Why is the volume so low on a Mac Mini 2018 minijack output audio when to connected to a receiver? I have a Mac Mini 2018 that I sometimes connect my headphones to. They work just fine. No issues with volume.

But if I connect the Mac Mini to a receiver (a Denon AVR-2106) via the minijack with a 3.5mm to RCA cable, then the volume on the speakers connected to the Denon is too low.

The volume on the Denon is in dB and starts in negative number (never understood why) and I have to crank it up to 0db and then I can barely hear the audio from the speakers. Also, I have to max the output volume setting on the Mac.

What gives?
This is because the jacks on the Mac are not line level inputs/outputs.

I wrote an answer to a question in a similar vane that addresses this in much more detail: Use 3.5mm jack as line in for a musical instrument

But, to summarize it here, the audio jack is putting out a signal in milivolts that enough to drive a simple microphone, a headset or some small speakers.  When you connect to the line input of a piece of AV equipment, it's expecting a much higher voltage - ~1.5V.  So, what you need is something that acts as a pre-amp.