turntable with low hum

What model turntable are you using?

Pioneer 1978. Mint condition.

Where’s the grounding wire? On the TT itself or on a preamp? 

Not sure what TT is but the grounding wire goes from the turntable to the amp. 

TT = turntable.

Amp or preamp? When connecting a turntable to a Five you’d just need a preamp. Kindly describe what’s connected to what. Is there a main amp still in the picture? Is there an external ground anywhere? 

The turntable is from 1978. Doesn’t have a preamp. Has grounding wire connected (not removable) coming from back of TT as well as the red and white connectors that are also going into the amp at the “phono” connection. Grounding wire to the amp as well. It is a Optimus SA-155 integrated stereo amplifier. Then amp is connected to the Sonos 5 via a “speakers” red and white (R/L) cord. 

You shouldn’t be using a ‘speakers’ output at all. It’s a wonder that the Five’s input wasn’t overloaded. The Five expects a line level signal.

Try using the Tape Out instead.

Ok. So using the “tape out” we have to turn the volume up to 88 to hear at a low/medium volume. The low hum is still there, just quite a bit quieter. Still annoying to the ear unfortunately. 

From the photo you appear to have the amp set for a ceramic cartridge. Is that really what’s fitted to the turntable? I haven’t seen one in decades. 

The amp itself is pretty ancient. The turntable is grounded to the amp but it doesn’t look like the amp is grounded to the electrical supply. 

By the way the Five’s Line-In does have an adjustable sensitivity. See Source Level in its room settings. 

Yes ceramic. If switched to the other it is terrible sounding. 
we did the source level per your instructions and it really just changes the hum but doesn’t eliminate. Unless there is anything else to do we will be returning. We love the other features of the Sonos 5 but want to continue using our current record player. Really appreciate your help trouble shooting this! 

You could try connecting the ceramic cartridge directly to the Five, removing the ancient amp entirely.  The level may be adequate, though you might need some manual EQ. 

The rest of your kit is decidedly ‘vintage’ so I can understand why you might want to stick with what you know.