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.
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.
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
Yours is "next to nothing". Enjoy some sweet music and the thought that you made it yourself.
Best regards
Ebbe
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...
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...
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.
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.
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.
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.
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!
PS: Nordic and Peter were right! thank you guys!
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.
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.
Andrew,
Lanchile07,
Did you keep the fixed resistor from input to ground as in the first schematic and then added a pot in front?
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?
What # resistor? are you talking #1?
.
should I take a resistor out if I used it with a potentiometer?
.
should I take a resistor out if I used it with a potentiometer?
He is talking about 22k input shunt resistor which is R2 on my board. That resistor needs to be installed, even if you are using potentiometer.
thank you Peter!
I always put all the parts that come in your LM3875 kit Peter!.
so,as you told me in your e-mail , the chips are the ones that are changing the dc offset.I changed a chip and the dc offset went down to 4mv. so I will not worry to much about 5mv difference.now what I do worry is a thump in one speaker when I turn off amplifier (one side is louder than the other).
I always put all the parts that come in your LM3875 kit Peter!.
so,as you told me in your e-mail , the chips are the ones that are changing the dc offset.I changed a chip and the dc offset went down to 4mv. so I will not worry to much about 5mv difference.now what I do worry is a thump in one speaker when I turn off amplifier (one side is louder than the other).
Re: thank you Peter!
Hi lanchile07,
Check you have put the right resistors in the right locations. Closely compare one amp with the other. Check resistor values with DMM.
BTW: This has nothing to do with DC offset.
regards
lanchile07 said:(one side is louder than the other).
Hi lanchile07,
Check you have put the right resistors in the right locations. Closely compare one amp with the other. Check resistor values with DMM.
BTW: This has nothing to do with DC offset.
regards
I am building a new LM3875 integrated amp from Peter Danile "Audiosector".but this time I will take one hour just to check all the parts that are in the right place lol. thank you guys for all your help. I will post some pictures when I am done with this "new proyect".
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