balanced line stage

[Alessandro]

Hi all, i have done the balanced line stage preamp but i have some problems with the power supply.
Probably the negative branch has an autoscillation.
Are there modifications or suggestions to solve the problems?

Thanks

 

[Alain Dupont]

Alessandro,

?? Probably the negative branch has an autoscillation

Use little caps to avoid this: 2* 0.1 microF 100volts plus 2* 100 microF 100volts
at bouth ends of the negative cable should do the trick.

A little picture of your design could help.

Regards.

Alain.

 

[Alessandro]

I have already tried this suggestions but the problem remain.
The positive branch go very well but the negative roasted all the zener and the mosfet after 2 second.
Is it possible that any one had the same problem ?
Only another question, i have used 0,5 watt zener, maybe this is the problem ?

thanks

PS
i have used the original PCB designed by Nelson Pass

 

[Alain Dupont]

Alessandro,

Did you check the 4.75 K R102 as for 16 Volts its about 4 milli Amps
so the 4.75 K is the problem
{4*4 milli * 9.1 volts == 0.1 milli watt for each zener even 1/4 watters
will be good} or Z116 is reversed. Or the mosfet short.
I did build this negative PS twice without any pb.

Alain.

 

[Nelson Pass]

Probably the negative branch has an autoscillation.

What symptom makes you think that?

 

[Alessandro]

Hi all, i have solved the problem.
It was the 221 resistor on the gate.
i have tested it, and moving this on the board, the component change its value if i push on the body.

I have substitute the component and now all it's OK.
After 20 years of electronics job this is the first time that i have found this type of problem.
We don't never finished to learn......

Thanks at all for the help