mosfet powered version
OK, I did a mosfet powered version to replace the output darlingtons. The mosfet are drived with the buffer set at low impedance in order to deal with the charge stored in the gates (wich is sustatial at high frecuencies 1nf cap). The numbers are slightly less (THD) than the darlington version, but I managed to deal with those HEXFETS IRF530 IFR9540 to obtain a decent distortion at high frecuencies (I want to hear all I can from a trumpet ie.). I will not put a servo becuase I obtained +/- 10 mVolts in the range of 30° to 100° (silicon temp), and I believe that 70° span is enough, only the fast protection needed to disrupt energy if something goes wrong. Note that I did not put a resistance in series with the gates, mainly because this topology is to be used by switching applications ??? (high dv/dt pulses to prevent mosfet instability), I guess not in a class A audio amp (there was a question about this, about a circuit without that resistor ???). The simulation showed that the inclusion of a resistor in series with the gate introduces spikes in the feedback loop, worsening a lot the response above 200KHZ, (in this topology), besides that, the gates HAS a resistance in series (the output resistance of the buffer), I dont know if it enough, I've heard values from 10 to 220 ohms ... comments please ...
THD's obtained at 65°
Frecuency THD Amplitude (Volts)
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10KHZ 0.00076 30.9884
100KHZ 0.00859 30.8988
200KHZ 0.02810 30.6306
400KHZ 0.08599 29.6109
I want to hear all ... the bad and the good
Cheers to all
Arturo