Hi all,
 
Microphones in the same room are quiet.  I think I will try making sure the guitar pickups and electronics are well shielded and everything first. 
 
With the guitar volume down and the preamps cranked there is only white noise.  No firehash. 
 
Yeah I agree lifting the grounds on everything is not the smartest thing to do.  I have not heard of the "pin 1" Firewire noise.  I'll look into that.  I also tried a regular guitar cable and a shielded TRS cable between the pre and the mixer input...no difference in the noise.  I did lift the ground on my headphone distribution amp.  (I had to to make it usable) 
 
My single coil guitars are going to remain problematic though.  I can correct the noise by moving the guitar in to just the right position, however, that is not really conducive to a natural performance. 
 
There is no Firewire/USB knob that I can see.
 
Anyone know a source for faraday cages for PCs?
 

Sounds more like RFI than ground problems (although your headphone distribution amp says that you do have a grounding problem - are you sure that you have a real ground?).  Just as a test, you may want to consider unplugging your video monitors from the back of the computer while you are experiencing the noise.  Monitor cables are a common source of RFI and often need additional ferrite filters to get the noise levels down.
 
Your case should be functioning as a faraday shield if it is FCC compliant (required for legal sale in US, but often 'missing' on cheap imports).
 
Glen

I tried the video with both HDMI (which I am currently using) and the regular VGA plug.
 
No difference.
 
Also no difference whether the monitor is on or off.
 
I know the ground is good. 
 
I just ordered a top of the line Firewire cable that is more heavily shielded and has ferrite beads on both ends.  I'll report back when I try it.  (I found a thread on Gearslutz where someone was having issues just like mine and this cable solved it.) 
 
Fingers crossed.
 
Thanks!

Hi Erik,
 
Well, from your extended description of the problem, now it sounds that it's clearly RFI. You said your guitar had humbuckers in your original post, but you may have just mentioned the culprit...single coil pickups are by nature really susceptible to RFI.
 
Putting a TRS cable between the Mesa preamp and the Zed wouldn't do much beyond lowering the noise floor if the preamp's output is balanced. To find a ground problem, you'd need to lift pin 1, which is the "pin 1 issue" Cactus Music mentioned above. That's what a "lift" switch does on a DI, which was why I was suggesting you try that. There's no "pin 1" to literally lift in a firewire or USB cable, but there are cables you can get that do essentially the same thing. As for AC lifting your headphone amp, you'd be much better off getting a pin 1 lift adapter if the inputs are XLR, or one of those Ebtech Hum Eliminators if they're TRS.
 
Anyhoo, looks to me like you have garden variety interference on ye olde single coils.

...a more heavily shielded firewire cable isn't a bad thing to have, but if it is just RFI through your pickups, it won't solve the problem. The only answer there is more cavity shielding on your guitar and/or using humbucking pickups. Or sitting in a horribly contorted position to avoid the stray magnetic field leaking out of your computer
 
Good luck!

I still have a similar problem with a API 500 series lunchbox. My noise is constant and it is tied to when a hard drive is being accessed.  I can hear a chirping sound when the hard drive is turning thru my audio.  I have to totally unplug the lunchbox to make the sound go away.  No other issues with any equipment in my studio (including my collection of about 18 different electric guitars).  I have tried everything including installing a line conditioner for all my studio equipment and PC.  No difference until I unplug (disconnect the power source to the lunchbox).
 
This too started happening when I switched to a new PC.  My "gut" is telling me my issue is with the HIGH END power supply in my DAW PC but never thought it could be related to a specific component (HD, video card, etc.) within the PC itself.
 
My problem has rendered my lunchbox hardware unusable in my studio (using the APOLLO as my preamp now as a substitute).  So, I read this thread carefully to see if there was some insight I hadn't yet tried. The OP tied this problem to a new PC and that was also the trigger to issue starting.
 
Thanks,
Jim
 

diasble C1 and EIST in the BIOS? can be an issue...
http://www.tomshardware.co.uk/forum/329790-28-processor-noise-coming-speakers
http://www.gearslutz.com/board/music-computers/473245-calling-all-gigabyte-mobo-users.html
 
 

I am a big believer in "process of elimination."   Have you tried the guitar through any other inputs?(e.g. direct into the Zed.) Or have you tried  other guitars in your preamp?  Have you tried plugging the Mesa into a different channel on the board? 
 
I really hope you get to the bottom of this. These kind of issues are a real pain in the butt,\.  Good luck!

tubeydude
 
My single coil guitars are going to remain problematic though.  I can correct the noise by moving the guitar in to just the right position, however, that is not really conducive to a natural performance. 
 

I have the same problem when I plug my Telecaster into my Scarlett 2i2… turn one way, noise… turn slightly another way, no noise.  I just assumed it was some kind of RF interference my pickups were…picking up.  Not even sure if it's coming from my computer or some other local electric device.  Rather than tear my hair out trying to figure out what's causing it I take the lazy man's way out and move the guitar until I don't hear it anymore…of course I get screwed if I move it the wrong way during playing.
 
Didn't see this mentioned in any other response, but if your new machine has built in wireless, you might try turning it off (if it isn't already).
 
Best,
Rob