Dear all, for the past 8 years or so I have been using an F4 as my daily amp. This winter I have notices that one half was definitely cooler than the other without any major sonic difference or problem. I checked the bias and it was about half of it should have been with offset also out for about 200mV. I have readjusted without any difficulty the bias and offset and the amp performed again flawlessly. Now again the same side is way cooler than the other! Should I check anything else? What could be causing the bias to drift?
Many thanks!
Many thanks!
The bias generator can be troublesome on these, relating to the TL431 and flavor-to-flavor variation of the "min cathode current for regulation" ratings..
I mention something here about it..
I mention something here about it..
Ok thanks. The fact that it was stable for so many years and now it’s not anymore is a sign of one of the tl431 getting damaged?
It's difficult to say. Any previous ESD event can result in weakened device reliability. This can range from subtle performance degradation to complete device failure. It's possible that such damage is not apparent until significant usage has occurred. Beyond that, are the trimpots retaining the set resistance value? Those are pretty good typically and failure is not common. Maybe other members have some input about a potential cause of your trouble.
Thanks. I guess that just by checking some voltages I won’t be able to diagnose the problem. The trimpots appear to be working correctly. Probably the easiest way is just to change the tl431 and see if it works I guesss
Another thing that provides a stabilising effect in regards to bias is the output stage Source resistors. Also on an "old" amplifier I would verify that the output devices are still torqued to the heatsink properly since thermal-cycling is a thing and can compromise the clamping pressure over time. Also the health of the thermal interface... grease eventually dries out, etc..