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Old 12th August 2009, 10:56 AM   #21
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I had such problem. The problem was in high-frequency hindrances from the SMPS. You need to use the high-frequency filter in the amplifier PSU or connect amplifier and laptop PSU in different power sockets. I was helped by the second variant.
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Old 12th August 2009, 11:24 AM   #22
Eva is offline Eva  Spain
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Quote:
Originally posted by AndrewT
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.
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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Old 13th August 2009, 12:49 AM   #23
wwenze is offline wwenze  Singapore
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Just to mention...

Laptops have no safety earth connection, unless I see one that has a 3-pin power connector.
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Old 2nd October 2009, 12:23 PM   #24
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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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Old 2nd October 2009, 01:19 PM   #25
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by danb1974 View Post
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?
The option components are not optional.
No mistake, it's National's poor choice of warning to those who don't yet know why those optional components are usually needed.
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Old 3rd October 2009, 09:51 AM   #26
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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?
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Old 3rd October 2009, 10:00 AM   #27
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Quote:
Originally Posted by AndrewT View Post
it's National's poor choice of warning to those who don't yet know why those optional components are usually needed.
The datasheet is probably intended for a different audience. Those ICs were not originally designed for first-time DIY projects, but to save engineering and assembly cost in mass manufacturing.
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Old 4th October 2009, 10:37 PM   #28
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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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Old 7th October 2009, 09:39 PM   #29
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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!
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Old 8th October 2009, 05:30 AM   #30
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Pins 7 and 8 on LM3875
Pins 9 and 10 on LM3886

http://www.national.com/ds/LM/LM3875.pdf
http://www.national.com/ds/LM/LM3886.pdf

If only one isolation transformer would cost less then the entire amp...
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