Phantom power voltage drop???

[bluebird]

Bare with me, there is a valid question at the end of this.

Using a typical regulated 48-52v power source, and 470 ohm ---> filter cap -----> 2X 6.8K resistors. There are different voltage drops for different mics. Some pretty dramatic. I noticed this when testing out a mic pre I was working on and two different mics had very different current draws. Measured voltage at the XLR for an old RODE NT1 (Schopes circuit) and a KM86.
The NT1 produced an almost 15V drop across all the resistors. 2X 6.8k = 3.4K+470 = roughly 4K. That's 3.7mA. The KM86 only has about 3 volts drop. Less than 1mA. The KM86 has a transformer out for the impedance converter and the NT1 is solid state. So that's that.

But before I installed my own phantom circuit, I was using a "Stewart" phantom power box that ran on a 24VAC wall wart. Measuring 36V with the NT1 on the XLR (with my regulated circuit)  bugged me a bit so I checked the voltage with the Stewart box and the drop was only a few volts.

Wow, so how different is the sound of the NT1 running on 36 volts or 47 volts? I mean this is capsule polarizing voltage...

I checked the Stewart box circuit as best I could, I could only see the circuit board from the bottom without un-soldering all the XLR's. It is a compact little box. The biggest difference I could see is that it only had 1K resistors to feed the XLR's. Hence the smaller voltage drop.

MY QUESTION:
If your phantom power supply can handle the current draw of 500ohms (shorted phantom)  almost 100mA, Should you use 2X 1K resistors because the phantom power will be more consistent and higher for all microphones?

 

[gyraf]

Aah, but I think  you misunderstand the idea in P48. The voltage is intended to drop depending on your current consumption - all the way down to zero volts at 14.1mA (i.e. all shorted, 48V across 3.4KOhms.)

The way we deal with this is that current-hungry microphones uses internal step-up-voltage-conversion, and those that do not, e.g. AKG C414-P48, Neumann KM84 etc, consumes very little power in order to keep phantom voltage high enough for direct polarisation..

Jakob E.

 

[ruffrecords]

The standard is 48V plus or minus 4V via 6K8 resistors in each leg. There really is no reason to change this. Mics like the NT1 do not use the phantom power directly to polarise the capsule - they have an internal dd/dc converter for that (as does the  Neumann TLM103).  Phantom is used mainly to power the interface electronics.

Cheers

Ian

 

[moamps]

If your phantom power supply can handle the current draw of 500ohms (shorted phantom)  almost 100mA, Should you use 2X 1K resistors because the phantom power will be more consistent and higher for all microphones?

You can kill some microphones that way. Also, the load of mics can be too high.

 

[mutterd]

Funny - I just came across this recently - here is phantom power as explained by Doug Ford - one of the former head  designers at Rode.

He explains the Schoeps circuit here.

https://www.eevblog.com/2014/05/15/eevblog-616-how-microphone-phantom-powering-works/

 

[ruffrecords]

Funny - I just came across this recently - here is phantom power as explained by Doug Ford - one of the former head  designers at Rode.

He explains the Schoeps circuit here.

https://www.eevblog.com/2014/05/15/eevblog-616-how-microphone-phantom-powering-works/

I have seen that before. Very good video. Highly recommended.

Cheers

Ian

 

[mutterd]

Agreed - I wound up watching all the videos with Doug, there is over an hours worth of them - all worth the time if you have it.

I have seen that before. Very good video. Highly recommended.

Cheers

Ian

 

[JohnRoberts]

Bare with me, there is a valid question at the end of this.

No but I will bear with you.... ;D

Using a typical regulated 48-52v power source, and 470 ohm ---> filter cap -----> 2X 6.8K resistors. There are different voltage drops for different mics. Some pretty dramatic. I noticed this when testing out a mic pre I was working on and two different mics had very different current draws. Measured voltage at the XLR for an old RODE NT1 (Schopes circuit) and a KM86.
The NT1 produced an almost 15V drop across all the resistors. 2X 6.8k = 3.4K+470 = roughly 4K. That's 3.7mA. The KM86 only has about 3 volts drop. Less than 1mA. The KM86 has a transformer out for the impedance converter and the NT1 is solid state. So that's that.

But before I installed my own phantom circuit, I was using a "Stewart" phantom power box that ran on a 24VAC wall wart. Measuring 36V with the NT1 on the XLR (with my regulated circuit)  bugged me a bit so I checked the voltage with the Stewart box and the drop was only a few volts.

Wow, so how different is the sound of the NT1 running on 36 volts or 47 volts? I mean this is capsule polarizing voltage...

I checked the Stewart box circuit as best I could, I could only see the circuit board from the bottom without un-soldering all the XLR's. It is a compact little box. The biggest difference I could see is that it only had 1K resistors to feed the XLR's. Hence the smaller voltage drop.

MY QUESTION:
If your phantom power supply can handle the current draw of 500ohms (shorted phantom)  almost 100mA, Should you use 2X 1K resistors because the phantom power will be more consistent and higher for all microphones?

The phantom voltage only matters if there isn't enough (or perhaps too much).

JR

 

[bluebird]

Aah, but I think  you misunderstand the idea in P48. The voltage is intended to drop depending on your current consumption - all the way down to zero volts at 14.1mA (i.e. all shorted, 48V across 3.4KOhms.)

The way we deal with this is that current-hungry microphones uses internal step-up-voltage-conversion, and those that do not, e.g. AKG C414-P48, Neumann KM84 etc, consumes very little power in order to keep phantom voltage high enough for direct polarisation..

Jakob E.

Thanks gentlemen, I get it.
At first looking at the NT1 circuit board it doesn't look like enough parts to have a step up converter. No coil or string of diodes. But I found this Schoeps schematic and I see the step up circuitry to 60V at the bottom of the circuit. Its looking like an oscillating transistor? Does this circuit actually regulate to 60V, or is just a chopper and voltage doubler for any incoming voltage?

 

[ruffrecords]

You may be right about the NT1. I was thinking of the NT1A which uses some CMOS inverters as an oscillator for its capsule supply.

Cheers

Ian

 

[bluebird]

It definitely doesn't have a cmos chip in there but I think you are still right, it pretty much has the same amount of parts as in the Schoeps schematic. I'm just wondering if that little circuit can actually regulate the incoming voltage or if it just doubles it. Looks like you can control the oscillation with the P2.