MIC Phantom Power Supply

[frank18]

Hi everyone! I'm new here. Basically i'm developing my final project and i have a big doubt:

I have 2 microphones AKG C417 which need a power supply as any condenser microphone. I have too a phantom power supply AKG B29L that works with a 9V battery, and my idea is to integrate o recreate this circuit into my board, but when i opened the B29L what i found was a complex circuit, with many capacitors and resistors, and actually looks very different with respect you can find on the internet about "Phantom Power Supply Interfaces".

So the question is, i need to feed my 2 microphones, i already have a regulated power supply of +/-9 V on my board, i attach the diagram of that, and i want to use it for my microphones, but i need a good interface from my source to the microphones, and from the microphones to the pre-amplifier.

I hope i have been clear with you about what i need.

Thanks

Best regards

[Buriedcode]

The reason the circuit inside the microphone cable/connector (assuming its a lavalier mic) looked different from your schematic is that it doesn't just contain a phantom power circuit - most condenser mics also contain a buffer/preamp, filtering and possibly pad switches.

I think I found a schematic of the inards: http://i909.photobucket.com/albums/ac293/Putcheen/Music%20Gear%20Repairs/file-page31.jpg 

It also shows how you actually power it - just a resistor and cap - no need to clone the internal circuit.  So for two mics you pretty much only need 2 resistors, 2 caps, and two XLR connectors!

[frank18]

Hi Buriedcode! Thanks for answer my post.

I was reading the documentation that you suggested to me and I had some doubts. The first one: in the photo that I attached called "Phantom Adapter" you see a quite complex circuit but I have the slight suspicion that it is what is implemented in the B29L source that I own. Where I basically believe that the unbalanced microphone input becomes a balanced output, correct me if I'm wrong.
The second: you suggest me that I only need 1 resistance and 1 capacitor per microphone only because of the figure that I enclose as "Power interface" and frame with the red circle ?.
The third: continuing with the photo "Power interface", what I marked with the green circle would be my pre-amplification stage right? and it suggests that it has an input impedance greater than 10kohm so I could start my preamplifier stage with a buffer type stage and thus achieve the Ri requirement?

Thanks

[Buriedcode]

It would indeed be prudent to provide a buffer/preamplifier stage (whether or not that has gain is up to you).  I think this would be the case for any microphone input - it should almost always present a reasonably high (as in >10x the output of the mic) input impedance.

You are correct in that the circuit provides a balance output, I am unsure if you have the power adapter, just the mic capsule, the phantom power adapter, I'm just going by what I found by googling.

Now, I assumed - perhaps incorrectly - that the schematic I found was the one in the XLR plug on the end of the mic cable, not the phantom power adapter.  It seems the mic requires another adapter for phantom power if it is to be run from batteries.

Are you ultimately asking - what kind of circuit should you design to power this mic?  As in, you want a box you can plug these two mics in to ?

[frank18]

Yes, the final idea is to create a board inside a box for example with only 2 conectors (Mini XLR) for the microphones, the conectors must to have the powe supply in their pins to feed the microphones, my question is if i only need to use the RC topology directly from my 9V supply or if i need a better supply interface. The better option for me if the use the simplest circuit, and that is why i'm a bit lost.

Thanks