testing phantom power delivery

ecc83

Well-known member
As promised over in Annyloggy...

The specification for phantom power is that it should deliver up to 10mA total current from the two "legs".

This does NOT mean you will measure 48V with a 10mA load. To test 48V you will need..

1) Voltmeter (with resistance measurement if possible i.e. almost any $30+ DMM)

2) Two 6,800 (aka 6k8) 1% resistors*

3) XLR 3 male plug

4) Soldering kit.

The 6k8s are soldered to pins 2 and 3 of the plug and the other ends both go to pin one. Leave the wires full length.

Plug into mixer, tape, whatever and clip meter across a 6k8. Power up. You should read 24volts across the resistor. If so this indicates that the circuit can deliver at least 7.6mA and that is enough for almost any capacitor microphone.

Now clip meter across pins 2 and 3. Ideally zero but a few mV indicates the lack of matching of the internal 6k8s (but see *) . Poor matching here will reduce PSU noise rejection but is rarely a problem.

If you wanted to check for the full 10mA you need a 1k4 load (2x 2k8) when you would get 14V across it but I doubt any modern mic would need anything close to ten mills?

*Buy a rake of resistors, cheap enough! Go through them and find the best match. Does not matter if they are "off" 6k8 a bit (you can use Ohms law to check the error but spook juice really is not that critical) what matters is that they are as close in value as possible.

Dave.
 
Last edited by a moderator:
Perhaps suggest checking voltage across both resistors to ensure approximately equal voltage on both legs.
"Plug into mixer, tape, whatever and clip meter across a 6k8." Implies just measuring one(?)

This setup might a good test of a questionable cable also by plugging the test jiggy into the end of a cable.

Good write up Dave. Helpful for someone who wants to check phantom and gives some hands on practice at soldering. :)

My most power hungry mic is a CAD M179.......

cad m179.jpg
 
Last edited:
Thanks for the nice comment Mark. I cannot see where my original text is unclear or lacking?

Perhaps you could 'splain it to this medd'ed up, one eyed old man?

Dave.
 
Where jugetthat from BSG?

I heard it somewhere and it stuck. I must have trusted the source. I've also read that the maximum allowable draw, according to IEC, is 10mA. If the supply is required to produce up to 14mA then it should never run short of current. Google doesn't turn up much other than an Apogee statement that all their mic inputs with phantom provide 14.1mA and a Shure page that indicates that one of their mic pres should do 7mA per pin.
 
I heard it somewhere and it stuck. I must have trusted the source. I've also read that the maximum allowable draw, according to IEC, is 10mA. If the supply is required to produce up to 14mA then it should never run short of current. Google doesn't turn up much other than an Apogee statement that all their mic inputs with phantom provide 14.1mA and a Shure page that indicates that one of their mic pres should do 7mA per pin.

That figure probably relates to a microphone type that operates as a current sink and uses an internal SMPS to generate polarizing and other voltages. Mic manufacturers are naturally very guarded about their internal circuit details.

Pulling 14mA from a phantom outlet would drop over 47V across the internal 6k8 resistors giving you buggerall external voltage!


Dave.
 
Back
Top