How to remove noise from power adapter
What you describe looks almost sure as an interference from your switching power supply to your amplifier. The design from previous reply does not help at all, sorry.

The origin of the problem is: the 24 V output of your PS (both wires!) is effectively "jumping" with respect to ground potential, most likely, at switching frequency, its harmonics and sub-harmonics.

The bad news are that it is very difficult to solve this problem. One (may be - easiest) solution is to use another 24 V DC source, based on traditional iron core 220 / 24 V transformer and rectifier.

If you wish to use this kind of switching supply, you can try:


Install a common mode choke on both DC 24 V wires
Use 2 common mode chokes on these wires and a pair of capacitors between them:








simulate this circuit &ndash; Schematic created using CircuitLab

In the second case, the mid-point of capacitors is connected to "true ground". You can try using the third line (0 or GND) on your Mains circuit, or some ground connection, if it is available.

Capacitors must be larger then 10 uF and rated for 50 V at least.

Chokes must be rated for 2 A DC current and their inductance must be no smaller than 1 mH, preferably much more.

And the last circuit to try is to add a differential mode LC filter. Hope it is not needed.