NEWBIE: Phantom Power question

[Guest]

Hi folks,

This is a general theory type question, I guess.

Looking at the power schematic for a seventh circle n72, the phantom confuses me. But, that's because ALL phantom confuses me (see signature below).

I the schematic, there are 48v coming off the ps. The 48 then hits a resistor (150 ohm), and is then wired in parallel with two capacitors. The first is polarized (positive terminal to +48), the second not polarized. Then, the positive output is split down the two signature 6k65 resistors.

I guess I'm wondering the following question:

Are positive polarized caps always connected to positive charged voltage? If so, is there a simple reason for this?

Why are the resistors and caps even there? To smooth any intermittent power issues on the 48? This seems redundant; the 48 is already regulated in the power supply.


Thanks!

Kelly

 

[alk509]

[quote author="thearnicasync"]I the schematic, there are 48v coming off the ps. The 48 then hits a resistor (150 ohm), and is then wired in parallel with two capacitors. The first is polarized (positive terminal to +48), the second not polarized.[/quote]

I don't have the schematic for this, but it sounds like an AC filter... The cap shorts any residual AC in the phantom line to ground. The resistor is there to limit the current through the cap when it first charges.

[quote author="thearnicasync"]Then, the positive output is split down the two signature 6k65 resistors.[/quote]

I've heard people say that the value of these resistors isn't critical (as long as they're matched, of course). Having said that, I've never seen any other value than 6K81, not 6K65.

[quote author="thearnicasync"]Are positive polarized caps always connected to positive charged voltage?[/quote]

Yes.

[quote author="thearnicasync"]If so, is there a simple reason for this? [/quote]

Reverse polarity wil damage them / blow them up.

[quote author="thearnicasync"]Why are the resistors and caps even there? To smooth any intermittent power issues on the 48? This seems redundant; the 48 is already regulated in the power supply.[/quote]

Regulated, but not decoupled from other circuits feeding off of the same PSU. Also, any EMI/RFI picked up by wiring will be reduced.

 

[NewYorkDave]

The resistor and cap(s) form a decoupling filter. This serves two purposes. First, it smoothes out any interference on the supply before it reaches the mic line. Secondly, it keeps the "low" side of the 6.8K resistors solidly at AC ground and prevents audio from coupling back to the supply line. A regulated power supply has a low source impedance, but it is never quite zero.

Actually, make that THREE purposes, since this R-C filter also causes the phantom power voltage to the mic to ramp up rather than slamming on all at once.

The time required for the (unspecified in your post) cap to reach 63% of full charge is called the time constant.

Tc = RC

Tc in seconds, resistance in Ohms, capacitance in Farads. It takes (5*Tc) seconds to reach full charge.

Polarized capacitors require the "+" terminal to be positive with regard to the "-" terminal. So, when there's a positive supply voltage, the "+" goes to the supply and the "-" to ground. If the power supply is negative, "-" goes to the supply and "+" to ground.

As for the nonpolarized cap in parallel with the polarized cap: that's a popular trick for fussy audiophile types who fret over the somewhat poorer performance of electrolytic caps at higher frequencies compared to their film-cap brethren. It's really not needed and the circuit will work fine without it.

 

[Gus]

The value of the phantom resistors does matter with microphones using the Schoeps microphone circuit.

The two 6.8K are part of the microphone circuit, they are emitter resistors for the output pair in the microphone. The Studio Projects C1 I have is like this as are the mxl 1006,603,2003 nady 950 and CM90. Alot of transformerless solid state microphones use this circuit.

 

[Guest]

Man. You guys blow my mind.

Funny, I forgot about the ramping phantom thing, and if I'd thought for a second I might have remembered that.

I just went and read about the differential calculus required to get time constants. It was funny. Everything on the screen just started flying around...like in Weird Science.

Hey Dave, don't feel even remotely obligated to answer this, but what are the "low" sides of a resistor?

Looks like I'll have to read about impedence (in my books) to find out why the source impedence of the supply is an issue.

Again, thanks so much!!!!!

kb

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