Quantum Thunderbolt cable - Computer noise conduit
Posted: Mon Dec 11, 2017 6:52 pm
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
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