Maplin GA28F mosfet disaster - Can anyone help me out here?

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#41
mosfet madness said:
After 10 minutes the mosfets (and the TR5 area) were above 45C and so I turned it off at that point. Current was still 100mA (give or take 2%) so I guess that this module is OK......Now on to the next! 🙂

MM
Click to expand...

45C is no problem for semiconductors (and the other parts).

100ma is the sweet spot for single die lateral FET's and is where the positive and negative temperature coefficients "cancel". The result as you have confirmed is that the current stays put... and all with no temperature compensation and additional circuitry.
 
#42
That module has 25mV on the speaker output, so all is looking well. Will not test with audio until both modules are looking good, but this is promising so far - thanks! 😉

I have learned a few things today, and who knows what tomorrow may bring (well, testing the other module for sure!) 🙂

I'll call it a day for now, but will be up in the morning to see if the other one still soldiers on......

MM 🙂
 
#45
Just thought I'd pop back in and say that after almost a year it's still going strong. The work done was worth the effort and a little cost as it improved the clarity in the top end whilst controlling the bass with more precision.

Thanks all and a Happy New Year to you all again! 🙂
 
#47
Interesting to note the values R13 and 14 are the same - for lower HF distortion they really should be different, due to the differing input capacitances on TR6 and 7.

TR6 - 2SK135 - i/p C = 600pF
TR7 - 2SJ50 - i/p C = 900pF

so take off a 3rd off the R14 value. 220R - 1/3 x 220 = 146 ohms, 150 ohms will do.
 
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