No sound input on iMac from Behringer UCA202

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I'm an absolute beginner trying to transfer some old 12" LPs to my MacBook Pro (2013) or iMac (2009). The MBPro is useless as it has no audio input port and a TRRS jack does not show in Preferences/Sound/Input. So let's focus on the old iMac which does.


I am connecting a Technics deck with low-level output and the most success I've had is to go from that through a Pre-Amp to the UCA202 when headphone monitoring reveals a good strong signal . . BUT . . I get nothing coming into Audacity on the iMac.


I've been to Preferences/Sound/Input and selected the USB Audio CODEC input and the Output setting is to internal speakers.


In Audacity my only Input choices are Built-in Microphone or Built-in Input and I've selected the latter.


Can't understand why I have no input - can anyone help?