LM3875 Gainclone Not Working and Stuck to Rail ...

[dannyjmf]

Hi ...

Today just build a gainclone LM3875 amplifier and found that it is not working ... output stuck to rail ... the circuit i used is i followed from the NS datasheet ...

BTW , i found that in the feedback path there is a capacitor in the schematic By NS. But i refer to some of the DIY projects schematics they are not included ... is it the cause of my problem ... ?

I need serious help here ... any feedback and suggestions are much appreciated and welcomed ...

 

[Bone]

So you used Fig 1.? What were your DC supplies?

 

[JMFahey]

Please post the schematic here so we all talk about the same.

 

[DUG]

Fig 1 or Fig 3?

 

[dannyjmf]

Fig 1 dual supply version ...

 

[JMFahey]

Ok.
Measure and post voltage on all 5 legs shown (1/3/4/7/8)
Measure from PS ground to each leg, straight at the chip.
Be careful not to let the multimeter tip slip and short to another leg.

By the way: I hope the chip is mounted to a heatsink and that you insulated it with mica+grease (and a screw-insulating nipple if needed).
Also, with power off, measure resistance to ground from leg 4 .

 

[dannyjmf]

ok ... get it work perfectly already ... caused by bad grounding issue ...

 

[JMFahey]

caused by bad grounding issue ...

Which was .....

 

[dannyjmf]

1st ... i didnt use the proper star grounding
2nd ... bad soldering joint from the ground to the 1k feedback resistor (which suppose connect to ground because of bad contact it didnt ...)

 

[dannyjmf]

completed amplifier

 

[Mihkus]

Hi everyone....
I have supply voltage of +-35V...
And i have +35V coming from output!!!!
How the hell is that possible?

 

[dannyjmf]

Hi everyone....
I have supply voltage of +-35V...
And i have +35V coming from output!!!!
How the hell is that possible?

Do you had the star grounding of the amp connected? if not likely the problem ... and be sure to include the Ci(optional capacitor) as well ... last time i also had the same problem with my gainclone LM3875 ... hopes this helps ...

good luck

 

[mrWagner]

Make a shot from the bottom and top of the amp plz.

 

[Mihkus]

Who me?

The problem was.. I didnt use input capacitor... if they got connected it just switched Vcc to output
My pcb... : [IMGHTTPDEAD]http://s6.postimg.org/4zogkvxg1/smd.png[/IMGHTTPDEAD]
image upload

And I think I have almost perfect grounding.