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JLH Help - Minimise dc offset drift
JLH Help - Minimise dc offset drift
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Old 15th November 2004, 02:57 AM   #1
CY is offline CY
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Join Date: Nov 2002
Location: Hong Kong
Default JLH Help - Minimise dc offset drift

I built the JLH Class A on Fig 2- The Final Circuit at www.tcaas.btinternet.co.uk/jlhupdate.htm

Works fine and sounds good.

The problem I have is output dc drift when temperature rises from room to normal operating temperature. The drfit is about 500mV downwards as temperature rises.

Transistors used are as per the circuit except Q3 is BD139.

I adjusted VR1 and VR3 of the CCS as per last paragraph under the heading "Removal of the Feedback Capacitor" in the same Article.

But the iterative process does not seem to substantially narrow down the drift to anything close to 50mV.

Can anybody share his/her experience in making this iterative process??

What should I look for more specifically in slightly adjusting VR3 after the amp has cooled down when making the iterative process.


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Old 15th November 2004, 05:58 AM   #2
Jonathan Bright is offline Jonathan Bright  Australia
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Join Date: Feb 2004
Location: Queensland Australia
cy, I suggest that you post your question in the "JLH 10 Watt class A amplifier" thread. That way all the JLH builders will see it and you will get a good response. At the moment it has over 1,000 posts. Just go to the search function and put "LJH 10 watt class A amplifier" in and you should get it. When I typed this it was currently on page four of the solid state section.
"It was the Springtime of the year when aunt is calling to aunt like mastodons bellowing across primeval swamps." P.G. Wodehouse.
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Old 16th November 2004, 04:25 AM   #3
destroyer X is offline destroyer X  Brazil
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Join Date: Feb 2004
Location: Recife - Brasil Northeast
Default CY, Jonathan suggestion is very good, as that thread, the JLH class A is famous here

And all people already read that thread.
Informs are concentrated there, and you will "wake up" all the class A guys writing there, a communication will be sent to everyone that will run there to see what is happening.

By the way, for a while, those 500 milivolts do not seems, to me, to be a big problem.... the power waste is minimum.

Some people used to say that this will create a lot of distortions here, there and everywhere.... i used to left of set misadjusted and them i return it to low off set voltage.... unfortunattelly i could not perceive that difference.

Go to JLH class A thread CY, you will have 10K guys reading it.


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