Humming noise between Mac Mini and Direct Stream USB


#1

HI,

I know there are few posts discussing about this problem. But my case is little unique I think. So thought of sharing if anyone of you have any clue.

I just bought a MacMini (with fusion i/o) to experiment with the USB input of DS, first time. Otherwise I am happy with bridge so far.

Here are the connection details I connect the MacMini to my TV over HDMI, the MacMini is also connected to my router which DS is connected with too. My USB cable is not high end - could get one of around $25 USD from radio shack.

I use only one wall socket to connect all my equipments. There is one high end stripper connected to the wall socket and then I use one more stripper from there as on one stripper I cannot connect all my equipments. In the main stripper I’ve my power amplifiers (two of them, monoblocks) plugged in. The MacMini is also connected to that. The DS, Preamplifier and TV are connected to the second stripper.

And I hear the humming noise from the speaker as soon as I connect any one of the USB ports of MacMini to DS’s USB port. The noise goes away as soon as I disconnect the same.

But to me more surprising are the following observations -

  1. If I disconnect the HDMI connection from MacMini to TV, the sound goes away. This is even if the USB ports of MacMini is still connected to DS’s USB port.

  2. Even if I disconnect power cables of all of three, MacMini, DS and TV, the sound still comes from the speaker. Only if I disconnect either the HDMI cable or the USB cable from MacMini the sound goes away.

Any idea ? Is it more a grounding problem in MacMini ? I need to return the MacMini if this does not work.

Regards,

Sourav


#2

It sounds like there might be a grounding issue between the two strips.

Dennis


#3

As Dennis says it could be a grounding issue.

If your TV is connected to a satellite receiver or cable, etc there are multiple grounds in your system and it can take work sometimes to break the groundloops. Also, for example my satellite receivers also has a connection to my phone lines so there’s yet another ground in the system.

When I was using as SVideo connection I had to get a SVideo ground isolator to get rid of the residual hum. Now I use a TOSLink connection from my video system to my audio system for the best isolation.

The problems can be further complicated if you have multiple power outlets connected to your audio system. Even tho we often are careful with the outlets for the audio system proper, having a computer in the system almost always involves another outlet with, perhaps, another level of ground which can cause more ground loop issues.

My laptop’s charger only has a two wire plug and by flipping the plug I can lower hum.

There are some helpful hints at the Jensen Transformer site: http://www.jensen-transformers.com/application-notes/ Especially AN0009. I just noticed that they now ask for an account to get to them, it’s worth it.


#4

Hi Dennis,

Just tried the same. No luck.

Also tried running 1 power amp, everything else disconnected. But still the same problem.

The weird thing is why the combination of the hdmi and usb port in the mac mini creates the sound. If I unplug any one of them the sound goes away.

Regards,

Sourav


#5

What happens if you unplug the TV from the power strip? There is some kind of ground issue at work here, but they can be notoriously difficult to find.

Dennis


#6

Hi Dennis,

I did unplug the TV too. Actually nothing is plugged now in the strip apart form one power amp. Even I changed the strip. No luck.

Regards,

Sourav


#7

I am not sure where else to look, as I am not an expert on finding grounding issues. I am sure there are others on the forum that can help.

Dennis


#8

Hello Sourav:

I absolutely agree that your humming is a grounding issue. Also, the noise level should be proportional to the volume set by your preamp. If the noise is content it is probably a power supply 60 Hz. hum. Different problem. I seem to experience the “humming” of ground loops more with tube based equipment. Anyway, I use grounded plug adaptors to eliminate the ground post on power cable of gear that want to create problems. These are readily available at Radio Shack or an equivalent. You can always take a cheap IEC power cable and snap the ground off for testing if you don’t have an adaptor on hand. The neutral of the 120VAC circuit feeding your equipment should be grounded, which is the ground you are using with a grounded plug adaptor (orange cheater plug). Good luck!

Mack Adams - Charlotte, NC


#9

Thanks Mack.

I found it is a typical mac issue when you try to plugin ti an external display. As you suggested I actually got a story of grounded power adaptors (hum) and that sort of solve the problem.

Regards,

Sourav