Mic Design/Powering...(not a phantom ?)

[Sleeper]

well, I lied- it's sort of a phantom power question...


I've got a 4 conductor + shield cable (shield, push to talk line,mic+, mic-, AND a spare into a 5pin XLR

want to run 9v to an electret mic for talkback.

HERE"S THE 2 QUESTIONs

if I run 9v in parallel- wired exactly as you would a standard 48phantom supply do the 6.81k resistors change value, and if so why or why not?

OR

Is this better (it seems simpler)
Run 9v up the spare line. royer runs 93v up its own conductor in the MXL mod, so this should be ok-right? I was thinking the transformer up at the mic head would let cmr eliminate any stray noise from this 9volts.

A LITTLE BACKGROUND

I have a talkback mic circuit in the headphone amp I'm almost done with. the mic is an old CB mic very nice, has a stand and a nice push to talk button. the crystal was trashed so I'm using a version of this mic

http://www.linkwitzlab.com/images/graphics/microph1.gif
with the wm61 capsule

I'm going to mod the design a bit and put a small transformer on the tail end of this circuit (up in the capsule area) so I'm running balanced to my headphone/talkback amp.

At the amp I have 9v supply that I'm getting off a voltage divider from my +18v rail.

I've got a 4 conductor + shield cable (shield, push to talk line,mic+, mic-, AND a spare into a 5pin XLR


Thanks for any input
Sleeper

 

[PRR]

This is a talkback mike?

How hi-fi does it need to be??

Unless your studio has extreme electrical noise, there is no reason to balance the output of these capsules. I used them as stage-mikes, unbalanced, no problem.

Unless you shout real loud, there is no reason to feed them 9V. Output clip-level hardly rises with supply voltage (unless you cut traces), clip-level is adequate for talk-back, the clipping is inoffensive if you do shout and will not harm talkback commnunication. And a single AA battery without a switch will power these capsules for a couple years. If the mike case opens, just put the battery in the mike.

So you really just need 3 wires. Ground, signal, and switch. Even if you want the power in the main box, use version C and it is still 3 wires.

 

[Sleeper]

Thanks PRR,

I'm interested partly as an experiment.
I'm curious how this batch of most likely modem transformers really sounds. but I'll try that once I decide on the powering issue. everything is nearly done including the 5pinXLR so I'd like to stick to the plan here, but this phantom question hangs me up a bit.

I would like to know about the phantom resistor just because I'm not clear why 6.81k.
I get how they balance the 2 sides of a phantom circuit, but I'm not 100% sure how having that particular impedence at that location works.

I see how it works as a load resistor on the dc going to the mic.

How does the relationship between 6.81 K and the impedence coming from the mic and the pair of 1k source resistors that feed + (with33k shunt to ground) and - (with 33k feedback) on a 5532/4 opamp that is the first stage of the talkback mic pre input


Sleeper

 

[PRR]

> I'm not clear why 6.81k

Because that is The Standard for 48V Phantom.

It is possible to run the P-Sonic mike capsules from a Phantom-standard supply. Seems kind of a waste. Phantom can supply almost 200mW, the P-Sonics only need 0.5mW. Or Phantom can supply 45V diaphragm polarization bias (if the mike is low-current); the P-Sonics are self-polarized and their amp-stage only needs a few volts.

The P-Sonic output impedance, a couple-KΩ, is much higher than modem transformers want to see. It will be quite gutless, a false impression since if driven from 100Ω the modem trannies will carry full male voice range.

If you must go this over-complicated way: put a 10K and a 5V Zener from transformer secondary to ground. Feed the P-Sonic capsule from the top of the 5V. I converted some mercury-battery Nakamichis to Phantom this way and it runs sweet. (Actually since the Naks did not have a center-tapped transformer, I used two 22K resistors to feed the Zener.)

 

[Sleeper]

sorry this was hard to find in the first post

i already have a 9v that I took off my +18v rail with a voltage divider.

Because that is The Standard for 48V Phantom

I was thinking more along the lines of a 9v supply run over the mic hot and cold-LIKE phantom, but NOT "phantom"

Anyhow, I'm going to try a bunch of variations here mostly coz I might learn something.

This is the first mic I've had a chance to make that wasn't filling some critical void in my studio situation, so I thought I'd try out some of the ideas I've been picking up along the road here and put to use.

The P-Sonic output impedance, a couple-KΩ, is much higher than modem transformers want to see

yeah, this is sort of what I thought, too,but some of these seem to have a 1:4 ratio and I thought I might give a try, backwards and see what shakes out.

I've made so many mistakes over the last year of reading this forum, but I haven't made THAT one yet :razz: ...and I've stopped making them twice, so I maybe should get that out of the way.

Thanks as always for the advice.
Sleeper