were proposed.
Since Moamps put up that section of schematic (I used to have a PR99 and unfortunately gave it away) there are any number of aspects to that design that will affect the overall performance. The Op amps originally used were not particularly 'fast' (I forget which they were now LM748?) and C11 is I think feedforward compensation. The output transformer isolates the amplifier stage from a possible strongly capacitive cable loading so in itself probably assisting stability of the amplifier stage.
The original design brief used to be a compromise between sounding good (with whatever it might be connected to), have good measurable specifications (to brag about) and be cheap enough to make a profit. There are of course many different ways to achieve these goals.
Playing a 'devils advocate' it would be possible, but not advisable to omit R4 and feed in some HF oscillation from the bias/erase oscillator so that the 'switching' of the output transistors gets blurred. This would then require output filtering ) see also 'class D' power amplifiers. Yes I know that class D is different altogether but having just written this I am reminded of my STC 'amplifier designs' booklet from the early 1970's which shows a design for a power amp (where they had bravely shunned a driver transformer to have a multivibrator (2 transistor) oscillator feeding the output transistors. Their 200 watt design (2 Ohm load) recommeneded the use of 3 phase mains because continuous 12Amps DC from rectifier/capacitor design was deemed impractical. Thus 3, smaller power transformers were proposed.
