Quite often, when a third party power supply is used with the RD-8, the unit will power up (if the voltage/polarity is correct) however you will not hear any audio from the main output. You may hear audio from the individual outputs. In this situation, ensure you are using the original power supply provided with the unit with the following specs:

Voltage: 18 V DC,
Ampage: 1000 mA
Polarity: Centre Positive