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Old 29th November 2005, 09:34 PM   #1
Nicks is offline Nicks  Sweden
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Default DC offset question

i just connected my P2P chipamp. Its an LM3886TF.

So i thought it would be wise to measure before connecting it to any speakers.

So when i first measured i got 20.2 VDC on the output!!!

Then i shorted the input and got 0.5VDC on the output.

Why is it like this?

it might be a stupid question, but i ask since i dont know

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Nicks
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Old 29th November 2005, 09:41 PM   #2
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Nicks,

What do you measure AC on the output ?

Jon
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Old 29th November 2005, 09:47 PM   #3
Nicks is offline Nicks  Sweden
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No, i measure DC at the output.

I think its supposed to come out clean and nice AC in the form of music from the output.
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Old 29th November 2005, 10:00 PM   #4
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Check the grounding. Check the grounding in the power supply.
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Old 29th November 2005, 10:22 PM   #5
boholm is offline boholm  Denmark
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Please post the schematics. the one, that you have build your GainClone after, where *all* the components are showed - also the "secondary" (and not so important) ones.

If you do not have a resistor from + input to ground, then you will have such problems. The inputs needs a reference, when nothing is connected. When connecting a cd-player or a pre-amp, this will provide a reference.
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Old 29th November 2005, 10:23 PM   #6
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The problem is two:

1) From each input the IC must "see" the same DC resistance (to signal ground, = real ground or output) if the value is "big" compared to the input bias currents. A large value is > 1 kohms (most likely).

2) If the DC gain is equal the AC gain the output offset will get the gain times more. If you have the gain set at 30 the offset will be 30 times higher which is 1-10 mV (according to datasheet) times 30 = 30-300 mV. On top of this you have input offset bias current also.

The input must ALWAYS be tied to ground via a resistor.

Have you checked the datasheet and application note AN-1192 carefully? If not do that because there you have almost everything you ought to know.
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Old 30th November 2005, 10:08 AM   #7
Nicks is offline Nicks  Sweden
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Thanks gentlemen,

I have made a slight error in my thinking.

Im building a BPA 200 but with 3 chips in parallell.

Then i use rod elliots bridge circuit instead of building 3 inverted and 3 non inverted.

So the input resistor is before the bridge circuit.

I only measured one chip without the bridge circuit.

I will measure again with the input resistor.


best regards

Nicks
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Old 30th November 2005, 11:38 AM   #8
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This was valuable information the you left out. We are not mind readers you know.
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Old 30th November 2005, 04:29 PM   #9
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Default DC offset

Input has to have a referanced to ground 1k input 22k to ground on each chip in parrellel. Make sure your inverted non inverted driver circuit is not introducing DC just one mv will be multiplied by the gain limits set by the amp.
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Old 30th November 2005, 05:48 PM   #10
Nicks is offline Nicks  Sweden
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Sorry guys...

i was so focused on the dc offset that i forgot the "golden rule" i.e present the WHOLE situation... not just the glitch...

THANKS anyway, this forum is really great!

Best regards

Nicks
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