Building a “hum-free” audio is not easy till you know what you are doing. It is not as easy as connecting wire into where it should be connected. Especially for grounds, if you connected it into wrong point. Noise usually come from ground loops or connecting a very sensitive signal ground line to a high current ground path. In most cases a star connection is very effective in eliminating the noise.
Layout consideration:
As I have said, star connection is one of the most effective way to eliminate noise on the system. Below is the correct way to minimize noise:
Figure 1
Wrong way – This layout did use star connection on the positive side but did not use on the ground.
Figure 2
Very Wrong way – This layout combines the input and output ground and did not use star connection
Figure 3
Where to use star connection?
Always use star connection on the negative terminal of the main filter capacitor. As you can see on my example on Figure 1, the input GND and output GND_ is connected directly to C4 negative terminal.
Example Layout with 5V output
The layout above is an example with a 5V regulator, notice how the GND pin of the regulator is connected. It is connected on the negative terminal of C4 instead on the GND_ terminal.
Using snubber
C1 and C2 functions as a snubber that reduces the high frequency oscillations of the circuit due to the parasitic capacitance and leakage inductance on the transformer. This sometimes can lead to white noise or middle tone hum on your audio systems.