If the pre-amp hums with nothing else plugged into it (except headphones), then you probably have a leaky capacitor in the power supply.
The only other thing I can think of is that you're equipment is right next to an extremely high level of EMI or RFI radiation.
This is easy to diagnose. Just set a portable AM radio next to the disconnected pre-amp and tune it off station to static. If the pre-amp and radio hum together it's not the pre-amp.
Now unplug everything from all your 120V electrical outlets and then plug in only the pre-amp and a clock radio into the same outlet. Tune the radio as above and listen for the hum. Both together means it's not the pre-amp. Just the pre-amp means it's the pre-amp.
Final final test, buy an emi/rfi filter at your local BB, Walmart, etc. Try it and see if the hum stops. If it does, keep it and call and see if you can find the cause of the interference. If it doesn't return it.
The only other thing I can think of is that you're equipment is right next to an extremely high level of EMI or RFI radiation.
This is easy to diagnose. Just set a portable AM radio next to the disconnected pre-amp and tune it off station to static. If the pre-amp and radio hum together it's not the pre-amp.
Now unplug everything from all your 120V electrical outlets and then plug in only the pre-amp and a clock radio into the same outlet. Tune the radio as above and listen for the hum. Both together means it's not the pre-amp. Just the pre-amp means it's the pre-amp.
Final final test, buy an emi/rfi filter at your local BB, Walmart, etc. Try it and see if the hum stops. If it does, keep it and call and see if you can find the cause of the interference. If it doesn't return it.