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AC439 26th January 2012 01:58 PM

repairing an all FET amplifier headache....
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Hi all...Long time no post, hope everyone is doing allright and enjoying great audio.

I have an all FET amplifier (schematic and info attached) that I built 30 years ago in Hong Kong. Definitely it is too old and not mainstream and it was there before the internet, I cannot find similar design. Current situation is the amp only has Left channel working. Right channel gives a hum and no audio. The power MOSFETs at the right channel run warmer than the left but not at burning hot.

I'm almost certain the driver JFET(s) on right channel is(are) broken. The design uses small signal JFET and parallel them together. I can feel the warm air rising above those small FETs. They are TO-92 package (like MPF102 size). The part numbers on the FETs were scrapped and changed (I think to protect the manufacturer of the kit). The driver JFETs are also complementary (N-ch and P-ch).

I have searched the forum and the internet but can't find a similar schematic that uses all FETs. All schems I can find are power MOSFET driven by PNP/NPN trannies or by an OP-amp. The search also did not come up with jfets bigger than anything like an MPF102. The good is I have a good channel so I can troubleshoot. But the bad is to find a pair of medium JFET
to drive the power MOSFET.

My first choice would be to find medium power complementary pairs of JFET to replace them but not sure if I'm able to. If not, then I have to find complementary pairs of small signal JFETs (probably 2sk/2sj combination) and then pair them (or even triple them) together and treak the resisters to lower the current passing through them. By guess-timation, I think each of the driver jFET has at least 100ma of current passing through D-S (30v across 2 jFETs with a 300 ohm at drain and another 300 ohm at source). The driver jFETs are TR3,4,5,6 on the schematic.

Any parts recommendation ? Source recommendation ?



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