Questions about how to decrease the PASS A-40 amplifier into a class AB amplifier.

[novtango]

Hi!
I had assambled the A-40 amplifier several years ago and found that it is real a power hungry amplifier; therefore, I decide to modify it into a class AB amplifier. I have read the technical manual of A-40 on PASS's web site about the analysis of the bias network; however, I still have no confidence in the modification. Is there anybody who could tell me clearly about how to proceed the modification, which components should be modified and what component value should be used? Besides, could you please also tell me how to make sure whether the modified A-40 is working at its best condition in AB Class or not. Thanks!

[paulb]

I don't think you'll get a reply with recommended components values and such. You'll need to experiment around with what you're doing. Try just reducing the bias (read the article as to how the bias current is set).
If you want an AB amp, why not build a new one? You can probably re-use most of the expensive parts. Or sell the A-40, that's a heck of an amp.

[novtango]

Thank you for your messages. Actually, I have already done the experiment on reducing the bias by using a SVR to trim the bias; I readjust the static bias current of A40 @ the 200mA/channel condition. However, I found that the static current is not stable; it keeps changing all the time. The bias current swings back and forth in the range of 250ma~150ma; that is, more than 100 mA from the original setting of 200mA/channel. I'm not sure whether it is normal or not for the A40 amplifier. As for building myself another new class AB amplifier, yes, I would think about it. Thanks.