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Author Topic: Laptop power supply causing hum through dj rig  (Read 6012 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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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.


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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.


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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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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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Brian Rodgers

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Re: Laptop power supply causing hum through dj rig
« Reply #10 on: March 06, 2015, 03:43:35 pm »

How?  Through what device/cable?

Sorry for not replying earlier. Our mains are all fed from the xlr out on the mixer. Some of our subs are fed from an unbalanced RCA output to xlr and others are fed through a crossover. I am thinking that it is an issue with cabling because some of the cables are unbalanced, but it also hums in the amps that use balanced xlr.
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Re: Laptop power supply causing hum through dj rig
« Reply #11 on: March 06, 2015, 08:42:25 pm »

Sorry for not replying earlier. Our mains are all fed from the xlr out on the mixer. Some of our subs are fed from an unbalanced RCA output to xlr and others are fed through a crossover. I am thinking that it is an issue with cabling because some of the cables are unbalanced, but it also hums in the amps that use balanced xlr.

The question is how/through what are you connecting the computer to the mixer...
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Rob Spence

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Re: Laptop power supply causing hum through dj rig
« Reply #12 on: March 08, 2015, 04:58:48 pm »

Sorry for not replying earlier. Our mains are all fed from the xlr out on the mixer. Some of our subs are fed from an unbalanced RCA output to xlr and others are fed through a crossover. I am thinking that it is an issue with cabling because some of the cables are unbalanced, but it also hums in the amps that use balanced xlr.

So, not identical as stated in the original post.

What else is different in these identical rigs?


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Ivan Beaver

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Re: Laptop power supply causing hum through dj rig
« Reply #13 on: March 08, 2015, 05:49:07 pm »

Sorry for not replying earlier. Our mains are all fed from the xlr out on the mixer. Some of our subs are fed from an unbalanced RCA output to xlr and others are fed through a crossover. I am thinking that it is an issue with cabling because some of the cables are unbalanced, but it also hums in the amps that use balanced xlr.
A drawing of what is being used (YES brand and model numbers HELP), and how it is wired would help to get a quicker response.

Until that happens-people are just guessing at how things are hooked up.
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David Allred

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Re: Laptop power supply causing hum through dj rig
« Reply #14 on: March 16, 2015, 03:34:16 pm »

May not apply, but an optical coupler (RS-232) stopped Driverack 260 communication noise from getting into the system through the laptops USB audio signal output.

David
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Janne Paul

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Re: Laptop power supply causing hum through dj rig
« Reply #15 on: June 27, 2016, 06:03:57 pm »

This is a common issue with Windows laptops. MacBooks which are used by the deejays don't have the problem.

Possibly you can remove it with a 60hz notch filter in your DSP.


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David Buckley

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Re: Laptop power supply causing hum through dj rig
« Reply #16 on: June 28, 2016, 12:38:02 am »

These are your laptops?  Easily solved: Don't buy laptops with a ground connection.
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Mark Dawson

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Re: Laptop power supply causing hum through dj rig
« Reply #17 on: June 28, 2016, 07:30:19 am »

Chop the earth pin off your laptop power lead.  Lots of power supplies link AC earth to DC negative, hence the laptop chassis. This basically creates an antenna surrounding hard disks, high frequency processors.  So yeah chop the earth off. Not really a problem on a double insulated supply.


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Re: Laptop power supply causing hum through dj rig
« Reply #18 on: June 28, 2016, 08:36:01 am »

Chop the earth pin off your laptop power lead.  Lots of power supplies link AC earth to DC negative, hence the laptop chassis. This basically creates an antenna surrounding hard disks, high frequency processors.  So yeah chop the earth off. Not really a problem on a double insulated supply.


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Nope.  Bad to do, irresponsible to suggest.

OP has not seen fit to reply with requested information in the last 10 weeks. Perhaps just ignoring the problem made it go away...
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David Allred

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Re: Laptop power supply causing hum through dj rig
« Reply #19 on: June 28, 2016, 10:13:06 am »

Nope.  Bad to do, irresponsible to suggest.

OP has not seen fit to reply with requested information in the last 10 weeks. Perhaps just ignoring the problem made it go away...

Neither of my laptops have 3 pin ac.  I didn't know they made them that way.
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Scott Carneval

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Re: Laptop power supply causing hum through dj rig
« Reply #20 on: June 28, 2016, 10:59:27 am »

All MacBook Pros that I've seen (I have 2, employees have them, many friends, colleagues, and clients have them) have 3 prong connectors. Never once had a buzz from any of them.

I think it's more to do with the specific power supply or laptop and not just the 'presence of a 3-prong adapter'.

Question though, I've seen many PC's without a ground prong on their adapter. Are these inherently unsafe, or is there something different about those power supplies?  In other words, why is it unsafe to remove the ground prong from a 3-prong supply when some PC's only come with a 2-prong supply in the first place?


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Don T. Williams

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Re: Laptop power supply causing hum through dj rig
« Reply #21 on: June 28, 2016, 11:38:47 am »

I've had several laptop noise problems that were cleared up by simply running them on the internal battery power.  One computer with a "short life" weak internal battery had the problem solved by using a 12V alarm system back-up battery wired to the computer's automotive power supply cable in an "emergency".
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David Buckley

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Re: Laptop power supply causing hum through dj rig
« Reply #22 on: June 28, 2016, 06:28:48 pm »

I think it's more to do with the specific power supply or laptop and not just the 'presence of a 3-prong adapter'.

Yes, of course it is: many manufacturers of power supplies for laptops carry through the ground from the mains to the negative of the output.  If there isn't an incoming ground then they cant do that.

Question though, I've seen many PC's without a ground prong on their adapter. Are these inherently unsafe, or is there something different about those power supplies?  In other words, why is it unsafe to remove the ground prong from a 3-prong supply when some PC's only come with a 2-prong supply in the first place?

If there is no ground pin, then the deice being powered must be designed and built to make it incapable of giving someone a shock.  This is most commonly done by making the thing "double insulated", which has the two square boxes symbol.

The other approach is to ground stuff, so if a fault occurs, then then effectively the fault current is shorted to ground causing a fuse to pop.  So, if you chop off the ground pin, there is no safe return path, and under fault conditions, someone can become shocked.

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Ray Aberle

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Re: Laptop power supply causing hum through dj rig
« Reply #23 on: June 29, 2016, 04:50:58 pm »

OP has not seen fit to reply with requested information in the last 10 weeks. Perhaps just ignoring the problem made it go away...

That's a YEAR and ten weeks... since the thread started in March of last year. :)

-Ray
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