"Center tap for phantom power" on mic output. How do I use this?

[midwayfair]

I picked up a Cinemag 4:1 mic output transformer with a center tap on the microphone's side for phantom power.

I can't think of a mic schematic off the top of my head that uses a center tap so I'm not sure how I'd use this, but I'm familiar with the pretty standard pair of 2k2 (or whatever) like in the U87 that go between the hot and cold and make a single DC point. If I'm using an existing layout with that setup, can I simply hook the center tap up where the junction of those two resistors go, or do I need to add a resistor between the transformer lead and that spot?

Side question: 4:1 is a little bit of an unusual ratio. Lower output than the 2:1 in the emitter follower circuits (which I'm fine with) and too low a ratio to use in a regular 87 or 84 style circuit. Anyone have a suggestion for other circuits for me to consider using with this transformer? Or can I maybe get enough current with a center tap to get down to 3k3 output impedance of a single JFET circuit?

 

[midwayfair]

Bump. Can anyone help?

The transformer is the CMO-2.65/200CT, but there's no documentation for it.

These circuits and others have the 2K2s I'm talking about:
http://recordinghacks.com/images/mic_extras/neumann/u87ai-schematic.gif
http://aurycle.com/manual/a460-diy-schematic.pdf

 

[PRR]

Ignore the CT until you find some circuit which would really be happier without another 1.1K in the DC feed.

I would not hesitate to try driving it with a single fat JFET. Will it be the world's best? I dunno. The experiment may be educational.

 

[midwayfair]

Ignore the CT until you find some circuit which would really be happier without another 1.1K in the DC feed.

I don't have a ton of mic schematics floating around in my head, but I can't think of one that I've seen. I'm more curious if bad things will happen without that extra 1.1k on the DC line. (p.s. thanks for reminding me that the 2k2s are in parallel.)

As far as "happier without it," that should help with ....

I would not hesitate to try driving it with a single fat JFET.

if I try to come up with something on my own. But 1.1K is hardly a big change in the available current department. Probably best if I breadboard a few things and take current measurements before I start changing layouts.

 

[midwayfair]

Would this get me to 2.65K output impedance?

(48V center tap)-----> Capsule polarization circuit
    |
(FET voltage reg circuit @33V)
  |
47K dropper-> big filter cap
  |
2K7
  |
(FET biased to ~12V)

That is, swapping R14 and R12 in the U87 schematic, except 2k7 instead of the 10K:
http://recordinghacks.com/images/mic_extras/neumann/U87-schematic.png

The center tapped transformer would give me more current to play with to make up for reducing the total drain resistance.

Would my output impedance in that case be the 2K7 on the drain? Or is it still higher? To be honest I don't really understand why it's only 18K in the U87 anyway, so maybe the drain resistor in my example would have to be more like 5.6K?

 

[rafafredd]

It should not be 18k output impedance on the U87. They simply load it with a higher impedance so that it works better.

It's the same with tube preamps. We load output stages that has a 1k to 3k output impedance with a 10k to 25k transformer primary. That means low distortion and good low frequency response.

 

[midwayfair]

It should not be 18k output impedance on the U87. They simply load it with a higher impedance so that it works better.

I guess I'm missing something fundamental. How do I know the source Z at the primary? Does getting low output impedabce from the amplifier stage only matter to conserve voltage when it divides against the resistance across the coil?

 

[gridcurrent]

I can't think of a mic schematic off the top of my head that uses a center tap so I'm not sure how I'd use this,

most (all?) 414's extract phantom via the center tap.
it is a simple and elegant solution.