Cool Site... My first question

[xtractor22]

Hello,
    I've been pouring over this forum for several days and have learned a LOT.  (gotta love the internet).  So first of all, thanks!

    My question revolves around phantom powering a driver device (i.e. something that will send a signal to the mixing board).  I have been looking at various schematics and reverse engineering circuits, coming back to this forum and saying, "oh that's what that RC filter is for" . 

    What I see is a lot of variation in the resistor values used to tap pin2 and pin3 for the common mode (phantom) voltage.  I have seen everything from a pair of 6.8k resistors to a pair of 1k resistors.  So my question is, is there some standard on what resistances can be used?  Or is it simply a matter of headroom (max current draw across these resistors) and output impedance (dominated by the output stage/transformer.  Don't want this resistor pair to have such a low impedance that it competes with the driver's ability to send ac).  Not sure about this, but the max current spec I have seen appears to be 10mA.  Assuming a pair of 6.8k resistors and this 10mA, the voltage drop would be 34V.  Assuming 48V sourced from the mixer, this would leave 14V for the common mode Voltage at the driver.

I appreciate whatever insight you can give me.
Thanks

 

[ruffrecords]

Basically yes. The standard is IEC 61938.

http://en.wikipedia.org/wiki/Phantom_power

Cheers

Ian

 

[xtractor22]

Basically yes. The standard is IEC 61938.

http://en.wikipedia.org/wiki/Phantom_power

Cheers

Ian

Ian,
    I appreciate that link.  I reread my post and realized it may not have been clear.  What I am asking about is on the driver side (i.e. phantom powered DI box) if there is a standard for the resistors used to tap off the phantom supply.

Thanks

 

[Andy Peters]

Basically yes. The standard is IEC 61938.

http://en.wikipedia.org/wiki/Phantom_power

Cheers

Ian

Ian,
    I appreciate that link.  I reread my post and realized it may not have been clear.  What I am asking about is on the driver side (i.e. phantom powered DI box) if there is a standard for the resistors used to tap off the phantom supply.

Thanks

On the device side (what you mean by the "driver" side), there are no resistors used to tap off of the phantom supply.

You'll find a DC block to isolate the audio from the phantom voltage. The phantom voltage may be regulated down to a lower voltage or it may be used as is, depending on the circuit.

-a

 

[xtractor22]

Basically yes. The standard is IEC 61938.

http://en.wikipedia.org/wiki/Phantom_power

Cheers

Ian

Ian,
    I appreciate that link.  I reread my post and realized it may not have been clear.  What I am asking about is on the driver side (i.e. phantom powered DI box) if there is a standard for the resistors used to tap off the phantom supply.

Thanks

On the device side (what you mean by the "driver" side), there are no resistors used to tap off of the phantom supply.

You'll find a DC block to isolate the audio from the phantom voltage. The phantom voltage may be regulated down to a lower voltage or it may be used as is, depending on the circuit.

-a

By "device" or "driver" I am referring to something like a Countryman 85 or J48 that is powered off of the phantom voltage supplied on pins 2 and 3 of the XLR connector.  It is the resistors in these devices I am referring to.

thanks

 

[Rochey]

the answer was in Ians link.

6k8.

The amount of current that phantom power can provide depends on its source - your mixing desk.

but it aint much!

/R

 

[Chris_V]

On the "device" side, there is no standards.
It will depends on the voltage you want, the current drawn by the "device" and the driving capability of the "device".

 

[xtractor22]

On the "device" side, there is no standards.
It will depends on the voltage you want, the current drawn by the "device" and the driving capability of the "device".

Thank you Chris.  That was my theory based on the research I did.  I appreciate the confirmation.

 

[JohnRoberts]

This is a well explored question and people have even powered a DSP from phantom power.

The classic trade off is the more current you pull from the phantom supply (through the 6.8k resistors) the less voltage you have left to do something with.  The maximum power in the load occurs at 50% voltage drop, Loading it further increases the power dissipation in the 6.8k resistors and current available, but less voltage for the load, so max power in the load peaks around 170mW at 24V.

This is enough to drive a modest current opamp with more than enough signal swing for many applications. For higher current lower voltage applications one could use a switching supply to convert that peak power at 24V down to 5V or less, while simply loading the phantom down to 5V can deliver more than 10 mA without tricks. 

JR

 

[ruffrecords]

By "device" or "driver" I am referring to something like a Countryman 85 or J48 that is powered off of the phantom voltage supplied on pins 2 and 3 of the XLR connector.  It is the resistors in these devices I am referring to.

thanks

OK, gotcha. The short answer is there is no standard for the way a device powered by a phantom supply should create its internal supply other than it should not consume more than 10mA. Manufacturers do it in very many different ways. Some use a zener diode, some use a swtched mode power supply and others neither of these.

Cheers

Ian

 

[PRR]

> on the driver side (i.e. phantom powered DI box) if there is a standard for the resistors used to tap off the phantom supply.

No standard. Do what you want, respecting the 2mA (pre-1980) or 10mA (later) limit.

Early phantom mikes drew a fraction of an mA, and used a CT transformer winding to extract the DC without loading the audio. Say 1mA and 20 ohm transformer DC resistance: 3.4V DC drop in the 2*6.8K and transformer, 45V to the FET and capsule.

OTOH I have used two 10K resistors, where the DC loading was small, I was replacing a small battery, the device couldn't afford any added audio loading, and transformer was overkill.

Two 100 ohm will give lots of power to the device but the 100+100= 200 ohm loading (in addition to 150-2K loading of the mike-amp and 13K loading of Phantom supply) may be distressing.

Note that historic "Phantom" has been interpreted many ways. 4K7 to 10K resistors, 50V to 30V (and lower) source. It is best to be gentle, tolerant, and check extreme cases. Once in a great while you may find true P24 and P12, which are different specs, but a clever implementation will work on all of them.