RCA to Optical Converter

It is not a pre-out.  It is the + and - for the left and right speaker.  Instead of using a bare wire terminal for each connection, they used RCA jacks.

So you need an ADC instead of a DAC.  An analog to digital converter.  Here's one. 

@erik_squires 

+1

Better, a complete rethink of what the OP is trying to achieve.  Had to check that it is not April 1

@jkampmeyer 

Something is not making sense here.  Here is a link to a Onkyo TX-8050 receiver currently on eBay.  Is this what you have?

https://www.ebay.com/itm/406013771468?_skw=Onkyo+TX-8050&itmmeta=01K1ENMJTDYR410M8VBE5NXHTB&hash=item5e884e7ecc:g:n6YAAeSwFWNoYsIr&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1dBZCneLj95Ml8n5aqNVNIxLqd3k3THPrvQLztbKdZ301s6kTb%2FleXmAXiYPCOynk1NneRXysap%2BhrK58mDAGv%2FNJ8h3qgSc2PE9SJr1kmAvIOvzOAubVrbnLARheAdPH1YspNul09OogyLsvw%2FJTWJIJWh3%2BEmoUOEeIpB8yQWt66Td0aG68Rvt1TT5wGhpEKc7VF3qiSgqfjuPDj3gBA%2FgoxMioKSNQFuL00sLJNnCJR7DXpUGyP138dq73HQNfr9sZHJdIH8v0zocuyVmR%2FecUYjd0ukS3%2BAiVPF4WT3xg%3D%3D%7Ctkp%3ABk9SR86t0tWLZg

These are speaker level outputs from the receiver and the soundbar needs a digital output in optical form. If I have described this correctly above I would start anew. This seems difficult to achieve and many better options should be possible like ditching the soundbar with its limitations.