High DC offset with no source (LM4780)

Tomten · 2nd February 2010, 02:32 PM

I run a paralleled Lm4780 config (audiosector) which dispalys an odd behaviour. With no source connected I measure ~2-3V DC on both channels, however if I short the inputs (or connect an preamp) the DC offset is ~16mV on both channels. This is with the preamp off, with the preamp on, the only addition is some minor fluctuations when playing music. I`m using the amplifier and it works great, I was just confused why it would do this with no source? I use 0.1% resistors.

I haven't had speakers connected when I had the 2-3V DC, so no worries

thetube0a3 · 2nd February 2010, 03:03 PM

TomTen : do you have a resistor going from the chips input to ground? The input of the chip needs a load. Typically 20K or so. This must be between the chip and any coupling caps.

Tomten · 2nd February 2010, 03:07 PM

Yes, it is a 22k resistor. This is the schematic: http://www.audiosector.com/lm4780%20amp.pdf

thetube0a3 · 03:15 PM

Hmm. I don't see any resistor going from your + input to ground. the - inputs on that when used in stereo are for feedback.

R4 and R0 should be 20-50K. if the channels are isolated.

Tomten · 03:22 PM

Do you mean the ci cap? If so, no.

Does it matter that the chips are paralleled?

Well, I only have R4 (22k) installed, not R0 beacause the chips are paralleled and the jumper in the schematic implies that they share R4 I Suppose?

thetube0a3 · 2nd February 2010, 03:22 PM

If they are paralleled and R4 is in place I'm out of idea's. Tho if you used the Cz caps in the feedback loop it should get rid of all or close to all of your dc offset.

Tomten · 03:44 PM

Ok, its just strange, I mean 16mv sounds just right (and quite good) and 2-3V doesnt

AndrewT · 2nd February 2010, 03:31 PM

That schematic shows a DC coupled amplifier.

The output offset is ~ gain * ( input offset current * input resistance) + residual output offset of the amplifier.

For your arrangement the input resistance changes from Rin to Rsource when you couple up the pre-amp. That CHANGES the output offset.

If you cannot understand why you should not do that then you should not be building a DC coupled amplifier.

thetube0a3 · 2nd February 2010, 03:53 PM

AndrewT : wouldn't using Cz in the feedback loop fix the offset problem? Or are the input caps still required?

I'll be building another lm3886 amp soon and was going to omit the input caps but keep the feedback ones.

AndrewT · 2nd February 2010, 03:59 PM

I do not recommend a mixed AC and DC coupled amplifier.
Either DC coupled with all the protections necessary to protect your speakers or AC coupled.

2nd February 2010, 02:32 PM	#1
Tomten diyAudio Member Join Date: Nov 2009	High DC offset with no source (LM4780) I run a paralleled Lm4780 config (audiosector) which dispalys an odd behaviour. With no source connected I measure ~2-3V DC on both channels, however if I short the inputs (or connect an preamp) the DC offset is ~16mV on both channels. This is with the preamp off, with the preamp on, the only addition is some minor fluctuations when playing music. I`m using the amplifier and it works great, I was just confused why it would do this with no source? I use 0.1% resistors. I haven't had speakers connected when I had the 2-3V DC, so no worries

2nd February 2010, 03:12 PM	#4
thetube0a3 diyAudio Member Join Date: May 2008	Hmm. I don't see any resistor going from your + input to ground. the - inputs on that when used in stereo are for feedback. R4 and R0 should be 20-50K. if the channels are isolated. Last edited by thetube0a3; 2nd February 2010 at 03:15 PM.

2nd February 2010, 03:19 PM	#5
Tomten diyAudio Member Join Date: Nov 2009	Do you mean the ci cap? If so, no. Does it matter that the chips are paralleled? Well, I only have R4 (22k) installed, not R0 beacause the chips are paralleled and the jumper in the schematic implies that they share R4 I Suppose? Last edited by Tomten; 2nd February 2010 at 03:22 PM.

2nd February 2010, 03:25 PM	#7
Tomten diyAudio Member Join Date: Nov 2009	Ok, its just strange, I mean 16mv sounds just right (and quite good) and 2-3V doesnt Last edited by Tomten; 2nd February 2010 at 03:44 PM.

2nd February 2010, 03:31 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	That schematic shows a DC coupled amplifier. The output offset is ~ gain * ( input offset current * input resistance) + residual output offset of the amplifier. For your arrangement the input resistance changes from Rin to Rsource when you couple up the pre-amp. That CHANGES the output offset. If you cannot understand why you should not do that then you should not be building a DC coupled amplifier. __________________ regards Andrew T.


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2nd February 2010, 03:03 PM	#2
thetube0a3 diyAudio Member Join Date: May 2008	TomTen : do you have a resistor going from the chips input to ground? The input of the chip needs a load. Typically 20K or so. This must be between the chip and any coupling caps.

2nd February 2010, 03:07 PM	#3
Tomten diyAudio Member Join Date: Nov 2009	Yes, it is a 22k resistor. This is the schematic: http://www.audiosector.com/lm4780%20amp.pdf

2nd February 2010, 03:22 PM	#6
thetube0a3 diyAudio Member Join Date: May 2008	If they are paralleled and R4 is in place I'm out of idea's. Tho if you used the Cz caps in the feedback loop it should get rid of all or close to all of your dc offset.

2nd February 2010, 03:53 PM	#9
thetube0a3 diyAudio Member Join Date: May 2008	AndrewT : wouldn't using Cz in the feedback loop fix the offset problem? Or are the input caps still required? I'll be building another lm3886 amp soon and was going to omit the input caps but keep the feedback ones.

2nd February 2010, 03:59 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	I do not recommend a mixed AC and DC coupled amplifier. Either DC coupled with all the protections necessary to protect your speakers or AC coupled. __________________ regards Andrew T.

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