Beyerdynamic msg 248 - Filtering DC from the outputs

[smilan]

Hi,
I have a Beyerdynamic msg 248 (a two channels +48V phantom power external power supply).
I would like to filter the +48V from the outputs (the XLR that goes to the preamp) since I have a mic preamp that can't accept +48V power at the input transformer.
Now pins 2 and 3 of both XLR connectors are simply connected to each other with a wire.
On one channel I've disconnected the wires that going from the input jack to the output jack and installed a 10uF/450V coupling capacitors between the input to the output.
After doing so there's around 30V on pins 2 and 3 of the output jack while the PSU is on and negative 3.5V after the PSU turns off (I've measured the voltages on the output jack while it was disconnected).
I don't have a schematic for this PSU but attached a photo of the PCB.
https://i.ibb.co/CBWCGD9/IMG-9393.jpg
Any ideas how to filter the DC from the outputs?

 

[RuudNL]

If there is an input transformer without a center tap, it is no problem to have the phantom power on the input.
(Since there is no differential DC voltage between the input lines.)
With an electronically balanced input or an input with a center tapped (and center grounded) transformer, it is another story.
Personally I would prefer a larger value than 10 uF for the DC blocking capacitors.  (Because with the two in series you only have 5 uF.)
And 450 V is probably an overkill....
As a safety measure, you could add two zener diodes in series (anode to anode or cathode to cathode) in each 'leg' of the output to ground.  (So: after the capacitor.) This will limit the DC voltage to a safe value at the microphone inputs.
Most inputs will survive 6 Volts DC on the input.

 

[Bo Deadly]

I know this isn't what you want to hear but that phantom supply is a joke if it doesn't properly filter DC from the outputs. A decent one would use transformers (like the Sony AC-148F).

But still, adding caps should work. Not sure what the problem is but note that if it's not connected to anything, a charge might build up on the unconnected end. This is why there are usually drain resistors. Specifically, add resistors between pins 2 and 3 to ground on the outputs to drain away any charge on the outside of those caps. Something like 22K or so is probably about right. So 22k between pin 2 to pin 1. And 22k between pin 3 to pin 1. Then try it with pin 1 connected to the chassis or not and listen to the noise level. Chances are that pin 1 to chassis would be better.

Also, if you get some mains hum, try replacing those old Frako caps.

 

[smilan]

Thanks guys,
I've added a 22k between pin 2 and pin 3 to pin 1 and now there's no DC at the output 

 

[RuudNL]

But you will still get a high voltage pulse when you switch the phantom power on...

 

[smilan]

So by adding two zener diodes in series the way you suggested I'll  avoid the DC pulse when  switching the phantom power on?

 

[RuudNL]

Yes. You will get a short pulse when you switch the phantom power on, but the amplitude will be limited by the zener diodes.
(You can leave the existing resistors in place, and put the zener diodes (in series) in parallel with the resistors.)
The pulse on the input of the amplifier without the zener diodes would be very high,- in fact 48 volts-,  if you (by accident) switch on the phantom power, without a microphone connected.

 

[smilan]

Thanks RuudNL, it make sence, I'll add the zener diodes today.