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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 8th March 2021, 04:34 PM   #11
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Nadeesh View Post
Without preamp lm3886 input connect 100 ohm with ground hum start, see image
As others have said: You have an issue with the amp, not with the preamp+amp. Fix that first.

The LM3886 circuit is copied directly from the data sheet. It'll work fine. Its HF stability can be improved, but you're dealing with LF stability. Fix that first.

Size Rmute such that you have at least 0.5 mA flowing in it. For the best THD, design for closer to 1.0 mA. There's no reason to go beyond 1 mA. I find 33 kΩ works well for power supply voltages above 20 V. Cmute should be at least 100 uF. A 6.3 V cap is fine here as it only sees about 3 V in operation.

Make sure you have proper decoupling. You need a minimum of 10 uF (tantalum or electrolytic) right next to the chip in parallel with a minimum of 100 nF ceramic (X5R, X7R, X8R dielectric types or better). In addition, you need 470-1000 uF where the power enters the LM3886 board. You can improve further on that if you'd like. If you haven't already, I recommend reading this: Taming the LM3886 Chip Amplifier: Power Supply Decoupling – Neurochrome

Tom
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Old 14th March 2021, 09:17 PM   #12
nigelwright7557 is offline nigelwright7557  United Kingdom
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If star grounding doesnt fix it then its possibly an earthing fault.
The system should have one earth connected to input.
No earth or multiple earths is not recommended.
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