10mv too much?

lanchile07 · 25th December 2007, 06:31 AM

Well.here I go again!.
I have been playing with my new int amp"LM3875".
I replace all parts for new ones.before there was a dc offset difference of 5mv between channels and one channel had a thump when I turn unit off.now the thump is a lot lower but,the dc offset went to 10mv between channel(one channel 5mv and the other 15mv)so now it is 10mv difference between channels!.
I have been checking and replacing parts here and there but I am so disappointed because I can never get it "perfect".the sound is good and there is no hum or hiss at all but,is it too much one channel 10mv higher than the other?.should I keep trying putting new chips(I have 5 new ones) to see if I can get it even or just leave like it is?.
when the volume is all the way down I get "0" mv one channel and the other I get "10"mv. and if I put the volume higher they both go higher but the difference between them is "10 mv".
I need your "expert" advice guys please.

es44 · 25th December 2007, 07:04 AM

You don't need to worry. My GC is close to 25mV in one channel, and the other is a little more than 15 mV.
Yours is "next to nothing". Enjoy some sweet music and the thought that you made it yourself.

Best regards
Ebbe

Nordic · 25th December 2007, 03:41 PM

There is great variation between chips even from the same batch, 10mV on a GC is nothing....

Hint, investigate the mechanism that causes the DC offset in order to cure the symptoms... hint, will likely include the resitors on the input pins...

Nico Ras · 25th December 2007, 08:24 PM

Show us your circuit, if you use the "volume control" as input bias resistor to the amp this could also be the cause. Varying the input while the NFB is fixed can cause offset.

However the offset you are talking about as the other participants indicate is negligible, you are actually lucky that it is this low.10mV offset represents only 12.5 μW in the speakers, hardly anything to worry about.

AKN · 25th December 2007, 08:37 PM

Hi,

Nico Ras, yes, using the pot as the only path for input bias is dangerous. Also, when the pot gets worn there will be serious noise when adjusting the volume.

AndrewT · 26th December 2007, 09:50 AM

sounds like there is no DC blocking capacitor to the inputs of the amplifier.
post a schematic.

lanchile07 · 28th December 2007, 05:34 AM

I am doing the one from "Audiosector" look.

http://www.audiosector.com/nigc_kit-users_guide.pdf

lanchile07 · 28th December 2007, 05:42 AM

Well,I changed the chip and the dc offset went down to 2mv.so I guess it is all in the chip...

PS: Nordic and Peter were right! thank you guys!

AndrewT · 28th December 2007, 08:08 AM

Hi,
the nigc schematic is DC coupled.
Getting 10mV of output offset from that circuit with variable source impedance due to changing the volume control position is superb.

AKN · 28th December 2007, 08:25 AM

Andrew,

Quote:

Getting 10mV of output offset from that circuit with variable source impedance due to changing the volume control position is superb.

Certainly when one consider the limited gain at disposal for DC feedback.

Lanchile07,
Did you keep the fixed resistor from input to ground as in the first schematic and then added a pot in front?

25th December 2007, 06:31 AM	#1
lanchile07 diyAudio Member Join Date: Oct 2007	10mv too much? Well.here I go again!. I have been playing with my new int amp"LM3875". I replace all parts for new ones.before there was a dc offset difference of 5mv between channels and one channel had a thump when I turn unit off.now the thump is a lot lower but,the dc offset went to 10mv between channel(one channel 5mv and the other 15mv)so now it is 10mv difference between channels!. I have been checking and replacing parts here and there but I am so disappointed because I can never get it "perfect".the sound is good and there is no hum or hiss at all but,is it too much one channel 10mv higher than the other?.should I keep trying putting new chips(I have 5 new ones) to see if I can get it even or just leave like it is?. when the volume is all the way down I get "0" mv one channel and the other I get "10"mv. and if I put the volume higher they both go higher but the difference between them is "10 mv". I need your "expert" advice guys please.

25th December 2007, 08:24 PM	#4
Nico Ras diyAudio Member Join Date: Nov 2005 Location: Amanzimtoti - East Coast of South Africa	Re: 10mv too much? Show us your circuit, if you use the "volume control" as input bias resistor to the amp this could also be the cause. Varying the input while the NFB is fixed can cause offset. However the offset you are talking about as the other participants indicate is negligible, you are actually lucky that it is this low.10mV offset represents only 12.5 μW in the speakers, hardly anything to worry about.

25th December 2007, 08:37 PM	#5
AKN No snake oil diyAudio Member Join Date: Aug 2005 Location: In the middle of Sweden	Hi, Nico Ras, yes, using the pot as the only path for input bias is dangerous. Also, when the pot gets worn there will be serious noise when adjusting the volume. __________________ / Anders

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25th December 2007, 07:04 AM	#2
es44 diyAudio Member Join Date: Apr 2005 Location: Fredericia, DK	You don't need to worry. My GC is close to 25mV in one channel, and the other is a little more than 15 mV. Yours is "next to nothing". Enjoy some sweet music and the thought that you made it yourself. Best regards Ebbe

25th December 2007, 03:41 PM	#3
Nordic diyAudio Member Join Date: Sep 2005	There is great variation between chips even from the same batch, 10mV on a GC is nothing.... Hint, investigate the mechanism that causes the DC offset in order to cure the symptoms... hint, will likely include the resitors on the input pins...

26th December 2007, 09:50 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	sounds like there is no DC blocking capacitor to the inputs of the amplifier. post a schematic.

28th December 2007, 05:34 AM	#7
lanchile07 diyAudio Member Join Date: Oct 2007	I am doing the one from "Audiosector" look. http://www.audiosector.com/nigc_kit-users_guide.pdf

28th December 2007, 05:42 AM	#8
lanchile07 diyAudio Member Join Date: Oct 2007	Well,I changed the chip and the dc offset went down to 2mv.so I guess it is all in the chip... PS: Nordic and Peter were right! thank you guys!

28th December 2007, 08:08 AM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, the nigc schematic is DC coupled. Getting 10mV of output offset from that circuit with variable source impedance due to changing the volume control position is superb.