This is probably USB noise, best reduced by balancing the signals. I would first try using standard TRS-XLR or TRS-TRS cables from the Saffire 6 1&2 out directly to the sub (not via the NAD), leaving the sub to monitor cables as they are. If that helps but does not cure the problem, you could try XLR-XLR cables going from the sub to the monitors as well. Have you tried the effect of the ground lift switch on the sub?

If the light bulb comes on, that could illuminate the buzzing sound.

Thanks heaps for the response!

I really want to keep my NAD Pre amp as it also connects to my Record Player and CD player.

Is using TRS from the Saffire the only way of getting rid of the usb hum?

I will try out the ground switch on the Subwoofer when i get home tonight.

Thanks heaps so far!

What you want to try using is balanced connections. The TRS outputs from the Saffire and the TRS or XLR inputs to the sub are balanced. The NAD is unbalanced.

You have to work on this methodically. Just to see if the problem disappears, try using TRS cables direct from the Saffire to the sub. If the buzzing is cured, you then try unbalanced RCAs from the Saffire to the sub. If there is still no buzz, it's the NAD that's the trouble. If it buzzes when using the RCA link (no NAD), you know you have to use balanced links, which means re-thinking how you connect your record and CD players. The sort of thing you should be looking at is an SM Pro Audio MPatch-2, which has balanced and unbalanced inputs with push-button selection.

Step 1, are you using a single outlet or is everything on the same circuit ? if so your problem may be a simple ground loop. you can use the various ground loop eliminators on the market or spend the 75 cents on a 3 prong to 2 prong ground plug adapter.

I personally don't think that you will get rid of the hum without going through some of the above suggestions. Part of the problem here is that computers utilize switching power supplies which have a tendency to both cause him & are marvelous at transferring clocking noise. Sometimes, the only solution is not so much balancing the circuit but utilizing a 1:1 audio transformer, to provide for electrical isolation by not connecting the ground of the transformer to the input of your NAD preamp/amplifier. All too frequently, someone will utilize a low impedance/high impedance, high impedance/low impedance transformer which will really mess up your gain staging causing either excessive noise in the step down process or, excessive distortion in the step up process. So while Transformers have been suggested the most typical ones are those impedance matching step up/step down types which really aren't appropriate in your application. And these 1:1 Transformers, which are generally intended only for professional applications, can get a bit pricey. As you indicated, this is strictly a monitoring problem where no hum actually has been recorded.

I can never remember the lyrics so I always hum
Mx. Remy Ann David

Remy are you referring to an isolation transformer? Such as the likes as this...?>>>
[="http://www.tripplite.com/EN/products/model.cfm?txtModelID=228"]Tripp Lite Isolation Transformer-Based Power Conditioner - Superior line noise reduction and spike suppression (IS1000)[/]="http://www.tripplit…"]Tripp Lite Isolation Transformer-Based Power Conditioner - Superior line noise reduction and spike suppression (IS1000)[/]

I have been meaning to get one of these doohickies so as to make it a true bench tech model (which involves removing the ground on the secondary coil). In this version the thing is still truly grounded on both sides of each transformer. I need to get one of these for use w/ checking for shorted out circuit boards in tandem w/ an oscilloscope. But in the case here they have versions of these for the like of HG "Hospital Grade" type equipment that needs some noise filtering. If this is what you are referring to then this youtube video explains how to remove the ground as to have a truly isolated 1:1 power transformer.
[[url=http://="http://www.youtube…"]ToddFun.com: Isolation Transformer - Tripp Lite IS1000 - YouTube[/]="http://www.youtube…"]ToddFun.com: Isolation Transformer - Tripp Lite IS1000 - YouTube[/]
[="http://www.tripplit…"]
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Yes my good doctor that is one of the ways to achieve electrical isolation and decoupling from the AC mains. I use a larger industrial version Signal Transformer Company unit rated at 75 amps. In an audio application actual audio 1:1 audio transformers such as the old UTC A-20. Modern-day units like the Jensen can virtually accomplish the same thing in audio applications. Basically, I utilize both with the CROW mobile remote truck. And it's amazing at how few problems I ever have because of that.

The electrical isolation transformer has a dual secondary where the 2 low sides are tied together (creating a center tap) to create my neutral. In a sense, everything in my truck has no electrical ground whatsoever. Most electricians look at it and are totally confused. So really the electrical isolation transformer is nothing more than a gigantic audio transformer that does not have to deal with frequencies much above or below 60 Hz in frequency response. On the primary side, I simply only require the need to connect to 2 hot phases of 208/220, sans neutral. Because without connecting to the AC power mains without any ground or neutral, no one understands how that could work? Which the electricians find twice as confusing. The body of my box truck is 100% wood. So there is no chance of any electrocution. The video basically describes exactly that with what you have posted from YouTube. And it was truly amazing between my direct electrical coupling and the electrical isolation transformer how much quieter everything that electrically. When necessary, I can bypass the electrical isolation transformer which then does require connection to AC power mains neutral. People have been confused between the difference of electrical neutral and ground. They both virtually have the same potential whereas the ground actually is ground and not neutral. I think even I'm confused? But then I've always been confused most of my life so... this just follows with my illogical logic.

In audio circuitry, sometimes lifting your ground is sufficient to prevent ground loop hum. This is basically translated also an electrical manner when using the AC power mains electrical isolation transformer. So I am actually electrically wired ground lifted with the AC power mains electrical isolation transformer. My AC power wiring is therefore wired as audio which is actually not to electrical code. But this is not a facility that has to live up to a permanently installed studio within a dwelling that does have to follow electrical code. And no inspectors were involved with the approval process of my Remote Truck because in and by itself, it did not require any state/county inspections. Yet there is still no danger from electrocution because of this complete isolation. KOOSTER McAllister from Record Plant Remote actually turned me on to this 20 odd years ago. So when I am connecting to AC electrical mains at certain jobs/venues an electrical neutral is taken but is not connected to my truck nor the electrical isolation transformer primary input side. It's wacky I know but it works great. And the same can be applied to a permanent dwelling studio in one's home or in one's warehouse after all inspections have been completed beforehand. Though this is not what one would call " balanced power " from devices when measured with reference to ground shows 60 V on one side of the Edison connector and 60 V on the neutral side of the Edison connector. But it still is truly a balanced power distribution concept executed in a slightly different manner. Maybe executed isn't a good word to use? LOL

After I posted this, I found this additional explanation that may further help everybody understand.

Note: you should still always utilize an AC voltmeter. Because even in this application, one can still have internal ground loop issues within your studio audio and electrical wiring.
Mx. Remy Ann David

Well wood is not always 100% non-conductive to electrical currents... If the wood were to get wet then that wood would equal execution. So now when I am eating beef lo mein and soaking my chopstick in soy sauce while tapping on circuit boards, I choose only the finest plastic versions of utensils. diddlydoo

Now time for something completely different...smoke

Right Doc. In my case, it doesn't matter how wet the wood gets, it's sitting on top of 4 foot high round rubber insulators a.k.a. truck tires. Everything is of course also checked with an AC voltmeter after tying into power and connection to the console/patchbay input panels, Stage boxes, for any existing current. It's also quite amazing that so many people don't use voltmeters because they find them confusing? You'd think that they would run out of gas all the time, get speeding tickets and wouldn't know how much they weigh when they get on the bathroom scale?

I like to smoke something better than cigarettes if I had any.
Mx. Remy Ann David