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Old 29th January 2013, 01:28 PM   #11
sraudio is offline sraudio  Lithuania
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Sorry mistake, R6 an C7 can't be the reason for oscillation
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Old 29th January 2013, 02:48 PM   #12
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As requested, this is the PSU:
Click the image to open in full size.

Would the grounds be ok like this:
Click the image to open in full size.
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File Type: png PSU.PNG (15.5 KB, 120 views)
File Type: png grounds.png (13.1 KB, 119 views)

Last edited by silver1omo; 29th January 2013 at 03:04 PM.
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Old 29th January 2013, 03:13 PM   #13
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Hi,
Attached it is a drawing of the start ground I used in my LM3886. All ground should be tie to one point. What are the red and green colors in the pictures means?
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File Type: jpg star ground layout.jpg (216.6 KB, 105 views)
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Old 29th January 2013, 03:37 PM   #14
JMFahey is offline JMFahey  Argentina
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I think red means points joined together and grounded through one (red?) wire, and green another similar net, using another (green?) wire.

I find the most "dangerous" return the speaker ground, and it always deserves its own dedicated wire, straight to the filter caps junction or the negative if single supply, because of 2 very good reasons:
1) it carries a lot of current and
2) it's audio, in phase with signal, and thus "excellent" to get positive feedback, instability and oscillation.
Or at least weird unexplained distortion.
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Old 29th January 2013, 05:39 PM   #15
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Correct, the green is intended as signal ground and the red as power ground.
I was checking in Elliot's page for his other implementation of the amp (Project 115) and noticed that the cap and resistor in pin 9 are inverted and the cap is a polar electrolytic.
Click the image to open in full size.
In the project I built (Project 19) it is shown as a non polar electrolytic.
I also checked other amp diagrams (chipamp.com and some others found with google) and it seems like the cap for pin 9 should be a polar electrolytic with the + side pointing towards the chip. Could you confirm that?
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Old 29th January 2013, 06:43 PM   #16
JMFahey is offline JMFahey  Argentina
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The RC in the feedback loop are in series, so order is not important.
The capacitor "should" be a bipolar or NP one, you are right, but a common prctice is to use a cheap polarized one there.
Why?
Because usually its value is quite large, so little peak voltage develops across it in normal use.
If it's less than 1 V under any circunstances, you are fine.
That's why the RC constant which defines the low frequency end must be the one at the input.
If not, you need a bipolar.
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Old 29th January 2013, 10:43 PM   #17
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Well, I guess I should have used a kit :P
I changed the grounding and still have problems, as soon as I connect that leg the DC goes from less than 4mV to anything from 9 to 190mV.
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Old 29th January 2013, 11:11 PM   #18
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Hi,
Connect a 220pf capacitor between pin 9 and 10 and see what happen.
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Old 30th January 2013, 01:08 AM   #19
JMFahey is offline JMFahey  Argentina
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Quote:
as soon as I connect that leg
Which leg?
Connected to what?
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Old 30th January 2013, 01:12 AM   #20
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I fixed it! It was a grounding problem. Checked a couple of point to point builds in the forum, notably http://www.diyaudio.com/forums/blogs...ne-part-1.html and realized that I had the feedback cap ground tied to the "zero volts" ground.

So based on wintermute's build, I moved the feedback cap lead to pin 7 and now I can turn it on with everything grounded and measure less than 1mV with no load.
I ventured to connect a source and a speaker and got music
That makes me happy, as you can notice this is my first amp build and now I can move to the other channel.
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