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Old 28th April 2013, 10:26 PM   #101
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Join Date: Dec 2012
Location: Michigan
Originally Posted by Mooly View Post
4 mv offset is negligible. Even if you work to 100mv the power dissipation in an 8 ohm load is still only 0.00125 watts. Now that won't heat anything, speaker or amp alike. It would represent only 12.5 ma "extra" current in the output stage.

No volume control ?

Why don't you try (just as a test) a simple R/C filter on each input. Be drastic, try something like a series 10K and 1nf to ground and see if that stops the problem. A filter like that would be audible but might give a clue.
Well, I probably should have tried a different source (other than a mp3 player) in the first place... because apparently it's only an issue with that one usb DAC. No problems at all if I use the headphone out of my iMac or the firewire Presonus interface... BUT everything is working perfectly now. It barely even gets above room temperature now, no dc offset and no turn on/off pops or thumps.

Hooked up to my Presonus interface it is completely silent. I can't even tell it's on with my ear against the speaker. No hum or hiss or anything. I'm very happy with how it sounds as well.

Thanks again for all the help and suggestions. If I get that tone section figured out and working, I'll post about it.

Also, I'll post my final schematic (minus tone section) soon so others can reference it if they'd like. A few parts changed compared to earlier ones posted.
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Old 29th April 2013, 06:13 AM   #102
Mooly is offline Mooly  United Kingdom
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Some MP3 players rely on the headphones to form part of the filter circuit for the class D output stage. You could always try something like two 33 ohm resistors to load the MP3 output and see if that helps.

Good to hear its all OK
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Old 28th May 2013, 04:23 PM   #103
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Join Date: Dec 2012
Location: Michigan
Finally got around to cleaning up my schematics and updating them with the final part values I used. I also included the calculations with final part values.

Still loving it after a month of use. I really couldn't be happier with how it sounds.

I have a machinist friend cutting the amp front panel for me, so once I get that back from him I'll post some pictures of the final build.
Attached Images
File Type: jpg Final LM3886 Stereo Amp.jpg (222.3 KB, 113 views)
File Type: jpg Final LM3886 Power Supply.jpg (158.3 KB, 104 views)
File Type: png Final LM3886 Amp Calculations.png (128.0 KB, 95 views)
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Old 29th May 2013, 08:47 AM   #104
AndrewT is online now AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
well done in differentiating between Signal & Power Grounds in your schematic.

Will you measure the resistance between SG and Chassis and PG and Chassis to double check you don't have an undetected third earthing point.

I recommend you fit inverse parallel diodes across the 10r and the 100r between Earth and the Grounds. These diodes should be able to pass Fault Current to the PE to keep you and other operators protected.
regards Andrew T.
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