BIAS wont stay still


2009-05-20 3:20 pm
My active monitors have a bias that just can't seem to stay still... :)

The HF amp bias is rock solid, but the LF amp bias keeps jumping around.

The target is 15mV, across the 0.3ohm emitter resistor but , even after being on for an hour, the bias keeps jumping between around 11 and 18 mV

*Schematics attached

What is the cause?


  • HF amp.jpg
    HF amp.jpg
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  • LF amp.jpg
    LF amp.jpg
    245.2 KB · Views: 130


2009-05-20 3:20 pm
all trimpots ( swapped to mutiturn), electrolytic caps, output transistors (very carefully matched), and rectifiers are new.

all heatsink compound, everywhere, is new..

re the 6 transistors on the LF vs only 2 on the HF, i do believe that at idle only the first 2 on the LF are being used
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2017-09-04 8:08 pm

The LF amp is configured like the old Crown DC300 where one pair of outputs is biased normally and the rest are driven from their emitters as higher current is needed. Those four devices won't e on at all at idle. Make sure you are measuring across the right emitter resistor!

It is odd that the base-to-base resistors for the outputs tie to the output node instead of simply spanning across "T" style. However, for the LF side this does not matter since no high frequencies are carried.

What is the crossover frequency? If Fc is <160Hz you might be able to idle the LF amp stone cold without audible effect.

My first suspicion for any errant behaviour is bad solder connections. In a vibrating assembly like this one, the connections need to be large.