Anyone Know Where I should Look to Figure Out This Hum?

I recently purchased a Lamm LP2. First couple days, silent.

But why the ground loop hum not happened in first couple days?

A *hum* is 60Hz *only*.
A *buzz* is based usually on 120Hz but can be heard in the tweeter.
So we might have a nomenclature issue. A ground loop would produce more of a buzz than a hum, at least in every case I've encountered!

Lamm tech said they found the LP2 to be incompatible with older home wiring and the cheater plug is the fix.

This is troubling; the cheater should only be used for testing. The chassis is grounded for safety reasons. If that is defeated and the equipment is damaged such that if the power gets shorted to the chassis, it can become a shock hazard.

I wonder what is meant by 'older home wiring'? If old enough, the wiring is knob and tube so there's no ground, so its not that. Slightly newer wiring but still quite old employs conduit. The conduit is usually the ground. This type of wiring works the same way that ROMEX type wiring does- the conduit is tied to the earth ground and the neutral side of the fuse/breaker box.

But why the ground loop hum not happened in first couple days?

My initial thoughts were how would this be solved by a cheater plug but....

It is likely a very good indication that something has failed or is on the verge of failing internally in the Lamm and was just very unfortunate timing on the ops part.
Did you buy it used?
If it was myself and it had started acting up after just two days I would be talking to the seller......

I’m not worried. Seller is a friend.

My wife is saying that our fridge problems started around the same time. Maybe it’s related to the fridge some how? Who knows. Maybe when the fridge is fixed on Friday the hum will come back and I"ll have to take the cheater plug off. That would be funny.

All I know is this phono preamp sounds incredible! 

So sorry to revive an older thread, but am curious about two things....

1.  Were you able to solve the hum/buzz problem?  Was it the fridge, the wiring from an older house, bad capacitor, etc.  Are you still using the cheater plug as the fix?

2.  I am considering getting a Lamm LP2.1 and am wondering if the LP2.1 is also incompatible with older home wiring as my house is over 76 years old now.

Thank you,
Don