Phantom Power

K

Killer B

New member
Hi Everyone.

Im just brushing up on the ol' phantom power, Its just a simple question.

If Phantom is 48v across pins 2(+ve) and 3(-ve), then if I have to use an unbalanced cable that grounds pin 3, its still going to supply phantom power to the mic, eh?

I'd try it out, but first I want to know if there are any dangers in it! And also, has anyone noticed any nasty noise or interference as a result?

Thanks.

-A
 
an unbalanced cable that grounds pin three. You mean that its got two wires and and pin three is soldered to pin 1?

You will probably blow your mic up.

Danny
 
Yep about the cable.

I dont plan on blowing my mic up! Im more concerned in whether it can be done, cuz I want to know if Im right or not.

Thanks
 
Killer B said:
Hi Everyone.

Im just brushing up on the ol' phantom power, Its just a simple question.

If Phantom is 48v across pins 2(+ve) and 3(-ve), then if I have to use an unbalanced cable that grounds pin 3, its still going to supply phantom power to the mic, eh?

I'd try it out, but first I want to know if there are any dangers in it! And also, has anyone noticed any nasty noise or interference as a result?

Thanks.

-A
Click to expand...

You can't use unbalanced cables simple as that
 
Killer B said:
If Phantom is 48v across pins 2(+ve) and 3(-ve), then if I have to use an unbalanced cable that grounds pin 3, its still going to supply phantom power to the mic, eh?
Click to expand...
Phantom is not 48V across pins 2 and 3 - it is 48V applied to both pins 2 and 3, referenced to pin1 (ground).
It probably won't ruin your mic, as it's applied though 6800 ohm resistors, limiting the current to 7ma, but since the mic's headamp circuit is expecting 48v at both pins, and is not getting it at pin 3 which you've grounded, it's not gonna work.

You could wire pins 2 and 3 together so you'd get 48v at both pins, and the mic's amp would work. Then, unfortunately, the signals coming from the mic into pins 2 and 3, which are of opposite polarity, will be summed to a resultant of zero, and you will only get the noise generated by the unbalanced run of cable at the preamp end. But the mic will be working. :)

Unfortunately, you need a cable with three conductors to make it work.
 
Well Doc, you certainy sound like you know your stuff!

Im a bit confused about how capacitance can work if the two plates dont have a potential difference?
 
Killer B said:
Well Doc, you certainy sound like you know your stuff!

Im a bit confused about how capacitance can work if the two plates dont have a potential difference?
Click to expand...

Nobody said the plates don't have a potential difference!

Pins 2 and 3 are both at 48V relative to ground - since they are both at the same voltage there is no potential difference between them.

There is still however a 48V potential difference on both pins relative to ground which is used to power the headamp.
 
I still dont get it. Does anyone know of a site with a good tutorial for condenser mics?

I guess I'm way down the wrong track.

Thanks
 
Killer

I couldn't find a tutorial that covered the bases in a concise and understandable way, so I'll give it a try. Refer to the attached schematic, which I've made as simple as possible, yet still realistic, of the electronics inside a FET condenser mic, powered by phantom power and transformer coupled to the ouside world. Ignore the unlabled capacitors and resistors - they're just there to develop and block voltages and currents.

C is the capacitor (condenser) that is the main element of the mic capsule, and consists of a membrane and a back plate. A is the FET that amplifies the low current produced as C responds to sound waves. T is the transformer that produces the balanced signal and isolates the phantom power from the rest of the circuit. The phantom DC voltage V+ is present on pins 2 and 3 of the connector. It is passed through the center tap of T to point X, polarizing the membrane of C.

When sound waves hit the membrane, it moves closer to and farther from the back plate, causing a small (extremely small) AC current to flow. This current is amplified by the FET, A, which also derives its current from V+, and is passed to the primary of T. This in turn causes an AC current to flow in the secondary of T, which is output to pins 2 and 3 of the connector. These currents will be equal but of opposite polarities, and are the balanced signal that is sent to the preamp.

So in this case, the phantom power is used to 1) polarize the membrane and 2) power the amplifier (FET) that amplifies the current of the small signal produced by the condenser membrane.

This got more involved than I wanted it to, but hope it helps.
 

Attachments

  • mic_schem.jpg
    mic_schem.jpg
    32 KB · Views: 62
Heck! - I really asked for it, didn't I!

Thats great, Thanks for that Doc. Took me a few minutes to digest, but without going into the extra capacitors and resistors, Ive got what I wanted to know. Very Much appreciated. :)

I see now that if I was to ground either pins one or two, Id be shorting the phantom supply from the preamp - Thats scary!

I'm glad I know. Thanks for the explanation. Perhaps I could put it on a free website for future surfers to find.

What do you reckon?
 
You're welcome - glad it helped.

Feel free to post it anywhere you like. :)

It probably wouldn't hurt the preamp at all to short the phantom power leads to ground - they're built with current limiting resistors that limit the short circuit current to under 10 milliamps.
 
The page looks nice, but my hokey schematic sort of junks it up. I'd have taken more care if I'd known. :)

Your link is wrong and doesn't work - it should be homerecording.com/bbs.
 
You must log in or register to reply here.
Back
Top