building a HD Audio front panel for my PC. Audio circuit?


I have a problem with my computer. the motherboard has HD-Audio but the front panel is the usual AC97 standard. You can change the BIOS config so the front panel audio connector acts as an AC97 one, but then the problem is the speakers connected to the back keep playing even if you connect a set of headphones to the front panel.

The reason is the audio jacks used for HD-audio are different from regular jack connectors (and very rare to find!). While the insertion of the plug in a regular jack connector results in breaking a circuit (and so, physically muting the loudspeakers), in an HD Audio jack, the insertion of the plug results in closing a switch (sending a signal to the driver ordering to mute the speakers by software).

Since I can't find this kind of jacks anywhere, I managed to create a sort of HD audio Jack by combining a regular jack connector and a zero pressure micro-switch so when the plug enters the hole, a tip inside the hole closes the micro-switch.

I wanted to make a good quality front panel so instead of just wiring the jack connectors (and micro-switches) to the cable, I decided to add an EMI filter.

I took the circuit from this doc:
and more precisely from fig 6 at page 25.

The circuit uses two 220 pF capacitors and two inductors for each jack connector. In this aspect the circuit is identical to that proposed for AC97. The problem is I have no idea what inductors to use or how to ask for them at the shop.
All the diagram says in a footnote is: ZL should be 600 Ohms or greater @ 100MHz with a low Q (broad Impedance curve over frequency)
but not even the guys at the shop knew what piece to give me.

So what I did was to cannibalize the inductors from two cheap promotional radio receivers with jack connectors for the headphones and use them for my circuit. After all, they perform the same function: filter the audio signal at the headphone connector so I supposed they should have the correct values I needed.

But my circuit is very noisy! headphones have a constant electronic background noise (that seems to react to things like mouse movements, hard disk activity or whiteness of the screen) and microphone has a lot of noise (of course I'm using very good quality mic+headphone). The cables are audio cables for front panel audio so they are supposed to be correctly shielded so the problem is in the circuit at my custom board.

I would like to make a tutorial (including HD Audio connector construction) once the front panel is working properly but I don't know how to fix it. Any ideas?

ruggb4 years ago
i have no idea when this was posted.
The noise u hear sounds like data noise. It is what the filter is suppose to block and short to grd.
If the 220pf caps are smaller than 220 less of that noise will be shorted to grd.
if the Inductor is too small more data noise will get thru

Zl=2πfL for the inductor - the bigger the I, the more noise is blocked
Zc=1/(2πfC) for the capacitor - the bigger the C, the more noise is shorted to grd

I think 100MHz is way too high to calculate for in an audio ckt. U sure it isn't 100KHz? But if the Q is not high enough and u calc for 50K or 100K u may have no audio.

I would increase the size of both till u get better results.
u can also try disconnecting one side of the cable shield. u may have a ground loop that is aggravating things. Use the side closest to ur main ground point.
jedikalimero (author)  ruggb4 years ago
Hi Ruggb,
Yes, there was a long time since I posted this and I already solved the problem. I still don't know the values for the inductors since I'm still using the ones I canibalized but the problem with noise was my circuit ground was connected to the shield ground so connected to the screws and metal frame of the computer.
So I isolated the ground of my circuit and only connected it to the motherboard through the audio cable and suddenly all noises disappeared.
Now it sounds much better than a commercial hd audio connector box since this boxes don't use to have filters (no inductors, no capacitors, just the connectors and the resistor that indicates the box is present)

In most cases like this you decouple the ground with a 100nf polyester capacitor it should fix the ground loop problem.

ruggb4 years ago
ground loops do it all the time. amazing isn't it?