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Phono preamp hum

I recently built my second SSE to give to my son. I ran it at my house for a few days and it sounded great, no issues.

I built it with two inputs. One is connected to a NAD phono preamp which it powered by a wall wart. The Project turntable connected to the preamp is also powered by a wall wart.

The other input is connected to a Sonos. When it is switched to the Sonos there is no hum, when it is switched to the NAD phono preamp there is a noticeable hum, particularly when there is no music playing.

I thought that a hum would be caused by an AC ground. I'm assuming that the wall warts are converting the power to DC, so why the hum?

Since he lives about an hour away, I ask him to buy a three prong adapter to see if there was an issue with the wiring in his apartment. It didn't help. I also sent him a Rolls Buzz off HE18 hum eliminator. It didn't help either.

I followed George's wiring diagram so that there is only a ground wire going to one of the inputs.

Would a different phono pre-amp help? I saw in an earlier post the suggestion that a 10 ohm resistor inserted between the input and the ground might eliminate the hum.

I am a newbie, so I'm trying to figure this out. Any other suggestions would be appreciated.

thanks, Jacques
 
Hey Jacques,

How is the turntable grounded? Is there a separate ground wire? If so, where are you connecting it? If there is not a separate ground wire, are you sure the tonearm is grounded through one of the interconnects (common with Rega tables)?

As W5JAG said, there could also be a ground loop created somewhere. The phono pre sounds like a two prong. If your SSE makes chassis ground at an input jack and both are plugged into the same set of outlets, this might be the source of problems. Try plugging the phono pre into a different set of outlets and see if that helps.
 
Thanks for your note.

There is a ground connection that goes to the pre-amp. We tried hooking the turntable directly to the SSE and there wasn't a hum, so it makes me believe that the pre-amp is causing the hum. Neither the pre-amp nor the turntable are grounded as they are powered by transformers that plug into the wall.

Per your suggestion, we will try hooking the pre-amp power to another circuit to see if that helps. Otherwise I can try disconnecting the ground in the SSE to see if that makes a difference. Any other suggestions are appreciated.

thanks for your help, Jacques
 
So it turns out that the phono cartridge was too close to the power transformer on the SSE. The turntable was right next to the SSE. Once we put the turntable on the other side of the amp, the hum went away.

I believe part of the problem is that the Grado cartridge is wooden and therefore not shielded.

I suppose a different problem cartridge would have solved the problem.

Thanks for others suggestions, they helped.