opamp questions

audiokid25 · 25th April 2005, 04:48 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?

Thanks

jan.didden · 25th April 2005, 05:16 PM

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

audiokid25 · 25th April 2005, 06:04 PM

The DAC is a V output. Would a pic of the power supply help? Its too large to attach, but I could email it to you. Let me know.

Thanks

Extreme_Boky · 26th April 2005, 01:31 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....

Extreme_Boky

25th April 2005, 05:16 PM	#2
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	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 __________________ It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts - Sherlock Holmes Check out Linear Audio Vol 5!

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25th April 2005, 06:04 PM	#3
audiokid25 diyAudio Member Join Date: Oct 2004	The DAC is a V output. Would a pic of the power supply help? Its too large to attach, but I could email it to you. Let me know. Thanks

26th April 2005, 01:31 AM	#4
Extreme_Boky diyAudio Member Join Date: Dec 2003 Location: Sydney	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.... Extreme_Boky