3.5mm jack sound card output

[rhyseyness]

So I'm looking to use the output from my sound card to "drive" a graphic equaliser I'll be making.

I have the circuitry for the equaliser sorted, but I have a problem.

I'm using an Asus Xonar DGX sound card.

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

 

I only have a multimeter, no scope, and I'm measuring off of the solder connection on the back of the card to the 3.5mm headphone jack.

If anyone has any useful information, I'd very much appreciate it (I've been trying to find something on Google for ages now).

Thanks in advance

[MayflowerElectronics]

Are you sure you are measuring the correct pin?

[EmoRarity]

Moose, come be manly and smart. 

[t0wer]

Are you sure you are measuring the correct pin?

 

I'm thinking it's gotta be this.

 

with nothing playing you should get very close to 0mv dc on output

[ShearMe]

I'm thinking it's gotta be this.

 

with nothing playing you should get very close to 0mv dc on output

 

Although, it is a soundcard.  :rolleyes:

[rhyseyness]

Are you sure you are measuring the correct pin?

I'm thinking it's gotta be this.

 

with nothing playing you should get very close to 0mv dc on output

Although, it is a soundcard.  :rolleyes:

 

 

This is what I thought, but I've tried all 3 pins which are connected to the headphone output. 2 sit at 0V and the other at 4.7V regardless of what I output.

I have today discovered that the pin which measures 4.7V is connected to the output of an op amp, so I assume it must be the correct pin, I just don't understand how volume is regulated without the voltage changing.

[t0wer]

there might be a resistor or cap in between but all 3 points probably come off an opamp.

what exactly are you trying to measure?

[rhyseyness]

there might be a resistor or cap in between but all 3 points probably come off an opamp.

what exactly are you trying to measure?

Yes they do come off of an op amp.

No cap or resistor in between, which surprised me.

I want to use the signal as the source for a graphic equaliser. i.e. the graphic equaliser works with what ever sound is coming out of that port.

It's a bunch of filters and LEDs.

In essence, I want to put the output into another op amp.

I'm trying to measure the change in voltage level of this output, so I know what resistors or caps I will need to use, but there doesn't appear to be any change whatsoever.

[ShearMe]

Maybe the opamp only outputs one voltage, and volume is controlled beforehand by the DAC, or afterwards by some series of resistors?

[rhyseyness]

Maybe the opamp only outputs one voltage, and volume is controlled beforehand by the DAC, or afterwards by some series of resistors?

This may well be the case.

However, what I don't understand is how this would cause the headphones to be louder or quieter at different volume levels (in windows).

I turn up the volume, the voltage being supplied to the headphones doesn't change, but they get louder.

I'm really stumped. The only thing I could think of was changes in current controlling the headphones, but the op amp feeding the port always outputs at the same current level (I checked the data sheet)

[rhyseyness]

If it's helpful, the data sheet for the op amp can be found here

The 4.7V is from pin 1 of that amp

[ShearMe]

Well, if volume is controlled at the DAC, that means it's output voltage is variable.

 

If it's controlled by resistors after the opamp (I'd bet on this) then the lowest volume will have all resistors activated, and the opamp output is basically shorted to ground. At max volume, all the resistors will be switched off, and the full voltage of the opamp will be delivered to the headphones (minus a tad for the soundcard's circuitry and headphone cable, of course). 

This is my understanding of it, though I'm no electrical engineer. Have you tried measuring across the actual headphone port pins? 

[rhyseyness]

Well, if volume is controlled at the DAC, that means it's output voltage is variable.

 

If it's controlled by resistors after the opamp (I'd bet on this) then the lowest volume will have all resistors activated, and the opamp output is basically shorted to ground. At max volume, all the resistors will be switched off, and the full voltage of the opamp will be delivered to the headphones (minus a tad for the soundcard's circuitry and headphone cable, of course). 

This is my understanding of it, though I'm no electrical engineer. Have you tried measuring across the actual headphone port pins? 

 

lol, I'm an electronic engineer and can't work this out

There doesn't appear to be any resistors in series between the op amp output and the headpone output... although, this was with the card unplugged.

However, even if a resistor network is how volume is controlled, I should still see chages in the voltage present on the o/p pin for the headphone jack when I change the volume.

I haven't measured across the headphone port pins... Do you mean the voltage across the headphone pins (e.g. pin 1 of headphone port to pin 3 of headphone port), or the voltage on each of the pins with respect to ground (e.g. pin 1 of headphone port to ground)?

 

Hopefully that made sense

[mr moose]

what does the voltage read on pin 7?

And can you tell me the voltage on pins 4 and 8 with respect to the chassis? <-these should be DC voltages

[rhyseyness]

what does the voltage read on pin 7?

And can you tell me the voltage on pins 4 and 8 with respect to the chassis? <-these should be DC voltages

I'll check when I get home (at work at the moment :'( )

I presume you're refering to the pins on the op amp.

Pins 4 & 8 being V+ and V-.

