DMX Audio Feedback Issues

[Gazza75]

Hi looking for some help here if anyone can contribute it would be greatly appreciated.

I have a new setup this year and I'm having some teething issues when it comes to putting it altogether. Basically I'm using the following and I'll keep this as simple as possible, with the opportunity to elaborate later if required:

Macbook pro
USB midi controller with in built sound card
2x speakers connected via XLR cables
KAM powerbar
USB DMX controller

Now the setup has been running without any issues, feedback or speaker hum for around 18mths without the dmx. This year I wanted to add DMX to my rig, low and behold we have an issue. As the faders in the software increase or decrease an audible hum can be heard through the speakers.

After a lot of internet searching I've been left with several industries professionals touting it's one thing or the other and to be honest, trying all of their solutions hasn't stopped the issue.

Solution one: it's the grounding. The audio PA must be connected to one circuit and the lighting setup another.
Solution two: it's because I didn't use DMX XLR cables. Well I've tried it with QTX DMX cables and still no change.
Solution three: it's the PA or other device interfering. I have painstakingly separated all of the cables and moved them a fair distance from power cables and other devices, no change.
Solution four: I need to buy a hum eliminator. Something like this http://www.sweetwater.com/store/detail/Hum/ which I haven't tried but hope I dont have to at this point.

I'm quickly losing patience and have tried multiple cables, connections and power combinations. Including running an extension from the neighbours to put the lights on a completely separate circuit, to no luck at all.

Am I missing something here or is this just not going to work off one computer? I would love to have a smaller setup but cannot afford the space of an additional laptop. I would appreciate any help I can get.

[DJ Jules]

You don't mention which USB DMX adaptor you've bought, but I'm willing to bet it's one where the outputs aren't electrically isolated from the USB supply. Basically, connecting the USB DMX controller to your laptop and then connecting this to your lights is creating a new ground in your laptop (from the lights, through the USB DMX controller, through your laptop, to your MIDI controller) which is then creating a ground loop back to your speakers.

The solution is going to be a Ground loop isolator between your MIDI controller/soundcard and your amp/speakers - fiver off eBay or go for an ART DTI if you want to be posh :D The other alternative is a better USB DMX controller with optically isolated outputs which will break the ground loop between your lights and your DMX controller (instead of breaking it between your amp/speakers and MIDI controller/sound card). The ground loop isolator is going to be the cheaper option, but a better USB DMX adaptor will come with other advantages (lower load on the laptop CPU, better DMX timing, less prone to compatibility issues with lighting fixtures, etc).

Julian

[Gazza75]

You don't mention which USB DMX adaptor you've bought, but I'm willing to bet it's one where the outputs aren't electrically isolated from the USB supply. Basically, connecting the USB DMX controller to your laptop and then connecting this to your lights is creating a new ground in your laptop (from the lights, through the USB DMX controller, through your laptop, to your MIDI controller) which is then creating a ground loop back to your speakers.

The solution is going to be a Ground loop isolator between your MIDI controller/soundcard and your amp/speakers - fiver off eBay or go for an ART DTI if you want to be posh :D The other alternative is a better USB DMX controller with optically isolated outputs which will break the ground loop between your lights and your DMX controller (instead of breaking it between your amp/speakers and MIDI controller/sound card). The ground loop isolator is going to be the cheaper option, but a better USB DMX adaptor will come with other advantages (lower load on the laptop CPU, better DMX timing, less prone to compatibility issues with lighting fixtures, etc).

Julian

Thanks Julian,

You are completely right, problem solved! Thank you for your very prompt response.

I am using ADJ MYDMX 2.0 primarily because I was restricted for Mac support initially.

I obviously haven't bought this hum eliminator yet but have managed to find a work around. I have connected my ADJ X MOVE 25R's in-line before the KAM powerbar which have eliminated any hum, still have the DMX terminator at the end of the 3 fixture chain. These must be earthing the units or have in built capacitors that are eliminating feedback? If only the USB ports weren't soldered together!

This will do until my hum eliminator arrives and thanks for the recommendation on the ART DTI it will be the option I go for moving forward.

Thanks
Gary