Excessive noise in amplifier circuit when switching from DC power supply to AC-DC adapter

Sounds to me (Pun intended) that this is a ground loop problem. If, for example, your supply current goes trough the same rail on the breadboard as the ground for the channels the current will create a small voltage differential that is directly translated as noise.

If your current-supply is a bit noisy as well this will also translate directly as noise, especially if ground connections or leads are shared between signal and power. This will be a continuous noisy tone at some frequency, often quite high.

Another issue is that the circuit may draw to much current, and the output from the power supply may oscillate. (The circuit draws too much current, the voltage drops, and the circuit switches off only to immediately restart, and draw to much current, and the cycle repeats.) With a speaker connected this often sounds like a helicopter or big fan. (fob-bo-bob-bo-bo-boo)

Could be several things:

1) The wall adapter is junk. Older SMPS are known to be noisy. Newer SMSP like Mean Well LED supplies are known to be very good and can be very quiet.

2) Even if the SMPS is the modern good variety, they may not "like" large capacitors directly on the output. The ideal scenario is to use a common mode choke that is as large as you can find for the current required and then follow that with whatever size cap you need to filter down below 20kHz. For example, you should be able to find at least a 1mH choke which in combination with 10uF cap will filter out everything above ~1.5kHz.

3) Even if the SMPS is the modern good variety, they may throttle switching which causes moduation at low frequencies if the output is not loaded with at least 10-20% of the load capacity. So you might need to just add a smallish resistor of appropriate wattage. But unless the SMPS is very large, my guess is that the quiescent current of your amp is enough to load properly.

So my guess would be that the issue is 1 - the adapter is some junk pulled out of a bin. There is no recovering from that. You can get a proper 24V (or 12V) Mean Well constant current LED supply for under $20 USD on Mouser.

looking at the datasheet, your dc power supply is not sufficient.

I would size it about twice the non repetitive peak Amperage for power headroom, but twice its repetitive peak output would be my recommended minimum.

If you go by typical practices, the maximum current draw should be 80% of the power supply so the rectifier filters in the power supply can recover.

But that is only the 1/3 of the issue, the main issue it has is its design. Unbalanced = 100% of power supply noise injected onto the signal.

Now if you create isolated power, or create a ground reference that has the inverse of the power supply noise on the ground, the noise would get subtracted by the output circuit.

Based on your description, I am assuming your schematic should be reading +12v and 0v, rather than +/-12v which, as mentioned above, would yield a total of 24v.

I am also going to assume that the noise is a buzz or a hum, rather than a hissing, or white noise.

If that is the case, the first thing I would suspect is that the wall adapters are switching power supplies, and your adjustable supply is linear. From my understanding, certain amplifier circuits can be more sensitive to the noise generated by the switching action, whereas linear supplies do not produce such noise. So, I believe you may also be amplifying the noise generated by your adapter.