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[ajmogis]

To anyone:

How much current capacity should a phantom power supply have for a 24 channel console?

Thanks,
AJ

[Andy Peters]

ajmogis wrote on Tue, 01 April 2008 19:50

To anyone: How much current capacity should a phantom power supply have for a 24 channel console?

24 channels times about 10 mA/channel so 240 mA.

-a

[drknob]

If you want to guard against the goofy possibility of driving a dead short on every channel, you should allow for 15mA per. At that point, you may have bigger issues.....

[ajmogis]

That's about what I had figured.  I've got an Acopian supply (narrow profile) that is rated for 500mA on an API 2488.  

I've been experiencing some weird behavior from a pair of Earthworks TC30Ks.  They sound like they keep shutting down on loud transients - audio drops out and they oscillate around 20-30kHz for a few seconds and the audio slowly comes back.  This doesn't happen with any of the outboard preamps we have, or the console in the A room.  All the other condensers seem unaffected.  

When I get a chance I'm going to see if I can measure the current coming out of the supply to see if maybe I've got something funny going on with on of the channels.

[sodderboy]

Did the mics always do this?  If one channel were maxxing the supply so that it sagged on two microphones, you would not have 48 volts at the pre's, and the supply would be way hot.
 
The focusrite isa 115 was particularly lacking in capacity for powering high current microphones, and the fix was to drop value on the current limiting resistor feeding the 6K8 resistors at the XLR.
Check the phantom distribution circuitry in the console as well.

another thing to try is an external phantom supply.  Every studio should have a couple of channels of external phantom.  
If not, the quick fix for a pre with no phantom is to patch one mic into two pre's, one with phantom and the amplifying pre.
And I know there are loading issues, rule books, math etc. but when in need, it works.  Many a phantomless pre has been powered by SSL or Harrison etc. phantom power.  This could be a quick test done at the patchbay.
Mike

[ssltech]

It sounds more to me like an Earthworks problem than a supply problem...

Are these the electret Earthworks or the externally-polarised ones? -I'm not 100% familiar with the model numbers.

Keith

[ajmogis]

I hooked my voltmeter up to an empty mic input while this was happening to see if the voltage was sagging.  It didn't budge, but that doesn't rule out a very brief drop.  I wanted to put my scope on it, but the scope went belly-up a couple days ago and I haven't had time to fix/replace it.

Quote:

another thing to try is an external phantom supply.

My work-around for this problem was to use an external preamp with its own 48V supply.  The mics worked fine like this.

Quote:

It sounds more to me like an Earthworks problem than a supply problem... Are these the electret Earthworks or the externally-polarised ones? -I'm not 100% familiar with the model numbers.

These are the phantom powered variety.  I'm suspecting they may be pulling some large transient current.  I've already emailed Earthworks and I'm waiting to see what they have to say.

[ssltech]

ajmogis wrote

These are the phantom powered variety.

 
They're ALL phantom powered... -even the electret ones.

ajmogis wrote

I'm suspecting they may be pulling some large transient current.

Impossible. They're fed through 6.8k resistors. even if they went DEAD-SHORT CIRCUIT, they couldn't possibly pull more than 7 milliamps per leg. Dig out your console schematics and confirm that the mics are fed with 6.8kΩ resistances from the 48V rail... or another way to check would be to meter the voltage between pin 1 and pin 2 with nothing connected. -It should be 48V of course. If you then connect a 6.8kΩ resistance between pins 1 and 2 and measure again, it should drop to 24V. -Given that these are confirmed by actual measurement, it would be completely inconceivable for any microphone to reasonably draw more than about 10mA... at 14mA (the absolute maximum current drawn by a dead-short on BOTH legs) the mic would be dead because while current would be at maximum, voltage would be at minimum...

Perform the 6.8kΩ resistor measurement test on your console outputs AND your external preamp outputs, if they agree, you're barking up the wrong tree.

-Has it been more humid when these instances have occurred, (is one question which springs to mind) -and were these difference in behaviour with different preamps noted within minutes of each other, or on different days...?

...the clue may be in the details.

Keith