hooking up PC to soundboard for playback

[elladine]

I want to hook up my laptop to the soundboard at my theatre so that i can play from it to record onto our minidisc. this is just how things work here, don't ask why we still do what we do. the owner is cheap and won't upgrade. anyway, when i plug my laptop into the soundboard i get this nasty high pitched hum through the speakers. it does it if i mute the sound and everything. the volume isn't up that high so i don't think it's a volume issue. could it maybe be a ground issue?

i eventually want to run sound effects and music off my laptop but i can't with that noise that it generates.

any ideas what may be causing it and what may help it go away?

 

[FunnyFellow]

Have you tried running it on battery power?

 

[avkid]

Is the laptop plugged into it's power supply when this is happening?

A stereo DI box with a ground lift switch can probably solve this issue.

 

[elladine]

hehe figures. yeah it does only seem to happen when i have it plugged in. i did try a DI we had sitting around but i couldn't get it to work. it's one we usually save for music instruments. would a regular ground lift help solve the problem? or should i just leave it unplugged when i want to play things?

 

[seanandkate]

Ground lift generally=bad. Something like a HumX might solve your problem without lifting the ground, but it sounds like since you're only recording things, you don't need to have 3 hours of battery power. Just unplug whenever you're recording and you should be golden.

 

[avkid]

If you have the budget the Whirlwind pcDI is the best stereo DI in a wide price range.
Whirlwind / Direct Boxes

If you can't afford it Audiopile/EWI makes a similar device.
DBRC-2A

 

[elladine]

thank you so much for your help =)

 

[len]

Not an expert, but I believe they make a DI box specifically for pc output.

 

[avkid]

Not an expert, but I believe they make a DI box specifically for pc output.

Hence pcDI.

Radial does make a more expensive one though.
Radial Engineering - JPC Computer / A/V Direct Box

 

[Soundrew]

I had similar noise issues a few years ago using my laptop to control my speaker processor. It was only when I lifted the ground pin on the inline transformer did the noise abate. As a rule, using ground lifts is a dangerous practice. It is the safety ground that will route current to earth if a device develops a fault. Seeing as the laptop's chassis was plastic anyhow, I figured it was not likely to get energized. In addition, the safety ground stopped at the transformer (in a sealed plastic housing). If this works for you and you want to do it safely, plug the lifted cord into a portable GFI power strip.

I use a DI made by Horizon/Rapco called the LTI (laptop interface?). It can do stereo, mono merge and lifting. Its a nice inexpensive interface for a laptop and it has an integral 2' cable with a 1/8" connector.

 

[Cashwalker]

I have noticed that laptop 1/8" headset jacks don't seat all plugs equally. An actual headset plug with no directional strain is usually fine. But the thicker plugs on the average inter-connect cord, either doesn't seat quite right, or because of its weight or being pulled in one direction or another, loses its connection with the pins inside the jack. I had the same problem trying to feed the output of my mixer to the laptop's line-in 1/8" jack. It didn't matter if I was on battery power or not - if I sat there and held the connector in JUST the right way, it seemed fine.

I dream of the day when have the budget to pickup a proper USB sound interface with XLR in and out.

 

[museav]

That could also be because a headset jack includes connections for a mic as well as audio out while a standard interconnect has audio out only. A 'phones' connection is audio out only but is not the same as a headset connection.

 

[museav]

I had similar noise issues a few years ago using my laptop to control my speaker processor. It was only when I lifted the ground pin on the inline transformer did the noise abate. As a rule, using ground lifts is a dangerous practice. It is the safety ground that will route current to earth if a device develops a fault. Seeing as the laptop's chassis was plastic anyhow, I figured it was not likely to get energized. In addition, the safety ground stopped at the transformer (in a sealed plastic housing). If this works for you and you want to do it safely, plug the lifted cord into a portable GFI power strip.

How does a laptop connected to a procesor for control purposes generate noise when it would not seem to be part of the audio path? Maybe there was actually some other problem. And it can't be any clearer, never leave a power safety ground lifted. The issue is not the laptop, which is usually DC powered, it is the power supply and power supplies with three prong plugs are designed to work with a ground.

 

[theatretechie19]

I use a regular DI box with my Mac Book Pro. seems to do the trick even when its plugged it. mayb that just an advantage of Mac, im not sure

 

[Soundrew]

How does a laptop connected to a procesor for control purposes generate noise when it would not seem to be part of the audio path? Maybe there was actually some other problem. And it can't be any clearer, never leave a power safety ground lifted. The issue is not the laptop, which is usually DC powered, it is the power supply and power supplies with three prong plugs are designed to work with a ground.

Because the laptop is connected to the PA via USB (on the pc end) to RS-232 (on the DSP end) it entered the system. Lifting the ground on the transformer (or running the pc on battery) made the noise go away. Since battery life was about 30 minutes, I chose to lift ground and run that off a GFI strip (thereby mitigating the ground lift safety issue). Yes, the laptop is DC powered and, now that you mention it, could not likely have had it's chassis energized by a fault in the transformer.

 

[museav]

Because the laptop is connected to the PA via USB (on the pc end) to RS-232 (on the DSP end) it entered the system.

