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Old 29th July 2009, 11:44 PM   #11
AndrewT is offline AndrewT  Scotland
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Originally posted by ormo
I've figured its definitely a ground loop problem: the hum goes away completely when I unplug the laptop AC adaptor.

Does the 220pF capacitor advice still apply here? Altering the PCB isn't really an option here so what else could I try?
is the Laptop AC adaptor double insulated? i.e. no safety earth connection.
regards Andrew T.
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Old 30th July 2009, 03:03 AM   #12
star882 is offline star882  United States
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Plug both the laptop and the amp into the same power strip.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 30th July 2009, 12:48 PM   #13
jaycee is offline jaycee  United Kingdom
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You were describing a high pitched whine, not a hum.

Yes, it is best to have that 220pF there. You can just solder it between the two pins on the back of the PCB.
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Old 30th July 2009, 10:19 PM   #14
SoliS is offline SoliS  United States
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Originally posted by ormo

V+ and V- : These go to the power supply (obviously)
GND : This goes to the chassis

IN : This goes to the signal of the 3.5mm plug
GNd : This goes to the ground of the 3.5mm plug

OUT : This goes to the positive wire of the speakers
GND : This goes to the negative wire of the speakers

wait for someone else to confirm this but...

shouldnt your INPUT GROUND also be referenced to your OUTPUT GROUND, using the star-ground configuration?

i.e. all grounds of the audio path should be connected to the same point in the star method. if your input ground (laptop) is isolated from your speakers, this makes the ground path go from speaker gnd through the power supply to the chassis through earth ground through your walls, back through your laptop plug and finally to the 3.5mm ground on your laptop card. if that ground has to go through the mains you could be picking up hum.

though if its high-pitched whining like someone above said, it might also just be your laptop SMPS not being properly isolated from its audio output
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Old 3rd August 2009, 08:34 PM   #15
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I had the same issue with my laptop and my LM3875. When using the laptop into the Gainclone (Dual Mono) with a Phono to 3.5mm jack cable and the laptop connected to the mains (Double insulated SMPSU/Charger), I had a high pitch whine from both speakers. With the laptop running on battery, it was fine.

Unfortunately I don't have the problem anymore, the laptop died on me (motherboard blew up)

I played with the idea of having a switchable input capacitor when using the laptop, but never got round to testing it.

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Old 9th August 2009, 06:09 PM   #16
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I have been using computers for several years now to record audio . Yhis problem is best cured by using transformer isolation to break the groung loop. Nice ones can be bought for big bucks, or, the cheap and dirty trick is to scavenge some from old computer modem boards. These are only rated for 200-4k response, but seem to work acceptably at lower signal levels. Size does seem to matter, I have found larger transformers have better bass response. A good one to try for somewhat better response is this Triad transformer from Mouser:
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Old 11th August 2009, 12:22 PM   #17
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Try to connect a power in different sockets in the house.
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Old 12th August 2009, 08:54 AM   #18
Eva is offline Eva  Spain
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Lift the ground in the amplifier, it's true that the metal case should be earthed but the internal ground of the amplifier is not required to.

In fact, amplifier ground shouldn't be earthed if you want to get some sound quality out of it (with earthed sources) without listening to all that earth loop noise and probably pops and clicks when appliances are turned on and off.

Leaving internal amplifier ground loosely coupled to earth is another solution (like 1Mohm and 1nF).
I use to feel like the small child in The Emperor's New Clothes tale
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Old 12th August 2009, 09:14 AM   #19
AndrewT is offline AndrewT  Scotland
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yes, you must permanently connect the metal chassis to the mains earth.

The Audio Ground can be directly connected to the chassis but this often leads to hum and/or buzz problems.
As an alternative the Audio Ground can be connected to the Chassis using a Disconnecting Network. This sometimes eliminates the hum problem and usually reduces it sufficiently to become acceptable.
regards Andrew T.
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Old 12th August 2009, 09:19 AM   #20
digi01 is offline digi01  China
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laptop's inside audio card is always high noise level.i use a usb soundcard(M-audio) to deal with it.

another way is add a high pass filter between the pc and gc.
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