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MBP Buzz when Plugged In

A

airkarol

macrumors 6502
Original poster
Nov 12, 2005
280
0
Hey,

I was doing a live recording last week, and whenever the MBP was plugged in with the power adaptor, there was a huge buzz/hum coming through the house PA and in the recording. This didn't occur when on battery. The setup was the MBP, with an external M-Audio Firewire 1814 (bus powered), with two microphones plugged into it going through a snake (for those who don't know, it's basically multi cable). Those two were not connected to the house PA at all. There were two others, going through the same snake, that were plugged into the mixer, and coming through the house PA. When I unplugged the two microphones from the firewire interface, the buzz went away.

Any ideas?

Thanks.
 
rezonat0r said:
You do recording but have never run into a ground loop? You must be incredibly lucky. :D

http://www.google.com/search?q=eliminate+ground+loop
Click to expand...

It's not a ground loop, that would be a constant 50Hz hum...wouldn't it?

The sound I'm getting when plugged in is a lot more random, like sort of interference. Running on the battery reduces it, and curiously unplugging my USB (music) keyboard 100% cures it! So I guess it's related to the USB for me.

Still, I need my USB keyboard for live use - so, any advice to get rid of the interference?
 
MarkW19 said:
It's not a ground loop, that would be a constant 50Hz hum...wouldn't it?

The sound I'm getting when plugged in is a lot more random, like sort of interference. Running on the battery reduces it, and curiously unplugging my USB (music) keyboard 100% cures it! So I guess it's related to the USB for me.

Still, I need my USB keyboard for live use - so, any advice to get rid of the interference?
Click to expand...

If there isn't a choke on the keyboard's USB cable, try adding one (or replace it with a cable that has one). The choke should trap the noise coming down the USB cable. (The choke is usually the cylindrical thing around the cable ends).
 
gr8tfly said:
If there isn't a choke on the keyboard's USB cable, try adding one (or replace it with a cable that has one). The choke should trap the noise coming down the USB cable. (The choke is usually the cylindrical thing around the cable ends).
Click to expand...

Good advice, thanks. I don't think any of my USB cables have chokes - what exactly would it look like?
 
For the OP it certainly sounds like a ground loop ("hum" or "buzz").

Your issue is being exacerbated by a ground loop - the fact that the noise decreases on battery would seem to prove that. A lot of isolation transformers cut down on the noise floor in general. Is your keyboard purely bus powered? Also, if you are using any unbalanced cables or connectors, that will probably be the main problem.
 
I'm guessing the snake you were using uses a single ground return? Many of them do...

If so, this may be the issue...
 
rezonat0r said:
For the OP it certainly sounds like a ground loop ("hum" or "buzz").

Your issue is being exacerbated by a ground loop - the fact that the noise decreases on battery would seem to prove that. A lot of isolation transformers cut down on the noise floor in general. Is your keyboard purely bus powered? Also, if you are using any unbalanced cables or connectors, that will probably be the main problem.
Click to expand...

No, my keyboard is mains powered. It just uses the USB for MIDI.

My cables are unbalanced yes, as the outputs on my controller/piano are unbalanced. But, it does just seem like a USB problem though - playing the piano directly into the PA (unbalanced) is perfectly clean, then when the USB is plugged back in, the noise comes back...
 
It sounded like more than one frequency. When the magsafe was plugged in, there would be one sound while the light was green, and once it changed to amber, there would be another, much worse.

Another thing-- the sound went away when the 2 XLRs were removed from the interface. Would the source of the problem be the computer, or the snake?
 
MarkW19 said:
No, my keyboard is mains powered. It just uses the USB for MIDI.
Click to expand...

Still sounds like there's an imbalance in the ground between your notebook and keyboard, and the power over the USB cable is completing the loop. Electricity is nasty stuff : (

Both of you should try to pickup a little conditioner/isolator (from Guitar Center or something), it would nip that in the bud. If everything is running XLR that probably isn't the issue, but it only takes one unbalanced RCA or something to cause problems.
 
rezonat0r said:
Still sounds like there's an imbalance in the ground between your notebook and keyboard, and the power over the USB cable is completing the loop. Electricity is nasty stuff : (

Both of you should try to pickup a little conditioner/isolator (from Guitar Center or something), it would nip that in the bud. If everything is running XLR that probably isn't the issue, but it only takes one unbalanced RCA or something to cause problems.
Click to expand...


Everything in my setup was running XLR, although there were snakes involved, I don't know if they're all sharing a ground inside.
 
airkarol said:
Everything in my setup was running XLR, although there were snakes involved, I don't know if they're all sharing a ground inside.
Click to expand...

Nah, those XLRs in the snake will all be balanced. They are transporting your unfortunate noise with pristine quality :D
 
Yeah, it's definitely the ground loop. I used to live over seas and I would get a more pronounced hum because the incoming frequency wasn't as doctored up as it is in the US. I think, too, if your ground is better, you should hear the sound less. Don't most houses just ground to the pipes? Obviously, when you don't have it plugged in, you're not completing the circuit.

I'm not sure if it's been mentioned, but I do think they have some devices to clean out the sound.
 
I'll pick up one of those when I have a chance, thanks for the help.
 
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