Giesberts HEXFET AMP Problem

[Brownstocker]

hi every one
i built up the Giesberts HEXFET AMP with the IRF pair and after i finished i started to measure the offset and till that it was fine ... then when i put my multimeter in series with the positive supply rail and strated to adjust P2 but i found that the quiscent current is fixed at 3mA not changing so i checked the transistors but it was all fine however i changed the TR7 with another one but the same result and when i tried to apply an input signal i got a clear good output sound but very low power arround 10 or 15 watt .. i don't have an osciliscope so ..... please help what my cause this ?????

and here is the link of the schematic :
http://www.mif.pg.gda.pl/homepages/tom/files/120MOS.gif

 

[egberttheone]

Are you sure T13 and T12 are fine? because without these or without the fuses the amp will make some output(as i remember).

 

[Brownstocker]

you mean that if one of the transistors is damaged it will work but not with the full power?

 

[egberttheone]

Yes, if it is not shorted out. without T12 and T13 there will be flowing some current trough R31 although it is not nearly 10-15 watt.

 

[Brownstocker]

thanks alot egbert i'm going to check them and may replace them .... i used the IRFP pair with the modifications that were posted in other threads and right now i was reading in the thread of the stability
http://www.diyaudio.com/forums/solid-state/89526-diy-hexfet-amp-total-disaster-6.html
and i realy want to have your final modification picture the other links are expired and thanks for support egbert

 

[egberttheone]

I would advise you to use non-inductive resistors, if you didn't use those already.

 

[Brownstocker]

i think i will rebuild the amp all again and use a non-inductive resistors.... does this omits the capacitors in parallel with r31 ?

 

[egberttheone]

Yes you can then leave the capacitor out of the circuit.

 

[Brownstocker]

sorry for late... i i'll give it a try and tell yo what's happened

 

[raise0]

Hi there. I just saw your message Brownstocker. What about T12 and T13? Are they getting hot or what? 3mA is quite strange Maybe the ~15W output you have is from the pre-amplification stage? Check all of your connections, PCB traces and make sure that the HEXFET's are ok. You have the same problem with 2 different boards, or is it just a mono amp?

 

[thimios]

thanks alot egbert i'm going to check them and may replace them .... i used the IRFP pair with the modifications that were posted in other threads and right now i was reading in the thread of the stability
http://www.diyaudio.com/forums/solid-state/89526-diy-hexfet-amp-total-disaster-6.html
and i realy want to have your final modification picture the other links are expired and thanks for support egbert

You can see an unmodified elektor hexfet amplifier here.hexfet amplifier - YouTube.
http://www.youtube.com/watch?v=v5ded-1ptbc

 

[thimios]

ELEKTOR HEXFET AMPLIFIER IRF540 IRF9540

Have a look here.post #74http://www.diyaudio.com/forums/solid-state/89526-diy-hexfet-amp-total-disaster-8.html