Causes of DC offset

[Heady]

I double-checked my previous builts of this amp and they have less than 10mV DC offset. Having torn off what's left of my hair, I have thrown away the board and will start all over again with a fresh built.

This current amp is slightly different from the rest, as I am using a chip for each channel rather than for both L/R channels. Does that make a difference?

Thanks to all for their help.

 

[nikongod]

i have found that "bad chips" are more inclined to cause dc offset than anything else. i have not used a bipolar chip in my own builds, but their use is well described.

in a fiew amps i have built, replacing the chip with one that isnt bad droped dc from almost 1/2 volt in 1 case to a couple milivolts.

 

[ECM]

when using a polarized ouput cap, did you install a resistor from the ouput to ground? i used a 1K resistor and that took care of any offset i had.

 

[Heady]

Quote:

Originally Posted by ECM when using a polarized ouput cap, did you install a resistor from the ouput to ground? i used a 1K resistor and that took care of any offset i had.

Before or after the cap, ECM? Thanks.

 

[ECM]

after the cap. tie the resistor from the output to ground. basically, it will be in parallel with the headphones. without headphones connected, the end of the cap is "floating." the resistor allows it to "see" the ground potential. (i'm sure someone else can explain it better.)

after installing the resistors, turn the amp on and wait a couple of seconds and measure the offset.

 

[Heady]

It's interesting. I managed to rebuilt the amp using a striped prototype board. It has the same type of DC offset behaviour measured with a DMM. The reading never settled down but kept on changing within a range.

When I use an analog meter, the offset rapidly decreased to near zero. If I then measure using the DMM, the reading is less than 20mV. Did I do something stupid, like using a DMM to measure offset?

And thanks ECM for your advice.

 

[ECM]

does your new build use a single or dual power supply?

 

[Heady]

Quote:

Originally Posted by ECM does your new build use a single or dual power supply?

I used a dual power supply, tapped from between the two AA batteries. Now I am thinking of using a resistors divider to provide the VG.

UPDATE:
Following advice from fa-schmidt in headwiz forum, I have changed the power supply to single. He said the chip is unreliable with a dual supply. I have found a slight drop in sound quality, just a little less transparent.

However, the single power supply did not fix the strange behaviour of having a lot DC offset at turn on and then dropping to near zero when measured with an analog voltmeter. With a DMM, the reading fluctuates but stay high. I am beginning to think it is just the output caps charging up and more current flows into the analog than the digital meter. Am I right? Anyone?

Thanks.

 

[ECM]

since you are using a single supply, you will have to bias the circuit to 1/2 the operating voltage. in other words, if you have a 12v supply, the output BEFORE the cap should be 6v. AFTER the cap, it should be 0v.

this can be accomplished by using a multi-turn potentiometer. apply 12v to one side of the pot and ground the other. then tie the wiper arm to the input of the circuit. measure and adjust for 6v.