I haven't yet checked pin 7, didn't even consider the other op amp on the chip.

I'll get those numbers to you ASAP (probably in about 8 hours time).

[ShearMe]

lol, I'm an electronic engineer and can't work this out

There doesn't appear to be any resistors in series between the op amp output and the headpone output... although, this was with the card unplugged.

However, even if a resistor network is how volume is controlled, I should still see chages in the voltage present on the o/p pin for the headphone jack when I change the volume.

I haven't measured across the headphone port pins... Do you mean the voltage across the headphone pins (e.g. pin 1 of headphone port to pin 3 of headphone port), or the voltage on each of the pins with respect to ground (e.g. pin 1 of headphone port to ground)?

 

Hopefully that made sense

 

Honestly, I don't know what an "o/p pin is"  :unsure:

 

As for the  headphone jack, the "sleeve" is the ground, and each other connection carries an audio channel. So, between the tip and the sleeve, you should get a variable voltage as you adjust volume, and the same for between the ring and the sleeve.

[mr moose]

I'll check when I get home (at work at the moment :'( )

I presume you're refering to the pins on the op amp.

Pins 4 & 8 being V+ and V-.

I haven't yet checked pin 7, didn't even consider the other op amp on the chip.

I'll get those numbers to you ASAP (probably in about 8 hours time).

yep,  I am predicting the chip is set up as a floating ground. so you should get 9-12v on the pin 8 and zero on pin 4. with respect to the chassis or PSU negative (black cable)

[ShearMe]

yep,  I am predicting the chip is set up as a floating ground. so you should get 9-12v on the pin 8 and zero on pin 4. with respect to the chassis or PSU negative (black cable)

 

[mr moose]

and how will I look if I am wrong?

[rhyseyness]

yep,  I am predicting the chip is set up as a floating ground. so you should get 9-12v on the pin 8 and zero on pin 4. with respect to the chassis or PSU negative (black cable)

That's interesting. I was just considering if it was something to do with the voltage floating to cause it to stick at 4.7V wrt ground.

The power of the  op amp changing dynamically is pretty impressive stuff.

Wouldn't that op amp need a + and - voltage though? 9 to12V on pin 8 and -9 to -12 on pin 4?

Non inverting inputs grounded?

I know op amps pretty well, but not when they're on sound cards. Is it different for soundcards?

[mr moose]

That's interesting. I was just considering if it was something to do with the voltage floating to cause it to stick at 4.7V wrt ground.

The power of the  op amp changing dynamically is pretty impressive stuff.

Wouldn't that op amp need a + and - voltage though? 9 to12V on pin 8 and -9 to -12 on pin 4?

Non inverting inputs grounded?

I know op amps pretty well, but not when they're on sound cards. Is it different for soundcards?

Nope, because the pc does not have a +/- power supply, but buy putting 10-12v+ on the pin 8 and 0v on pin 4 at zero signal output the op amps output will be exactly half way between pin 4 and 8 exactly like a class A amp.  which would be around 4.7V allowing for losses.  the output of the amp will then swing up and down from 4.7 (effectively making 4.7v the new ground reference hence the term floating ground) if the other side of your headphone is connected to a 4.7v source then the headphone will see an AC signal with 4.7v peaks. 

[rhyseyness]

Nope, because the pc does not have a +/- power supply, but buy putting 10-12v+ on the pin 8 and 0v on pin 4 at zero signal output the op amps output will be exactly half way between pin 4 and 8 exactly like a class A amp.  which would be around 4.7V allowing for losses.  the output of the amp will then swing up and down from 4.7 (effectively making 4.7v the new ground reference hence the term floating ground) if the other side of your headphone is connected to a 4.7v source then the headphone will see an AC signal with 4.7v peaks. 

Ah yes, I see.

That makes sense.

I think you're probably right.

More complex electronics than I expected.

I was hoping for an op amp which just swung either side of a 0V reference (that's what I'll be using )

Do you know the reason they use a floating ground?

[mr moose]

Ah yes, I see.

That makes sense.

I think you're probably right.

More complex electronics than I expected.

I was hoping for an op amp which just swung either side of a 0V reference (that's what I'll be using )

Do you know the reason they use a floating ground?

It's because they only have single rail supply. So by making half of the supply voltage the ground point you then create a dual rail supply.  effectively turning a 12v and ground into a -6v 0v +6v supply.

[rhyseyness]

It's because they only have single rail supply. So by making half of the supply voltage the ground point you then create a dual rail supply.  effectively turning a 12v and ground into a -6v 0v +6v supply.

Oh yea, I understand that. I'm using the same system for my op amps which I'm using for filters.

... actually, I should have confirmed, I'm measuring 4.7V AC on OP1 of the op amp, not DC

[mr moose]

Oh yea, I understand that. I'm using the same system for my op amps which I'm using for filters.

... actually, I should have confirmed, I'm measuring 4.7V AC on OP1 of the op amp, not DC

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?