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Old 7th November 2007, 09:33 PM   #1
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Default DC offset without coupling cap

Hello,

in an amp after removing the input coupling caps there is 2V DC on the output and the amp going into protection, but only with preamp connected, that preamp does only have 0,05mV DC offset.

Can the output impedance of the preamp cause a DC shift in the amp ?

Rather old design...
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Old 7th November 2007, 09:54 PM   #2
gni is offline gni  United States
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Put the cap back. . .new one if you can. . .the input cap is there
to protect against DC being amplified. . . there are more qualified
DIYers but that is the long and short of it.
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Old 7th November 2007, 10:08 PM   #3
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I know but there is nothing to be amplified.

There is 20mV offset on the open input of the amp.

And I get output offset when I short the input.

Question is if I can adjust output offset to zero with preamp connected
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Old 7th November 2007, 10:10 PM   #4
Nordic is offline Nordic  South Africa
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yes!!!!!... just put a cap between the two
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Old 7th November 2007, 10:24 PM   #5
quasi is offline quasi  Australia
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This is a risky situation. The input cap provides DC isolation under normal operation but also during power up (particularly of the pre-amp) while the DC voltages settle.

The static input offset voltages you are observing will depend on the input stage filter configuration and the impedance.

All pre-amps have (or should have) an output capacitor, but after that they may have some circuitry that causes a small output offset when open or connected to a high impedance. Removing the power amp input capacitor will have an undefined effect on this small pre-amp offset because we now have a DC path to the power amps signal ground (or similar).

You can adjust the output of the power amp with the pre-amp connected, but I would look at the rational of removing the power amp input capacitor first. Maybe you should consider replacing the input cap with a higher value / quality before you decide to remove it.

Cheers
Quasi
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Old 8th November 2007, 09:34 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the inputs to your power amp each have two connections (four in total).
Usually two of these have a DC blocker and one on each input is just a resistor.
These two resistors are usually close in value to balance the voltage across them. these two voltages are the input offset voltage. The same offset voltage appears at both inputs and cancels. This cancelation results in near zero output offset voltage.

If you remove one of the DC blocking caps then that input now has two DC current paths and the effective resistance is both of those resistors in parallel. You no longer have matching resistances on the two inputs. This gives an unbalanced voltage at the inputs and the amplifier amplifies this to become an output offset voltage.

You really must understand the schematic before you start making willy nilly modifications that ruin the performance of the amplifier (in this case it shuts down due to DC output detection and isolation, instead it may have blown your speakers).
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