36 V DC offset when source connected

[revell]

The problem has probably been disussed before but I couldn't find it:
when no source is connected I have 54 mV dc offset on my GT kit when I connect a source (DVD player) the offset jumps to 36 volts

anyone who knows what to do?

 

[jaudio]

Make sure signal ground is connected

 

[peranders]

Do you have any resistor from the input down to ground?

 

[jan.didden]

Meneer Revell,

Gij zoudt kunnen trachten de source met een condensator in the koppelen, als het dan goed is weten we dat het ofwel de source is, ofwel de ingangsschakeling van de versterker.

Jan Didden

 

[revell]

Do you have any resistor from the input down to ground?

I'm not sure what you mean, isn't the BrianGT kit an all-in package? I connected SG to - of the rca connector and IN to the+ of the rca connector

 

[revell]

Ok I tried a different source (very old walkman) and the dc remains low but when nothing is playing and the source is still connected I get a very loud hum. Maybe because it is not yet in a case?

 

[jan.didden]

You may want to post the circuit. Then we can check. Not everyone here is familiar with that particular kit you mentioned.

Jan Didden

 

[revell]

a diagram and more can be found here: http://www.chipamp.com/lm3875.shtml

 

[Tube_Dude]

Hi Revell

Use a cap at the input or a cap in series between the 620 Ohm resistor and ground.

 

[paologatto]

Probably a ground-loop problem.
Try connecting the 2 devices chassis/ground together with a wire.

I had a similar problem (70 V AC between 2 audio devices, also not pleasant to feel on my fingers!) and resolved with the chassis connection.

Bye

 

[carlosfm]

when no source is connected I have 54 mV dc offset on my GT kit when I connect a source (DVD player) the offset jumps to 36 volts

anyone who knows what to do?

36V?
Did you check the polarity of the PSU caps?
BTW what trafo are you using?

PS: without an input resistor to ground, DC would be high with no source connected, so I suppose you have that resistor. But did you install the right resistors on the right places?

 

[jan.didden]

Re: Re: 36 V DC offset when source connected

36V?
Did you check the polarity of the PSU caps?
BTW what trafo are you using?

PS: without an input resistor to ground, DC would be high with no source connected, so I suppose you have that resistor. But did you install the right resistors on the right places?

Carlos,

The cap polarity and type of transformer has nothing to do with his DC problem, I'm sure you know that.

He has no problems with another source, that is the giveaway. Apparently, he cannot focus on this particular problem, because now it's all about hum.....

Jan Didden

 

[carlosfm]

Re: Re: Re: 36 V DC offset when source connected

Carlos,

The cap polarity and type of transformer has nothing to do with his DC problem, I'm sure you know that.

He has no problems with another source, that is the giveaway. Apparently, he cannot focus on this particular problem, because now it's all about hum.....

Jan Didden

The problem with the caps may appear under heavy load... but yes, it's not likely to be the problem.
What kind of DC-offset has the problematic "source"?
More than 1V?