LM3886 Hum

danielnaveen · 19th December 2007, 05:46 AM

Hi,

I just built 2 LM3886 based amps and am powering it with a powersupply based on carlosfm's design. When i switch it on the speakers just buzz but it goes away when an input is fed. The amps are not in a cabinet and i haven't used a DC blocking cap on the input and the amps are built on strip boards. Is this buzz normal?

Daniel

tangmonster · 19th December 2007, 06:01 AM

does it buzz with no input connected?

or does it buzz with input connected but no music played?

if it buzz'es with the input shorted to ground then you need to source the problem.

How does your lm3886 circuit look?
How much does it differ from the national semi application circuit?

danielnaveen · 19th December 2007, 06:24 AM

I've posted a picture of the circuit i used, i dont use Ci and i dont use R IN or R2 instead i use a 56k resistor between pin 10 and pin 9 before R1.

It buzzes when no music is played when i first start it up and then once i start play'n music it is ok even when i stop the music and turn off the pre.

tangmonster · 19th December 2007, 06:38 AM

You could try doing it like national's application.

You don't need the Rin 25k pot , but try the R1 and R2 just like that and see.

danielnaveen · 19th December 2007, 07:09 AM

I'll try that, but is this normal? will it go away if i put in a box

tangmonster · 19th December 2007, 07:26 AM

I don't think the box will make any difference.

I haven't tried the circuit in the way that you did , so i won't know if it is normal.

But i think if you tried the circuit the way that application note shows it might be better.

just do it to one channel en see what happens.

At this stage it could be anything that brings on the noise , even your preamp.

you need to try and cut out and narrow down where the noise are created.

disconnect the input,

short out the input to ground , does it quiet down.

is the noise louder with the preamp plugged out?

aletheian · 19th December 2007, 07:56 AM

Quote:

Originally posted by danielnaveen
I've posted a picture of the circuit i used, i dont use Ci and i dont use R IN or R2 instead i use a 56k resistor between pin 10 and pin 9 before R1.

It buzzes when no music is played when i first start it up and then once i start play'n music it is ok even when i stop the music and turn off the pre.

So you have a 56k reference resistor tying the inverting and non-inverting inputs together and no ground reference? That might be the problem if I am interpreting what you mean properly.

tangmonster · 19th December 2007, 08:47 AM

I agree about the ground reference.

danielnaveen · 19th December 2007, 09:39 AM

Is there a ground ref in the schematic i've used? and if not where do i put it?

tangmonster · 19th December 2007, 10:12 AM

R2 is the ground ref resistor.

19th December 2007, 05:46 AM	#1
danielnaveen diyAudio Member Join Date: Oct 2006	LM3886 Hum Hi, I just built 2 LM3886 based amps and am powering it with a powersupply based on carlosfm's design. When i switch it on the speakers just buzz but it goes away when an input is fed. The amps are not in a cabinet and i haven't used a DC blocking cap on the input and the amps are built on strip boards. Is this buzz normal? Daniel

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19th December 2007, 06:01 AM	#2
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	does it buzz with no input connected? or does it buzz with input connected but no music played? if it buzz'es with the input shorted to ground then you need to source the problem. How does your lm3886 circuit look? How much does it differ from the national semi application circuit?

19th December 2007, 06:38 AM	#4
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	You could try doing it like national's application. You don't need the Rin 25k pot , but try the R1 and R2 just like that and see.

19th December 2007, 07:09 AM	#5
danielnaveen diyAudio Member Join Date: Oct 2006	I'll try that, but is this normal? will it go away if i put in a box

19th December 2007, 07:26 AM	#6
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	I don't think the box will make any difference. I haven't tried the circuit in the way that you did , so i won't know if it is normal. But i think if you tried the circuit the way that application note shows it might be better. just do it to one channel en see what happens. At this stage it could be anything that brings on the noise , even your preamp. you need to try and cut out and narrow down where the noise are created. disconnect the input, short out the input to ground , does it quiet down. is the noise louder with the preamp plugged out?

19th December 2007, 08:47 AM	#8
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	I agree about the ground reference.

19th December 2007, 09:39 AM	#9
danielnaveen diyAudio Member Join Date: Oct 2006	Is there a ground ref in the schematic i've used? and if not where do i put it?

19th December 2007, 10:12 AM	#10
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	R2 is the ground ref resistor.

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