Laptop Audio Hum
June 16, 2005 1:27 PM   Subscribe

I use my laptop in conjunction with midi controllers as a live instrument. When running off the battery it sounds great through the PA system, but when I plug in the AC adapter I get a very loud humming sound. Is there any way to get rid of this?

I usually just run the laptop off the battery and hope it lasts through band practice, but lately the battery life has been shorter and I would like to run it off AC power. Is this hum simply a grounding problem? If so, how could I go about wiring the laptop and the PA to a common ground?
posted by monsta coty scott to Technology (20 answers total) 1 user marked this as a favorite
I'm not that familiar with electrical systems but from my own brief research on this (I have the same problem) the laptop and the PA are probably on the same circuit. This is going to cause noise. You need to ensure their outlets are not on the same circuit. I believe there's something you can do to the ground on one of them to help prevent this, but I suggest using Google instead of what I could drag out of the depths of my cloudy mind.
posted by melt away at 1:34 PM on June 16, 2005

You've got a ground loop, and you need a ground loop isolator.
posted by zsazsa at 1:44 PM on June 16, 2005

Use a Furman Power Conditioner.

(BTW, What softsynths are you using? I do this with Native Instruments B4 Hammond Organ Modeler w/two Leslie Simulator pedals and a Volume pedal.)
posted by sourwookie at 1:45 PM on June 16, 2005

I've had the same problem. I do the same things- live shows with a laptop and midi controller. The problem is a ground loop, and the simplest thing to do is to get a power strip and cut the ground plug off of it, or to use a "cheater" plug that isnt' grounded.

You can do it more elaborately, but that solution has worked perfectly for me for over 20 shows, and my laptop and gear don't complain.

I'm sure people will complain about not grounding your laptop, but whatever. There are also ground loop isolators, and that is the professional (and somewhat expensive) solution.
posted by fake at 1:46 PM on June 16, 2005

If you run the laptop output signal thru a DI box (you'll need two or a dual-channel one if it's stereo), these often have a "ground lift" or "earth lift" switch that may diminish or eliminate any ground hum.

In the Wikipedia article linked, you can see a picture of a dual-channel DI by Behringer, the third dipswich from the right is the ground lift.
posted by goodnewsfortheinsane at 1:47 PM on June 16, 2005

You'll get much better sound quality if you use a separate interface like the M-Audio DMP3, and it won't be affected by the ground loop, but you should fix it anyway.
posted by abcde at 1:47 PM on June 16, 2005

Also, don't use a groundless plug or anything. That's only acceptable for testing, if anything. The chance of fatal electric shock is notable.
posted by abcde at 1:49 PM on June 16, 2005

If you haven't you might also want to keep the power cords as far away from the audio cords as possible. Or get shielded audio cables.
posted by mnology at 1:51 PM on June 16, 2005

You could also buy one of the car adaptors, and run the laptop off a lead-acid car battery. Takes it off the mains completely.
posted by -harlequin- at 2:28 PM on June 16, 2005

Grounds confuse me, but as for ground-lifts.. they are on some XLR connectors as well. You'd have to rewire your plugs, but they might hae pre-made coupler plugs with ground lift switches as well. I believe Neutrik makes XLR plugs with a ground lift switch, but I can't find them at the moment, I'd call Mouser electronics.

Batteries are nice, but heavy and pretty pricy. Lead acid is going to cost $100+, and weigh 3+ lbs, lithium will beat the weight, but battery + charger would cost $200+.

Zsazsa's link looks like the best bet.
posted by Jack Karaoke at 4:00 PM on June 16, 2005

I had almost the exact same thing happen to me. I figured it was a ground loop problem, but first I tried switching out the no-name power strip I was using with a nicer Belkin strip and that eliminated the hum.

On the other hand, a new power strip would be more expensive than that ground loop isolator zsazsa linked to, so unless you have one sitting around, I'd go straight for the isolator.
posted by cloeburner at 4:11 PM on June 16, 2005

There's another source of noise, and it has nothing to do with ground loops, and it's incredibly common with laptops.

DC offset.

Laptops run on DC, and the power supplies for them are often fairly lousy switching supplies, so they're running on noisy DC. This often leaks out onto analog ports, and causes buzzing noises.

The way to tell, since you are a musician: Does the noise have tone, if so, how close to 60Hz or a harmonic is it?
If it is that, then it's AC hum, and a ground loop. Normally, you hear the 120Hz harmonic (100Hz for you euro folks.)

However, I'm willing to bet it isn't -- instead, it's DC hash from the power supply. The reason you don't hear it on battery is that batteries put out very pure DC.

If the laptop is actually generating sound, what you need to solve the problem is an isolation transformer. Many DIs are also isolation transformers, but not all. A transformer will pass the AC signal that has your music, but is a solid wall to DC, thus, the DC hash doesn't happen.

If the laptop is just generating control signals, something as simple as an inline capacitor can fix it. The problem with that in the audio chain is that it'll act as a filter, when combine with the various components in the audio output, will seriously change the sound of the output, which you don't want. But for control circuits, the frequency bandwidth often isn't critical. Ideally, though, for digital signals, you'd use an opto-isolator, which uses an LED and a photo transistor to completely isolate the two circuits.

If you want to check for DC hash before dropping money on parts, that's simple, if you're comfortable with a little cable hacking. We just use a cheaper DC blocker. You'll need a cable you can sacrifice (or wanted to shorten) and two or three AC capacitors (namely, not normal electrolytic or tantalum caps.) Cut the cable, and solder in a cap to reconnect each wire, matching the color. Make sure they don't short to each other. Now, use that from the output of the notebook to your mixer and beyond, plug in the notebook, and listen.

