Hum/Buzz from one channel of ART DTI Box

[BigRedX]

I'm using an ART DTI box to isolate the outputs of my Focusrite Scarlet 18i20 interface from the PA when we play live. I recent problem with the wiring has led me to look critically at the complete signal path, and now I am detecting a slight hum/buzz from channel 2 of the DTI box.

The normal signal path is balanced out from the Focusrite TSR jack to XLR socket in on the DTI box. XLR out on the DTI box to PA. Now that I have fixed the original wiring fault, I can hear a faint hum/buzz on channel 2 only of the DTI box when using a balanced line headphone amp to check the signal. Connecting directly from the interface to the headphone amp using all the same cables eliminates the hum so it is definitely the DTI box that is the culprit, and it is only happening on channel 2.

Any ideas?

[Hugh Robjohns]

There are several possibilities. Buzzes can be induced magnetically into the transformers or can be caused by a ground loop, or by the total absence of a ground.

The DTI box breaks ground loops by design, so we can rule that out. But if your headphone amp has a class2 mains power supply (double insulated, without an earthed mains cable), it may well require grounding via the input cables which it won't get through the DTI.

Regarding the possibility of induced hum, is the DTI box located close to something with a mains transformer? If so that could induce hum into the audio transformers, and probably more into one channel than the other depending on relative proximities.

Also, is the DTI box case touching any other metal work? Channel 1 input ground is tied to the case. In some unusual cases you could end up with induced ground noises on channel 1 but not on channel 2, and hence one clean channel and one buzzy channel.

[Martin Walker]

Also, is the DTI box case touching any other metal work?

This caught me out a few years ago, when I developed a ground loop without changing any of my gear or its interconnections.

I eventually traced it to a 19-inch rack unit case, one of whose screwheads had eventually worn through the paintwork on the unit beneath it, thus completing the loop. Placing a piece of sticky tape between the two cases solved my nasty noises.

[Wonks]

Depending in the connectors used, you might be able to use a multimeter to check and see if there is a low resistance reading between the ground/screen connections on both sides of the DTI box.

[BigRedX]

Thanks for all the replies. The DTI Box is not touching anything else metal in the rack (It's an EMS ABS modular rack case) other than by the cables attached to it and unplugging all of them other than the Channel 2 output makes no difference.

Moving it about will in some instances cause the hum to get slightly louder but no location eliminates it completely, and Channel 1 is hum free at all times. There are two mains-powered devices in the rack - the Focusrite interface which has a built-in PSU (this is one of the reasons why it was chosen), and a MacBook Pro plus it's external PSU. It have done everything possible (given that I only have 450mm depth and 3U height to play with) to keep the mains an signal leads as far apart as possible.

Given what Hugh has said about the possibility of the Headphone amp having an unearthed PSU, my next action will be to try it on batter power when I get time later today. If this eliminates the hum, can I assume to connecting to a properly grounded PA system will also be hum-free?

[ef37a]

Also, is the DTI box case touching any other metal work?

This caught me out a few years ago, when I developed a ground loop without changing any of my gear or its interconnections.

I eventually traced it to a 19-inch rack unit case, one of whose screwheads had eventually worn through the paintwork on the unit beneath it, thus completing the loop. Placing a piece of sticky tape between the two cases solved my nasty noises.

Can I mention once again "Rack Snaps"? These are a plastic rack screw system with a moulded in M5 threaded nut and come with M5 screws wih plastic washers thus totally insulating any rock mounted gear from the frame. I am sure my old firm, HellermanTyton still stock them but I shall check.

Red, does the hum move channels if you swap the cables over? Could give further clues one way tother?

https://www.hellermanntyton.co.uk/produ ... /854-40074
Seem to have dropped the bright Nickel screw? Shame.

Dave.

[BigRedX]

The rack case itself is an EMS ABS plastic one, and the design is such that the even the fixings are isolated from each other. The DTI box is located behind all the other items in the rack (to get it as far away from the mains cables) and does not touch anything metal in the rack other than the conductors of the cables attached to it.

Hum is most definitely being generated by channel 2 of DTI box only. Swapping cables over makes not difference.

I'm suspecting that the culprit is my testing method using a headphone amp with an Class 2 unearthed PSU. I'm hoping that when I try again later today running the headphone amp off the battery the hum will have disappeared.

[Mike Stranks]

Just picking-up on the isolation within a rack theme...

I tend to use plastic washers, but there's also these which seem widely available:

https://www.adamhall.com/shop/gr-en/19- ... 4/humfrees

[ef37a]

Just picking-up on the isolation within a rack theme...

I tend to use plastic washers, but there's also these which seem widely available:

https://www.adamhall.com/shop/gr-en/19- ... 4/humfrees

Thanks Mike. I knew of those but I think you will find their unit cost higher than RSnaps and they look fiddlier to use?

Dave.

[BigRedX]

Success!

Running the headphone amp using the battery option rather than the PSU has eliminated the hum/buzz completely. So it looks though the problem was entirely down to the method I was using test the set up in the absence of an actual PA to connect to.

Hopefully that means when connected to the PA any hum present will be down to the inadequacies of the PA equipment and not my set up.

Thanks to all who replied, and extra thanks to Hugh, whose suggestions pointed me in the right direction.

[Martin Walker]

Hooray!