turning +- 24 into +48

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arkestra
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turning +- 24 into +48

Post by arkestra » Sun Jun 22, 2008 7:41 pm

got a power supply giving + and minus 24v to a pre. is there a way to get +48 out of this for phantom?

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Post by Chris_Avakian » Mon Jun 23, 2008 12:20 am

the +48v for phantom power is dc, so simply? no...

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RodC
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Post by RodC » Mon Jun 23, 2008 6:56 am

FRIZEYED wrote:the +48v for phantom power is dc, so simply? no...
Yes but, it needs to be +48v in realation to ground. If the same supply is used for the preamp, even if you use the +24 and the -24 connections, he will not get +48 in relation to ground. Unless I have totaly misread his question.

Prob would be easier to build your own phantom PS with a transformer. After 60+ channels of preamps without phantom, I gave up and bought myself a 6channel supply.

If you only have DC to work with:
http://www.national.com/ds/LM/LM2586.pdf#page=23
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Post by Jim Williams » Mon Jun 23, 2008 7:40 am

Take the unrectified AC off the power transformer. Run it into 2 or 3 stages of voltage multiplication which consists of large 50v El caps with rectifier diodes. That will push the AC above 50 volts and then you can use a TL783 regulator to smooth it out.
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RodC
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Post by RodC » Mon Jun 23, 2008 7:58 am

Jim Williams wrote:Take the unrectified AC off the power transformer. Run it into 2 or 3 stages of voltage multiplication which consists of large 50v El caps with rectifier diodes. That will push the AC above 50 volts and then you can use a TL783 regulator to smooth it out.
crap, one of the links I was going to post:

http://www.coolcircuit.com/circuit/voltage/

Guess im out of posting shape, been in the studio too long.
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Post by nclayton » Mon Jun 23, 2008 8:33 am

If the preamp is fully transformer balanced, both in and out, potentially you could lift the preamp's ground connection to the chassis and then connect the -24 from your power supply to chassis and +24 to phantom. You have to be careful doing this, though, as the PS 0 volt and preamp ground are now at +24 in relation to earth. You just have to be absolutely certain both the preamp and power supply are totally isolated from earth except for that -24 volt connection. Otherwise, if you're just running a couple of preamps, I don't think there's any particular reason not to do this.

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Post by Jim Williams » Mon Jun 23, 2008 10:13 am

+24v DC + -24 v DC = 0v DC. You would need to invert the - rail if you want it to add.
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Post by ashcat_lt » Mon Jun 23, 2008 7:51 pm

Nope.

24VDC - (-24VDC) = 48V.

What matters is the potential difference between the 2 rails, aside from those things mentioned above about the ground reference...

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Post by nclayton » Tue Jun 24, 2008 11:04 am

As far as my other post up there goes, I know it's true not only cause it's obviously true, but also cause I've done it myself several times. It's not the absolute best way to get 48 volt phantom, but it works fine, especially if you don't have a large or complicated ground system in the box, which, if you're just powering one or two preamps, you probably don't.

To try to make it a little clearer, here's what you connect to each rail of your power supply:

-24 volt rail: connect to preamp -24 volt, the chassis (including XLR pins 1 for the phantom return), and AC line ground

0 volt rail: Connect to preamp 0 volt and nothing else. Don't connect the preamp ground to the chassis or anything else.

+24 volt rail: Connect to preamp +24 volt and phantom +48 volt.


Basically now (in reference to chassis and earth) you're running your preamp from 0, 24 and 48 rather than -24, 0 and 24, but as long as the preamp is isolated from the outside world it won't know or care about the somewhat semantic difference between these two situations. Note that if you've got a preamp or power supply with its 0 Volt connection hardwired to it's own chassis (as opposed to your rack mount chassis) you'll either have to break that hardwired connection or else insulate the preamp or power supply chassis from the rack chassis somehow. In this situation it might just be easier to make a separate 48 volts supply.

I think this is probably already clear, but we're not trying to add +24 volts to -24 volts....they're already in reference to each other, ie, they've already been statically added by the power supply manufacturer. What we want is the difference between the two voltages that are already stacked up, and difference is subtraction not addition: 24 - (-24) = 48. But I know this probably isn't news to anyone. Obviously if you touch the -24 volt rail to the +24 volt rail you get sparks, so it should be totally obvious there is something other than 0 volts there as Jim, I think, mistakenly implied.



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Post by Skipwave » Tue Jun 24, 2008 12:22 pm

For schematic examples of how to run on a single rail by means of biasing at the half-rail:

http://jlmaudio.com/Baby_Animal_Mic_Pre.htm

Note the 1/2V tied to the input of the opamp, and how that circuit is not referenced to ground, as Ned pointed out. The circuit must "think" that +24V is ground. We can do this because the circuit does not know what 0V is, and in reality there is no such thing as 0V anyway, so we can just tell the circuit that +24v is 0V. The circuit happily swings up and down around +24V.
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