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**Amp Kat** · 08-04-2007, 03:16 AM

Acouple of 6.8k to 10k resistors should do it. I don't think the cap is neccessary. Check out http://jensentransformers.com as every thing you can do or need to do is there.

**DarthWombat** · 08-04-2007, 04:17 AM

Originally posted by Amp Kat View Post

Acouple of 6.8k to 10k resistors should do it. I don't think the cap is neccessary. Check out http://jensentransformers.com as every thing you can do or need to do is there.

Thanks, Amp Kat!
OK, then: Could I use a pair of 1% 6k8 resistors between hot & cold on every channel in the stage box (both those with live phantom power and those without) and link the junction of each pair beween all channels? The cap was suggested for noise immunity. I'll probably still put a single one in for the whole lot. Does this sound OK?

Can you see any potential problems at the mixer if it sees the phantom power (which it originally provided via other XLR connectors) at a balanced TRS input?

**Enzo** · 08-08-2007, 12:29 AM

Those resistors go from the two signal lines to the +48v bus. How you gonna do that from the XLR jacks?

You could put the phantom in the stage box, but you still need to get the 48v there. I would not tap off one of the existing phantom XLRs because that 48v is not the power supply, it is on the receiving end of its own isolation resistors.

The 48v is supposed to get to the XLR pins of the mic, but the isolation caps are there to keep that DC out of the mixer input circuits. I'd use them.

**DarthWombat** · 08-08-2007, 03:19 AM

Originally posted by Enzo View Post

You could put the phantom in the stage box, but you still need to get the 48v there. I would not tap off one of the existing phantom XLRs because that 48v is not the power supply, it is on the receiving end of its own isolation resistors.

Sorry, Enzo, I don't quite follow why I can't tap off one of the existing phantom XLRs. I presume the isolation resistors you refer to are the ones back at the mixer behind the XLR with the phantom power. Why would it be a problem to run this (via another set of isolation resistors) to a +48V bus in the stage box?

Sorry, but you'll just have to bear with me on this - I'm still trying to come to grips with the wheres and why-fors of all of this. I'm probably just not grasping something blindingly obvious. Thanks for your help with this!

**Enzo** · 08-08-2007, 08:05 AM

Where will you get that 48v bus in the box? The 48v bus does not appear on any connector on the mixer, and by extension any connector on the stage box.

The 48v is connected to any pin on the XLRs through isolation resistors, just as you plan to add. So the 48v you might want to grab at one of the channel connectors would be coming THROUGH an isolation resistor already.

If you have an extra wire in the snake, you could send the 48v to the box and wire these extra phantoms from it.

If it might help, let me make a parallel example of my point as a hypothetical tube circuit. All the 12AX7s in an amp have 100k plate resistors. Now suppose you wanted to add a couple more 12AX7s. You'd need a couple more 100k plate resistors, and they would have to be tied to B+. You could not reasonably connect these new resistors to one of the original 12AX7 plates instead of to B+.

My concern here is that you want to add phantom to some stage box jacks, you need 48v to do it, and you have no source of it at the box.

Unless I am missing your plan.

**DarthWombat** · 08-08-2007, 09:54 AM

G'day Enzo...
I may not have expressed my plan clearly enough, so hopefully this makes more sense:

Inside the stage box (because that seems to be the most convenient place for extra wiring) I wanted to take the 48V from the 8 XLR-connected mono channels which our mixer already supplies with phantom power to share with the 3 pairs of (balanced) TRS-connected inputs which the mixer does not supply. All of the channels are XLR female at the stage box.

I figured if I ran very high isolation resistor values on all of the 8 powered channels (say, 54k9) to a bus, and more "normal" ones (6k8) on each of the unpowered ones from the bus, I could source current from every one of the powered channels while minimising any signal attenuation across the resistors. Does this sound feasible? the 48V is already getting to the box via the channels supplied from the mixer, so all I want to do is extend it to the ones it doesn't already supply.

What I'm trying to work out is whether the DC offset is likely to cause problems at the balanced TRS inputs at our Behringer mixer. All of the channels on the stage box are connected back to the one mixer. Does this all make sense?

Thanks for bearing with me. I do appreciate your input.

**Steve Conner** · 08-08-2007, 11:18 AM

Well, any problems with the DC offset can be eliminated by putting DC block capacitors before the mixer line inputs. (which I'd recommend: while the mixer should have DC block caps on its inputs already, I doubt they will withstand 48V)

What I'd be more worried about is:

the possibility of the line-level signal from the DI box leaking back through the resistors into the mic inputs. A large electrolytic capacitor from your "bus" to the ground of one of the XLR inputs should help that.

the DI box outputs not being hot enough to drive TRS inputs without distortion, as they are designed to work with mic inputs.

