Audio source from pc or smartphone to the SGTL5000?

[niolp]

Hello all!

I want to use the audio board LINE IN to receive sound from either a smartphone, pc, mp3 player or fm radio and save it to the sd card.

The issue i'm facing is to understand how these output signals look like, as i have no scope i can't really see the waveform output and Vpp levels.

So, can i connect a 3.5mm jack to the SGTL5000 LINEIN_L LINEIN_R and GND signals directly?

Or amplification, dc offset and impedance match are required?

Thank you!
Regards

 

[Blackaddr]

There is no DC offset, audio is AC coupled both on your phone's headphone out jack, and the LINE IN on the PJRC Audio board. Be careful with the VGND on the headphone output from the PJRC audio board, don't connect it to any other ground other than the ground connection on headphones themselves. I know that's not your question, but if you don't know what I'm talking about, it's a good idea to look that topic up as well.

Strictly electrically speaking, no, you are not supposed to plug a headphone signal into a line-level input. You only plug line-level sources into line-level inputs.

In practice though, we do it all the time, but there are some important considerations:

- DAMAGE. Your headphones put out a voltage designed to drive the speakers in your headphones. It is capable of producing a voltage large enough to damage the Audio board. That said, if you start with volume at zero, and bring it up carefuly, you can safely send a headphone signal into a line-level input.

- AUDIO QUALITY. The headphone amp is designed to drive headphones with most likely a 16-ohm load. A line input usually presents a 10K or so load. If the headphone amp directly AC couples the output of something like FET drivers, you will have no significant impact to the sound profile, no unintended EQ filtering, no distortion, etc. If the headphone amp has a transformer that drives the it's output, it will react with the input impedance of the LINE IN and will cause the sound to be altered as if you EQ'd it. Typically it loses bass and sounds more tinny. It is unlikely a smart phone uses an audio transformer for it's headphone jack.

So, understand the two concerns above, then proceed with due care and expectations.