Interesting problem with Amp and turntable

@buzz you nailed it. I hear no audio at all from the Amp when I switch it to mono. 

I'm a bit confused as to what this tells me though?

Check the cartridge wiring.

“Mono” adds the left and right channels of Line-In. If (+) and (-) wires are are mixed up for one channel, the sum is zero. I’m surprised that you have no sound when in mono, I would expect minimal sound and possibly some instruments cancelling, but we now know that there is an input issue for the turntable signal.

Worth testing that the sound is fine when the Amp is set to Mono mode.

If that works, and the TT works when played through a conventional amplifier,  then it would seem to suggest that the TT is ok.

Have you tried using the AMP as the source of a streaming service playing that through the passive speakers and then grouping the amp with your other devices. Next test would be to go to one of your other devices and select the line-in of the amp and see what the sound is like (do not play through the Amp).

You can generate diagnostics at any time just keep track of the scenario you were testing for each diagnostic. If you post the diagnostic numbers here Sonos staff may (but only may) look at them before you have a chance to call them.

I probably can’t add anything that can resolve your issue. However, I’ll share this information:

The Thorens TD318 was introduced in 1984. That said the Technology is 39 years old. All reviews I’ve read about the Thorens TD318 state that is an excellent TT and well worth keeping. The reviews also mentioned that it may perform better with a more modern cartridge and possibly different type of Pre-Amp vs the Project Mono Box and/or the LD Systems PPA2. More on that later.

I assume your basic setup is TT leads to Pre-amp In-put > Pre-Amp RCA-OUT > Sonos AMP RCA-IN > Passive speakers wired to L/R Sonos Amp Terminals. The Sonos Amp is set as a room/zone. If so by all that is Sonos your TT is properly setup and the sound should be great.

When Playing Vinyl the Sonos AMP with passive speakers should be set as the default room with Line-in  set to auto play in the Sonos App.

For reference my setup is a Sonos Project Debut Carbon TT with Sonos Amp, Cambridge Duo Pre-Amp, Definitive Technology D7 Series Book Shelf Speakers and Sonos Sub (G3). I’m able to start the music and group it to any Sonos speaker (Era 100’s in Stereo pair with sub, Arc HT with Era 100’s and sub, Arc HT with Era 300’s and sub & Era 300’s in stereo pair and sub) with no drops and/or static.

The only major difference I assume in our setup is the Cambridge Duo Pre-Amp. Click this link for info on Pre-Amp. As has been suggested submitting a diagnostic to Sonos may be your best course of action.

If this was the case would it not affect how the passive speaker pair wired to the Sonos Amp speaker terminals sounds when played in stereo mode?

SOLVED:

I'm feeling very dumb about this, but after examining the turntable leads for probably the third or fourth time, I noticed the tiniest piece of solder coming off the center pin of one of the coax connectors and just resting on the outer rim. Honestly, it was so small it just broke off when I blew on it.

That was super frustrating, but thank you all for your perseverance because I was convinced it was not an issue with the turntable- as I grew up in the CD era, I probably should have known better...

Side question- Is the Amp meant to run relatively hot? Because mine is.

Thanks.

Well spotted! And now, it’s sorted. How are you enjoying the sound? 

It sounds quite good overall. After this episode, I suspect there might but some other issues in the turntable, so I will probably give it a thorough once over now that I know there's nothing wrong with my actual set-up. At least it sounds the same on all devices now. And I'm still surprised at how differently each individual Sonos product got thrown off by some bad wiring. Especially considering it sounded fine on the Amp... 

The Era 100, which is quite far away from the Amp and router, seems to sometimes be cutting out intermittently (only on line-in). Is there any way to improve the signal without moving it closer?

That flake could have moved around, changing the symptoms, as you fussed with things.

Try the compressed setting for line in.