Gooday,
Is it possible that an intermittent faulty bias pot can cause intermittent Dc offset on the output of an amplifier ? When cold, there is no problem but when the amp heats up there is some intermittent DC offset now and then.🙄 It is a slight scratching on the speakers, when measured it is between 80 and 120mv .
Is it possible that an intermittent faulty bias pot can cause intermittent Dc offset on the output of an amplifier ? When cold, there is no problem but when the amp heats up there is some intermittent DC offset now and then.🙄 It is a slight scratching on the speakers, when measured it is between 80 and 120mv .
By "bias pot", I typically think of the pot that sets quiescent current in the output stage. While a faultybias pot that sets output stage current could change the offset voltage, I'd more likely suspect the offset voltage correcting pot.
Does the amp have two adjustments, one for bias current in the output stage, and a second for offset voltage?
Akitika GT-101 Audio Power Amplifier Kit
Does the amp have two adjustments, one for bias current in the output stage, and a second for offset voltage?
Akitika GT-101 Audio Power Amplifier Kit
djoffe,
No, there is no DC offset adjusting pot. I have replaced all the electrolytic caps on the amp, thought it could be some of them. It is a Nad 214. The bias pot is very sensitive and the bias drifts a bit with 2-4mv +/- in both directions on the problematic channel, that's why I suspect the bias pot.
No, there is no DC offset adjusting pot. I have replaced all the electrolytic caps on the amp, thought it could be some of them. It is a Nad 214. The bias pot is very sensitive and the bias drifts a bit with 2-4mv +/- in both directions on the problematic channel, that's why I suspect the bias pot.
When you say, "the bias drifts a bit with 2-4 mV +/-", do you mean bias as measured across the emitter resistor of an output transistor, or are you referring to the DC output voltage at the speaker terminals?
It is across the emitter resistor. Bias should be 20mv but it changes slowly, takes a couple of minutes maybe 5 to change between 18mv and about 22mv. Seems compensation is working.
that kind of drift is very typical for bias setting. I don't think it's a problem. So, does leave us back to the occasional scratchy sound question?
Sakis have suggested a while ago that the op amp can be problematic,the op amp is an NJM 072d, but they are obsolete where I live, They are still available on fleabay, I will try to get some, they are cheap.
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