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NAD receiver hum: cause and cure

 

John A.
Posted on   
Having had occasional problems with hum on an NAD receiver, only on surround sound, never on stereo, I have come to the conclusion that the DSP holds onto AC interference by "learning" to generate it itself.

My guess is that hum in an NAD receiver is induced initially by an external, alternating electromagnetic field. That AC signal can be picked up in the first instance by a long subwoofer cable (as in my case), perhaps by other components. Once the hum is induced in the DSP/amp, it does not go away, even after the original source of interference is turned off. Possibly there is an internal feedback loop set up, which the DSP sustains. That is why the hum is absent in stereo, and sustained even on standby or "off".

What to do.

1. Reboot the DSP. On any surround mode (DD, DTS, Prologic, EARS), with the receiver on, hold down the channel selector (e.g. "Video 1") on the front of the receiver. Keep it held down while switching the unit off (big green button). Keep the channel selector button depressed whilst pressing the power button a second time. The unit comes on without the hum, and on the channel whose button you have depressed. Release the channel selector button. You may lose settings (channel delays; radio preset; etc.). There may also be a thump when you power off.

2. If the hum returns after a while, it is being induced again. Unplug a neighbouring electrical appliance which you suspect may be the original cause of the induced hum.

3. With the suspected source of interference disconnected and turned off, reboot the DSP again as in 1. If the hum comes back, as in 2, repeat 1 with a different suspected source of interference turned off. If not, go to 4:

4. When you have found, and disconnected, the original source of interference, after 3, the hum does not return.

In my case, the initial source of interference was a 240 V 250 W uplight with a slide control. The lamp's supply cable, at line voltage and frequency, crossed over the subwoofer cable, and drew from the same supply socket as the sub. The AC field from the lamp (IKEA) is tangible when you touch it lightly, and it has a large metal base to radiate AC field: it is a badly-designed lamp. The clue was when the hum re-appeared, as in 2, when moving the lamp slide control.

I think NAD equipment is designed for fast transient response, part of the quality sound. The disadvantage is that it is susceptible to induced hum, and holds on to it, by generating its own source of hum in the DSP. Probably the engineers who designed it had good clean power supplies with no strong AC electrical fields nearby.

My home-spun theory behind 1-4 could be nonsense, but the procedure worked for me. Apply it at your own risk. Also, there may be other sources of hum (grounding problems etc.) for which this is not a cure.

Note: updating the firmware or installing a new DSP chip may be expected to give a temporary cure, but the hum may return if you retain the original source of interference. Just eliminate that, and then zap the DSP (as in 1) to restore excellent surround sound.

To any NAD techs/reps reading, if this is generally useful and makes sense, you may save a lot of warranty replacement DSP chips. My choice would be a complimentary 240V T763, please....
 

Railbait
Posted on   
Nice diagnostic. Kudos, John A. :)
 

John A.
Posted on   
Thanks, Railbait.

It would be interesting to know if this works for anyone else.

This is not a fault with NAD: it is a fault with an IKEA floor-standing lamp.
 

Robert Gatt
Posted on   
Good one John
 

John A.
Posted on   
I have spent part of my Sunday afternoon sorting out a return of the hum. Not as bad, but there is no such thing as a little hum.

The culprit was a low-grade, thin, single co-ax cable with RCA plugs I was using for composite video connection between the receiver and the DVD-player. Always use well-screened 75 Ohm interconnects for digital audio, composite video, and the sub cable. All of these will act as antennae for mains hum if they are not properly screened, and I do think NAD receivers are particularly sensitive to interference. As in my original post, the hum "sticks": it is not enough simply to disconnect the rogue cable; you have to reboot before the hum is cancelled. You know you have nailed it when the hum does not then come back.

(Originally posted on Receiver quality and NAD, my mistake!)
 

John A.
Posted on   
Not sure if my personal "hum" diary is useful, but last traces now eliminated by turning the power plug on the sub-woofer through 180 degrees. Where I live there is no distinction between Live and Neutral polarities on AC supply: even when earthed, you can put the plug in either way. This should not be a problem in US, UK, Canada, Australia, NZ [why only English-speaking countries....?] where three-pin plugs are triangular, not round, and will only go in one way. All assuming your house wiring is correctly phased.

So I think the receiver and sub were out of phase. I had narrowed it down to the sub/receiver combination. You have to be systematic with sources of hum. A long sub cable acts as an antenna.

As before, the cure is not instant. You have to reboot and see if the hum ("Buzz") returns eventually. If it does not, you have fixed it. I think that is worth repeating. It sure ain't obvious.
 

John A.
Posted on   
Sorry about that - I kept getting "Timed out" splash screen, and just pressed "post" again. My internet connection is like my domestic power supply
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