Line-In

Are you using the Connect:Amp (Gen 1)? If so, it is not compatible with the Sonos S2 app. Read more about it here: https://support.sonos.com/s/article/4786?language=en_US

No. I’m using the S2 app.

@Zigurds I think that's what @GuitarSuperstar is trying to tell you: a Gen 1 Connect:amp (built before 2016 - if that's what we are talking about) is not visible in the S2 app, because it can only work with the S1 app.

Hi. I’m not sure what you’re telling me. Are you saying that any external line-in equipment bought before 2016 (such as my CD player) cannot be recognised by the Sonos 2 app? If so, that adds insult to injury. I have just had to spend over £500 on the new Sonos amp. Am I now going to have to spend £x00 pounds on a new CD player, just because Sonos has arbitrarily rendered non-Sonos equipment obsolete? 

I think there's some misunderstanding about what Sonos device you are using. In you first post you talk about “the old Sonos 1 amp”. Since there's no such Sonos device both @GuitarSuperstar and myself assumed you where using an Connect:amp (Gen 1, so built before 2016). These devices cannot function with the S2 app.

I now understand you own a Sonos Amp, that you had on S1 software and are now running on S2 software. Am I right?

It seems strange to me an RCA-cable ordered from Sonos would not fit the Amp. Can you post a picture?

Well that escalated quickly!!  We are trying to help here, please see 106rallye’s post so we can establish what Sonos kit you have.

Thank you. There obviously was a misunderstanding. I have both the Sonos 2 app (gold) and a new compatible amp (like the one in @106rallye’s post). What I was trying to say was this: whereas my CD player was recognised by the Sonos 1 app and old amp as a line-in device, the same device is not recognised by the new app, whether I connect it to the new amp via the old cable or the Sonos RCA cable.

Here are the cables in question:

The lower ones should fit according to RCA-kabels voor analoge apparaten | Sonos

Your Amp (not the app, because the app is only a remote to the Sonos devices) does not recognise the CD-player. Where do you go wrong in the steps on this page Lijningang gebruiken op Sonos | Sonos?

@Zigurds Make sure your new Amp is set up correctly in the Sonos S2 app. Follow these instructions: https://support.sonos.com/s/article/4717?language=en_US