Line out or headphone out?
Both are often a 3.5 mm jack. One is built to send a signal, the other to drive a speaker on your head.
A line out sends a signal
A line output is meant to feed another box: an amplifier, a receiver, powered speakers. Those inputs have a high impedance, often 10 kΩ or more, so they draw almost no current. Consumer gear usually sends about 2 volts RMS at full scale, the level set by CD players. A line out is often fixed at full level, and the next box does the volume control.
A headphone out drives a load
Headphones are tiny speakers with a low impedance, commonly 16 to 300 Ω. Moving them takes current, so a headphone output is a small power amplifier with its own volume control.
What happens if you mix them up
- Headphones in a line out: often too quiet, sometimes far too loud with no volume control, and the output may distort trying to deliver current it was not designed for. It can work with high-impedance headphones, but it is not what the jack is for.
- Headphone out into an amplifier: this usually works. You are stacking two volume controls and adding the headphone amp's noise, so set the source fairly high (without distortion) and use the amplifier's volume.
A rule of thumb for headphone outputs
A headphone output's own impedance should be small compared with the headphones'. A common guideline is at least eight times smaller. Above that, the output can change the headphones' tonal balance, especially with low-impedance models.
On this site's streamer
The Black Lion lite has RCA jacks and a 3.5 mm jack, but both are line outs carrying the same signal. They are for powered speakers and amplifiers. A headphone amplifier would be a separate project.