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Old 8th October 2009, 10:32 AM   #31
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Originally Posted by gaplessophile View Post
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?
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?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 9th October 2009, 12:51 AM   #32
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Join Date: Mar 2006
Location: New Zealand
Actually, I should have added that my LM3875 amp has a tube buffer - it's a JLTI, built according to this design:

Click the image to open in full size.

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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Old 15th December 2009, 07:14 AM   #33
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Join Date: Mar 2006
Location: New Zealand
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
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Old 15th December 2009, 07:34 AM   #34
Key is offline Key  United States
diyAudio Member
Join Date: Jan 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.

Last edited by Key; 15th December 2009 at 07:36 AM.
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