What if my XLR connected box gets a jolt of phantom power up its output?

[Skruffyhound]

Fingers crossed, my long term project of a harmonica stompbox is reaching a close.

Mixer > XLR > Effects/switchbox > XLR > Dynamic microphone.   (you can turn the arrows round the other way if you like)

Of course, just as I'm about to box it up, it occurs to me that if my (not always very careful) friend attaches it to the mixer on a channel where phantom power is enabled
then my lovingly crafted little box will get 48 V up it's output. Presumably not a good thing.

I can't be the first person to run into this problem. What can I do?

[jpwilksch]

Caps in series with pins 2 and 3 of the output XLR should stop the 48V DC ruining your stompbox. Bear in mind your audio is then going through these caps, though.

Searching on "block phantom power" returned a few useful suggestions for the cap approach:
http://sound.westhost.com/project96.htm -- Suggests 10 or 22 uF caps. Includes zener diodes to prevent current spikes.
http://taperssection.com/index.php?topic=76866.0  -- Suggests 10 or 47 uF 100V Mylar caps.

Alternatively, you could use an isolation transformer on the output. This sort of thing:
http://www.altronics.com.au/index.asp?area=item&id=M0707

[R.G.]

Protection circuits are all about two things;
(1) clamp the voltage to safe limits
and
(2) limit the current flow.

I would install clamp diodes to force the incoming phantom power to some "safe" voltage limit with diodes and/or zeners to prevent it from getting far enough from ground to damage things, and I'd use resistors to limit the current.

Since the intended use is to drive a balanced XLR input, I'd make an opamp balanced output circuit, and install the clamps and limiters on the two outputs.

For extra credit, one *could* set up the protection circuit to light up an LED when Bad Things are happening so the problem is obvious, not just ugly sounding.

[Perrow]

You could've used it to power the box

I suppose that input and output caps are in place so all you really have to worry about is that first jolt of power. A zener diode to ground?

[Perrow]

Hot damn, I type way to slow on my mobile.

[Skruffyhound]

Ok after yet another hiatus I'm back on this.
Thanks for the tips, I read the links two weeks ago and again now. It makes some sense to me today but I'm still confused about the zeners and I like to hear what you think about the rest of the set up.

I have opamp balanced output circuit (as R.G. suggested)
Between that and the XLR cable I need caps, resistors and possibly zeners.
As I understand it the caps should be closest to pins 2 and 3 and in series, next, resistors also in series and then Zeners to ground. Please correct me if I am wrong here.

What is the polarity orientation of the Zeners? I just don't understand the flow of current.

Since I can't use mylar 10uF caps (for lack of space/price considerations) I am considering using two electrolytic 22uF's back to back in series to produce a non polarized 11uF cap and then bypassing that with a 0.1uF poly box
to cut down on loss of signal transients (pretty much as described in the links above, if you put all the suggestions together).

Next would be resistors, again, in series, but what value?

Finally, Zeners, 10V, but like I said I'm not sure what the orientation is.

Just to be clear, this is what I think the signal chain should look like :

Input to Mixer < XLR < ( Caps < Resistors < Zeners < Balanced output from the rest of the guts)

Please set me straight on the configuration/values of this output section so I can get finished with this box and get back to playing guitar

[Skruffyhound]

@ Pelle, not enough amps, phantom power can be as low as 10's of mA (as I understand) and I've got tube heaters to power in this box

[Skruffyhound]

Bump

[Skruffyhound]

OK, sorry, but another bump, I'm actually putting it in the box as I write and I would really appreciate some suggestions at least about what resistors to use and Zener and resistor orientation.
Thanks

[tempus]

Hey Skruffy

If you look at the westhost link given above, just build Fig. 2 (you can leave out the 6.8K resistors' the switch and the 10uF cap; they'll already be in the mixer's phantom power circuitry). Read the explanation below the diagram. The caps will be closest to the XLR out, and the diodes will be closest to your opamp out. The resistors are 10 ohms. Check here http://www.eleinmec.com/article.asp?29 for orientation.
I hear ya about getting back to playing guitar - I'm usually kicking  myself for ever starting a project when it drags out too long and takes me away from playing.

[Skruffyhound]

Thanks Tempus, I made this part last night with the back to back electro's bypassed by a 0.1 uF poly box with matched 10 ohm resistors and 12V zeners. There's almost no room left in the box so I just soldered them nose to tail and wrapped them in heat-shrink  
Another twelve unplanned for components.