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Old 13th March 2013, 11:14 PM   #1
Bando is offline Bando  Canada
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Default lm1875 psu issues

Hello, making my first chip amp that I am actually spending money on .

Well, as a newbie, of course I have some issues.

Disclaimer, I do apologize if this has been covered. I didn't find anything by google. Probably because my word choice is poor.

So I have a normal out board psu, +18 -18 and earth. 25 amp bridge rectifier and 0.12 farad soothing caps (yes farad not uf, I got them for free at work).

The issue I am having is that my amp is "popping" when on and the sound is distorted. The positive rail goes to +28v and the negative rail drops in voltage to about -8v.

When in open circuit the voltage is steady at +18 and -18 respectively.

LM1875 is the exact same as what TI calls for in its normal dual power supply circuit.

So thats where I am at,

Look forward to any and all replies

Thank you!
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Old 13th March 2013, 11:16 PM   #2
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Are you 100% the psu caps are orintated correctly for polarity?
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Old 13th March 2013, 11:19 PM   #3
Bando is offline Bando  Canada
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Yep for the positive rail its on the + side and the - is on the other side for the other cap. Its a screw down cap with + and - marked on it.
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Old 13th March 2013, 11:21 PM   #4
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So each cap has a + and - connected to ground (0v)?
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Old 14th March 2013, 12:01 AM   #5
Bando is offline Bando  Canada
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So for the cap on the positive rail it is paralleled in and has the negative grounded. So conversely the negative rail has the negative side paralleled, and the positive side to ground. Like all other psu's I have seen.
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Old 14th March 2013, 12:20 AM   #6
Tajzmaj is offline Tajzmaj  Slovenia
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Do you have center tap of the transformer connected to the ground? It seems to me that your ground is floating.
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Old 14th March 2013, 12:22 AM   #7
Bando is offline Bando  Canada
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This transformer doesn't have a centre tap....

Is that not a good thing?
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Old 14th March 2013, 12:28 AM   #8
Tajzmaj is offline Tajzmaj  Slovenia
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It is the reason of strange behavior. You need to fix that ground somehow. You can borrow circuit used in newer Quad306/520/606....family. Other way is to use transformer with center tap I'm afraid.
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Old 14th March 2013, 12:31 AM   #9
Tajzmaj is offline Tajzmaj  Slovenia
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p.s.
Quad's psu is also quite good dc protection for speakers.
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Old 14th March 2013, 12:37 AM   #10
Tajzmaj is offline Tajzmaj  Slovenia
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Ah...one more thing. Just for test. You can use 2 let say 180E/5W resistors (value is not critical though), and connect one from + to gnd and the other from - to gnd. It will probably work good enough to see if amp is working or you have some other problems.
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