Aleph3 clone offset problem

[prakarn]

Thank you again .

R108 is 25 turns pot. I noticed the good one has dc offset about 300mV at mid span POT but the Bad one is start at -3.5V. It is too far for adjusting.

How can I check input mosfets by not take them out?
And Is it possible that Q103 be a malfunction part?

 

[Zen Mod]

voltage across R106 is OK < so Q3 is OK

it's easiest and fastest to just replace input mosfets ..... behavior of circuit is exactly checking procedure

 

[Dennis Hui]

How were your input mosfets matched?

 

[prakarn]

I bought a quad matched from eBay then checked again before put it in.

 

[Dennis Hui]

How did you check? What matching criteria?

 

[prakarn]

How did you check? What matching criteria?

By using below diagram. They are very close matched group, as remember ,less than 0.02V.
Is it okay for this job ?

 

[Dennis Hui]

Should be fine.

 

[prakarn]

Hi Zen Mod
After replace 3rd input mosfet set I got it.
Now Happy with new amp.


Thank you very much for your nice support. I almost gave up when changed second input mosfet.
This time I found PCB spoil out then I fixed. It may be damaged since replace first time.


thanks for everyone who advised in this post.


and also be grateful to Mr pass for honorable amps but DIY can affordable.