Phantom stuff

[Rob Flinn]

Hello folks.

Thought.

I seem to be making a low current 48v supply for each mic pre I make & often wondered about the idea below.

A friend gave me a 48v 250mA regulated PSU. Obviously this has excess current available for a single stereo mic pre.

Has anyone ever used a single 48v PSU to supply several different preamps in separate boxes. I`d thought about this in the past, maybe just taking the power from an XLR on my desk & feeding it to the various mic amps I have at my disposal.

The issue as I see it is the dreaded hum !!

Has anyone done this, or have any thoughts on the matter ?

 

[SSLtech]

Consoles do this. As long as you can quietly sonnect multiple audio grounds without the loop (which you should easily be able to do as long as audio and power ground are kept seperate inside the boxes!) then you can do it. -If you have 12 points where audio ground and power ground meet, this might become an issue.

Keep church and state seperate (where EU regulations permit) and you'll be fine. This might mean power-grounding some equipment cases and removing any links between pin one and the chassis...

btu the easiest way is to try it and see if there's even a problem to solve! -As long as there's not a large loop area between power ground runs, you can still have low -or no- noise.

Keith

 

[jrmintz]

Why not make a 1-rack space multi channel phantom distribution/mic input box with 6.81k ohm resistors and switches and use short jumpers from the outputs to the mic pres in the rack, rather than bringing the phantom into each mic pre.

 

[Rob Flinn]

(which you should easily be able to do as long as audio and power ground are kept seperate inside the boxes!)

Keith,

By this do you mean ground the case with mains earth & ground the audio via XLR outs ? If not could you elaborate on this please, since earthing isn`t something I would want to get wrong.

 

[clintrubber]

Perhaps of interest:

http://sound.westhost.com/project96.htm

 

[rafafredd]

Or this simpler version...

http://sound.westhost.com/project30c.htm

 

[SSLtech]

In Europe (as I understand it), it's now a requirement that every piece of electrical equipment has a grounded case if the case is conductive, or that the equipment has to be double-insulated.

In the case of audio installations where you don't want dozens of linked paths to ground making loops, keeping the audio ground on XLR pin 1 and the case unconnected is a good strategy, so long as there are no metal ¼" sockets making the case connection for you...

Numerous amplifiers have links which can be lifted to facilitate this seperation of church and state, and it's a good reason not to take everything you build and connect the audio ground to the case...

That having been said, AKG and other manufacturers used to build 48V PSUs with XLRs in and out, all sharing a 48V supply through some 6.8k resistors. (and either transformers of capacitors to block the DC path) Usually the cables alkl went to the same destination (preamp rack or console) so there was little chance for a big loop area to build up.

Keith