**DarthWombat** · 08-08-2007, 12:46 PM

Originally posted by Steve Conner View Post

Well, any problems with the DC offset can be eliminated by putting DC block capacitors before the mixer line inputs. (which I'd recommend: while the mixer should have DC block caps on its inputs already, I doubt they will withstand 48V)

What I'd be more worried about is:

the possibility of the line-level signal from the DI box leaking back through the resistors into the mic inputs. A large electrolytic capacitor from your "bus" to the ground of one of the XLR inputs should help that.

the DI box outputs not being hot enough to drive TRS inputs without distortion, as they are designed to work with mic inputs.

Thanks, Steve!

How large do you think the the DC blocking caps should be? Would they compromise the low frequency response at all? I presume I would use a pair for each channel: one in series with each of the hot & cold lines on the mixer side of the isolation resistors - does that sound right? I had thought of doing this, but I wasn't sure how kosher it was.

Shouldn't the hot & cold pretty well sum to zero at the connection to the bus for each channel? I was going to use 1% tolerance resistors (or better) plus the nice big cap between the bus and ground to take care of any residual signal due to component imbalances. Any suggestions for values here?

This is all really helpful for me - thanks for your input!

**Enzo** · 08-08-2007, 11:57 PM

Get the schematic of most any phantom mixer and see how they do it. They are all about the same.

My concern for you is that what you are calling 48v is not what I call 48v.

There is phanton power in the existing 8 XLR jacks. It comes up the cable from the mixer. THat is not a 48v supply, it is 48v coming through the isolation resistors for the channels. Since there is no current draw, the voltages are the same.

Your plan will result in having 48v, yes, but instead of getting it from the power supply, you will now be pulling it from the pins of a microphone. So any signal at that pin will be impressed upon your faux 48v. While your sea of resistors might even that out, I still feel it is bad practice.

What I am saying is that you do NOT have 48v already in the stage box, you have the RESULTS of 48v there. I'd much prefer if you used a spare wire in the snake and fashioned a 48v buss up the pipe.

**Steve Conner** · 08-09-2007, 11:27 PM

DarthWombat: Yes to the DC blocking caps like you said, and I'd guess "About 10uF". I'd make the big ground bus cap 470uF.

Enzo: I agree that DarthWombat will not get the full 48V, but I think most phantom powered thingies are specified from 12 to 48V, so I'd guess it would have a fair chance of working, especially if he taps off both hot and cold of all 8 mic lines. Yes, it will affect isolation, but if he uses that capacitor, it won't affect it any worse than the phantom power network inside the mixer itself does already.

It's a strange way of doing things though: if it were me I'd just hack the mixer to supply 48V out of a RCA socket into a horrible 5-headed home-made adaptor cable. (it's only a behringer after all

) Or better still buy a phantom power supply for �25. Then if it didn't work I could call up Studiospares and whine instead of having to try and fix it myself...

http://www.studiospares.com/pd_44855...%20CHANNEL.htm

**Enzo** · 08-10-2007, 12:11 AM

Maybe I am over anal about stuff.

I wasn't worried about a lack of voltage, I was concerned about taking that voltage from the load end of the supply resistors of the other channels and potential crosstalk issues resulting. The 48v bus in the mixer offers a low impedance to prevent channel interactions.

**DarthWombat** · 08-10-2007, 12:20 AM

I have to admit that I'm beginning to lean towards the idea of an independent phantom power supply as well, rather than hacking the stage box. I've never used one before, so I need some pointers on how best to use them. If I bang one in somewhere between the TRS-XLR adaptor from the input on the mixer and the XLR output of the snake, would I need to worry about the mixer seeing the 48V? Also, I need to apply power to at least 3 of the lines at once (to power three two-channel DI boxes) - is there a single phantom power supply out there which will do this?

At the very least, this exercise has really aided my understanding of how phantom power is used. One school of thought I've encountered is "avoid phantom power unless absolutely necessary" because of the possibility of a dodgy cable accidentally putting 48V across an expensive microphone and frying it. The independent supply would at least allow me to select which of the channels should carry it, where the mixer option does not (it's global across all mono channels).

Thanks once again for all of your help on this.

**Enzo** · 08-10-2007, 04:23 AM

Avoid flying in airplanes in case a mountain pops up in front of you. COmmercial mics are not usually bothered by phantom as it is a fact of life in the audio game.

COmmercial phantom supplies are invisible to the mixer. They have the isolation you need. They come in little single ones, dual ones, and as many as 8 in a box as far as I have seen.

**Steve Conner** · 08-10-2007, 10:19 AM

DarthWombat, the phantom power supply ought to have DC block capacitors inside. But if you're in the UK why not just call Studiospares and ask. I think they sell a 4 channel unit, too.

I used to greatly enjoy hacking stuff up to be non-standard. For a while I had an XLR on my guitar instead of a 1/4" jack because I figured it wouldn't come out if I trod on the lead. But then just try borrowing a replacement guitar lead at a gig!

Nowadays I think there's a lot to be said for just getting boxes off the shelf and sticking them together like Lego bricks. If they break or get stolen, you can easily replace them in a hurry, and since they're standard parts, they have resale value. However, I'm still stuck with a guitar amp that has a female XLR as a speaker jack

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Phantom power - how to share between channels?

Phantom power - how to share between channels?

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