Oscillations or Improved SQ? A Class A Bias Question - diyAudio
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Old 1st March 2012, 10:09 PM   #1
Monago is offline Monago  Canada
diyAudio Member
Join Date: Mar 2012
Default Oscillations or Improved SQ? A Class A Bias Question


First time poster!

I have a Lovely Cube which I've recently discovered sounds vastly better in 0 gain mode than any of the other optional gains (10, 18, and 20). I purchased a class A 3.49mA bias mod with OPA627 opamp modification when buying my lovely cube, as I heard the SQ improvements were well worth the cost.

My question, is whether I'm hearing oscillations because of my amp being at 0 gain; it being potentially impossible that my OPA627 is stable with the class a bias mod in 0 gain mode (at 3.49mA); or if I am indeed hearing a stable opamp at 0 gain.

I've read that with a class A bias modification, you need gain to make the chip stable; however my confusion comes from the chip sounding better at 0 gain in class a than any other gain setting. Any clarification here would be greatly appreciated.

Oh, and my headphones are the HD650's if that makes any difference.

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Old 1st March 2012, 11:48 PM   #2
diyAudio Member
Join Date: Jun 2011
>I've read that with a class A bias modification, you need gain to make the chip stable
Not true, OPA627 may very well be stable at gain of 1x (0dB) with or without class A output biasing.

If the opamp is used in non-inverting mode, 0dB gain subjects it to full common mode voltage on its input, while 627 handles this remarkably well, you might find further improvements in the 3x - 6x gain range. For 10x gain, you might as well use OPA637.
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