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
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
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.
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.
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
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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