Acoustic 270 - trouble in preamp

[Drummer4gc]

Got it. There is a lot of conflicting and vague information/opinions about biasing SS amps online and I just wanted to make sure I was taking an appropriate approach. Sounds like I'm well within the ballpark.

Adjusting R404 didn't appear to change the symmetry of the clipping, which is why I left it at the point where it sets the output cap voltage at half the supply. I'm pretty sure this issue is originating in the preamp.

Thanks again for sharing your thoughts and experience, I've learned a ton through this board and through refurbishing this amp.

[Enzo]

You think the preamp is clipping? Find out. Scope those LINE OUT jacks, is the signal clipping asymmetrically there?

If it is indeed preamp clipping, well most preamps clip asymmetrically. it's not a hifi.

Just my opinion, but I think too many people get hung up on "right" and "wrong". There are various methods to bias up an amp, sure, but they are all trying to achieve the same thing. And they won't really wind up all that different. I like to adjust bias to the performance of an amp. But some factories determine that when an amp is adjusted the way they like it, then the currents will be thus and so. SO they just tell you to adjust for that current. But if your 10mv turns out to be 6mv or 12mv, the sky won't fall.