YOU EXPERIENCE HUM WHEN CONNECTION TWO TURNTABLES TO A PREAMP USING THE LOW NOISE SWITCH

OR

YOU EXPERIENCE HUM WHEN CONNECTION JUST ABOUT ANY DEVICE TO ANOTHER

 A few facts about the Low Noise Switch.  There is no connection between L & R shield (we call it ground) . The input (or Common) shield only connects to the corresponding Source 1 or Source 2 to which it is switched.  The chassis or earth ground is not connected in any way to any of the shields.  This eliminates hum loops that usually occur when trying to use ordinary switches to connect two turntables to a preamp input.

 
One must connect the appropriate earth or chassis ground of each turntable to the switch box, and run another from the switch box to the preamp being fed by the switchbox. 
 
There are variations that might be tried, such as, connect the earth of each turntable to the preamp earth and also the earth of the switchbox to the earth of the preamp.  Other last resorts may include reversing AC plug polarities.
 
When totally baffled, I suggest, trying one turntable at a time.
 
* Connect one turntable to Source 1.
 
* Connect Common to the preamp input.
 
* Connect the turntable earth to the SW earth and connect the SW earth to the preamp earth.
 
Now, the above electrically represents almost no difference than connecting the turntable directly to the preamp. The only circuit intervention is 4 electrical contacts that connect the L, the R, the LG, and the RG.  The earth connection of the turntable directly connects to that of the Preamp.  The chassis of the the SW simply surrounds and shields the switches. There is no connection from the SW earth to anything else.  There should be no hum.  If there is  Something is extremely bizarre.

RCA CONNECTOR COMPATIBILITY 
 

Nowadays there are often mixtures of English and of Metric connector parts.  The simple RCA connectors can sometimes look connected, but are not.  To explain.  Sometimes, the diameter of the shield connection of an RCA connector may actually be a minute fraction of a mm larger than the ring of the jack into which you are plugging.  You will feel normal resistance when pushing it in, but what you are feeling may simply be that of the center post connecting.  The shield may actually be held by that center post directly in the middle and the shields are apart by that minute fraction of a mm.  The fix is to pull the plug out and squeeze the connector's shield.  You may have to use pliers to make that circle a tad oval.  Then, when you plug it in, it will definitely make a good ground connection.