Oh, ok. The TDA8920 has a differential input. This is good for two things:
- Cancelling out interference in the cable
- Cancelling out common-mode ground loop voltages
For #1, you'd need a balanced line feeding it. This means the source impedance for the positive and negative signal pins needs to be equal, so that any interference creates identical voltages on each, which can then be cancelled out by the diff amp. (The signals do not need to be equal and opposite, despite popular opinion.) But since your signal is already unbalanced, you won't get this benefit.
The #2 case only matters if the source is grounded through the third prong on the power cord. If the amp and source are grounded through different outlets, there can be significant differences in their reference voltages. Then you can get 60 Hz power line hum in your amp, because ground loop currents flowing through the cable's shield will cause the amp to measure relative to a fluctuating ground. If so, you want to keep the power amp's ground disconnected from the source's ground, except the unavoidable connection through the wall. The ground from your source should connect to the - input of the amp, and nothing else.
On the other hand, if the source is floating, this will cause problems because the common mode voltage will be too great to cancel. To handle both cases (floating source and grounded source), you can use a ground lift switch on the input, or compromise with a smallish resistor (100 Ω) between the amp's ground and the source ground:
And follow the NXP schematic and PCB layout as closely as possible, including all the filter components shown above, or you'll get distortion, noise, EMI, etc.