Not sure if non Sonos adapters exist for this, but you need an RCA to USB C adapter,  not USB C to RCA adapter. They are not the same thing.

Ah that could be it - this is defo a USB C to RCA - just assumed would work both ways at it fits fine at both ends. Thanks for quick reply

Yea, converting USB C to RCA is DAC (Digital to Analog Conversion) while RCA to USB C is ADC (Analog to Digital Conversion).  I suppose a device could be created that could sense which end audio is coming from and perform the appropriate conversion, but have seen such a device.  As well, I imagine it would need external power.

Honestly, I wouldn’t be too surprised if  the adapter Sonos sells doesn’t actually do an ADC conversion itself, but just transfers the RCA analog signal (left and right channels) along two specific pins in the USB C cable, and the Era speakers themselves perform ADC.  I have no idea.

Many thanks. Very helpful. Now to source the adapter. Sold out on Sonos site. Lesson Learnt 

Marto69, were you ever able to source the RCA to Type C cable. I’m having the same issue. 

I think that the ADC converter is in the USB adapter and this is why the generic adapters don’t work. I think the generic adapters are simple pass throughs. In the expected mass use cases (surround sound or a Room), no analog input processing is necessary. Yes, ADC is required for voice processing, but this is a different type of animal from high quality stereo audio processing.

Okay, so do I need to get a ADC converter to connect to my ERA 300 to the T1 phono sb?

You need the ERA 300 USB adapter in order to connect an analog source. A generic adapter will not work.