LM3875 outputing DC

h3ad · 29th December 2007, 09:38 PM

hey all, recently got hold of two LM3875's and wired one up as per the diagram of the basic circuit in the datasheet how ever with out an input with -+17v power (for testing will get bigger trans) the amp outputs around 15-16v DC! what have i done wrong ?

Juergen Knoop · 29th December 2007, 09:48 PM

wiring error, re-, double-, triple-check your work!

regards
Jürgen

h3ad · 30th December 2007, 12:32 AM

i think if i double, tripple and so on check it anymore my eyes will start to bleed!!!!

anyone had similar probs b4??

thanx haydn

Nuuk · 30th December 2007, 11:13 AM

Yes, and it was a poor/broken connection of one of the power rails.

Walk away and leave it overnight. Then come back and start checking again.

AndrewT · 30th December 2007, 03:12 PM

If a chip amp loses a power rail, does it output DC?

Nuuk · 30th December 2007, 05:15 PM

Quote:

If a chip amp loses a power rail, does it output DC?

That's a good question Andrew! It measures DC on the mulitmeter but it doesn't fry the voice coil on the speaker!

Juergen Knoop · 30th December 2007, 05:56 PM

haydn, have you drawn the schematic yet?
Sometimes I just begin building a circuit by just having a more or less similar circuit from a book or article on my mind.
That is a bad habit and intodruces all kind of errors.
Take pen and paper and draw the circuit yourself. This helps focusing and maybe you will recognise the error.
You can also post your drawings here.
Regards

Peter Daniel · 30th December 2007, 06:01 PM

Quote:

Originally posted by AndrewT
If a chip amp loses a power rail, does it output DC?

It outputs DC only without a load. If you connect the load, the DC disappears.

h3ad · 6th January 2008, 01:11 PM

no i havnt drawn a scematic, i am bulding it simple point to point using the "typical application" in the datasheet, and even with a load it still shows high DC voltage
i have tried 2 chips, and both do the same thing... and both power rails are working fine.... im stuck please help

thanx heaps haydn

AndrewT · 6th January 2008, 01:17 PM

are you using fig1 of the Aug 2000 datasheet?

29th December 2007, 09:38 PM	#1
h3ad diyAudio Member Join Date: Sep 2007	LM3875 outputing DC hey all, recently got hold of two LM3875's and wired one up as per the diagram of the basic circuit in the datasheet how ever with out an input with -+17v power (for testing will get bigger trans) the amp outputs around 15-16v DC! what have i done wrong ?

30th December 2007, 11:13 AM	#4
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Yes, and it was a poor/broken connection of one of the power rails. Walk away and leave it overnight. Then come back and start checking again. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

30th December 2007, 03:12 PM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	If a chip amp loses a power rail, does it output DC? __________________ regards Andrew T.

6th January 2008, 01:17 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	are you using fig1 of the Aug 2000 datasheet? __________________ regards Andrew T.

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29th December 2007, 09:48 PM	#2
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	wiring error, re-, double-, triple-check your work! regards Jürgen

30th December 2007, 12:32 AM	#3
h3ad diyAudio Member Join Date: Sep 2007	i think if i double, tripple and so on check it anymore my eyes will start to bleed!!!! anyone had similar probs b4?? thanx haydn

30th December 2007, 05:56 PM	#7
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	haydn, have you drawn the schematic yet? Sometimes I just begin building a circuit by just having a more or less similar circuit from a book or article on my mind. That is a bad habit and intodruces all kind of errors. Take pen and paper and draw the circuit yourself. This helps focusing and maybe you will recognise the error. You can also post your drawings here. Regards

6th January 2008, 01:11 PM	#9
h3ad diyAudio Member Join Date: Sep 2007	no i havnt drawn a scematic, i am bulding it simple point to point using the "typical application" in the datasheet, and even with a load it still shows high DC voltage i have tried 2 chips, and both do the same thing... and both power rails are working fine.... im stuck please help thanx heaps haydn

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