adjusting the bias on an amp YBA

I have an older YBA Integre Single Transform. It must be 10 years old. About 3 years ago it developed a hum/buzz in one channel that progressively turned into distortion. I thought it was hosed. I sent it in and they said "we adjusted the bias" and sent it back. Now it works perfectly.

I had assumed that something major was wrong (like a cap or something going bad). I don't want to look a gift horse in the mouth but what exactly is "adjusting the bias" and how might it have solved my problem? Are there some screws or something one would turn? Is it a user serviceable adjustment?

Thanks in advance.

Showing 2 responses by asdf7e16

That sounds to me like there is something amiss with your DT. I'd suggest getting it checked out. Everybody else I know that's heard both, including myself, likes the DT better.
Adjusting the bias refers to the operating point of the output transistors. If the transistors aren't always "on" slightly, there will be a discontinuity at the zero transition point which would actually be visible on a scope with a small signal. The audible result would be high distortion with low signals, while loud ones wouldn't be as bad. If the bias is too high, power is wasted doing nothing but heating. The bias is adjusted using an aptly named "trim pot" which has a screw slot. Sometimes they are multi-turn pots, but not in a YBA Integre for some reason. It is user serviceable depending on the user and his equipment of course. Sometimes the trim pot gets dirty and just needs to be moved to clean it. You can adjust it if you know the spec. and have a millivoltmeter, or are good at reading a scope trace.