Building an external phantom power box

[Phil Gorey]

I'd like to build a couple of external phantom power boxes that wouldn't add any color to the mic signal. Anyone have any cool resources for a DIY project?

[tINY]

Look on the Jensen transformers site - there is all kinds of info on there for center tapped transformers, basic feed resistors, and full servo-balancing designs.




-tINY

[tINY]

Look on the Jensen transformers site - there is all kinds of info on there for center tapped transformers, basic feed resistors, and full servo-balancing designs.




-tINY

[Helsing]

Easy project so long as the mic pres the box feeds are transformer balanced or dont mind seeing 48vdc at their inputs so that you can omit the series DC blocking caps that will color your signal.

Check out the Jensen schematic ASO16.

******//www.jensentransformers.com/as/as016.pdf

Omit R4, R5. R6, R7, R8, S2, S3, S4

Match R2 and R3 to .1%

Send pins 2 and 3 from the female input xlr to a male xlr's pins 2 and 3 (output). Ground the output xlr's pin 1 in the same fashion as the input's pin 1.

Good Luck

Jon

[Phil Gorey]

Thanks for your quick replies. I've got two 30 series Berlant mic pres that Andy Barret did some modifications on for me. They have MX-10 mic input trannys which I am fairly certain don't have any balancing circuitry...I'm also not sure if they would mind being hit with 48V. Anyone familiar with the MX-10 transformers?

[Helsing]

Your pres probably has those little Beyers which I believe wont have any problems with the 48v.

Jon

[Phil Gorey]

looks like a pretty easy project....is there a specific reasonably priced 48V power supply that you would use?

[brianroth]

Look for a Power-One/International Power/Condor "open frame linear" module. The smallest in 48V is rated for 0.5 Amps. The module does need to be mounted inside of a metal enclosure.

Bri

[moogus]

Just wondering what you mean by a phantom power box that wont color the signal?

If your mic pres have input transformers all you need is 2x 6.8k resistors joined together at one end and connected to 48V, and with the other ends joined to pins 2 and 3 of your mic input XLR. It might be worth checking to see what kind of PSU is already in your mic pre as there is a small chance that theres a 48v rail sitting there unused.

Theres no danger in switching on the phantom supply to your input transformer BTW, despite what some people say on the internot.



M

[Helsing]

There are a few a few unconventional circuits like the the Telefunken V76 dont like to see an immediate spike of 48v at its input. Oliver Archut over at Tab Funkenwerks suggest a ramped phantom supply. For your purposes Id just go with the standard wiring scheme like the Jensen schematic. As long as you have a decent 48v supply and the matched 6.81k resistors at each channel, you should be good. I second Brian's suggestion of getting a Power One or Condor. Look on Ebay first. You might score one for $10. Just be sure its linear and not switching.

Jon Erickson

[Phil Gorey]

this place is awesome and all you guys rock!!! Thank you for your input.
ramped phantom power? Is this something you'd run for your studio...for instance, I was working up at Rolling Thunder in San Diego and the Neve there seemed to automatically supply any mic plugged in with phantom....no switches...this made me wonder about the tube mics and ribbons that I was using....anyone ever see this before? Thanks again...a truly beautiful place to learn and hopefully i'll be able to give back some helpfull input.

[moogus]

No, so called ramped phantom power isnt needed. Its only real purpose is to stop the loud pop you might hear when turning it on with the mic pre gain up.

The Neve you describe just has phantom perminantly wired to all of its mic inputs. Its the same circuit, just without the switch for each channel. I would definitely advise having a switch in case you ever use one of the ribbon or moving coil mics with a grounded centre tap on its output transformer. Its a good idea to have phantom switched off while youre plugging the mics in.


M

[brianroth]

Oliver Archut has run into blown input transformers on Telefunken V modules, hence his suggestion of a ramped power supply:

******//recforums.prosoundweb.com/index.php/t/3244/4437/

Bri

[Phil Gorey]

Originally Posted by brianroth

Look for a Power-One/International Power/Condor "open frame linear" module. The smallest in 48V is rated for 0.5 Amps. The module does need to be mounted inside of a metal enclosure.

Bri

Brian, I put the phantom power project on hold for a bit while working on my ssl clone compressor. I just started going back at it again and I'm looking for a clean 48V supply. I'm looking through my Mouser catalogue...specifically at the Condor Linear Power Supplies section. The power supply I'm thinking of using is Mouser stock number 675-HC48-1A and Condor Part # HB48-0.5-A+

I'm wondering if you think this would be a good choice....
Mouser says I need a cooling fan for this unit, which is not ideal as it will be in the control room. I don't want a fan, but the power supplies that don't require fans seem a little too large for a standard 1U box. Should I go for the 1A model which is a bit larger and go for a 2U enclosure?

This will be for a 2 channel phantom power unit...

Also, can I use one single output power supply for two mic channels or do I need a dual output power supply?

Sorry if these questions seem stupid...I don't have much experience to draw from.

Thanks for all your help so far.
Hopefully I'll be able to get this thing finished soon.
-Phil

[tINY]

If you are only using this for two pre-amps, you are not likely to draw more than 25mA. That supply is rated for 500mA.

You can probably get away with not using a fan. You will need to allow for convection cooling, though.

Hook all the "+48v" nodes to the supply in parallel - no need for separate channels (unless you are a complete freak about lowering cross-talk).



-tINY

[Tim Farrant]

Here's a circuit that regulates down to 48V and also has a soft start up. The 55V does not need to be regulated, but well filtered with minimal ripple. It will easily power 2 channels if necessary. The switch could be moved to the output side of the regulator just before the series diode, and repeat that part of the circuit for the second channel.

Cheers
Tim.

[nosebleedaudio]

Thats close to what I use on my JM 130 Pre, except I use the 7824 with a 24 volt zener. I would put the soft start after the regulator.
Most phantom mics only draw around 3 to 4 milliamps, I have seen as low as .5 milliamps. So it does not take much current to drive several mics.
Also most regulator circuits work better with a little load across them all the time, say around 33K.

[sysexguy]

Thanks for the info. I have a pair of GT Mp-1's that for some reason have the phantom switches on the back (perfect for those times when one tracks a band from behind the console )

If I choose to do mod, would I be better to "extend the switch " or set them to off and build a circuit as above (with a switch within my normal grasp)?

Anyone have the schema btw?

Andy