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Topic # 24118 15-Jul-2008 18:04
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Hi, 

Having 'hum' when connecting macbook pro via the line out jack to the RCA inputs of a peavey powered portable mixer.
Anyone know of any good ground loop isolators and where to get these from?

The hum lessens when the laptop is powered by battery and Im not going to be disconnecting the earth lead at all ! 


Cheers



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222 posts

Master Geek
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  Reply # 148223 15-Jul-2008 18:24
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Disconnect the power charger, they cause all sorts of earthing issues.




Masterpiece Calibration Ltd, isf certified

 

www.mastercal.co.nz

 

 

"I'm not a robot!"



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  Reply # 148227 15-Jul-2008 18:34
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HI, 

thanks, for that... however there is still a slight hum. I suspect this is an earth loop / balance line problem. There are line filters avalible for the audio signals. Quite hard finding them online.     Besides, theres only so much battery power you can use before having to plug in again! 



 
 
 
 


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Ultimate Geek
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  Reply # 148257 15-Jul-2008 19:45
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You could try this from Dick Smith. One worked quite well for me if not a bit uneven between the channels. Also they don't necessarily stop the high frequency (induced Noise) from getting through.

What really solved my ground noise issues was connecting the chassis's of the components together with a good sized cable.

How you do that with a laptop I'm not sure. USB has an earth line but might not be advisable to try that.

Also plug all gear into the same sockets if possible and check no other connected equipment is introducing noise.



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  Reply # 148261 15-Jul-2008 19:56
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stuzzo: You could try this from Dick Smith. One worked quite well for me if not a bit uneven between the channels. Also they don't necessarily stop the high frequency (induced Noise) from getting through.

What really solved my ground noise issues was connecting the chassis's of the components together with a good sized cable.

How you do that with a laptop I'm not sure. USB has an earth line but might not be advisable to try that.

Also plug all gear into the same sockets if possible and check no other connected equipment is introducing noise.

cheers
The noise source is the apple power supply and secondly the laptop itself... like you say how to you strap the laptop to the amp ! 
Power is all on the same circuit......and seperating the power sources just to see if there was more noice etc etc etc      I could really do with a inline DI from the audio out to the audio in (laptop to amp)

Im yet to test this in another venue all together as the current venue's power supply might just be stuffed !



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  Reply # 148266 15-Jul-2008 20:01
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I'm remember having this problem with my old Dell Inspiron. I also tried running the amp and laptop off different circuits but could never get rid of that hum. The solution was to charge up my laptop before plugging it into the amp, and running it off battery while it was plugged in.

Once I sold my Dell and got my current HP Compaq laptop, the hum went away, even with the power cord plugged in.  Have no idea what is different between the Dell and HP, but no more hum.

No idea if this would help, but would a USB soundcard solve this issue?  Maybe worth a try.

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Master Geek
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  Reply # 148274 15-Jul-2008 20:15
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Alot of these modern power supplys can cause earth noise according to my electronics man, they are a cheap(but effective) to build switchmode design.



They cause all sorts of problems at Uni when we are setting up dataloggers with sensitive sensors.




Masterpiece Calibration Ltd, isf certified

 

www.mastercal.co.nz

 

 

"I'm not a robot!"



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  Reply # 148275 15-Jul-2008 20:17
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Masterpiece: Alot of these modern power supplys can cause earth noise according to my electronics man, they are a cheap(but effective) to build switchmode design.



They cause all orts of problems at Uni when we are setting up dataloggers with sensitive sensors.

so true....

lead shielded room anyone?



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  Reply # 148301 15-Jul-2008 21:24
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This is a really common problem (another hat I wear is as a sound engineer)

Take a look at this thread on another forum I frequent for more info.

Cheers
David




------------------------------------------------------
David Elsbury
Freelance Lighting, Sound and AV Technician
"Technician like ninja... live in shadow, move in silence"


222 posts

Master Geek
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  Reply # 148345 15-Jul-2008 22:33
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That reminds me, from a past hat....sound engineer in UK no less.....you could try a DI box inbetween.
Check out music stores for interfaces for PC to audio devices, quite often you can find devices with earth loop toggles, ie DI box.




Masterpiece Calibration Ltd, isf certified

 

www.mastercal.co.nz

 

 

"I'm not a robot!"

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Ultimate Geek
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  Reply # 148357 15-Jul-2008 22:52
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I always use DI's for this sort of thing. Most of them have a ground lift.






cisconz
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  Reply # 148418 16-Jul-2008 09:01
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rhysb: I always use DI's for this sort of thing. Most of them have a ground lift.


I have found for some laptops with cheap sound cards not even a DI will work.

To get around this there are 2 options.

1) Spend about $200-$500 on a toroidal power supply (you will need to make part of this)

or

2) Get an opto isolated usb sound card.




Hmmmm



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  Reply # 148571 16-Jul-2008 17:40
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Cheers guys.

For an experiment, I have succumed to buying somthing from DSE which looks like it has been designed for car audio. In the same breath, good decent DI's cost dollars.....  I think ultimatley I will have to save for a decent multi channel DI with hum lift. 
In the meantime the nice guys @ strawberry sound are making me up an inline transformer circuit to test. 

This weekend will tell if it works, or I may just have a crappy soundcard in the macbook!  In which case I would like to try a firewire audio I/O (but they cost $$$), unless I can get one to 'try' first.

cheers
goose



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  Reply # 148707 17-Jul-2008 01:16
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Out of interest I searched for a circuit diagram for a switch mode supply similar to those that would be used on computers.

One is here but you need to scroll down to the title "Off Line SMPS circuits".

You can see that though the DC output is isolated, earth is railed right through and will be the reference for the connected device (PC) signal ground. I guess hence the susceptability to ground hum.

I think from memory computers have to have an earth and it's something to do with USB, possibly hazard protection, whereas a lot of CE equipment has no earth connection.

I guess computers such as Apple that expect a lot of pro-audio use have a design strategy to minimise ground loop effects.

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Wannabe Geek


  Reply # 149955 21-Jul-2008 17:49
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You didn't happen to go to Magnum Mac to find an answer to this did you?

I'm guessing your refusing to go with the ground lifter idea...
How did it go, have you cured your buzzing issues?

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  Reply # 150013 21-Jul-2008 20:59
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All the apple adapters I have seen have being a 2 pin, so chances are its getting way too much leakage because of the obnoxious number of capacitors that the bloody euros make manufactures put on things, but there is no ground to try and lift on the supply side.

I had the same problem with my laptop and stereo, problem was solved when my power adaptor blew up and I had to replace it with another one.




Richard rich.ms

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