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Old 30th September 2012, 03:42 PM   #1
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Default LM3886 DC offset and hum

Hi, I am new to the community and just built my first chipamp. A LM3886 stereo kit from chipamp.com. I followed all instruction and did not install the Ci cap. I use a 400va 2*24v toroid. I've put everything together on a piece of wood for testing purpose. I like the sound of the amp very much. It is detailed, kinda smooth, but powerful.

Here are my two questions.

Firstly, DC offset is 126mv on one channel and 97mv on the other. Do I need to worry? Have I made a mistake ?

Secondly, I hear a slight hum with my ear to the speaker. 10 inches away from the speaker it is inaudible. Again, do I need to worry? Have I made a mistake?

Any input is appreciated.

Regards from Switzerland.

Michael
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Old 30th September 2012, 04:01 PM   #2
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that DC offset is okay.
Do You use any volume controll device? like a potmeter?
(turning the volume down in Your computer's operating system -like windows - is not a good solution)
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Old 30th September 2012, 04:24 PM   #3
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I use a little external box, a line driver, which has an Alps pot.: ABACUS electronics | Line Driver variable

The line driver has one input, to which an Airport express is attached at the moment. But the hum is the same with or without the line driver attached.
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Old 30th September 2012, 04:54 PM   #4
WSJ is offline WSJ  United States
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Remove the input wires and short the inputs with a 1/2" wire. This will eliminate external hum sources from the inputs, to determine the hum source. Used a hand drill to twist the speaker wires, the twisted wires limit noise going into the amp. from external sources. Twist the input, output and speaker wires at one twist per 1/2". Move the orange and gray transformer wires away form the amp.
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Old 30th September 2012, 04:58 PM   #5
AndrewT is online now AndrewT  Scotland
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You missed a capacitor.
That missing cap could be the cause of excessive output offset !
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Old 30th September 2012, 05:07 PM   #6
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I have not used the Ci capacitor, as the instructions said it is optional.
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Old 30th September 2012, 06:57 PM   #7
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Quote:
Originally Posted by zimmer64 View Post
I have not used the Ci capacitor, as the instructions said it is optional.
Try whit it.
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Old 1st October 2012, 08:59 AM   #8
AndrewT is online now AndrewT  Scotland
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Zimmer,
If I recommended that you cross the road without looking, it's only optional, would you accept that?
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Old 1st October 2012, 09:15 AM   #9
bcmbob is online now bcmbob  United States
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This is a monoblock but the goal was to make all wires as short as possible. I didn't install Ci and both amps are dead silent. You might get better results with a metal chassis.

There is always the phantom of nearby noise generators with longer internal untwisted wiring. Check for light dimmers, halogen and neon lamps in the area. Try a diferent location or a different room. I get some noise when the amps are near a computer/monitor..

Also check section 1.7 in the user guide for optional placement of R3. You might consider swapping that placement
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Last edited by bcmbob; 1st October 2012 at 09:33 AM.
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Old 1st October 2012, 12:08 PM   #10
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Thanks a lot guys. As I said, this is my first amp. It seems that my AC wires are too close to the amp boards. My test setup also does not provide any shielding. bcmbob's design makes more sense. I guess I will also need to revisit some of the wires and twist them. I will also need to work on my soldering skills.... bcmbob's soldering looks a lot cleaner :-)

Michael
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