12V Phantom Power (P12)

[fjgaston]

Hi,
I recently purchased an old Schoeps CMT34 microphone, it's great but it should be powered by a 12V phantom power (P12)
I looked everywhere for a P12 power supply schematics but I never found one.
The two informations I have are :
- standard IEC1938 says : No load voltage 12V (+/-1V), supplied current up to 15mA max, R_phantom 680 ohms (respictively 48V, 10mA and 6.8k for a 48V phantom power)
- I found plenty of 48V schematics including this one on http://sound.westhost.com/project30a.htm

If I use this schematics and just change the 48V with 12V instead and 680 ohms instead of 6.8k will it be ok ?

 

[fjgaston]

nobody ?

 

[JohnRoberts]

No... not like that...

JR

 

[moamps]

I recently purchased an old Schoeps CMT34 microphone, it's great but it should be powered by a 12V phantom power (P12)
I looked everywhere for a P12 power supply schematics but I never found one.
........
If I use this schematics and just change the 48V with 12V instead and 680 ohms instead of 6.8k will it be ok ?

I think it will work. Similar powering uses CMC3 (12V-2x680ohms).

http://www.sdiy.org/oid/mics/Schoeps.gif

 

[JohnRoberts]

Hi,
I recently purchased an old Schoeps CMT34 microphone, it's great but it should be powered by a 12V phantom power (P12)
I looked everywhere for a P12 power supply schematics but I never found one.
The two informations I have are :
- standard IEC1938 says : No load voltage 12V (+/-1V), supplied current up to 15mA max, R_phantom 680 ohms (respictively 48V, 10mA and 6.8k for a 48V phantom power)
- I found plenty of 48V schematics including this one on http://sound.westhost.com/project30a.htm

If I use this schematics and just change the 48V with 12V instead and 680 ohms instead of 6.8k will it be ok ?

Sorry... yes... if you change the 48V to 12V and change the 6.8K resistors to 680 ohm that should work...

I just looked at the schematic  and ASSumed the clamps at the right were dropping the 48V phantom voltage down to 12V.

I shouldn't post before drinking my coffee....

JR

 

[fjgaston]

Sorry... yes... if you change the 48V to 12V and change the 6.8K resistors to 680 ohm that should work...

I just looked at the schematic  and ASSumed the clamps at the right were dropping the 48V phantom voltage down to 12V.

I shouldn't post before drinking my coffee....

JR

Haha, don't worry and thank you J.R.

Yes Moamp, it uses the same powering as CMC3, CMT3 is like the older version of the CMC3.

 

[pH]

Maybe make a T-power adaptor box like this:

http://www.uneeda-audio.com/phantom/t-power.htm

I made one for a Sennheiser 416T and it works with any of my preamps with 48v phantom.

Paul

 

[abbey road d enfer]

Maybe make a T-power adaptor box like this:

No! T-power is a different animal, where one leg receives +12V via a suitable resistor and the other leg goes to 0V via an identical resistor.
Best case the mic won't work, worst case the transformer fries.
The microphone the OP cites was designed for 12V phantom, i.e. with 680r resistors from a 12v source.
That was the case of some AKG 451's, but they operated perfecly on P48.
In fact, because of the current limit of 14mA on P48, the risk of damaging a mic designed for P12 is almost nil. Some P12 mics had a higher current consumption that made them unusable on P48.

 

[ricardo]

The microphone the OP cites was designed for 12V phantom, i.e. with 680r resistors from a 12v source.
That was the case of some AKG 451's, but they operated perfecly on P48.
In fact, because of the current limit of 14mA on P48, the risk of damaging a mic designed for P12 is almost nil. Some P12 mics had a higher current consumption that made them unusable on P48.

Is the  CMT34 really P12V only?

All the "P12" mikes I know .. work well on P48 too.  The exceptions might be modern P12 Schoeps but I've never seen these in real life.

Similarly, all modern "P12" preamps are actually P48 with their 6k8 resistors instead of the 680R mandated by the standard.  The most common are the  Sound Devices that can be switched to "P12" to nearly double battery life.

Of course a lot of prosumer stuff with "phantom power" don't meet P48 specs.  My MOTU Traveler is P39  :-[

Anyone know of true P12 devices ... mikes & preamps?

 

[fjgaston]

Hi,
I finally built a little prototype P12 power supply.
Here some photos :

Inside :

I used what I had at home, a metal tea box. As I didn't have a male XLR connector, I just soldered a wire for the moment.
Here is the complete schematic.

It uses two 9V batteries, I measured 3.8mA (loaded or unloaded it's the same) I don't know if there is an easy way to improve.
Any comment or suggestion is welcome 

 

[PRR]

As there "must" be 5mA in R7 alone, your measurement is dubious.

 

[abbey road d enfer]

I measured 3.8mA (loaded or unloaded it's the same) I don't know if there is an easy way to improve.

That's the problem with standard regulators; they draw more quiescent current than the load they are powering.
I don't know if there's any low-quiescent regulator suitable for 12V (they all seem to be limited to about 5V).
You may have to use a discrete design - it may not be too difficult because you don't need über precise regulation - or add a string of diodes in the reference connection of one of these low-quiescent regulators (with a by-pass cap, of course).

 

[JohnRoberts]

There are some CMOS VR that have low quiescent power... I don't have any PN on the tip of my tongue but used a CMOS Microchip VR  in my battery powered tuner.

JR 

 

[PRR]

more quiescent current than the load

It is possible this P12 with 680r per leg could pull 20mA. I don't know if that is likely.

Dead-short on output is 27mA.

I would not fret a 5mA idle.

I can't imagine any "P12" device which wouldn't be happy with 18V of battery through two 1K resistors.

I do wonder how 1.25V in 240r resistor measures 3.8mA.

I can't imagine any "P12" device which wouldn't be happy with 18V of battery through two 1K resistors.

 

[abbey road d enfer]

I can't imagine any "P12" device which wouldn't be happy with 18V of battery through two 1K resistors.

  Assuming that part of the circuit is identical in the CMT and the CMC: That would draw about 20mA. I think the resistors could be increased to 3.9k.
Schoeps recommends 3.3k for P18 for the CMC series. There is no good reason to increase phantom current; it's all wasted in the zener.