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Old 31st August 2010, 02:43 PM   #1
.rhavin is offline .rhavin  Germany
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Default How to *use* phantom power?

I have a simple balancing circuit using half a LT1365 opamp. as long as it is powered by a 9V battery all goes well. now i want to modify my circuit to get power from phantom (+48).

Click the image to open in full size.

but i dont know how to get it to work. i connected both lines with a resistor to a cap and divided the resulting line with two resistors to get a virtual ground. when i simulate in ltspice, the amount of the signal on my power line is still to large and the negative signal line now has the positve signal on it, because the virtual ground has a small part of it and that is fed into the non-inverting input of the lower opamp.

so how do i get a stable DC from my signal lines without feeding back my signal into the opamps?
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Old 31st August 2010, 05:07 PM   #2
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Normally, no power is available from a phantom supply. I'm not sure what you are trying to do. Does the 9V supply float, or is it on ground? I don't see any power line destinations on your diagram.

You can't ground the negative power line on the amplifiers if you expect to ground the input as well. (You should also add an impedance in the input lines to balance the offset current.)
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Old 31st August 2010, 05:38 PM   #3
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Those op-amps use too much power. You can only use 10mA or so and have 9V left for your circuit and yes you will need to convert it to single supply operation.
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Old 31st August 2010, 05:50 PM   #4
Artie is offline Artie  United States
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The amount of power available from a phantom power supply is simply dependent on the design of the supply. In other words, there's no inherent limit. In reality, most phantom power from commercial mixing boards, et al., will be fairly low. But, you can also build your own.

Study this circuit from www.jensen-transformers.com, to get an idea how to implement this.
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Old 31st August 2010, 05:52 PM   #5
.rhavin is offline .rhavin  Germany
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Hi bob91343, thx for helping. I post an updated circuit to show power supply…

Click the image to open in full size.

On the left side, you see the phantom power from 9V battery. In the middle, you see the power decoupling. Above C2 i have phantom supply of ≈8.946V. The circuit runs well if i use battery V4 in simulation, but when i leave it out and connect to phantom supply (by closing the little gap), all bets are off ;-/
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Old 31st August 2010, 06:36 PM   #6
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V2 should be 48V and the op-amps draw 7.5mA each so they simply collapse the supply (even at 48V)
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Old 31st August 2010, 06:38 PM   #7
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You forgot to ground the signal input.

And you are indeed trying to draw too much to run the opamps. I think. How much current do they take?
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Old 31st August 2010, 07:09 PM   #8
.rhavin is offline .rhavin  Germany
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Quote:
Originally Posted by bob91343 View Post
You forgot to ground the signal input.
er, no;-) Its ground virtually(<-correct term?) at half dc phantom power via R25/R26, C1/C3.

Quote:
Originally Posted by bob91343 View Post
And you are indeed trying to draw too much to run the opamps. I think. How much current do they take?
In simulation is about 13mA each.

scotts hint gave me a clue to a now working solution – at least in simulation: i changed R1, R2, R12 & R17 to 1kΩ each. But, as far as i know, phantom is normally driven tru 6k8… so this means i cant drive LT1365s via standard phantom power, right?
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Old 31st August 2010, 11:11 PM   #9
Minion is offline Minion  Canada
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Phantom power is 48V through two 6.81k resistors which act to limit the current , it can only supply 14ma when shorted , so if you draw 13ma you will collapse the voltage to practicly nothing ... so no you can"t power this from Phantom power unless you use a very low power Opamp .....
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Old 1st September 2010, 01:17 AM   #10
benb is offline benb  United States
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Mic buffers running on phantom power usually use discrete transistors as the active elements so the current draw can be controlled to work well with phantom power. They're usually arranged as a long-tail pair with collectors going directly to the two signal/power lines, and the base(s) as the input. Also, while the standard voltage for phantom power is 48V, cheaper equipment may use 24V or 12V (with the same 6.8k series resistors), so it's prudent to design a commercial product to run on those lower voltages.

A little googling finds this op-amp-based circuit designed to run on phantom power (5th page, page number in text is 100). Like most of these, this is made for an electret element, but it's easy enough to add a capacitor between an unbalanced mic signal and the input, and to change those bias resistors to set the input impedance:
http://www.edn.com/contents/images/83001di.pdf
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