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48V phantom power on multiple channels - how??

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Yairisan

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I'm confused.

I've always assumed that phantom power feeds to mixer mic pres need to be isolated from each other, i.e., each strip has its own dedicated 48VDC supply. However, my Soundcraft 6000 desk PSU has just one global feed that supplies all 24 channels. How do they keep them isolated from one other??
 

Yairisan said:
I'm confused.

I've always assumed that phantom power feeds to mixer mic pres need to be isolated from each other, i.e., each strip has its own dedicated 48VDC supply. However, my Soundcraft 6000 desk PSU has just one global feed that supplies all 24 channels. How do they keep them isolated from one other??
Click to expand...

No need to isolate then
You can even run the FOH and monitor desk in parrallel (both suppling 48V)

Joost
 

vaneenbergen said:
No need to isolate then
You can even run the FOH and monitor desk in parrallel (both suppling 48V)

Joost
Click to expand...

So those two matched balancing resistors used in phantom power end up doing the isolating (aside from their noise limiting function). I guess they must do, otherwise you are paralleling your microphones together...
 

It's high impedance to a low impedance PSU. so no cross talk.
 
"High impedance" refers to the input impedance of the mic pre (in comparison with that of the PSU)? And in saying "low impedance" of the PSU, you are looking into its output, so to speak (so I guess that would be output impedance of the PSU)? Sorry, but it's one of those 'I may be about to learn something useful' moments...
 

I mean the PSU will not be influenced by the mic, so the mic signal will not go onto a different channel.
The XLR of the mic is connected via a few k-ohms to the PSU, while the internal resistance of the PSU is very low.
 
vaneenbergen said:
I mean the PSU will not be influenced by the mic, so the mic signal will not go onto a different channel.
The XLR of the mic is connected via a few k-ohms to the PSU, while the internal resistance of the PSU is very low.
Click to expand...

Dammit, I never could grasp the impedance thing in this kind of context (internal resistances of circuits). I still can't grasp what's actually happening with the signal flow, but your answers at least inform me that I can do what I was hoping; that is, to run phantom to multiple channels from one PSU - thanks for that. Sorry, my interest in electronics far outweighs my understanding of it a lot of the time. Many thanks for putting me straight on the practical side of the issue, at the very least.

Robin
 

A simple reason, why a phantom power supply can't be isolated is the fact, that it uses the cable screen (connected to XLR pin 1 respectively mixer common ground) as return. Crosstalk through the bias resistors will be in the first place suppressed by the input amplifier's symmetrical design. But the 48V bus should have sufficient bypass capacitors in addition.
 

FvM said:
A simple reason, why a phantom power supply can't be isolated is the fact, that it uses the cable screen (connected to XLR pin 1 respectively mixer common ground) as return. Crosstalk through the bias resistors will be in the first place suppressed by the input amplifier's symmetrical design. But the 48V bus should have sufficient bypass capacitors in addition.
Click to expand...

whell you could have transformer isolated inputs. then the screen would be isolated. but these inputs are rare (expensive)
 

I have worked with transformer input microphone amplifiers, but they also connected the screen to common ground and used a common phantom supply. I don't see a purpose of isolated microphone inputs.
 

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