3.5mm jack sound card output

[rhyseyness]

hmmmmm,, that's different. your multi meters not stuffed is it?

 

EDIT: I might need to see a schematic because I can't think of a good reason to have an AC signal there. 

 

Have you measured the voltages of the headphone jack itself? Have you checked for continuity between the headphone jack ground pin and the pc chassis?

Yea that's what I thought.

I don't see you could possibly have an AC voltage there without a sound playing.

My multimeter is cheap, but I tested it on the mains, and it measures that correctly.

I wish I had a schematic, I looked for a while the other night but couldn't find one

If I had one I could probably work it out too, lol.

I shall check continuity between the ground and chassis when I get home.

The measurements I've taken so far have been off of the headphone jack itself, yes (the solder joints connecting the jack to the card).

[rhyseyness]

@mr moose

Ok so I got some real numbers, and now I'm really confused.

Ignore all the numbers I've said up to know, they were from my awful memory, these are what I'm measuring right now.

All measurements taken with a 1KHz tone playing through the output

 

V+ on the op amp: +12VDC, as expected, but also 26VAC wrt ground

V- on the op amp: not really accessible, but I'ma keep trying

Pin 1 & 2 on op amp: 0VDC, 0VAC wrt ground

Pin 3 on op amp: -10mVDC, 0VAC wrt ground

 

All headphone jack pins: 0VDC, 0VAC wrt ground

 

Pin 1 (output) wrt pin 3 (non-inverting input): 20mVDC, 0VAC

 

Pins on the other side of the amp aren't really accessible. I was gonna take out my gfx card,  and move the sound card up, but I have no video out compatible with my monitor on my motherboard.

I'm pretty much more confused now... AC voltage on the power pin. No AC voltage between O/P and non inverting I/P?

What do you think?

 

p.s. the 4.7V from earlier came from a number I was reading off of the mic in port, which I accidentally probed thinking it was the headphone port

[rhyseyness]

Two of the headphone pins have continuity with pin 1 of the op amp.

These two also appear to have continuity with the case.

The other pin on the headphones is not common to the case.

[mr moose]

It does sound to me like a floating ground.  which means I wouldn't use the headphone out to drive another circuit unless you power that circuit from batteries and have no common earth to the chassis or power supply.

[rhyseyness]

It does sound to me like a floating ground.  which means I wouldn't use the headphone out to drive another circuit unless you power that circuit from batteries and have no common earth to the chassis or power supply.

I'll be powering the other circuit from elsewhere (fan header), so no need to worry about that.

I only want the signal, not the power behind it.

I.e. the signal from the jack will feed into an op amp, which is powered from the fan header (potential divider), and then is processed through the rest of the circuit, also powered from the fan header.

I think that should be ok, do you agree?

 

I'm still baffled as to why I can't measure an AC level on the output pin, and I don't understand why it's grounded either

[mr moose]

I'll be powering the other circuit from elsewhere (fan header), so no need to worry about that.

I only want the signal, not the power behind it.

I.e. the signal from the jack will feed into an op amp, which is powered from the fan header (potential divider), and then is processed through the rest of the circuit, also powered from the fan header.

I think that should be ok, do you agree?

 

I'm still baffled as to why I can't measure an AC level on the output pin, and I don't understand why it's grounded either

 

It will only work if the potential between your signal path and actual ground is maintained.  that is you continue to keep the ground floating (I did some research and it looks like they call it a virtual ground now).  All supply and signal potentials have to be maintained or you will get either noise or a very bad DC bias.

 

To determine the rest I'd really need to see a schematic, and probably an oscilloscope.

[rhyseyness]

It will only work if the potential between your signal path and actual ground is maintained.  that is you continue to keep the ground floating (I did some research and it looks like they call it a virtual ground now).  All supply and signal potentials have to be maintained or you will get either noise or a very bad DC bias.

 

To determine the rest I'd really need to see a schematic, and probably an oscilloscope.

Oh yea, a scope would make this so much easier

I'll see what I can do regarding "using" the floating ground.

Thanks for all your help

I'll let you know how it goes (I'll be posting a "build log" here any old how)