Now we're getting to the best stuff. Let's discuss some parts:|
As you can see, the input stage is quite similar to the previous types - it was originally developed for this one. There's only one new thing - the servo feedback amp OA1. It subdues the DC voltage on the output below 1 mV.
The real news come in the power stage. The fets are completely different than bipolars. A problem is how to eliminate the gap causing crossover distortion. This is, however, not an easy thing to do. FET's need a bigger bias current, about 100 mA per transistor. A way to solve this is an error-correction circuit, which decreases the FETs' nonlinearity. This error correction circuit is based on a design invented by Mr. M. J. Hawksford and originally published in several issues of volume 1981 of the Journal of Audio Engineering Society. In fact it's the transistors T17 to T20 and the additional passive devices. Those transistors are the same types as T11/T12, fast switching ones.
Then there's some other junk - the passive devices sitting around the FET's which compensate their tendency to oscillate - R49 to R52, R53 to R56, C19 to C22, C2 3 to C26. These devices have to be as close as possible to the transistors, the author recommends to solder them directly from the side of copper at the PCB.
As you must've noticed, there are three power supplies instead of one. This is because the low-power stage needs higher voltage to completely open the power FETs. The fets have lower grade / amplification in comparison with bipolars; they already need significant voltage for the bias current and they need a lot more to get completely open.
Also the input stage must have a bigger slew rate than the whole. That's why there are two more supplies, floating on each branch of the main power supply. They are fed by two separate additional windings on the main transformer, which also feeds the main supply.
The output coil is 15 turns of 1.5mm wire on an 8mm diameter.
Here you can read more about the power supply. The amp needs 2*6 V and 2*41 V AC.
One more change : two resistors were added - R60, R61 - for better security during wiring. If you use the securing block described above with this amp, as you should, you must replace resistors R25, R26 on the securing block PCB with 47k ones.