Dc offset

Vrystaat · 14th May 2013, 03:43 PM

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 .

djoffe · 14th May 2013, 04:16 PM

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

Vrystaat · 14th May 2013, 05:07 PM

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.

Vostro · 14th May 2013, 05:22 PM

Maybe servo, for offset since its a Nad

djoffe · 14th May 2013, 05:22 PM

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?

Vrystaat · 14th May 2013, 05:34 PM

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.

djoffe · 14th May 2013, 05:38 PM

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?

Vrystaat · 14th May 2013, 05:48 PM

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.

Vostro · 14th May 2013, 05:56 PM

TL072 should be fine for servo opamp

Vrystaat · 14th May 2013, 06:16 PM

Thanks Vostro,
The TL072 is still available. Think I need to get two magnifying glasses in series and check for dry joints. Thanks for help guy's !

14th May 2013, 03:43 PM	#1
Vrystaat diyAudio Member Join Date: Jan 2012 Location: Bloemfontein	Dc offset 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 . __________________ Regards Jan

14th May 2013, 05:07 PM	#3
Vrystaat diyAudio Member Join Date: Jan 2012 Location: Bloemfontein	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. __________________ Regards Jan

14th May 2013, 05:34 PM	#6
Vrystaat diyAudio Member Join Date: Jan 2012 Location: Bloemfontein	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. __________________ Regards Jan

14th May 2013, 05:48 PM	#8
Vrystaat diyAudio Member Join Date: Jan 2012 Location: Bloemfontein	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. __________________ Regards Jan

14th May 2013, 06:16 PM	#10
Vrystaat diyAudio Member Join Date: Jan 2012 Location: Bloemfontein	Thanks Vostro, The TL072 is still available. Think I need to get two magnifying glasses in series and check for dry joints. Thanks for help guy's ! __________________ Regards Jan


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14th May 2013, 04:16 PM	#2
djoffe diyAudio Member Join Date: Oct 2008	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

14th May 2013, 05:22 PM	#4
Vostro diyAudio Member Join Date: May 2012 Location: Other side of the fence	Maybe servo, for offset since its a Nad

14th May 2013, 05:38 PM	#7
djoffe diyAudio Member Join Date: Oct 2008	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?

14th May 2013, 05:56 PM	#9
Vostro diyAudio Member Join Date: May 2012 Location: Other side of the fence	TL072 should be fine for servo opamp

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