ashok said:I am not sure if you are talking about the 6DJ8 used as a differential input stage in a class A amp . I saw this on the web with IRF9540/540 mosfet output transistors.
I rigged up the circuit with the 6DJ8 at the input and followed by a bipolar stage using BD139/140. I also tried an IRF 9540 in the voltage gain stage position. The output stage was a Darlington
TIP142/147 combination. It seemed to be fine without any input. I then touched the input to see how it overloaded at low frequency (50Hz!). It seemed clean and then the output tr's blew.
Not dead short but gone bonkers. Nothing semed to be over heated and there was no load.
I replaced all the transistors (including the good ones) and tried again. It seems to be fine. I am now building a second stage to test it out in my hi-fi system. I already use a hybrid power amp and no preamp. I will test this circuit with the ECC82 and the 6SN7. Will measure the spectrum of the distortion to see how they compare.
Upupa Epops said:To Lauris : Idiot, who designed this circuit, don't give there protection zener diode to the gates of both power mosfets. By the overload is gate broken by driving voltage and transistor is destroyed.
Upupa Epops said:What I think ? This amp will be precious slow - if you will use ECC 88, output impedance will be app. 10 k Ohm. With input capacity of this mosfets you don't get acceptable SR, respectively distortion at high frequency will be high. Temperature sensing with this type of fets is not mandatory ( they have very favourable Tc ) .
I think that long tailed pair is not necessary on the input. I would use common cathode stage with parallel connected triodes. It works much better, and drive the mosfets easily.
I made some hybrid amps, and I think that DC coupling is not good way. You have to use output relay to protect your speakers, during the heating up period.
Mosfets are not easy to drive due the large input capacitance. This is true for common source connected devices. Followers are better.