In need of a GOOD 3:1/4:1 passive input selector

If your switching unit is a passive preamp, and if you are handy with a soldering iron, try bypassing the volume pot.

Hum could also be simply poor earthing arrangement.

You can also short any unused inputs with a "shorting plug" - put a resistor across the +- inside an RCA connector - this is better for eliminating noise intrusion than the "audiophile" shorting RCA's sold that are simply a dead short.

When I had four tonearms and one phono stage input, I used one of these:

https://www.amazon.ca/gp/product/B085W1PPHL

No hum.

Go balanced. But how are you feeding 3 different phono outputs to the passive pre? Do you have 3 discrete phono stages?

I would try a Humbuster on the turntable.

I use a Line Router from Mapetree Audio Designs.  Custom for inputs and output.

Not sure if it will remove the hum.

I also have a CS-2 from Fidelity Research that was in the system previously.  Three in three out selectable.  It might be for sale.

Decware has one that quite nice 

Follow-up to this problem.

So in looking at other grounding/hum issues with my system ( and after listening to some advice from Bill at Fred's Sound of Music om Portland, OR), I purchased a bag of these little devils Brass noise caps for RCA connectors.

IMMEDIATE IMPROVEMENT! The hum on the phono selector is gone,  and things are pristine again. I still have another intermittent hum issue I am working on, but I am pretty close to getting that solved.

Thanks for the suggestions/advice. Should these caps no work, then I have some good ideas of where to go.

If those caps work to reduce the noise you’re hearing, then the source is most likely not “hum” in the strict sense of the term, which is a 60hz or 120hz (in countries with 60hz AC)  noise due to a ground fault. As I understand those caps, their noise reduction effect is due to blocking RFI. I’m still wondering what you use for phono amplification and RIAA filtering.

I have a builtin phono stage on my Bryston

I see now that your Bryston 1B-MC is a preamplifier.  Up until now, I thought it was a power amplifier.  My bad.  So as I now understand it, you feed your passive switch with each of your 2, 3, or 4 turntables, Then the output of the passive switch, which is unamplified un-filtered phono signal, goes to the Bryston.  Is that correct?  If so, that is a very suboptimal way to go, if you want to get the best out of each of those sources. Ideally, you would want to do any switching AFTER RIAA equalization and amplification of the phono signal to linestage level (e.g., mV signals coming from cartridges then equalized and amplified to signals at 1V to 2V). I am sure your present set up "works" but you are losing some fidelity in the process. Ideally you want either a phono stage with multiple switchable inputs to feed your selector (these days there are many excellent choices with 3 or fewer phono inputs that obviate the need for any selector switching) or a discrete phono stage for each TT output, which you could feed to a switcher.

More good news on this front.

I started looking "behind the curtain" if you will. While the last vestige of noise was rattling my brain, I shifted one of my power cables and the hum went away. Hmmmm...

After contemplating things a moment, a slash of inspiration like a bolt of lightning from Zeus hit me! Basic electrical wiring knowledge! So I remounted my power strips,  rerouted and secured the cables along a horizontal shelf, oriented the cabling to be at 90 degrees to the power cables and wrapped them in rfi shielding. Everything is crystal clear!

I knew keeping those old circuits books would pay off.

Well done!

Another thing I did (which I failed to elaborate), I utilized the Bryston's two switched outlets (and soon 3rd once I fix the BIC). I think that was the biggest fix that was, in all actuality, the simplest fix. Common ground between the turntables referenced a single reference.

You gotta love electricity. 

😁

Thanks for all the words of wisdom to everybody!