Doug, as @onhwy61 said, you can connect directly from the 3.5 mm analog output from your iMac using a
3.5mm to RCA adapter/cable. It will be using the DAC built-into your iMac, so it is not optimal.
You also do not need to spend anything close to a thousand dollars to get a perfectly good DAC for your use case. I believe a simple Audioquest Dragonfly Black $99, Red $199 or Cobalt $299 or a Schiit Modi 3+ $99 will do the trick. You should get better sound quality using an external USB DAC than you will directly connected via analog from you iMac.
You also do not need to spend anything close to a thousand dollars to get a perfectly good DAC for your use case. I believe a simple Audioquest Dragonfly Black $99, Red $199 or Cobalt $299 or a Schiit Modi 3+ $99 will do the trick. You should get better sound quality using an external USB DAC than you will directly connected via analog from you iMac.