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DC offset drift
DC offset drift
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Old 16th December 2002, 01:11 PM   #1
BBB is offline BBB
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Join Date: Apr 2002
Question 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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Old 19th December 2002, 09:59 PM   #2
GRollins is offline GRollins  United States
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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.

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Old 20th December 2002, 12:13 PM   #3
BBB is offline BBB
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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.
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Old 20th December 2002, 12:24 PM   #4
chris ma is offline chris ma  Canada
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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.
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Old 20th December 2002, 03:51 PM   #5
Nelson Pass is offline Nelson Pass  United States
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DC offset drift
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.
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Old 20th December 2002, 04:42 PM   #6
BBB is offline BBB
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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!

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