Strange problem with simple power supply

[rock soderstrom]

Ruud, What DC-DC IC does your board use?

I am not Ruud, but he mentioned it in this thread:

"EDIT 29/08; Today I looked with 'better eyes', the chips are CXW8509 and MAX1771."

 

[rock soderstrom]

The operation with a standard 9V DC wall wart is very tempting.

This of course raises the question of where to get a PE connection? An effects unit could be set up with protective insulation case, but not a tube microphone.

You would have to do it the way Ruud did.
Is there a qualitative difference for B+ whether the module is supplied with only rectified + RC filter or a regulated DC voltage?

 

[RuudNL]

At first I used the +210 output of the DC-DC converter directly, but this resulted in a very noisy signal from a ('real') U67. After adding a 4.7K/47µF filter, there was no audible noise anymore.
Unloaded voltage is now 213V, loaded voltage (with a U67) is now 208V.
(208 V. is the same voltage the original Neumann power supply delivers.)

 

[rock soderstrom]

After adding a 4.7K/47µF filter, there was no audible noise anymore.

That’s totally ok for me. Look what the postman just dropped at my door!

 

[elskardio]

The output voltage of the DC converter drops, so no excessive dissipation in the resistor.

I have swapped the two DC converters; the one I used for the +210V now only delivers 6.3 V and vice versa.
To my surprise this solved the problem. So it seems one DC converter had a problem.

Still it is strange that the 'faulty' DC converter delivered 210 V without a load, and did the same with a 10 mA load, but dropped the voltage when an R-C load was conneted. Well, problem solved it seems...

View attachment 122813

Hi RuudNL,

how did you connect the chassis ground and the DC converter's ground?

Are they tied together? I know some DC converter only works well when the output ground is floating...

Thanks