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
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
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
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
no.
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.
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.
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.
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.
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.
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 said:
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
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.
Chris.
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.
Chris.
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:
http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMv0IfuNuy2LUX/9G5MznyA%2bcTcy0wGzVq4=
http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMv0IfuNuy2LUX/9G5MznyA%2bcTcy0wGzVq4=
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).
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).
Hi,
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.
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.