High pitched whine when using laptop input

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Eva

Eva

Member
Joined 2003
AndrewT said:
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.
Click to expand...

Hi Andrew, I know you are very strict about these subjects, but most audio gear is "double insulated" and has no earth connection, which just means that some creepages and clearances (and insulation thicknesses) between mains live parts and secondary/isolated or user accesible parts are met. A diyer can easily meet or exeed these requirements, and any good transformer will meet them too.
 
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danb1974

Member
Joined 2005
Just finished building a lm3886 amplifier after the datasheet schematic and hit more ore less the same problem. When connecting the amp to my diy pcm2702 dac, the amp started oscillating (some high pitch wine audible, radiators way too warm, power consumption 300+mA instead of normal 100mA for two idle channels). Amp has no ground loops, lm's isolated, radiators connected to ground.

Happened even I shorted the input, unplugged usb from dac, leaving just the dac powered on external power adapter (no earthing - so no ground loops). Seems like hf noise enters through ground. DAC uses ground plane (not the best ideea). Amp has star grounding.

The 220pF capacitors did the trick - but I am curious, did I make a mistake in the dac design or the lm3886 has too much bandwidth?
 
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danb1974

Member
Joined 2005
Right. Lesson learned. Always check if consumption with no input matches the datasheet.

I've also seen in a couple of schematics and input RC filter (680ohm+470pF in one place, 680ohm+47pF in another place). Would this also be a good ideea? If yes, which values would be more appropiate?
 
N

Nathanshort

Member
Joined 2009
What you are hearing is the noise from the SMPS in your computers charger as it changes AC to DC. Noisy little buggers.

Noise when plugged in, fine on battery.

We deal with this all the time in pro audio. Use a 1:1 isolation transformer on your RCA cables.

I highly suggest Jensen Transformers. Or the EBtech Hum Eliminator.

Make a box and take it with you, no one likes noise when you decide to play DJ.
 
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gaplessophile

Member
Joined 2006
I'm having the same problem with a LM3875 amp.

With my laptop connected to power, I get high-pitched whine and noise. It's not there when the laptop isn't connected to power.

Would the 220pF capacitors help me?

I assume I should use them on pins 7 and 8 on my LM3875, since they're Vin+ and Vin-, same as pins 8 and 9 on the LM3886?

Thanks!
 
A

AndrewT

R.I.P.
Joined 2004
gaplessophile said:
I'm having the same problem with a LM3875 amp.

With my laptop connected to power, I get high-pitched whine and noise. It's not there when the laptop isn't connected to power.

Would the 220pF capacitors help me?
Click to expand...
Why does anyone build an amplifier without an RF attenuation filter on the input.

Why do experienced designers rarely mention that they have deliberately deleted the RF attenuation filter and forewarn the builder what symptoms may result and what solutions exist to cure the problems that ensue?
 
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gaplessophile

Member
Joined 2006
Actually, I should have added that my LM3875 amp has a tube buffer - it's a JLTI, built according to this design:

JRGC_DIY_T-Net-use.gif


Would the 220uF caps still help under this design?

If it helps, I previously had a non-inverting GC without the buffer, which had the exact same problem with noise from the laptop.

If no one tells me it's a bad idea, I'll just go ahead and try it :)
 
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gaplessophile

Member
Joined 2006
In case anyone's referring to this thread, I just tried the 220pf capacitor approach and it did not work. I was still getting the same high-pitched "static" from my laptop.

I then tried a disconnecting network like this, between the chassis and the circuit ground (centre tap of transformer). (Chassis remains earthed.) It worked perfectly. No more noise for me :D
 
K

Key

Member
Joined 2009
Haha this is really common on laptops. Nothing electrical. There is a tiny little microphone in most laptops. Mute it.

Hmm didn't read the whole thread. Glad you fixed it. But uh just in case someone else reads this that is usually the first thing to check.
 
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