I'm trying to create a clean 60hz 18V 190mA signal to drive a turntable motor using a TDA7396 to amplify a small 60hz reference. Since 18Vac is 50.9Vpp and the max TDA7396 supply voltage is 28Vdc, I'm considering using a 1:1.8 turns ratio transformer. The ratio can be higher since the TDA output can lowered. I have an old 25Vdc power adapter which could work since 120/25 = 1:4.8 ratio.

When loading the transformer with a clean 60hz from a signal generator a large amount of impulse(?) noise starts appearing. Is this expected from a cheap ac adapter.

 

AC transformers do not introduce noise spikes as shown by the oscilloscope trace.
The noise spikes are originating from some other place.

 

Is this expected from a cheap ac adapter.

Switch-mode power supplies can be noise sources. However, the noise shown on the scope trace is unlikely to have any noticeable effect on the turntable motor performance.