DC offset when turning amp off....

ghisdal · 26th August 2007, 05:45 PM

Hi,

Just finished BrianGT's LM3886 amp. I am using the feedback resistor, and I get 1mV when measuring DC offset.

But, when turning it off the DC offset reaches almost 4V before quickly falling to 0mV again. What is the reason for this, and is it dangerous?

Also, I have connected the CHG points to AC ground, and all of these are connected to chassis ground. Does anything else need to be connected to ground?

Glenn

Nordic · 26th August 2007, 05:50 PM

How did you connect your mute pin? Are you measuring that without a load? try putting a 10ohm resistor of the output... do not apply any signal.. just short the input... now put voltage probes over the resistor and switch off... you will likely see that the current is not much when its delivired into a load.... you can work out the current useing ohms law the measured voltage and the resistor value...

ghisdal · 26th August 2007, 05:51 PM

Where and what it that? I'm using a kit....

Nordic · 26th August 2007, 05:54 PM

In kit form I would deal with the problem elsewhere if it is a problem... do the little test suggested above... if results look ok, try with a speaker and see if it makes too loud a pop when powering down...

Peter Daniel · 26th August 2007, 06:44 PM

Quote:

Originally posted by ghisdal
Hi,

Just finished BrianGT's LM3886 amp. I am using the feedback resistor, and I get 1mV when measuring DC offset.

But, when turning it off the DC offset reaches almost 4V before quickly falling to 0mV again. What is the reason for this, and is it dangerous?

Also, I have connected the CHG points to AC ground, and all of these are connected to chassis ground. Does anything else need to be connected to ground?

Glenn

When you measure the offset without speakers connected, 4V value is normal at turn off. You won't see it though, when speakers are connected.

If you are not sure, measure the offset behaviour with 8 ohm resistor connected to the output.

26th August 2007, 05:45 PM	#1
ghisdal diyAudio Member Join Date: Nov 2006	DC offset when turning amp off.... Hi, Just finished BrianGT's LM3886 amp. I am using the feedback resistor, and I get 1mV when measuring DC offset. But, when turning it off the DC offset reaches almost 4V before quickly falling to 0mV again. What is the reason for this, and is it dangerous? Also, I have connected the CHG points to AC ground, and all of these are connected to chassis ground. Does anything else need to be connected to ground? Glenn

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26th August 2007, 05:50 PM	#2
Nordic diyAudio Member Join Date: Sep 2005	How did you connect your mute pin? Are you measuring that without a load? try putting a 10ohm resistor of the output... do not apply any signal.. just short the input... now put voltage probes over the resistor and switch off... you will likely see that the current is not much when its delivired into a load.... you can work out the current useing ohms law the measured voltage and the resistor value...

26th August 2007, 05:51 PM	#3
ghisdal diyAudio Member Join Date: Nov 2006	Where and what it that? I'm using a kit....

26th August 2007, 05:54 PM	#4
Nordic diyAudio Member Join Date: Sep 2005	In kit form I would deal with the problem elsewhere if it is a problem... do the little test suggested above... if results look ok, try with a speaker and see if it makes too loud a pop when powering down...

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