Please help me to solve the Unbalanced Bias voltage and The DC Offset

Hi there, Please provide a solution to the issue. I know very little about amplifiers. I purchased an amplifier board a few days ago and tried to sketch the circuit diagram. Both the original and the suggested diagrams are attached.
Regards -- opu


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R3 / R6
just make same value 22k

R7 is little high
make 680 or 820 ohm

R9 / R10
make same value 3.3k

R9/R10 and R5/R4
are current sources
simple resistor values are set
to be correct current at 40 volt rails

long as your testing amp around 35 to 40
volts. If test voltage is much lower
current sources be off a little
and cause DC offset.

lowering R7 should help
most the problem.
Hi opusadat

As WhiteDragon said, some other things I also added.

R3 & R6 must be equal.

Q1 & Q2 needs to be a matched pair and thermally coupled.

C2 & D1 are drawn wrong, Anode of D1 connects to ground & C2+ connects to Cathode of D1.

By adding VR1 this will adjust the DC offset, however it also changes the GAIN BANDWITCH of the input stage and may change the stability of the amplifier.

VR2 needs a resistor in series so Q3 never can never be turned off.

@Audio1Man , I appreciate your insightful feedback and will update the R3 from 10k to 22k. Q1 and Q2 have the same hFE, and I did tie those transistors together. Oh, and the capacitor is drawn incorrectly by the software, but that was correctly drawn in my notebook :)

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Q5 doesn't appear to be conducting. Your test supply voltage is so low that there may be insufficient current flowing in Q1 to bias Q4 into conduction. The emitters of Q1 and Q2 look suspiciously the same.

In view of the low supply voltage, temporarily make 470 ohm. If the amp still doesn't show signs of life, report all the voltages you noted in the schematic above and:

base of Q1
base of Q2

base and collector of Q5 with respect to Q5 emitter.


BTW, I don't see any bypass caps on the supply rails. Adding them would be good practice.
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Hello to all of you. There is activity on the amp. Without your suggestions and comments, it would not be possible to run the amp properly. Many thanks.

For of this modification, I was quite hassle to couple Q1 and Q2 Transistor. Rather, I make a bridge out of copper wire to allow the two transistors to share heat. It's a dumb idea, but I have gave it my best shot.


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