Turntable via Sonos audio input adapter cable

If the Era 300 are being used as surrounds, then the answer is no. Line-In is disabled on surround speakers.

https://support.sonos.com/en-gb/article/surround-sound-guidelines-and-limitations

Yes, although you’ll likely need a pre-amp between the turntable and the Sonos, in order to get the signal to line level. 

But Sonos does turn off a line in on a device connected as a surround speaker, so if these Era 300s are surrounds for your Arc Ultra, you’ll need another device.

Thanks. Indeed, I use my 2 ERA 300, my ARC ULTRA and my SUB in a group and my 2 ERA 300 are configured in surround. Is there no way to connect it to the ARC ULTRA or is the only solution the preamp from SONOS?le preamp de SONOS?

Sonos disables the LINE-IN feature when speakers are setup in Home Theater Mode. Switching between a setup where two Era300s are in stereo utilizing the LINE-IN and back to Home Theater Mode is slow, inconvenient and impractical. 

The least expensive and most practical solution is to add a Sonos DAC like the older Sonos Connect S2 which run from $50-150 on ebay. (Keep in mind there’s an S1 version to avoid which is likely too old and incompatible with with the current range of software.)

pro-tip: if you like or click the 🖤 on a Buy-it-Now item prior to purchasing something on Ebay which adds the item to your watch list, Ebay will encourage the seller to offer you a discount. Sometimes these are pretty good 10-20% off and you might get the offer within in minutes or in a day or so. Worth remembering. 

I purchased a Sonos Port S2 this way for $150 and now my TT is available anywhere in my setup
You can also purchase an Era100 simply to use it’s LINE-IN but that’s often not the least expensive solution.

Additionally if your turntable doesn’t provide a Line-out, instead Phono-out you’ll need an RIAA phono pre-amp between the turntable and Sonos Connect S2 unit. Phono pre-amps run the gamut price-wise but a host of capable ones can be found on Amazon for between $15-100 for those on a budget.  
 

Which preamp do you suggest to use my turntable with my current SONOS setup?

​@David0961 

The solution is not just a preamp… first of all you need a Sonos device with analogue audio input option that isn’t used as a surround speaker. If the turntable hasn’t got a line out you will also need a phono preamp. 
As an alternative you could use the Arc Ultra as a bluetooth audio input option. To do so connect a bluetooth audio transmitter to the turntables line out or to an additional phono preamp if the turntable just supports phono out. 

No, there isn’t an extra line in on the Arc Ultra, unfortunately. As I said, you’ll need to get another device as a line in, but the signal can be sent anywhere in your Sonos system.

Pre-amps are cheap. If your turntable doesn’t have on built in, search Amazon for ‘turntable pre-amp’, or go to your local electronics store. Some pre-amps are matched to the type of stylus on the turntable, some are not. 

All suggestions are valid. However, IMO it really depends upon how serious you are about the vinyl audio quality you’ll receive. A pre-amp is required for all options either built-in or 3rd party outboard.

1. Bluetooth regardless of how you send it to the ArcUltra via an adapter and transmitter or direct via the Technics (if so equipped) is lower audio quality versus a a line-level which the ArcUltra can’t accept.
2. You have a Play 5 that will accept a line-in via 3.5mm (RCA to 3.5mm); however, it may not be in close proximity to the ArcUltra. *
3. There’s also the option of purchasing a Sonos Port and setting the ArcUltra as the auto-play room. **
4. You could purchase a Sonos Era 100SL and connect direct via the optional Sonos Line-in cable. * ^
5. Finally, you could purchase a Sonos Play and use its Line-in via the optional Sonos line-in cable. You would also gain an excellent = portable speaker. *

 * Once the music is started you can group the speaker to the ArcUlta. You also have the option to lower the volume on the speaker connected to the Technics independent of the ArcUltra.

 ** Cost prohibitive IMO

^ Least cost option

A bit of background.

A phono pre-amp raises the low level cartridge output to a higher, line-in level, and also applies the RIAA equalization needed to get decent sound.

The Sonos adapter changes the line-in analog signal (from any source) to a format that Sonos can process. Most other-brand converters will not work.

I think my comment is pending but an S2 Connect or Port from Ebay is a relatively economical option. Lots of installers sell off S2 Connects for well under $100. I got my Port S2 for $150 this way. Might take some time to wait on a deal but they are absolutely out there.