lm1875 psu issues

Bando · 13th March 2013, 11:14 PM

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!

tom wesley · 13th March 2013, 11:16 PM

Are you 100% the psu caps are orintated correctly for polarity?

Bando · 13th March 2013, 11:19 PM

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.

tom wesley · 13th March 2013, 11:21 PM

So each cap has a + and - connected to ground (0v)?

Bando · 14th March 2013, 12:01 AM

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.

Tajzmaj · 14th March 2013, 12:20 AM

Do you have center tap of the transformer connected to the ground? It seems to me that your ground is floating.

Bando · 14th March 2013, 12:22 AM

This transformer doesn't have a centre tap....

Is that not a good thing?

Tajzmaj · 14th March 2013, 12:28 AM

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.
Cheers

Tajzmaj · 14th March 2013, 12:31 AM

p.s.
Quad's psu is also quite good dc protection for speakers.

Tajzmaj · 14th March 2013, 12:37 AM

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.
Cheers

13th March 2013, 11:14 PM	#1
Bando diyAudio Member Join Date: May 2012 Location: Stoney Creek, ON	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!

13th March 2013, 11:16 PM	#2
tom wesley diyAudio Member Join Date: Jun 2010 Location: Pickering, North Yorkshire	Are you 100% the psu caps are orintated correctly for polarity? __________________ British Dept. for Unlikely Audio.

13th March 2013, 11:21 PM	#4
tom wesley diyAudio Member Join Date: Jun 2010 Location: Pickering, North Yorkshire	So each cap has a + and - connected to ground (0v)? __________________ British Dept. for Unlikely Audio.

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13th March 2013, 11:19 PM	#3
Bando diyAudio Member Join Date: May 2012 Location: Stoney Creek, ON	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.

14th March 2013, 12:01 AM	#5
Bando diyAudio Member Join Date: May 2012 Location: Stoney Creek, ON	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.

14th March 2013, 12:20 AM	#6
Tajzmaj diyAudio Member Join Date: Mar 2011 Location: Ljubljana	Do you have center tap of the transformer connected to the ground? It seems to me that your ground is floating.

14th March 2013, 12:22 AM	#7
Bando diyAudio Member Join Date: May 2012 Location: Stoney Creek, ON	This transformer doesn't have a centre tap.... Is that not a good thing?

14th March 2013, 12:28 AM	#8
Tajzmaj diyAudio Member Join Date: Mar 2011 Location: Ljubljana	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. Cheers

14th March 2013, 12:31 AM	#9
Tajzmaj diyAudio Member Join Date: Mar 2011 Location: Ljubljana	p.s. Quad's psu is also quite good dc protection for speakers.

14th March 2013, 12:37 AM	#10
Tajzmaj diyAudio Member Join Date: Mar 2011 Location: Ljubljana	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. Cheers

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