Noise with bus-powered audio interfaces

I have posted similar material previously but given the recurring nature of the problem and that some of the advice I've seen posted exposes owners of these devices to both serious physical and financial risk, I'd like to reiterate the points I've made previously.
 
NEVER EVER use ground-lifting adaptors or disconnect ground wires on the AC mains side. To do so exposes you to two risks. The first is simply that in disconnecting the protective ground you are disabling a vital safety measure intended to ensure that if a power adaptor etc. fails, then any live mains voltage that makes its way through to the equipment chassis will effectively be shorted to ground and blow a protective fuse or trip a circuit breaker before any harm is done elsewhere.
 
If you have never had the experience of being across either 110VAC or (elsewhere in the world, up to 240VAC) then I can assure you from personal experience that it is at best excruciatingly painful. Should you be unlucky enough to trigger ventricular fibrillation, then you will also have several seconds of remaining consciousness during which you will be aware that you have just killed yourself. (there are plenty of reports of such experiences, narrated obviously by onlookers). You may also be surprised to learn that much smaller voltages can be fatal if the contact points are close to the heart and/or the skin is moist. One researcher died after applying only 12V to his body during a research project.
 
The financial risks are twofold. Firstly that an electrical failure is more likely to trigger a fire if the ground path is interrupted, and secondly that when grounds are lifted, equipment tends to 'float' and there is a substantial voltage between chassis and ground. This voltage exists because of capacitive  leakage paths within power adaptors (particularly via transformers, which have capacitance between the primary and secondary windings). This voltage is associated with a high impedance so it isn't immediately dangerous - you'll probably have experienced this when touching an improperly grounded device; the surface feels like it's vibrating when you run your finger across it.
 
But this voltage, while associated only with a very small current, poses a serious danger to your equipment. If you connect or disconnect signal cables associated with an improperly grounded device, it's quite possible for a hundred volts or more to appear at sensitive preamp inputs or outputs. Since inputs, in particular, have quite high impedances, this voltage can go on to destroy semiconductor devices or degrade them so that they either fail prematurely or exhibit seriously reduced gain or noise performance. I've seen all manner of equipment damaged or destroyed this way, including laptops where a projector was plugged in to the VGA port and destroyed the laptop.
 
So, what to do?. Well, the basic problem with bus-powered devices is that an already noisy power source (taken from the computer) is being fed into a digital device whose power consumption is also 'spiky' as various digital devices inside the audio interface switch on and off.
 
Although the USB cable ground connection is fairly substantial, due to the high currents flowing in the cable (up to half an amp), a voltage will be induced between one end of the cable and the other. Although the spikes and noise associated with this fluctuating voltage are small, a few millivolts at most, this is of course the kind of sensitivity that mic preamps are designed for. So when another piece of equipment is plugged into the interface, it's quite possible for this voltage to reappear across the preamp input, leading to hum, buzzes, and various noises, which not uncommonly change as other equipment such as a mouse is moved around. (this is the contribution from the already noisy power supply inside the computer).
 
When this happens, the safest mitigation is to connect a thick cable - and by this I mean something with a conductor size of 3mm or thereabouts, between the audio interface and an appropriate ground reference, which might be the ground on the mixing desk it's plugged into, or the computer, or possibly a ground directly at the mains plug itself (you make a cable with just a ground wire that plugs into the electrical outlet). The best kind of cable I've found is the flexible insulated copper braid cable sold to wire up car audio systems - it can be obtained in nice solid thicknesses but is still flexible and easy to deal with.
 
What you have done here is to give that noise current a much lower impedance path than the USB cable. This should substantially reduce the induced noise.
 
You may also - if the interface supports balanced inputs/outputs - use special cables for the ins and outs where the ground is lifted at one end. This is perfectly safe since you are only dealing here with audio level signals and there are no safety implications.
 
To be honest however, none of these mitigations works all of the time. Bus powering an audio interface is a fundamentally flawed idea. Cut your losses and purchase an interface that uses an external power adaptor, such as the FocusRite Scarlett 6i6 etc. Note that some of the smaller FocusRite units ARE bus powered. The Roland quad capture is bus powered, hence the prevalence of posts regarding noise issues. It's just a plain stupid idea and although it works in a lot of cases, it's just guaranteed to cause problems in other cases.
 
 
 
 
 
 

The reason I would never use a bus powered interface is there's no way your going to have proper Phantom power for condensor mikes. There's just not enough juice in that cheap little supply. Then you use the 2 prong wall warts and you get hum.. No look for an interface with a A/C power cable or next best a gounded/ earthed wall wart. 

i had one plugged into the hub built into my laptop cooling station just to test it out.  how does all that hum show up in the audio through the usb??? lmao!

You know, these threads are why I really love this place!!
 
Thanks guys.