Amplifed Ear Dc on output

Calebay · 27th October 2005, 02:33 PM

Hi

i build this thing you can see it here: i get about 0.68 volt DC on the output??! the Ac signal looks fine on the scope. from what i can see there must be a mistake in the last stage (Q4) or have i done something wrong? i like to know how i get rid of this DC

THXHere

anatech · 27th October 2005, 08:06 PM

Hi Calebay,
This has a class A output stage in the oldest sense of the term. The DC current passes through the load (speaker or earphone). The amount of DC across the output is determined by the DC resistance of the load and the standing current in Q4. This will change a fair amount with temperature.

-Chris

MikeB · 27th October 2005, 08:27 PM

This DC-offset is by design... unavoidable without redesign.

Mike

anatech · 27th October 2005, 10:58 PM

Yes, and a coupling cap would break it.

Smile. It is what it is.

-Chris

johnthetweeker · 28th October 2005, 04:41 AM

Yeah, the headphone coil's DC resistance is the load resistance for the output transistor. We used to do such things in the valve lab those days.

Calebay · 28th October 2005, 09:25 PM

it works now i added a coupling cap and a resister..

27th October 2005, 02:33 PM	#1
Calebay diyAudio Member Join Date: Jul 2003 Location: Denmark	Amplifed Ear Dc on output Hi i build this thing you can see it here: i get about 0.68 volt DC on the output??! the Ac signal looks fine on the scope. from what i can see there must be a mistake in the last stage (Q4) or have i done something wrong? i like to know how i get rid of this DC THXHere

28th October 2005, 04:41 AM	#5
johnthetweeker diyAudio Member Join Date: Jan 2003 Location: Hyderabad, India	Yeah, the headphone coil's DC resistance is the load resistance for the output transistor. We used to do such things in the valve lab those days. __________________ John (not the baptist)

28th October 2005, 09:25 PM	#6
Calebay diyAudio Member Join Date: Jul 2003 Location: Denmark	Tnx it works now i added a coupling cap and a resister..

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27th October 2005, 08:06 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Calebay, This has a class A output stage in the oldest sense of the term. The DC current passes through the load (speaker or earphone). The amount of DC across the output is determined by the DC resistance of the load and the standing current in Q4. This will change a fair amount with temperature. -Chris

27th October 2005, 08:27 PM	#3
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	This DC-offset is by design... unavoidable without redesign. Mike

27th October 2005, 10:58 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Yes, and a coupling cap would break it. Smile. It is what it is. -Chris

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