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Topic: Line out definition (Read 11706 times) previous topic - next topic
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Line out definition

Reply #25 –
Quote from: saratoga on
Quote from: Fedot L on
Quote from: saratoga on
line out should be fixed at line level and have a high output impedance.

A LOW output impedance. Well explained in “Impedances” chapter of the above-cited link
http://en.wikipedia.org/wiki/Line_level


No, high impedance. As your link notes, 100 ohms is common.  Compare that to ~1 ohm typical for a good headphone output.  The point of line out is to drive a low power signal into an amplifier, hence the impedance must be much higher then headphone out where one is concerned about power transfer.


I thought we were concerned about impedance bridging when it comes to audio voltage levels. Power transfer is for the RF and HF guys is it not?

Line out definition

Reply #26 –
Quote from: d_headshot on
I thought we were concerned about impedance bridging when it comes to audio voltage levels. Power transfer is for the RF and HF guys is it not?


Headphones are mechanical transducers that push air by taking power from a battery.  Hence, portable devices care about efficiently transferring power, and so the output impedance of a headphone jack must be very low relative to headphones.  For a line out we want to accomplish something similar, but we don't care about total power transfer, and so we can tolerate poor efficiency by simply transferring less power.  Thus a line out is also bridged, but at much higher impedance so that amplifiers are easier to build and less likely to introduce distortion.

Line out definition

Reply #27 –
My two cents, since this topic asks about people's opinions and expectations.

For a portable audio player, I actually don't have any conception as to how a line-out should behave versus the normal headphone jack. I've never had any portable player with multiple output jacks. However, if I were to buy a DAP and discover it had two 3.5mm jacks labelled "Headphones" and "Line Out", I would expect them to behave the same (that is, volume control and EQ applied). It just seems more intuitive to my mind, to see two outputs as being identical, having no experience to the contrary.

Now if it's a stereo receiver, or some other large piece of hardware that sits stationary on a shelf, I would NOT expect the "line out" to behave the same as a Headphone jack. I have plenty of experience with stereo receivers, and TVs, where the line out is not affected by the volume knob. And that is how I would expect such a piece of equipment to behave.

Now, if I had a DAP that had a 3.5mm jack, and RCA jacks, I would expect the RCA jacks to not have any DSPs or volume controls applied, because all the RCA jacks I have experience with (receivers, TVs) have behaved that way. However, I would expect that the 3.5mm jack DID have DSPs and volumes applied to it, since all the 3.5mm jacks I've used to this point have behaved that way.

To recap -
If a DAP had two 3.5mm jacks labelled "Headphones" and "Line Out", I would expect them to behave identically.
If a DAP had a 3.5mm jack labelled "Headphones", and RCA jacks labelled "Line Out", I would expect them to behave differently.

It just depends on what you have experience with, and how you are used to various types of hardware and interconnects behaving.