ASUS Motherboard Front Panel Audio Issue

[Dedpoet]

Hello everyone. I'm having a problem connecting the front audio panel connectors on an old version Coolermaster Praetorian to an ASUS M4A78 Plus motherboard.

According to the motherboard manual, the header can act as HD Audio or AC97, depending on how you toggle it in the BIOS. This is how it describes the pinouts. The "O" is the spot without a pin, where it would key if this were a solid connector:

Edit: Yes, the manual shows "PORT1 L" twice. I assume the bottom one is "PORT2 L". It's 2009 and still no one checks motherboard manuals for errors...

HD Audio:

PORT1 L     XX  GND
PORT1 R     XX  PRESENCE#
PORT2 R     XX  SENSE1_RETUR
SENSE_SEND  XO
PORT1 L     XX  SENSE2_RETUR

AC97:

MIC2        XX  AGND
MICPWR      XX  NC
LINE OUT_R  XX  NC
NC          XO
LINE OUT_L  XX  NC

Unfortunately my case connectors are not labeled either way. They are:

Ground
MIC Power
MIC In
L OUT
L RET
R OUT
R RET
Ground

Google searches have led to 404'd pages, unanswered posts, or answers that don't make any sense. Does anyone know which pins should go to which connectors, and whether I should be using AC97 or HD Audio in the BIOS? Thanks!

 

[Mohonri]

I ran into this issue about a year and a half ago, so I ought to be able to help:
First, set your bios to use AC97.

Next, here's the pin translation: L Out is LINE OUT_L and R OUT is LINE OUT_R. MIC Power is MICPWR and MIC2 is Mic in.

The R RET and L RET should be placed on the pins opposite the LINE OUT_R and LINE OUT_L. The motherboard may not actually use those pins (see note below), but it'll keep those connections tidy. You'll have to test the two Ground connectors to see if they are tied together at the front panel (use your multimeter--you have one, right? ) If they are indeed connected, go ahead and plug either one into the AGND pin. If not, then you'll need to connect them somehow.

So here's the deal: AC97 was designed to put the two-channel output on the R OUT and L OUT pins. The headphone jack is of a type that returns the audio signals back to the motherboard if no headphones are plugged in. So without headphones, the signals go back into the motherboard through R RET and L RET, and then out through the back-panel speaker jack. Thus, if you plugged in headphones, it would break that return path, automatically cutting output to the speakers.

The Intel HD Audio spec, on the other hand, uses a discrete switch inside the front-panel jacks, which just sends a present/absent signal through the SENSE1_RETUR and SENSE2_RETUR pins. The sound chip, in conjunction with the driver and software, can then decide what to do when headphones are plugged in (or not). With AC97, there's an increased chance of picking up noise if headphones aren't plugged in, since the signal goes through three mechanical connections and some cable before even getting to the back-panel jack.