phantom power resistors, additional components? (newb)

[kato]

So imagine I'm racking an SSL9k board and have no desire for pad or phase switches - just phantom power.

Two 6k8 resistors are required. I know that much.
Looking at other people's schematics (jensen, fabio), I notice there's often a cap of varying values tween +48 and ground - I presume to cut down on the popping sound?? (really wild guess.)

So I incorporated a cap (c1 on screenshot) on my phantom go b'tween.

PP1 and PP2 are phantom power resistors.
PP and PP2 molex in lower right corner is 48v input from PSU.
C1 and R1 are just there because it seems like people generally use them. (I would be interested in knowing why.)
and C69, C55, L1 and L2 are the front end of the SSL9k and should be considered the front end of the preamp in this example.

See any problems with this?

-----
edit to remove the word "blocking."

 

[Rochey]

Kato,

the resistors are there to limit the current, so you don't blow your power supplies if you connect your microphone to ground.

Capacitors should be put in the signal chain after the phantom power resistors to block the path of phantom power going into the mic pre. You want phantom power to go up the cable to the mic, not further into your system.

I'll dig around to see if I can find a basic schematic.

/R

 

[Rochey]

Something like this:

Would be your phantom connection, then followed by 2 capacitors in the signal chain i.e. not connected to ground... the signal flows into one side, and out the other into the mic pre.

 

[kato]

Thanks Rochey! I don't know why I said "phantom blocking" about the resistors. I guess the phrase "phantom blocking caps" was on the tip of my brain.

 

[tomelectro]

The 6k81 resistors also keep the phantom supply from shorting out the signal (if you run pins 2 and 3 directly to +48, then you are also tying them to each other, shorting out the signal). The cap between the junction of the 6k81 resistors and ground, along with the resistor to +48v act as a filter on the phantom rail.

 

[kato]

[quote author="tomelectro"]The 6k81 resistors also keep the phantom supply from shorting out the signal (if you run pins 2 and 3 directly to +48, then you are also tying them to each other, shorting out the signal). [/quote]

Now that you mention it,
it's a bit mysterious to me that we connect the +48 to both pins 2 and three. I suppose that's a "no matter how your mic is wired" assurance.

[quote author="tomelectro"]The cap between the junction of the 6k81 resistors and ground, along with the resistor to +48v act as a filter on the phantom rail.[/quote]

What do they filter?

Thanks TomElectro.

 

[tomelectro]

Now that you mention it,
it's a bit mysterious to me that we connect the +48 to both pins 2 and three. I suppose that's a "no matter how your mic is wired" assurance.

Someone else can probably explain it better, but to the best of my knowledge it's wired to both 2 and 3 so that it is a common-mode signal which disappears at your input transformer (hence the name phantom power). With both sides biased to 48v, there is no net DC across the winding, which means no DC current, core saturation, etc. With transformerless inputs, you have to add capacitors to keep the phantom from messing up the input device biasing (since phantom is generally a higher voltage than the audio supply voltage) and/or blowing them up.

The cap filter on +48 is to ensure clean DC, diminishing any leftover ripple from the PSU or any junk/noise accumulated along the way.

 

[kato]

Thanks for the common mode rejection explanation. It helps little-by-little as this stuff all sinks in.

[quote author="tomelectro"]

The cap filter on +48 is to ensure clean DC, diminishing any leftover ripple from the PSU or any junk/noise accumulated along the way.[/quote]

Further smoothing, less noise. I get it.

I figured C1 and R1 were optional, but probably somehow useful components. I know somebody around here - I think it's DrPat - simply goes +48 to one side of a SPST, and the other side of the switch straight to matched 6k8's, straight to XLR with no problems.

This morning I discovered I don't have enough of the 5mm input caps to do more than 1 channel so I stripped the phantom blocking caps and inductors off the board.

Now there's nothing on my board except molex connectors and resistors. Which seriously reduces the need for a board at all. But flying leads still make me uncomfortable so here goes...