Safely Interfacing to PC Headphone Port

[Wirth's Law]

Hello all,

I have a system in need of a PC audio source by way of a headphone jack or possibly a line out jack. The intent is to plug my device into a PC audio jack, have it show up as an ordinary set of phones, and push audio signals to an analog to digital converter (ADC). You can see my diagram attached, and this is mostly concerning the blue shaded area.

The idea is to of course do this without damaging the PC side in any way. After doing some preliminary searching, I've found no real standard range for headphone impedance: or if there is one, there are many headphone products in conflict with it. So I guess my initial questions for how to connect an ADC to a PC are:

1. Do I need a transformer at my input? If so: are there specifications it should meet?
2. What voltage range should I expect from a typical PC headphone port if headphones were plugged in?
3. Does a headphone waveform cross the PC's 0-volt reference/ground?
4. Are there any unexpected traps/pitfalls to watch out for when connecting electronics to PC audio jacks?

 

[NsrMagazin]

1. No. If your input can take the voltage and current from the PC (2.5W).
2. You have to measure, can be a little different, but it depends on the signal(how loud the song is, because its different for the same settings).
3. I don't think so, but I have not checked this one.
4. No! Its just a jack, if you don't want to risk use an MP3 player. Its about $2.20 USD
Mp3 players.

 

[Nigel Goodwin]

1. No. If your input can take the voltage and current from the PC (2.5W).

Where did you get that from?.

PC's haven't included audio amplifers since probably the 286/386 days, the audio output socket is just a line level or headphone socket, and can feed any standard sort of line level input. Hence the fact that computer speakers have had in-built amplifiers for decades now.

 

[rjenkinsgb]

I have a system in need of a PC audio source by way of a headphone jack or possibly a line out jack. The intent is to plug my device into a PC audio jack, have it show up as an ordinary set of phones, and push audio signals to an analog to digital converter (ADC)

The output is line level / headphone compatible.
The impedance can be just about anything, ten ohms to 10K or more.

Aiming somewhere around 10K is probably most practical for an audio input.

The load needs probably needs a DC path (resistor across it) for the PC auto switching system to detect it, if that's enabled.

The signal will be symmetrical to ground.

Downsides:
PC analog audio is generally not that good quality and frequently has power supply noise superimposed on it.
If the external equipment is separately grounded, earth loops and yet more noise on the input are likely.
It may need isolation transformers to eliminate ground loops, but they can then reduce frequency response and do nothing for source noise.


You say you are going to feed the signal to an ADC - so you want digital audio...

The better option for that is to take a direct digital signal and use that in your project - you avoid all the noise and grounding problems and retain maximum possible quality.

Most motherboards have an S/PDIF digital audio output, even if there is no rear panel connection for it.
It's a 5V logic level signal and can connect directly to a TOSLINK transmitter for optical out, or a resistor divider for a coax output.
You can buy add-on connector plates for desktops, with the connectors and appropriate cables to suit different makers S/PDIF headers.

If your computer does not have S/PDIF, you can get an external USB sound card with optical or coax out for about 10- on ebay.


For info:
For serious music / high quality audio use, you can get USB (or firewire etc.) Audio / MIDI interfaces that are vastly superior to PC sound card audio, eg. 24 bit at 96KHz or 192KHz on multiple simultaneous channels is not unusual.

Not knowing your intended application, such devices may possibly be relevant.
Examples - Focusrite Scarlett 2i4, Presonus Studio 192 (and many, many others).
https://www.amazon.com/Focusrite-Scarlett-Audio-Interface-Tools/dp/B01E6T50LY
https://www.amazon.com/PreSonus-Studio-192-Interface-Command/dp/B00XVIN776

 

[NsrMagazin]

Where did you get that from?.

PC's haven't included audio amplifers since probably the 286/386 days, the audio output socket is just a line level or headphone socket, and can feed any standard sort of line level input. Hence the fact that computer speakers have had in-built amplifiers for decades now.

I got it from opening the PC and checking whats inside. You have a speaker of 2.5W AC audio signal. Your auido jack is not more than the internal speaker's power of "2.5W".

 

[rjenkinsgb]

You have a speaker of 2.5W AC audio signal

That's presumably a laptop or all-in-one, as desktops do not generally include speakers.

The audio out socket on a laptop/all-in-one machine will not be the same level as the speakers - most basic headphones are only rated for a fraction of a watt & 2.5W into headphones is enough to rupture eardrums...

 

[NsrMagazin]

Its a desktop Fujitsu-Siemens.
Also it will not be more than "2.5W" so you can use this for your calculations and expected measurement.

 

[rjenkinsgb]

ts a desktop Fujitsu-Siemens.
Also it will not be more than "2.5W"

That is an oddball and definitely an exception to normal PC standards, so not a useful reference for general designs.

I'd still bet that the headphone out socket is taken before the power amp!

 

[Nigel Goodwin]

Its a desktop Fujitsu-Siemens.
Also it will not be more than "2.5W" so you can use this for your calculations and expected measurement.

And has not the slightest thing to do with the audio socket output, which will either come from before the powe ramp in the laptop, or via an attenuator after the power amp (much less likely).

 

[NsrMagazin]

My guess would be in the US you have different standards. I have never seen a desktop computer without a speaker.

 

[rjenkinsgb]

I have never seen a desktop computer without a speaker

I've worked on and/or built hundreds of PCs - I've never seen a "separate" desktop PC WITH an internal speaker, other than for the BIOS "beep" output, which is nothing to do with the main audio system.

Standard desktop PC cases have mostly not even had mountings for a BIOS speaker for some years.

(Speaking as a professional electronics designer and repairer in the UK).

I'm not responding re. this any further, it's irrelevant to the original question.

 

[Nigel Goodwin]

My guess would be in the US you have different standards. I have never seen a desktop computer without a speaker.

Must be a new record, for the most ludicrous thing you've posted yet

Not to mention quoting the USA, when most members aren't even from there.

Perhaps you care to post a picture of a desktop PC with internal speakers?.

 

[NsrMagazin]

https://forum.ts.fujitsu.com/forum/viewtopic.php?t=12401