Hi there,

I am working on a circuit which would allow me to connect a headphone output to a pc mic input.
When asking for advice about the design a question came to mind (thread can be found here if anyone's interested: https://forum.allaboutcircuits.com/...-output-microphone-input.175995/#post-1591786).

What is the DC current range in a 3.5mm aux output and input jack, and what relevance does the DC current have anyway?
I cant seem to find this online for some reason..

At the moment, I have a capacitor to block the DC current (presumably to protect the mic input), but the circuit doesn't actually work yet. The computer doesn't identify my input as a source, and I suspect that maybe this has something to do with me blocking the DC current. Is it unsafe to have the current going into my mic jack if I still make sure to use a voltage divider to reduce the voltage?

Here's the circuit:

Best regards,
Axel

 

The computer doesn't identify my input as a source

I replied in your previous thread. Really didn't need to start a new one.

What is the DC current range in a 3.5mm aux output and input jack, and what relevance does the DC current have anyway?

Zero and not relevant. There are no DC voltages on the headphone or input jacks.
Besides all that if you plug in the 3.5mm cable to the PC mic input with nothing attached does the computer give a notification that something has been plugged in?

 

In your other thread I said that a pc uses an electret microphone that has a Jfet in it that draws 0.3mA to 0.5mA of DC current. Maybe that is what the computer senses to activate the mic input. I recommended adding a 3900 ohms resistor to ground at the mic input to draw 0.4mA DC so the pc senses "a mic".

You wrongly show a DC voltage of 1V from your headphones output but it is probably a 1V peak audio signal which is not DC.
Your pot is drawn wrongly and will be a VERY poor level control. A level control has 3 pins; an input, a sliding output and a ground.

 

I replied in your previous thread. Really didn't need to start a new one.

Zero and not relevant. There are no DC voltages on the headphone or input jacks.
Besides all that if you plug in the 3.5mm cable to the PC mic input with nothing attached does the computer give a notification that something has been plugged in?

Perhaps you're right, but the question seemed independent enough to be asked by itself. Besides, I asked a few "return-questions" in the other thread that wasn't answered which suggested that the thread was dying

Oh I see..
Interestingly enough though, I don't get a notification when plugging an "empty" 3.5mm into my "test laptop" (less expensive if I break something), but my main computer does show a new mic input when plugging it in.. Could I have damaged something in my sound card on my test laptop if I have ranged between 0 to max 100mV input in my testing?

Best regards

 

In your other thread I said that a pc uses an electret microphone that has a Jfet in it that draws 0.3mA to 0.5mA of DC current. Maybe that is what the computer senses to activate the mic input. I recommended adding a 3900 ohms resistor to ground at the mic input to draw 0.4mA DC so the pc senses "a mic".

You wrongly show a DC voltage of 1V from your headphones output but it is probably a 1V peak audio signal which is not DC.
Your pot is drawn wrongly and will be a VERY poor level control. A level control has 3 pins; an input, a sliding output and a ground.

Yes, and I asked if that would even make a difference since I have a capacitor blocking the DC current. I was afraid to take it away because of me not knowing the DC properties of a normal 3.5mm aux.

Aha, I see! A 3 pin level control is the type of pot that I have; I was just sloppy when putting that component in the schematic. Sadly, a 0K-10K was the only variable resistor I had lying around.

Best regards

 

You need the capacitor to prevent DC in the mic input from going into the audio level control.
The level control should have an audio (logarithmic) taper.

 

You need the capacitor to prevent DC in the mic input from going into the audio level control.
The level control should have an audio (logarithmic) taper.

Cool, I had no idea!
So would a schematic like this be more reasonable with a logarithmic 0-100k pot? I simulated with an audio input with max 1V this time and the values looks like they range between 0-10mV when changing the pot values.
https://www.falstad.com...


Best regards

 

1) Your inputs are not stereo anymore.
2) Your level control is still wrong. Please look in Google for Volume Control Schematic.

 

1) Your inputs are not stereo anymore.
2) Your level control is still wrong. Please look in Google for Volume Control Schematic.

1) Yes, I left the other stereo channel out in the schematic since they would be mirrored and joined in the end anyway.

2) Ohh I see, my third pin is not connected to the other pole like shown in this circuit. I see now that I misunderstood the arrow part of that component in Falstad aswell..