The point was that the connection has nothing to do with the audio path, it is a control connection. How does lifting the ground on the laptop do anything that should affect the audio when the laptop is not part of the audio path? Since there was no audio path between the laptop and the DSP, might there be something wrong with the grounding of the DSP that results in the control connection from a laptop introducing noise into the audio? Did you try lifting ground on the serial line, lifting the audio grounds on the DSP inputs or outputs or simply plugging the laptop into the same power source that the DSP uses? If you have not already done so I would make sure to at least look at all of those possibilities before using lifting any power safety ground as a solution.

 

[Soundrew]

Brad, this is not an uncommon problem- there are numerous threads on PSW on this very subject. I promise I'm not making it up!

There doesn't have to be an audio path open- its noise from the laptop's transformer (a high pitched sound much like the OP here described). All my rack components (including the laptop) were running off the same distro in the rack and same AC feed from stage distro. The only thing that remedied this (althogh I'll admit I didn't try lifting ground on the serial line) was either running the laptop on battery or lifting the laptop's power cord. By plugging the lifted cord into a GFI power strip (which senses when hot and neutral vary) the shock risk associated with that lifting (which on a plastic housing transformer is minimal to begin with) was properly mitigated. I have since changed laptops (my new one doesn't even come with a grounded cord) but if this happens again, I will investigate lifting the ground line on the serial connection.

 

[Chris15]

The consensus has been for some time that devices like the Hum-X ought to be illegal.

All it takes is one cold solder joint and you have no earth path for your device and where you would notice something like that in active or neutral, you will never know until it's too late that your earth path is dead...

Defeating a safety ground is not ever an acceptable solution. Ever. The designer of that power supply must have intended it to have a ground, otherwise they would have put a 2 pin plug on it. It may be to comply with EMC requirements but I know that my laptop supply carries the earth connection through to the computer...

An RCD does not negate the necessity of earthing. They are designed to be complementary solutions. An isolation transformer may solve the issue and remain safe.

 

[SHARYNF]

Oh well I am sure this will start a major controversy, BUT my experience with notebook powersupplies and also usb devices connected to them leads to a very different conclusion.

One is IMO one of the issues is that some of the power supplies/charger are actually designed to have an " double insulated" and are designed for a two wire NON ground connection. Part of this is due to the fact that there are some countries where typically the notebook connection is only two pin. What I have found is that some of these powersupplies are in fact improperly supplied with a three pin cable, while it could be argued that it is "safer" in fact this provided DUAL ground to neutral connections in a system, causing all sorts of noise problems. It is also evident on the USB connection since the standard USP port will output 5 volts.

So A few suggestions, one is to find a powersupply for your notebook that has a double insulated setup with just a two wire connection.

The other is JUST for this application, eliminate the ground.

i know others will disagree but on some notebooks (HP and Dell) with some of the three prong powersupplies, there is NO way to eliminate the noise at some locations unless you :
Get another notebook
Run off the battery
Get another powersupply with a two prong connection
or eliminate the ground connection.

The problem is most of the notebooks are designed by one group and then someone goes out and basically buys a "specedoff the shelf powersupply"
In notebook testing there is AFAIK no on who is testing to see if there is any ground noise from a ground loop connection to another system. The Notebooks are tested in an isolated environment. So the issue never comes up.

Basically again from what I know is that this is an issue where a double insulated system is then attached to an external ground via the three pin ac connection.

 

[museav]

Brad, this is not an uncommon problem- there are numerous threads on PSW on this very subject. I promise I'm not making it up!

I never said it wasn't happening, but rather that connecting a communications/control cable to the DSP should not cause audio noise and that it doing so indicates that the problem may be with the processor as that is the only place interaction between the laptop and the audio signal path could occur. If the laptop and processor are off the same power then there should be no ground loop, something that seems easy to test, and since you are not sending audio from the laptop then any noise on the laptop audio should not be relevant. Any other cause would seem to be processor rather than laptop related, which would suggest looking for solutions related to the processor and not the laptop power supply.

However, this situation does not really seem directly relevant to the application being discussed as that is related to a laptop audio connection and not to a communications/control connection. That is quite different as it is not uncommon for there to be some noise on the laptop audio signal without even having any connection to the audio system.

The other is JUST for this application, eliminate the ground.

I agree with all your other comments, but why for this application? Just because it solves the problem does not make it an acceptable solution. While some power supplies are double insulated and operate without a ground, you cannot assume that a power supply was intended to operate without a ground and lifting a safety ground should never be seen as an option for a permanent solution.

I have a Dell laptop with a three prong power supply and have never had noise problems with either the internal sound card, a PCMI sound card or an external audio interface. So that seems to show that a grounded power supply may often be associated with laptop audio noise but is not inherently a problem or the cause. That it is not neceesarily the source of the problem was recently seen in a class where we ran into noise when any of the student's laptops was run off AC power. Everyone was tied via a system distributing the instructor's audio and everyone was using external FireWire or USB interfaces, but any of the laptops being on AC rather than battery resulted in noise being seen at all the laptops. We later discovered that the culprit was noise on the AC line powering all the student laptops, the instructor's laptops and equipment were on a different circuit which is why we did not get the noise from those sources. However, that is one example of another cause of the noise, one for which tha brand of laptop or the power supply being double insulated or grounded did not matter. The point being that while it may be a common issue, one should not assume that a grounded power supply will be or is the problem and focus only on that aspect.