Phantom power blocking

[ficelles]

I need to block phantom power to safely allow direct connection via XLR out to desk etc in a compressor/preamp I've built.

Searches find that DC blocking via capacitors in series from pins 2 & 3 should work and 100uf electrolytic is suggested so I got myself some suitable voltage 100uf caps, but I'm still reading 48v either side of the cap on pin 3 (0v on pin 2)... can anyone suggest where I'm going wrong or provide a (simple) schematic?

Tia,

[iainpunk]

you need pull down resistors, 1M, from the - side to ground.
the + side should go to the plug.
if you want a schematic, its basically a High Pass Filter (at a way lower cutoff frequency than audio) just google

cheers

[antonis]

If you use XLR to unbalanced Mono, you only need one cap (in series with pin 2) and one grounding resistor..

[ficelles]

Based on that schematic I now have this, but am still reading DC after the cap & resistor...

Although part of the problem could be that I'm measuring 50v on both pins 2 & 3 with just an XLR cable connected to the mic pre?

[PRR]

You need a pull-down *on the side where you want zero Volts*.

For 100uFd e-cap (seems large) in a Phantom link, the pull-down can be 10k. (The big e-cap will leak more than the puny film caps in pedals.) Any source that can seriously drive mike inputs should tolerate 1K pull-downs.

[merlinb]

Although part of the problem could be that I'm measuring 50v on both pins 2 & 3 with just an XLR cable connected to the mic pre?

That's how phantom power is supposed to work. But when you short pin-1 to pin-2 you will then find 0V on pin-2 (obviously), leaving 48V only one pin-3, which is what the cap blocks.
For reference, the inside of the mic pre will look something like this. Phantom power is supplied to pin-2 and pin-3 through the 6.8k resistors:

[ficelles]

You need a pull-down *on the side where you want zero Volts*.  should tolerate 1K pull-downs.

That is what @antonis diagram shows, yet voltage still measured.

[ashcat_lt]

How much do we actually have to worry about this?  I'm not arguing, I'm legitimately asking.  I plug pedals and other active devices into mic preamps all the time.  I'm always very careful to make sure that the phantom power is switched off when I have control of the preamps, but sometimes in live situations I'm not at that end of the cable.  Part of me feels like the phantom power is just not stiff enough - can't flow enough current, and will get divided down enough - to hurt anything, but I'd love it if somebody who knows more than me could clarify the situation a bit.

[PRR]

... yet voltage still measured.

How much "voltage"? 37V? 0.05V? 230V??

[ElectricDruid]

How much do we actually have to worry about this?

In theory, if the input of the pedal has a DC blocking input cap, you shouldn't need to worry at all. Say there's a 1M pulldown resistor outside the cap. I=V/R, so I=48/1M= 48nA, not enough to fry the puniest resistor in your drawer.

All that said, you're connecting a 48V source to a 9V pedal. If that doesn't make you nervous, there's something wrong with you!

[ficelles]

How much do we actually have to worry about this?  I'm not arguing, I'm legitimately asking.  I plug pedals and other active devices into mic preamps all the time.  I'm always very careful to make sure that the phantom power is switched off when I have control of the preamps, but sometimes in live situations I'm not at that end of the cable.  Part of me feels like the phantom power is just not stiff enough - can't flow enough current, and will get divided down enough - to hurt anything, but I'd love it if somebody who knows more than me could clarify the situation a bit.

Resistors are not a worry.

Caps, depends on what you use (in the build in question I used 25v and 35v caps).

The transistors and ICs I used are specced for max 30v supply voltage. Not sure if 48v will instantly kill an IC but I don't want to take the risk of phantom power killing a pedal just when I want to use it.

Anyhow I decided to give up on this kind of power blocking and use an isolation transformer, hopefully that will work.

[ficelles]

How much "voltage"? 37V? 0.05V? 230V??

Same as the input voltage.