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Old 28th September 2004, 10:08 PM   #1
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Default opamp 'class A' biasing??

I just came across another incident of someone modding a cd player by biasing the output stage opamps into class A by inserting a voltage divider on the output rail from V+ to ground. I saw this same thing a few months back in an Audio Xpress article about an RIAA phono preamp.

I just don't get it. How does adding a DC offset to the output rail of an opamp bias it into class A?? The output stage of the opamp is not having its bias changed by this is it?

Are these people insane or am I?
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Old 28th September 2004, 10:23 PM   #2
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You are insane

This is a know fact to have current drawn from the output down to V-. With this you will convert the class AB stage to a signel ended stage in class A... but... only for small currents. If this has any effect or know is not totally certain because it takes some knowledge about how the output stage is designed. The best effect is achieved for low loads like where the opamp is followed by a buffer, like in this design.
/Per-Anders (my first name) or P-A as my friends call me
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Old 28th September 2004, 11:24 PM   #3
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Cool, I get it. I do recall the Audio Xpress article mentioning that better results would be achieved with a FET based current source similar to the circuit you sent uses. This seems like a poor mod to make on an existing circuit, in particular the output analog stage of a CD player as I saw. The mod was to a Marantz CD67se btw.

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