Stepping down phantom power voltage?

Discussion in 'General Electronics Chat' started by Pure L, May 23, 2006.

  1. Pure L

    Thread Starter New Member

    May 23, 2006
    4
    0
    Hi. I just registered and this is my first post. Here's the situation:

    I bought these strange Shure teleconferencing (STM30W) mics off ebay. Strange thing about them is that--supposedly--they take 18 volts to power them. The guy who sold them to me sent along two 9volt batteries wired in series to do so. FWIW, I have no other info on these mics and Shure hasn't written me back with my request for the info so I'm kinda shooting in the dark here.

    Anyway, I hooked them up and used them for a few days before the power ran out (I accidentally left them plugged in.....I know, I know....).

    So, what I'd like to do is to power them using the phantom power of my Soundcraft 200. As I'm sure you know, phantom power is supplied on pins 2 and 3 of an xlr plug and gives 48 volts.

    Now, the strange thing about these mics is that--from what I can tell (i.e. looking at how the mics are wired to the xlr pins)--these mics were getting their 18 volts on pin 3. In other words, pin 3 was getting the "+" connection to the 9volt "brick" and the pin 1 was getting the "-" connection to the 9volt "brick".


    What I'd like to do is to build a little adaptor like this:
    https://home.comcast.net/~elmastero/48Vto18Vadaptor.jpg


    So, here's what I need to know:
    Is there any way I could stepdown the 48v on pin 3 to 18v? Maybe a zener or something?

    Also, could I step down the 48v on pin 2 to 0v?

    Perhaps I'm just plain confused? Thanks for your time, all.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    It seems you actually misunderstood the voltage in XLR. Both pins 2 and 3 are +48V (but not connected together), pin 1 is ground - 0V.
    the adapter could be like this: two resistor dividers between pins 2 and 1, the second between 3 and 1.
     
  3. Pure L

    Thread Starter New Member

    May 23, 2006
    4
    0
    Thanks for the reply, Kubeek.


    I was aware that phantom power supplied 48 volts to both pins 2 and 3.

    From looking at the way they are wired though, sending 48 volts to pin 2 probably won't be needed as the way they are right now, only pins 1 and 3 of the actual mic plug itself are hooked up to the battery pack (the two 9 volts wired in series) with pin 3 taking the "+" lead and pin 1 taking the "-" lead.

    That would lead me to believe that turning on phantom power and, therefore, sending power to BOTH pins 2 and 3 might create an unwanted situation (i.e. noise, etc) as pin 3 is the only pin taking power.

    That's why I was wondering if there was a way to totally cut off the 48v to pin 2.

    By the way, what was the equation you used to knock the voltage down to 18v from 48?

    Again, thanks for your reply.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    1) the voltage on both pins must be the same, otherwise you could destroy the microphone.
    If you power it only from batteries, then it can be connected to only one of the pins, but when powered from mix, the voltage has to be either connected to only one pin - you would have to use insulation transformer and connect the lower voltage to only one pin, or the same on both pins.
    -->Do you have some link to the microphone, or datasheet?

    2) the equation is like this: 48V=R1+R2, 18V=R2. It´s just Ohm´s law, but I had to guess the value of hte resistors. You should keep the ratio between R1 and R2, but the right values can be different, since I don´t know the microphone type - dynamic/condenser/lavalier/....., also the output impedance of the mike would help.
     
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    If its a Shure mic, then surely it is already configured to run on Phantom power..?
    I'm guessing the previous owner was powering using the 9v batts simply because his mixer didn't have phantom.....
     
  6. Pure L

    Thread Starter New Member

    May 23, 2006
    4
    0
    I did some research on the board that these mics were meant to be powered with. You can read some info here: http://shure.com/pdf/discontinued/ST6000T2...lers_manual.pdf

    Actually, here's the link to the mic info:
    http://shure.com/pdf/discontinued/stm30.pdf

    Looks like the board gives out 14 volts. Under the "Balanced Microphone Input Phantom Power" section on page 3 of the board-link it says, "14 volts, 2.2k source". I'm not too sure what the 2.2k source is referring to. The impedance, I'm assuming?

    So if these mics take 14 volts (and not 18) would that change the resistor values that kubeek gave out?

    Thanks again.
     
  7. Pure L

    Thread Starter New Member

    May 23, 2006
    4
    0
    El bumpo.
     
Loading...