Lyra Lydian Beta, Rega hum?

This rig is all new. Bought the cartridge from overseas seller. Last night I shut my amp off and listened with headphones through my Sonic Frontiers line 2SE headphone jack. There is a hum in stereo, but when I switch to mono the hum is at such a low level it is not a problem. Why would it hum in stereo and not in mono? When I use mono through amp and speakers hum is lower than stereo, but not really tolerable.

Problem solved! Had a repair guy check it out and he found the pins on the cartridge were labeld wrong! Never would have found it myself.

I did a little troubleshooting today. I hooked the Rega/ Lydian up to an old Yamaha A700 integrated I had in the attic. The A700 has an MC phono stage. At first there was only a very low level hum. Then I realized I had the A700 in mono mode. Switched to stereo. Big hum. So I put the Rega/Lydian back in the main system and tried mono; low level hum. Switch to stereo; big hum. Did I get a bad cartridge? Or is the cartridge picking up the hum from some other source? I am just getting back into vinyl and am not having fun yet. Please help me before I bail (excuse the pun) out!

Thanks all for responses so far.

All contacts are tight and on the correct pins. Hum is from both channels and is heard all the time. Jimbo3, how would I ground a Rega turntable?

Today I put a B&O cartridge from my circa 1982 beogram RX on the Rega. The hum is now a factor only at high volume. It would be something I could live with if it were a decent cartridge. I don't think this proves the problem is with the Lyra. The low level hum with the B&O is the same as with the Lyra, only at a much lower level. Maybe the Lyra is more sensitive?

I clearly don't know what I am talking about.

Jfrech, I wish I could comment on the characteristics of the Rega/ Lyra combination, but so far it is unlistenable.

I bet even a Basis 1400 sounds bad with a loud hum.

Thank you mmrevolver. I can see a radio tower on a hilltop from my window. It is about 3 miles away. Could be the problem.

Yes Franks, The hum is too loud to listen to and it starts as soon as take phono preamp off mute. Guess I need to get it checked out.