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Author Topic: Laptop power supply causing hum through dj rig  (Read 5094 times)

Brian Rodgers

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Laptop power supply causing hum through dj rig
« on: March 04, 2015, 10:56:07 am »

The company I work for has four identical DJ racks. While testing them before our busy season we noticed a loud hum through at least two of our racks and possibly the other two that haven't been tested yet. It always goes away when we unplug the laptop power supply. The supply is grounded and runs through the same power conditioner as the mixer. The laptop connects through USB to the mixer and then from the mixer to the amp rack. The hum does not come out of the headphones connected to the mixer and only comes through the amps. Any thoughts on how to fix this?
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dick rees

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Re: Laptop power supply causing hum through dj rig
« Reply #1 on: March 04, 2015, 11:07:48 am »

The laptop connects through USB to the mixer and then from the mixer to the amp rack.

How?  Through what device/cable?
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Ivan Beaver

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Re: Laptop power supply causing hum through dj rig
« Reply #2 on: March 04, 2015, 01:00:42 pm »

The company I work for has four identical DJ racks. While testing them before our busy season we noticed a loud hum through at least two of our racks and possibly the other two that haven't been tested yet. It always goes away when we unplug the laptop power supply. The supply is grounded and runs through the same power conditioner as the mixer. The laptop connects through USB to the mixer and then from the mixer to the amp rack. The hum does not come out of the headphones connected to the mixer and only comes through the amps. Any thoughts on how to fix this?
I had to remove the ground from the grounded supply in one of my old computers to get rid of the hum.

I am not suggesting removing the ground-that could make it unsafe.

But simply saying what I did.
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Len Zenith Jr

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Re: Laptop power supply causing hum through dj rig
« Reply #3 on: March 04, 2015, 10:04:11 pm »

http://www.musiciansfriend.com/amplifiers-effects/ebtech-hum-x-hum-exterminator

Use it on your laptops power cord, or run on UPS, or get a DI box with ground lift, or replace your 3 prong laptop power supply with a double insulated 2 prong power supply (has square logo on transformer).

Either of those 4 ways will solve your problem.
« Last Edit: March 04, 2015, 10:21:46 pm by Len Zenith Jr »
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Mark Cadwallader

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Re: Laptop power supply causing hum through dj rig
« Reply #4 on: March 04, 2015, 11:30:18 pm »

I believe that a 600 ohm isolation transformer (1:1) between the mixer output and the amp input should do the job safely, too. Mark C.
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Rob Spence

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Re: Laptop power supply causing hum through dj rig
« Reply #5 on: March 04, 2015, 11:43:34 pm »

I believe that a 600 ohm isolation transformer (1:1) between the mixer output and the amp input should do the job safely, too. Mark C.

But his connection from laptop to mixer is USB.


Sent from my iPad using Tapatalk HD
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Mark Cadwallader

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Re: Laptop power supply causing hum through dj rig
« Reply #6 on: March 05, 2015, 12:08:18 am »

But his connection from laptop to mixer is USB.


Sent from my iPad using Tapatalk HD

But the mixer to amp is not USB. The hum is not in the headphone out, so (I think) placing the iso downstream of the mixer would provide a fix. Maybe I'm missing something, however.  I agree that ideally the iso would be upstream of the mixer.  Mark C.
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Len Zenith Jr

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Re: Laptop power supply causing hum through dj rig
« Reply #7 on: March 05, 2015, 04:32:37 am »

Since your company has 4 separate rigs going, I'd stay away from my suggestion of a UPS as then you are dealing with battery power and weather someone forgot to charge it, etc. Same thing with the new laptop power supply. Although the most elegant solution, what if you have to use a fill in DJ and he brings his own laptop and he is unaware of the 3 prong problem.

To me the DI or the hum-x would be the most reliable solutions. The DI could be mounted right inside of your amp racks and is a versatile tool for any sound company to have. Just make sure it has a ground lift on it. The hum-x could also be mounted to your powerbar and taped in place.
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dick rees

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Re: Laptop power supply causing hum through dj rig
« Reply #8 on: March 05, 2015, 06:34:25 am »


OP has not checked in  since posting, so be patient.  My question still stands and an answer should clear it up.
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Canute J. Chiverton

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Re: Laptop power supply causing hum through dj rig
« Reply #9 on: March 05, 2015, 09:55:33 pm »

The company I work for has four identical DJ racks. While testing them before our busy season we noticed a loud hum through at least two of our racks and possibly the other two that haven't been tested yet. It always goes away when we unplug the laptop power supply. The supply is grounded and runs through the same power conditioner as the mixer. The laptop connects through USB to the mixer and then from the mixer to the amp rack. The hum does not come out of the headphones connected to the mixer and only comes through the amps. Any thoughts on how to fix this?
Were all the racks tested from the same Wall Outlet/Circuit?
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