Audible ground loop before windows starts

**willmeyer** · 04-22-2009, 12:18 PM

I have a USB sound card with audio outs hooked to my head unit. I don't use the pc to control any amps because I want the audio system to work separately from the pc (for obvious reasons ;-)). The problem is that there is a ground loop on the audio line in while the pc is starting up (meaning I hear a high-pitched whine in the otherwise silent audio signal while the pc is starting). Once windows starts and the usb sound card is turned on/activated, the loop stops and all is well.

Any ideas on what I can ground to avoid this? I get the problem in theory -- the audio input line is effectively just carrying noise from the system/soundcard case/etc until its circuitry actually turns on, but it is kind of a bummer.

Can I ground the audio inputs somehow? Or the sound card or something?

Thanks for any tips

**bratnetwork** · 04-22-2009, 01:05 PM

Maybe try plugging the sound card into a powered USB hub and have the hub turn on with the pc power. that way the sound will be powered and grounded from the beginning.

**Sonicxtacy02** · 04-22-2009, 01:29 PM

the problem is when windows is starting up your sound card is powered but the drivers aren't initialized. There's nothing that you can do about that really.

**willmeyer** · 04-23-2009, 10:48 PM

yeah seems like it basically. The thing I don't get is that before the drivers and such are initialized, there is a whine with the alternator, so there is some signal on there from somewhere that shouldn't be, and it seems like i should be able to ground that out somehow. The USB card is powered off of the pc supply, so it is not grounded to chassis directly. I am still not sure why someone told me to run both the ground and the positive line from the pc supply, rather than just the positive and then using the chassis ground. Should the carpc's PSU output lines share the chassis ground, or is it a totally disconnected/decoupled circuit?

Going to have to start pulling devices out and see what happens.

**Curiosity** · 04-24-2009, 12:47 AM

The sounds card might be in a high impedance state when not initialized. Some kind of line level filter might work. Another option would be to control the amp remote-on with both sources (I'm assuming HU and PC) by using diodes. Either or both would turn it on at the appropriate time. example:
HU-->|--\
PC-->|-- --AMP

**97luder** · 04-24-2009, 01:45 AM

How about relays to bypass the aux in and use a timer to control it

**Curiosity** · 04-24-2009, 06:09 AM

Oh, AUX! Duh, ok. Can you use relays with audio? I see some circuits on the net that do it. Get a pair of small 5V signal relays, an old USB cable and maybe some stereo jacks, route the stereo signals through them and power them off the USB hub. I'd expect a little click when it goes on, but no timer needed.

**Crinos** · 04-24-2009, 06:42 AM

Originally Posted by Sonicxtacy02

the problem is when windows is starting up your sound card is powered but the drivers aren't initialized. There's nothing that you can do about that really.

Yes there is.. delayed start of USB drive... been discussed before on this forum

**Sonicxtacy02** · 04-24-2009, 09:33 AM

Originally Posted by Crinos

Yes there is.. delayed start of USB drive... been discussed before on this forum

The problem is the usb initializes too late as it is. Delaying it wont help.
Unless you meant delaying the power to the usb drive.. then we're talkin

**HiJackZX1** · 04-24-2009, 11:41 AM

Originally Posted by Curiosity

Oh, AUX! Duh, ok. Can you use relays with audio? I see some circuits on the net that do it. Get a pair of small 5V signal relays, an old USB cable and maybe some stereo jacks, route the stereo signals through them and power them off the USB hub. I'd expect a little click when it goes on, but no timer needed.

I think the easiest solution is to have the AMP turn on with the PC. Problem is the PC would have to be on or the AMP wont be on. This is why I did away with my HU. If you use radio then just get a HD Radio that intergrates into the PC.