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Quantum Thunderbolt cable - Computer noise conduit

Posted: Mon Dec 11, 2017 6:52 pm
by jamesbarber2
I recently bought a Quantum and am having a terrible time getting and keeping computer noise out of my rack. The rack contains the Quantum, a XLR patch panel, (not currently used) a Behringer active DI and a Behringer 4-channel headphone amp. The Thunderbolt cable is the only physical connection between the computer and the rack other than the common 120V mains and safety ground.

When I first connected the Quantum main outputs to the headphone amp, (via 1/4" TRS cables) the noise in the cans was terrible. Lifting the shield at the headphone amp knocked it down by probably 20 dB or better, but you can still hear it in the background. Adding any additional outputs from the Quantum to the unbalanced 1/4" AUX inputs on the headphone amp (for "more me" monitoring) brings back the same horrible level of computer noise, regardless of how you wire it. The only thing I've found that works is using the active DI box to isolate the Quantum from the headphone amp. That isn't a great solution, because I need the DI box for instruments. I *could* buy another one, but I'm not ready to give up on a "real" solution first.

What's worse than the previous paragraph is that if I use a very high impedance (unbalanced) acoustic guitar pickup on the Quantum ports 1 or 2, (in instrument mode) I get low-level computer noise in the recording. I haven't bothered trying it with the active DI box, because as above I'm holding out on that being the "last resort" solution.

The PC is a hand-built i7-6700 with a steel case, ASUS motherboard, the ASUS Thunderbolt EX 2 PCIe card, and the ubiquitous Startech Thunderbolt 3 to 2 adapter dongle. The Thunderbolt 2 cable between the dongle and the Quantum is a no-name Thunderbolt cable I bought in the Apple section at Best Buy. I can certainly buy a different one, but since I haven't found any yet with common-mode noise suppression (like ferrite cores) I haven't done it yet.

So... Any suggestions? I'm an old, cranky EE/audio engineer and I've tried all the normal "tricks" for breaking ground loops. This seems to be a case of RF noise from the PC being conducted common-mode over the Thunderbolt cable, and spraying all over the audio rack.

Thanks,
--jim

Re: Quantum Thunderbolt cable - Computer noise conduit

Posted: Sat Jan 19, 2019 8:14 pm
by joekenny1
Did you sort out your problem? I'm curious if it was your cable. I'm running a 2m Thunderbolt startech not having any trouble. I'm thinking of switching to a 3M startech cable. I'm hoping I won't develop problems.

Re: Quantum Thunderbolt cable - Computer noise conduit

Posted: Sun Jan 20, 2019 10:58 am
by Jim Roseberry
Running a 3m Thunderbolt cable here connecting Quantum to custom PC... no noise
I'm wondering if it's a ground issue.

Re: Quantum Thunderbolt cable - Computer noise conduit

Posted: Sun Jan 20, 2019 11:11 am
by Skip Jones
I have a Q and Q2, no noise issue although I have seen some folks reporting issues.

If you ARE having an issue with your.Quantum and noise and have not opened a service ticket, please do so now. Be sure to include your system specs as this seems to be an issue only on some setups.Opening a ticket will help identify any issue needing resolution.

https://support.presonus.com/hc/en-us/requests/new

Re: Quantum Thunderbolt cable - Computer noise conduit

Posted: Tue Jan 22, 2019 11:06 am
by TerjeNorway
When I had my speakers power, pc power and audio interface in the same power strip I got a strange ground computer noise. When I put my active speaker power in another power strip it solved my problem. So what I learned:
Don't put the speakers and audio interface power in the same power strip
Don't put the pc power and audio interface power in the same power strip
Don't put the pc power and speaker power in the same power strip

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Re: Quantum Thunderbolt cable - Computer noise conduit

Posted: Tue Jan 22, 2019 9:50 pm
by jamesbarber2
The issue turned out to be a ground loop. Huge surprise, right? :-)

In my application at least, you can either run the Quantum out of the rack or run it in the rack with the other gear... If you choose the latter, however, you'd better be prepared to do a significant amount of case-to-case ground bonding. At the risk getting flamed, I also cut the safety ground on the Quantum's IEC power cord.

The combination of the above, a (yet another!) new computer and changing out the DI box took care of the issue. I still get classic pickup buzz, but that isn't the fault of the rack. The mic channels are dead silent, or at least less noisy than the studio room rumble. I could put a 600-ohm resistor across an XLR cable end and measure it, but I haven't been that motivated now that the rack is working.

Getting back to the safety ground, I did order a couple of industrial power strips with 12AWG internal wiring and 10AWG cords. When and if I get the time and spare change, I want to:

(1) Install a new electrical panel that has lower resistance neutral and ground busses. (current one isn't the shiniest)
(2) Pull a dedicated 240V run from the new panel to the studio
(3) Put my (unrelated) test lab on one leg of the new circuit and the (related) recording rack on the other

Nothing new under the sun, but I will say that now that I've been able to put a hundred or so hours on it, I really do like the Quantum.

HTH.