DC offset drift

BBB · 16th December 2002, 02:11 PM

I have recently rebuilt my A75 and BOSOZ in order to eliminate
some buzz/hum problems and to my suprise it worked very well
both are now quite as a mouse.
the only side effect is when the balanced lines are connected between the A75 and the BOSOZ the DC offset increases by approximately 65mv I am able to adjust the offset down to <10mv on the left channel and it will hold steady but the right channel wants to drift. I have made these adjustments after both units have been running for several hours.
without the balanced lines connected the A75 will easily hold at <10mv both channels with no drift.
Any ideas on the drift problem would be much apprecitated.

GRollins · 19th December 2002, 10:59 PM

Any chance that there's a draft that cools one side more than the other? Near a window, for instance? Or a heat/air conditioning vent?
Changes in temperature will make the operating point drift.

Grey

BBB · 20th December 2002, 01:13 PM

Thank You, for the reply
The A75 has a fan cooled output section which seems to hold the temprature steady except of course when i take the top off to make an adjustment at which time I do see a change in DC offset.
so solution was make an adjustment put the top and leave it for an hour or so for it to stabilize.
The problem is when I connect the BOSOZ to the A75 via XLR connectors the DC offset jumps approximately 65mv and despite adjustments back to 0vdc it drifts 60mv or so.

chris ma · 20th December 2002, 01:24 PM

One of my experience(very limited ones) with DC offset drifting was due to mechanical factor. Solder joint was good normally but at certain temperature the solder joint becomes not good enough, some thing happening at the joint intermediately casues the drift.
Good luck with your hunting.
Chris

Nelson Pass · 20th December 2002, 04:51 PM

The DC gain of the A75 is increased at DC due to the resistive
source of the BSOZ. As to drift, if you can keep it below 50
mV after warmup, I wouldn't think twice about it.

BBB · 20th December 2002, 05:42 PM

Thanks, everyone for the suggestions
I will double check the connections and then live with it and see how it holds up after some use, in other words quit fiddling with it and enjoy it!

BBB

16th December 2002, 02:11 PM	#1
BBB diyAudio Member Join Date: Apr 2002	DC offset drift I have recently rebuilt my A75 and BOSOZ in order to eliminate some buzz/hum problems and to my suprise it worked very well both are now quite as a mouse. the only side effect is when the balanced lines are connected between the A75 and the BOSOZ the DC offset increases by approximately 65mv I am able to adjust the offset down to <10mv on the left channel and it will hold steady but the right channel wants to drift. I have made these adjustments after both units have been running for several hours. without the balanced lines connected the A75 will easily hold at <10mv both channels with no drift. Any ideas on the drift problem would be much apprecitated.

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19th December 2002, 10:59 PM	#2
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	Any chance that there's a draft that cools one side more than the other? Near a window, for instance? Or a heat/air conditioning vent? Changes in temperature will make the operating point drift. Grey

20th December 2002, 01:13 PM	#3
BBB diyAudio Member Join Date: Apr 2002	Thank You, for the reply The A75 has a fan cooled output section which seems to hold the temprature steady except of course when i take the top off to make an adjustment at which time I do see a change in DC offset. so solution was make an adjustment put the top and leave it for an hour or so for it to stabilize. The problem is when I connect the BOSOZ to the A75 via XLR connectors the DC offset jumps approximately 65mv and despite adjustments back to 0vdc it drifts 60mv or so.

20th December 2002, 01:24 PM	#4
chris ma diyAudio Member Join Date: Sep 2001 Location: Pickering, Canada	One of my experience(very limited ones) with DC offset drifting was due to mechanical factor. Solder joint was good normally but at certain temperature the solder joint becomes not good enough, some thing happening at the joint intermediately casues the drift. Good luck with your hunting. Chris

20th December 2002, 04:51 PM	#5
Nelson Pass The one and only Join Date: Mar 2001	The DC gain of the A75 is increased at DC due to the resistive source of the BSOZ. As to drift, if you can keep it below 50 mV after warmup, I wouldn't think twice about it.

20th December 2002, 05:42 PM	#6
BBB diyAudio Member Join Date: Apr 2002	Thanks, everyone for the suggestions I will double check the connections and then live with it and see how it holds up after some use, in other words quit fiddling with it and enjoy it! BBB

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