If the noise goes away, you've got a DC hash problem. Alas, with cheap caps, most of the musicality of the output will be gone as well, but now you know you need to isolate the DC out of chain, namely, you need an isolation transformer. Note that cheap ones will cause the same sorts of musicality problems, and the component ones that Radio Shack sells are optimized for phone/ham use, and only pass 300-3Khz. Great for phone, lousy for music.

If it doesn't, then it is AC noise, and you need to look at ground loops and other strangeness. But given that, with a notebook, you're effectively isolated from ground anyway (most notebooks don't even have grounding plugs), I'm betting DC hash.

(Note: I spent 10+ years doing live sound work, and many more interfacing computers and radios. Both have the same issues with bad supplies putting out noise on DC.)
posted by eriko at 4:32 PM on June 16, 2005 [1 favorite]

Can you go fibreoptic? My Fujitsu laptop has an optical out. That would have a good chance of defeating an earth-loop, and might even sort out other hum problems. It would be my first step.
posted by krisjohn at 5:16 PM on June 16, 2005

I had this problem with my Dell Inspiron 7500 - turns out it was a DC power thing as eriko described so well - I now use the car adapter (cigarette lighter) power plug and a DC converter box that accepts a cigarette lighter attachment to power my laptop whenever I'm doing sound work - it's a ridiculous looking setup but it runs perfectly quietly now
posted by soplerfo at 6:00 PM on June 16, 2005

Response by poster: Thank you very much for your responses. Eriko, I will hack up that cable this week and test that out.

I was in a hurry at the time I posted the question and completely forgot to mention some important facts. I assume they stear away from the ground loop and more towards the DC noise...

1) It is not a constant hum, but gets louder when the LCD is in use, and quieter when the LCD is blank (screen saver mode).

2) I can hear the hum change frequencies when the hard drive is reading or writing to the disk.
posted by monsta coty scott at 9:40 PM on June 16, 2005

I would just like to point out that Macs don't have this problem; they're engineered with this in mind.

Get yerself a PowerBook!
posted by chota at 10:15 PM on June 16, 2005

Wow, I've been having this problem forever as well. Everyone kept telling me that it was a ground loop problem but that didn't make any kind of sense to me at all. The sound wasn't a constant 60hz and it's a rough buzz. The buzz changes when you do different things on the computer. If you move your mouse, or get network traffic, if the monitor is on, or if the HD is being accessed, you get different sounds. I can turn my levels way up and hear noises appear when I move my mouse!

This happens both on my Dell Laptop and on my Shuttle XPC.

I read Eriko's post, but does anyone have an easy solution? I don't have a Car Adapter for my Shuttle desktop and I don't want to hack up my cables. I don't want to risk ruining my precious PC because of my lack of electrical knowledge or soldering skill.

Great question, monsta coty scott!
posted by redteam at 8:39 AM on June 17, 2005

I read Eriko's post, but does anyone have an easy solution?

What you need is something that will isolate DC, but allow AC to pass. Many passive Direct Injection boxes will do this. (some one.) Some active ones will as well. All tube DIs will, because of the nature of tubes. Best way to make sure it'll work-- take the notebook into the store, try a few.

However, redteam, your problem sounds different. You have gainitis -- inflammation of the output gain. Your notebook has an amplifier. It will pick up noise. When you crank the gain, the signal gets amplified -- but so does the noise. Noise *in* the signal is there forever -- you can't remove it without removing the signal.

What you need to do is maximize the signal you are trying to output, before the final amplification stage. How this is done depends on the software, but in general, you'll want to push up the "PCM" or "WAVE" gain, while reducing the master or "output" gain. I'd drop everything to zero, push the master to 100%, and then back it off until the noise is gone. Then, push up and fade one channel at a time, until you find your musical source. Push that up until you get a good signal.

Don't try for maximum loudness. That's not your job, that's the PA's job. The PA has vastly more amplification than you do -- which means he can make things vastly louder -- both signal and noise. Your job is to present him as much signal as you can, and as little noise as you can. Indeed, you must -- with enough amplification, I can pick up noise with a bare piece of wire. Add a diode, and you start picking up AM radio.

Finally, "clean sound" and "computers" is a real problem, esp. with cheap computers. There's so much noise (in audio terms) in a computer that needs to be there to make the computer work. (The CPU, the Video card, the various oscillators, etc. etc. etc.)

Chota: It happens with Powerbooks as well. Apple audio is, as a class, better than most, but power supplies are hard to make clean and cheap.

The cheapest way to fix power problems in performance is batteries, and a notebook that will allow you to change them without rebooting. A popular musician's notebook was the Wallstreet Powerbook, with the CD-Rom pulled out and replaced with another battery during performance (and a third battery ready to go, just in case.)

posted by eriko at 8:28 AM on June 18, 2005

Again, using an external interface (I mistyped before, something like the FireWire or USB Audiophile from M-Audio would be appropriate) would solve any problems with a laptop's internal interference, which is even a problem on a desktop.
posted by abcde at 9:55 PM on June 18, 2005

abcde - I use an external audio device and still have the problem with the hum - I assure you, it's got somethign to do with the power AC vs DC - the only thing that's quieted it is to use a converter with the car adapter...
posted by soplerfo at 1:16 PM on June 19, 2005

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