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ormo 29th July 2009 01:36 PM

High pitched whine when using laptop input
I am running a stereo amp with one LM3886TF on each channel. It's nothing special, definitely not audiophile quality, but it fits my needs and works perfectly most of the time.

The input to the amp is a standard 3.5mm stereo jack so I can plug in my MP3 player, iPod, laptop, phone, etc.

Recently the amp emits a very high pitched whine through the speakers under certain conditions:

iPod plugged in:
Silence until I play music

MP3 player plugged in:
Silence until I play music

Laptop plugged in:
High pitch whine until I play music, high pitched whine as well as music when I play something

Laptop plugged in BUT only signal ground connected (ie. left and right signals not connected:
Same as above

Evidently the problem is something to do with the signal ground on the laptop, but is there any way I can fix this?

Thanks :D

ormo 29th July 2009 02:08 PM

If it helps (and I think it does), the input ground, output ground and power ground are all kept separate from each other.

ormo 29th July 2009 02:19 PM

Just realised I've been a bit dumb - this is a ground loop, right? The reason I only get a buzz from the laptop is because thats plugged into the same mains circuit as the amp but the iPod/MP3 player are battery powered and not connected to the mains ground.

Correct me if I'm wrong (quite likely) but I need a ground isolator? At the moment I'm using a star ground - all grounding wires meet at the same point on the metal case which is connected to earth in the plug, so the isolator needs to go between the star and the actual earthing pin, correct?

Please correct me is the above is wrong :)

AndrewT 29th July 2009 02:28 PM

the chassis must be directly connected to the earth wire in the mains cable. This is the Safety Earth. Do not disturb it.

If you decide to add a Disconnecting Network, then fit it between chassis and the main star (Audio) Ground. You can fit a switch across this Disconnecting Network to short it out if it helps with some installations.

Never fit a switch between the chassis Safety Earth and the mains cable. This must be a permanent mechanical fixing.

ormo 29th July 2009 02:42 PM

Sorry if I was unclear - I was trying to say the same as you suggested: an isolation transformer between the star and chassis. Now that I read it back the previous post is kinda garbled.

I'm still not sure on where to put the isolator... There is one PCB for each channel (the amp is effectively 2 mono amplifiers). Each PCB has 7 connections on it in 3 groups... Below is how they are labelled and how they are wired up.

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

Is this correct or should the three ground connections be somehow different? Where does the isolator need to go in this configuration?

Thanks a lot for this, it's got me pretty confused at the moment.

pacificblue 29th July 2009 04:16 PM

It should be on the PCB with power and output ground on one side of the resistor and the input ground on the other side.

Is the 3,5 mm jack isolated from the chassis?

jaycee 29th July 2009 07:08 PM

What you might also be hearing is the switch mode regulators in all those devices - especially if your LM3886 amp does not include a lowpass input filter. The laptop may be particularly bad as onboard sound chips in laptops typically have very poor isolation between the analogue and digital sides.

Try soldering a 220pF capacitor (a good polypropylene is best, but a NPO ceramic will do) between pins 9 and 10 on the LM3886 chip. This might well cure the problem.

ormo 29th July 2009 11:29 PM

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?

AndrewT 29th July 2009 11:32 PM

post the schematic of the power amp.

imix500 29th July 2009 11:41 PM

It's actually a fairly common problem with laptops. In the theater we use a Radial JPC or equivalent to isolate the laptop.

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