High grade switch in a box with high grade RCA jacks. Goldpoint switch, or better. etc.
There will be degradation from the extra cable, switch and RCA jacks on the given box, on the desired turntable, vs the one not connected by the switch.
The y splitter is probably among the worst sounding. as the second ’leg’, which runs back to the unused turntable, will inject notable amounts of unwanted and almost ’floating’ LCR residuals, as noise.... into the tiny phono signal and make a mess out of it.
As an analogy, think of a racing vehicle on the track, with really good capacity to get around the track, and be competitive. This is your turntable, the one being listened to. Then add two or three 20 foot composite rods that stick out of it and weigh about 250lb each (some unspecified flubbery-floppy great mass), and float above the ground (at various angles, like some really big porcupine quills) and are moved about as the car moves around on the track. This is the shut down and not listened to but still connected turntable. Imagine how this fluttery, discombobulated, flexible and self resonant mass, will heave the car about, and totally ruin it’s capacity to get around the track. It will get around the track, sure, but it will be notably compromised. That is what a parallel connected but not used turntable lines, connected via a y-splitter bit of wiring -- will do do the tiny phono signal on the ’desired to be listened to’ turntable.
With the switch, that extra noise injection is all neatly separated, but some can still come in via the separate turntable ground, unless even that is made independent via three switched aspects, not two. (isolate the positive, negative and the independent ground wires of the two turntables via a three contact switch, not the normal two.
Actually..what is required in this passive arrangement...is a 5 point switch. Neg-pos for each channel and the ground as well. So the switch has to be a 5 contacts switched’ kind of arrangement. Everything lifted and separated. That darned phono signal is so small, it is easily affected by any connectivity in any way. Just like trying to measure micro signals in a ted bed set up. All this switching and/or separation would be the norm. Not even a question. It is simply done as it is known that this would be a serious and critical issue. This turntable connectivity issue...is no less critical.
You’re looking at about $200, or more, to get it done right (parts cost alone, never mind the build). At least, as right as can be, relatively speaking.
