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Old 12th September 2012, 10:03 PM   #1
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Default dc offset help!!!

Hello All,

I'm new on this forum and is for me a pleasure to partecipate at this community.
I need some help for understand if my power amp is good or have few problem:
My amp when is cold give around +60mV of dc offset on output, then after around 20-30minutes, when heatsink reach 27-30įC the dc offset drop down until 5-10mV,when is very hot less than 5mV
What do you think about this?
Can someone let me know if my amp is ok or if there's some problem?


Many thanks for help
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Old 12th September 2012, 11:31 PM   #2
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After warm up, 5 to 10mv is good. I would not worry about it.
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Old 13th September 2012, 10:38 AM   #3
Bone is offline Bone  United Kingdom
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Nothing wrong with that.
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Old 13th September 2012, 12:05 PM   #4
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behavior like that will mean one amplifier that is depending very much on the hfe of the transistors that form the circuit .

most of elektor amplifiers are like that

also will mean that it could be one amplifier with plenty of output transistors but with marginal or low bias so its expected that before amplifier equalize to an ""operating""temperature both bias and offset values will not be correct .

If the amplifier hasn't plenty of outputs it could be a minor error in the thermal design

Would be nice to know what are we talking about ...

Kind regards
sakis
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Old 13th September 2012, 12:58 PM   #5
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Quote:
Originally Posted by sakis View Post
behavior like that will mean one amplifier that is depending very much on the hfe of the transistors that form the circuit .

most of elektor amplifiers are like that

also will mean that it could be one amplifier with plenty of output transistors but with marginal or low bias so its expected that before amplifier equalize to an ""operating""temperature both bias and offset values will not be correct .

If the amplifier hasn't plenty of outputs it could be a minor error in the thermal design

Would be nice to know what are we talking about ...

Kind regards
sakis
Hello All, and many thanks for help!!

Sakis, if i try to increase a little bit the bias curruent (F.E.from 12 to 13mV) the temperature of heatsink increase exponentially and i don't know if there's some risk to burn my PA
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Old 13th September 2012, 01:01 PM   #6
Mooly is offline Mooly  United Kingdom
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Altering the bias current will not affect the DC offset. They are two different things.

I doubt bias would be stable to 1 mv anyway. Measuring with the amp hot and then cold will give different results. Always use the manufacturers setup procedure and values as a starting point.
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Old 13th September 2012, 01:14 PM   #7
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Yeah that is true..another strangers is if i measure the bias on mosfet N (4in parallel) there's always some difference of bias current..12,18,22,16mV...and the same for P rail
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Old 13th September 2012, 01:26 PM   #8
AndrewT is offline AndrewT  Scotland
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or does that difference reflect the 5% tolerance of the source resistors used in the build.

You cannot rely on 5% tolerance resistors to give better than a 10% spread of source currents.
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Old 13th September 2012, 01:48 PM   #9
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or does that difference reflect the 5% tolerance of the source resistors used in the build.

You cannot rely on 5% tolerance resistors to give better than a 10% spread of source currents.
Yes the power resistor have 5% tolerance
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Old 13th September 2012, 10:00 PM   #10
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Originally Posted by GiulioSL View Post
Yes the power resistor have 5% tolerance
And if i add for example an utomatic dc offset null circuit, i can ontain best performance::?,there's someone with experience with this kind of "problem"? Or is bettere don't touch nothing?
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