A half-wave power supply produces 50Hz ripple but is never used with a power amplifier. Instead, a full-wave power supply is used to produce much less ripple at 100Hz. You might be able to measure the ripple if your voltmeter is set to measure AC but the waveform is not a sine-wave that the voltmeter is calibrated to measure so you will not know the actual amount of ripple.
With a supply of only 9V then a single-ended power amplifier (LM386 IC) produces only 0.45W into 8 ohms and it rejects power supply hum very well if it has a filter capacitor to ground at pin 7.