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LM3886/LM3876 Hum and motorboat sound come Ground line
LM3886/LM3876 Hum and motorboat sound come Ground line
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Old 3rd March 2021, 10:41 AM   #1
Nadeesh is offline Nadeesh
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Question LM3886/LM3876 Hum and motorboat sound come Ground line

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problem LM3886 input and ground interconnect with audio source

no source connect no any hum or motorboat sound but preamp or source connect hum and moterboat sound start

lm3886 input to ground add 100 ohm resistor same hum and motorboat sound come, connect preamp or any audio source LM3886 input to LM3886 Power ground interconnect and start hum and moterboat sound come

this error LM3886 power supply or LM3886/LM3876 IC or some thing, see lot of thread hum and motorboat problem but not real solution, this is not ground loop error

please help
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Old 3rd March 2021, 02:50 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Use a star ground to avoid instability.
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Old 3rd March 2021, 03:01 PM   #3
Nadeesh is offline Nadeesh
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use star ground problem not solving this is not ground loop error
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Old 3rd March 2021, 03:13 PM   #4
joensd is offline joensd  Germany
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You could have a ground loop (green line and then via the PE from the preamp to the power supply).
Is the power supply earthed (PE)? Is ground from the power supply directly connected to earth (PE).
What about the preamp´s power supply?
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Old 3rd March 2021, 03:31 PM   #5
Nadeesh is offline Nadeesh
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Without preamp lm3886 input connect 100 ohm with ground hum start, see image

Preamp have another power supply
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Old 3rd March 2021, 06:43 PM   #6
ctrlx is offline ctrlx  United Kingdom
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post some pictures of your build.
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Old 3rd March 2021, 07:09 PM   #7
Markw4 is offline Markw4  United States
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Datasheet says: "Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines."
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Old 3rd March 2021, 07:53 PM   #8
Nadeesh is offline Nadeesh
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Quote:
Originally Posted by Markw4 View Post
Datasheet says: "Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines."
no source connect no any hum or motorboat sound but preamp or source connect hum and moterboat sound start

you see image
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Old 4th March 2021, 11:50 AM   #9
Markw4 is offline Markw4  United States
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Your schematic doesn't show 0.1uf bypass caps in parallel with the electrolytics?

Also, just because input has to be connected to something in order to notice motorboating, that doesn't mean the underlying cause is not due to power supply and or decoupling problems.

Agree with poster who asked for pics of the build. They are likely to help the most if they are hi-res and in good focus. Could be a layout problem related to bypass or other things, can't tell from the information you have provided so far.
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Old 4th March 2021, 03:01 PM   #10
adason is offline adason  United States
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Quote:
Originally Posted by Nadeesh View Post
Without preamp lm3886 input connect 100 ohm with ground hum start, see image

Preamp have another power supply
if the input is shorted with 100 ohm and it hums, you have amp problem, not ground loop with preamp
any amps input shorted or almost shorted should not produce any hum
there is something wrong with your amp, wiring perhaps, hard to know without pictures
solve the amp problem first
there will be no preamp ground loop problem
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