Ground loop with an external USB card?? Help! (LONG)
Anyone want to take a crack at this perplexing problem? I have been running my carpc for a couple of months now and it works well except……I have this alternator whine that is driving me crazy! Anyway, I have been trying to troubleshoot this using some sort of logical approach, and as best I can tell, the problem lies with my USB sound card, which I bought because of the great recommendations it received from other carpc owners (and it does sound awesome when the whine isn’t there!).
So, I have a Shuttle-type AMD64 Sempron w/Nvidia 5200, Audigy 2 NX USB, SIR-ALP1 Sirius receiver, wireless card, USB Bluetooth dongle, and Xenarc touchscreen running off of a 400W Coleman inverter. It feeds into an Audiobahn 6601X 6-channel amp which is then wired back into my factory harness to output to the 6 factory speakers on my Chrsyler 300C.
Here is what I’ve tried and what I know, starting with the USB sound card connected, but the Sirius disconnected to minimize possible sources):
1) engine off, no whine, sounds AWESOME, even with the cheapo factory speakers
2) engine on, whine present, remove RCAs in to amp and no more whine (this means the amp and speakers have no loop, anyway)
3) plug in RCAs, whine present again. NOTE: changing cables (I have Monster cables 3.5mm to RCA) doesn’t help. When plugging in, if only 1 cable (pair of RCAs) feeding any two channels is present, I hear a LOUD hum in amp….sound drops to the standard high-pitched whine when 2nd RCA pair is plugged in. Is this evidence of the ground being worse until the 2nd pair permits additional flow THROUGH the amplifier? Dunno, I’m EE challenged…)
4) All RCA ends plugged in, I next remove the 3 3.5mm ends from the Audigy 2 – no whine. I plug them into 3 turned off mp3 players I have just for grins to see if it is some strange electrical ghost – STILL NO WHINE. Looks like it is the PC or USB adapter…. Plug it back in to USB adapter, whine returns….
5) Screw wire into outside of case of PC where it also screws into PSU and ground to car…..no improvement. Screw 2nd wire into chassis of PC and then to same ground as before – no improvement.
6) Disconnect Audigy 2 NX from PC and disconnect power from USB sound card as well. WHINE STILL PRESENT! As best as I can tell, this is a plastic case, and it is not making contact with anything metal that might be an alternative ground, so what the heck????
I don’t know what to do next, folks. I guess I could try ground loop isolators but I didn’t want to because a) I want to figure this out, b) I understand they can adversely affect sound quality, and c) I would have to buy THREE of them to cover the 3 RCAs used to drive my 6-channels.
Does anyone have any other ideas? FWIW, I have also previously plugged the 3.5mm jacks into the onboard sound of the PC motherboard (it supports 6-channel, just not powerful enough for good loud sound) and the whine was there, so it almost seems to me that it is something about the amp flowing current back through any device plugged into it with more than one cable…..I know, I don’t know jack about this, maybe it is just more likely that the PC has some ground issues too which I’ll have to address later, but I can’t see how. I have been testing with the PC sitting on cardboard as I’ve removed the mounts while I troubleshoot this grounding issue.
Okay, enough for now – any help would be HUGELY appreciated!