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[David Aberdeen]

In our church sound system, we use a Macbook for the presentations, which sometimes include video/audio clips.  About half an hour into the service, this channel develops a buzzing noise.  I'm trying to work out where this noise may be coming from, and how to prevent it.

The laptop is connected through the headphone port (3.5mm stereo) , through to 2xRCA plugs at the mixing desk input.  The headphone to RCA cable is about 3m long, and not shielded.  It is run along a cable tray with lots of other cables, including power.  When first setup, it is fine, with no noise, but at some random point during the service, it develops a constant buzzing noise (not 50Hz), and requires that we turn off the laptop sound channel when it's not needed.  The laptop is not connected to power, and just runs off battery.  At the mixing desk, we have options for XLR inputs.

Is it possible to get (or build) a passive shielded cable to take the 3.5mm headphone jack through to XLR inputs?
Is this likely to help?
Is there a better way?  I know we could take the headphone signal through a stereo DI box and convert to balanced, but I'd like to avoid that extra cost/complexity if all it needs is a cable.

[Dave Pluke]

Is there a better way? 

Probably. First, Check the Rules, Change your ID to your real name.  Then, we might be able to offer some suggestions.

Dave

[Tracy Garner]

In our church sound system, we use a Macbook for the presentations, which sometimes include video/audio clips.  About half an hour into the service, this channel develops a buzzing noise.  I'm trying to work out where this noise may be coming from, and how to prevent it.

The laptop is connected through the headphone port (3.5mm stereo) , through to 2xRCA plugs at the mixing desk input.  The headphone to RCA cable is about 3m long, and not shielded.  It is run along a cable tray with lots of other cables, including power.  When first setup, it is fine, with no noise, but at some random point during the service, it develops a constant buzzing noise (not 50Hz), and requires that we turn off the laptop sound channel when it's not needed.  The laptop is not connected to power, and just runs off battery.  At the mixing desk, we have options for XLR inputs.

Is it possible to get (or build) a passive shielded cable to take the 3.5mm headphone jack through to XLR inputs?
Is this likely to help?
Is there a better way?  I know we could take the headphone signal through a stereo DI box and convert to balanced, but I'd like to avoid that extra cost/complexity if all it needs is a cable.

On the 3.5mm does it have 1 or 2 black rings? You can usually do it without going to a balanced DI Box but in a church, I would rather go the DI route - even passive. If you search around the forums here, you will see several recent threads with suggestions for boxes from $20 to a couple hundred depending on what you are trying to accomplish.

[Nathan Riddle]

Purchase a Peavey USB-P USB Playback Device and be done with the whole ordeal.

Everything else is a compromise.

[Mac Kerr]

Purchase a Peavey USB-P USB Playback Device and be done with the whole ordeal.

Everything else is a compromise.

^^^^^THIS^^^^^

[Mike Caldwell]

It's not the cable itself but needing isolation between the the computer / video system and the sound system.
The DI would work chances are you'll need to flip the ground switch.

The mentioned USB-P works great, plug it into your computer and then set up the audio out to use it as the default audio output device.

[Jonathan Johnson]

I can't address Macs specifically, but I've noticed this behavior with other PCs when they go into sleep mode. The sound card is somehow put into a different mode that puts noise onto the line.

You might need to disable sleep mode during the service.

Or get a dedicated USB audio interface, such as the aforementioned Peavey USB-P, and avoid noisy onboard audio devices and unbalanced lines altogether.

[David Aberdeen]

Thanks for the responses.  I will look into these possible solutions.

[Ole Anderson]

Unplug your wall wart and try running it on battery alone.  That is often your source of buzzing. Edit: I see you are already running on battery.

[Stephen Beatty]

This is the best thing to do as suggested above. The Peavey USB-P USB Playback Device is a plug and play device. It has 2 XLR balanced outputs and can be run in mono or stereo mode. I have used it on Windows, Mac and Linux driven machines. So who ever brings in a laptop to play can hook up easy.

You will start to see USB-C only machine so get the appropriate USB cables. 

[John Schalk]

Another vote for the Peavey USB-P, it's a great piece of kit and Guitar Center has them for only $53.19 this morning.  We bought one for the karaoke DJ's laptop at a local club and liked it so much that, when we swapped out the X32 at FOH for an M32, we bought another one to use for the house music coming from a computer at FOH.  Got rid of a whole nest of dirty patch cables. 

[Jonathan Johnson]

One word of caution with the USB-P: there were some units that shipped with one (or both?) of the channels' polarity inverted.

This causes cancellation when mixing to mono (or selecting mono output), and if playing back on stereo speakers could result in unacceptable phasing issues.

So when you get it, check to see that it doesn't go silent when playing back in mono mode. (You'll need a source that has the same waveform in left and right channels. If using a stereo source, mono content such as centered vocals may cancel.)

To identify the bad channel, you'll probably need to record the output and analyze the waveform, comparing it with the waveform from the original source.

* * * * *

Mine is, unfortunately, one of those with a problem. Also unfortunately, I identified the issue after the warranty period expired. I wired up an adapter that flips the polarity on the bad channel. Even so, I would not hesitate to buy another; I'd just make sure it worked properly.

[Stephen Beatty]

Wow. That's not good. The two I have are fine. A good thing to know.

[ProSoundWeb Community]