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.


A POSSIBLE LAST RESORT MOD FOR SOME SITUATIONS

This simple modification to the circuit board has proven to solve nasty hum problems.  The situation has usually been when one or both of the turntables have the chassis ground connected to one of the signal cables' shield.  Two short insulated wire leads of just about any gauge that is convenient can be used.

 

HOW ABOUT ONE TURNTABLE TO TWO AMPS?

Connects either of any two sources to any one input or any
   one source to either of any two inputs.  Either direction.
 
So, the two inputs become the two outputs, and the one output becomes the one input for the turntable.
 
However in that case, it can get a bit more complicated.  One would hope that both amp chassis are at the same potential.  If the two amps are powered from different power outlets, this may not be true due to how the house wiring was done.  Then in that case, you might experience hum from one or the other or both. 
 
One solution could be reversing the power line plug polarity of one of the amps or the other, even both.  Just pull the plug out turn it round and stick it back in.  
 
Another solution could be running a length of wire (any small gauge) from the chassis of one amp to that of the other.  Often there are ground connectors near the phono inputs.
 
Actually, the first thing I would do would to be connect the turntable to the box and connect one amp.  See how that works.  Hopefully, when you connect the other amp, it would also behave.
 
It won't hurt to try.  The one thing to keep in mind is that all chassis be electrically connect, and that includes that of the box and the turntable.  There is the ground wire from the the  turntable it should go to the box, a wire from the box should go to the chassis ground of one amp, and another wire (you'll have to did up that one (any gauge-no big heavy wire is necessary) from the box to the other amp.