-24 and phantom from one supply

[sardonic_z]

Hey there everybody. I have been tasked with powering some Scully 280 mic pres, and I want to equip them with phantom power, cause, you know, it's 2013. The Scullys run off of -24VDC. I saw a post from Jakob of Gyraf on a similar thread (someone was trying to get +24 and +48 off a single supply) where he suggests using a ±24v supply, running the phantom between the positive and negative terminals, and then just getting the +24 by referencing ground as the positive voltage.

Will this type of idea work if I need -24? IE I could get the 48v from the pos and neg added together, and then take the -24 from either the -24 terminal and ground, or from ground and the +24. I just keep getting confused about whether that screws up the grounding for the whole module or not. I see no compelling reason why the two power sources need to share a ground, EXCEPT maybe the way phantom power works, EXCEPT maybe that's not such a big deal etc etc.

Please, help out a relative noob.

 

[Nele]

I recently racked some old EastGerman RFT pre's which also run on -24v. I ended up using two separate transformers and powersupplies for the -24v and the phantompower...

 

[abechap024]

Yes it will work.

 

[Harpo]

...depending on input transformers shield connection. Phantom voltage is +48V above mic shield. If this shield also connects to your 0V reference voltage on pcb, you'd have to interrupt this connection (might cause hum) or build a supply 24V below this reference. Two separate supplies without common reference seems a safer bet.

 

[abechap024]

You could isolate the 2 feeds from the transformer with AC coupling caps. That way you could reference both to the same ground.

 

[Silvas]

I´ve done this many times with at least 5 Scully 280 units. Everytime i used 2x 24v transformers. One for the - and other for phantom.

Another cool mod i do is popping out the red pilot, and putting a grayhill rotary switch (you´ll need a washer so you can tighten the grayhill switch on that hole) , so i can supply +48v to either mic or line input transformers.  The line input trafo is lovely for kick drum with Neumann FET47 !

Don´t forget to make a -20 pad for the mic input on hotter signals using the mic input. And if you want a bit less gain, just rewire the min input transformer primaries to paralell instead of series.

Best of luck.