opamp questions


2004-10-01 6:28 pm
1)Ok, heres my question. Im swapping out the opamps in my dvd player which are currently 4560's. Im using AD8620's. Ive attached a image of the circuit. Shouldnt the PS decoupling be the same for the +8 and -8? They are using 100uf for the +8 and 100nf for the -8. Shouldnt these be the same value?

2)What would be the best value to use as decoupling with the 8620's?

3)What is the best way in this circuit to bias the 8620's into class A?



  • opamp.jpg
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Hard to tell, there a whole supply system we don't see. maybe either the +8 or -* have regulated supplies? Is the load assymetric? That should be known first before deciding on improvements.

Class A can de done by a resistor or current source to one of the supplies, but I doubt it will be effective in this circuit. I would start to look at the electrlytic input and output coupling caps, they do much more harm than the opamp non-class A. And look to the whole circuit. What you show is just the anti-alias filtering. Is there an I/V upstream, or does it have a V-output DAC?

Jan Didden


2003-12-07 11:57 am
Hi audiokid25,

+8v rail is probably loaded much heavier than -8V rail - this is why they used 100uF cap - pretty bad design, if I may say.

AD8620 might be too fast for your application and might emphasise the design or components in your player. Start with op275, proceed with AD826, but definitely try AD8620...

These OP's are already running their output stages in class A. No need to rape them further....