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Front of case jacks

post #1 of 7
Thread Starter 

As far as I can tell, a sound card amplifies the front panel jacks of a computer case just the same as plugging headphones straight into the sound card in the back. Do most people use the front panel jacks or plug directly into the back?

post #2 of 7

The front panel output is actually not always the same as the rear one. For example, the motherboard in my PC has only a line output (with huge output impedance and only 10 uF capacitors) at the rear, while the front panel uses the built-in headphone amplifier of the Realtek chip, and is not as bad at driving headphones. Unfortunately, however, the front panel is often poorly implemented in PCs, and is prone to interference and grounding problems.


Edited by stv014 - 12/30/12 at 1:30am
post #3 of 7
Thread Starter 

The integrated Realtek audio of my Asus motherboard sounded pretty bad when plugging in to either the front panel or back. When I bought my X-Fi Titanium and installed it however, the X-Fi seems to be now powering the front panel. So plugging into the front panel or directly in to the soundcard sound good. I can still plug into the Asus motherboard in the back and get the substandard Realtek sound. Is this how it works?

post #4 of 7
Thread Starter 

So I looked inside my case and discovered that the front panel is connected directly to my X-Fi Titanium soundcard with a HD audio connector (it is a cable with both the AC'97 and the HD audio connectors on it). There is a separate connector on the motherboard to connect the front panel to the integrated Realtek sound.

post #5 of 7
Quote:
Originally Posted by TwelveTrains View Post

The integrated Realtek audio of my Asus motherboard sounded pretty bad when plugging in to either the front panel or back. When I bought my X-Fi Titanium and installed it however, the X-Fi seems to be now powering the front panel. So plugging into the front panel or directly in to the soundcard sound good. I can still plug into the Asus motherboard in the back and get the substandard Realtek sound. Is this how it works?

 

Yup. I have the same setup with my Asus Sabertooth Z77 + X-Fi Titanium HD.

 

Naturally, I don't use the on-board Realcrap codec. Only have it enabled because it gives me an extra line-input so I can send in my phone's output until my 3.5mm to RCA cable shows up.

 

Quote:
Originally Posted by TwelveTrains View Post

So I looked inside my case and discovered that the front panel is connected directly to my X-Fi Titanium soundcard with a HD audio connector (it is a cable with both the AC'97 and the HD audio connectors on it). There is a separate connector on the motherboard to connect the front panel to the integrated Realtek sound.

 

You will still have all the issues with poor shielding, and depending on your case, poor grounding.

post #6 of 7

I don't think that shielding is relevant here - a half meter cable is not enough of an antenna to drive headphones.


The real fault of (some/many?) front panels is that they connect audio ground to the case.


There is always a few mV of voltage between motherboard ground traces and case due to lots of devices dumping current onto motherboard ground. Front panel cable has nonzero resistance, so it cannot fully even this difference out (and it shouldn't, it's not its job to be a current return from motherboard to PSU).


Since there is a voltage drop across panel connector's "ground" wire, the soundcard and headphones "see" different "ground levels" and the signal which is correct relatively to "soundcard ground" isn't correct relatively to "headphone ground". Now, if the voltage between motherboard ground and case was constant, this would only cause a small DC offset. However, this voltage varies with devices (mainly CPU) changing their power consumption many times per second, so it produces hissing in audible frequencies.


I've once fixed such panel (by disconnecting jack ground from case) and it became dead silent.


Edited by mich41 - 1/6/13 at 5:49pm
post #7 of 7
Quote:
Originally Posted by mich41 View Post

I don't think that shielding is relevant here - a half meter cable is not enough of an antenna to drive headphones.

You'd be surprised at what can happen when amped, even with a cheaper amp like the PA2v2.

Otherwise, thank you for explaining exactly why front-panel jacks have poor grounding.

I suspect PC audio also depends to a certain degree on the PSU quality and the associated ripple voltage. A good PSU will likely lead to a cleaner sound, but I personally haven't been able to test that theory: I switched from a crummy laptop on-board sound all the way up to an X-Fi TiHD with a Corsair AX850 (one of the best performing desktop computer PSUs out there... besides its rather higher than normal 12V rail (12.288V for some reason, within ATX spec), other voltages are within 1% with very low ripple on all voltages).
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