Thank you!

 

Could I have damaged something in my sound card on my test laptop if I have ranged between 0 to max 100mV input in my testing?

I doubt it.
What model is your laptop?
Possible the external mic is not enabled.
I just checked my Acer laptop and it doesn't give a notification either. But the external mic input works.
In fact I connected from the headphone jack on my PC speakers directly to the mic input on the laptop and controlled the level using the volume control on the speakers.

 

I doubt it.
What model is your laptop?
Possible the external mic is not enabled.
I just checked my Acer laptop and it doesn't give a notification either. But the external mic input works.
In fact I connected from the headphone jack on my PC speakers directly to the mic input on the laptop and controlled the level using the volume control on the speakers.

It's an OBSIDIAN pc with windows OS. I cant find my specific model now though (probably since it is quite old by now)

Wow, really? And it did not cap early or anything? Did you measure the peak voltage before-hand?
Wouldn't voltages over 100mV risk damaging the input?

 

Wow, really? And it did not cap early or anything? Did you measure the peak voltage before-hand?
Wouldn't voltages over 100mV risk damaging the input?

Voltage was around 50 to 60 mv peak to peak.
I don't think a 100mv signal would damage anything.
Have you tried connecting your PC to the the laptop like I did?

 

Voltage was around 50 to 60 mv peak to peak.
I don't think a 100mv signal would damage anything.
Have you tried connecting your PC to the the laptop like I did?

I tried connecting my PC headphone output to my laptop and it actually picks up the sounds I am playing!
However, the quality was quite bad.. I guess I can tinker around with some sort of quality preserving circuit solution without having to worry about my sound card now though!

 

I tried connecting my PC headphone output to my laptop and it actually picks up the sounds I am playing!
However, the quality was quite bad.

You must turn down the headphones output level to microphone level.

 

You must turn down the headphones output level to microphone level.

Yeah, I made sure that I started on 0 and turned it upwards until there was decent volume in the laptops mic playback, but the sound had quite poor quality for some reason..

 

but the sound had quite poor quality for some reason..

I noticed that as well when recording. Turning up the volume from the headphone jack actually sounded better.

 

"Poor sound quality" does not describe the problem:
Hiss, hum, distortion, acoustical feedback howling, no low frequencies, boomy low frequencies, no high frequencies, shrieking high frequencies??
Are you hearing the poor quality from the laptop's tiny little speakers?
Are you playing back a recording from the laptop to a poor quality sound system?

 

"Poor sound quality" does not describe the problem:
Hiss, hum, distortion, acoustical feedback howling, no low frequencies, boomy low frequencies, no high frequencies, shrieking high frequencies??
Are you hearing the poor quality from the laptop's tiny little speakers?
Are you playing back a recording from the laptop to a poor quality sound system?

I found it a bit hard to describe. Here's a Vocaroo link from when I recorded a spotify song from one computer to the other.
(WARNING: The sound gets louder after a few seconds, so dont have a loud volume setting from the start)
https://voca.ro/19445UTPDmrT

I recorded this through a mono channel in Audacity. It sounds like I am listening on a recording from a phone recording another speaker. It lacks the lower frequencies, and its pretty distorted. The song is Diffus, by JEREMIAS, in case you want to hear the reference.

 

I recorded this through a mono channel in Audacity. It sounds like I am listening on a recording from a phone recording another speaker. It lacks the lower frequencies, and its pretty distorted. The song is Diffus, by JEREMIAS, in case you want to hear the reference.

Couple of thoughts. My guess is that a mic input on a laptop is not going to be a full bandwidth circuit, but rather optimized for the frequency range of the human voice. Noise is related to a circuit's bandwidth and gain, so again, if you're the maker of the laptop you severely limit bandwidth so no one complains about the mic input being noisy. That likely explains what you are hearing.

You're also going to pay a pretty high noise penalty attempting this as you've got to cut a high signal from the headphone amp significantly only to boost it back up again. It's almost guaranteed you'll end up with audible hiss no matter how good the headphone amp and mic preamp are.

 

The Vocaroo under-water sound is like the Automatic Gain Control on the laptop mic input is made poorly for speeech, not music.
A loud sound is produced then the AGC reacts too late to reduce the level, then the level is too low so it increases it too late, over and over.