48v Phantom Power Supply Questions

oswcall · 7th December 2012, 10:32 PM

Hi friends, I have a Rolls PB23 Phantom Power Adapter, there are 2 versions, the second is made since 2004, I think. I have the old Version 1.

Here The Schematics:

Version 1: http://i.imgur.com/LdOcp.png

Version 2: http://i.imgur.com/nrabx.png

Questions:

1. Since I have the Version 1, I'd like to know which is the function of R8 and R9 (in Version 2, inside the red circle), because these resistors are absent in Version 1.

2. In Version 1, when I Select 12v, then I measure the voltage in the XLR jack, and is 12v. I guess is 12v because comes directly from the 7812. But when I Select 48v, the voltage measured in the XLR jack is only 40v... it's possible to change a resistor to get 48v? or at least close? Which one?

I hope you can understand me. Thank you in advance and sorry for my english.

Oswaldo.

JMFahey · 7th December 2012, 11:03 PM

1) those resistors are "anti-pop" by providing a DC path to ground for C3 and C4.
You can easily add them.
2) the multiplier design in theory should reach 48V but real world parts have normal losses and rob a little from that.
By mental calculation it should have around 44/45V so having around 40 is still within parameters.
Don't worry, it will work perfectly

To have exact 48V it should have 1 extra multiplier stage (reaching roughly 50/52V) and then regulate it down to 48V.
In practice it's not worth it.

Simon B · 7th December 2012, 11:10 PM

Hi Oswaldo,

R8 and R9 in the second version are there to provide a path to take the DC leakage currents coming through the two electrolytic capacitors, C3 and C4 to ground. Electroytic caps are always at least a little bit leaky, and if there's a difference in how leaky the two caps in the pair are, that would put a dc voltage into the mixer, which might cause problems. They would also help stop a dc level remaining on the outputs if feeding into a mixer with capacitor input coupling and no dc discharge path.

If you want to add these to your unit, just for peace of mind - standard 0.6W, metal film 100k types should be fine - check resistance with a meter before connecting.

The voltage multiplier circuit shown for version 1 will never reach 48v when run from a 12v supply. It's nominally a 4x multiplier, but a small voltage is dropped across each of the diodes in the chain. Because 48v phantom power is quite 'saggy', ie, when you plug an actual mic into it, the voltage drops a lot anyway (because each leg is fed through a 6k8 resistor), I wouldn't worry about it unless you have to - I don't think it's going to cause you problems.

Particularly, leave R3 and R4 alone, they should be a closely matched pair.

oswcall · 16th December 2012, 12:53 AM

Thank you guys, i'll put those resistors. I'm thinking to replace the 7812 with a LM317 (with its resistors) to rise the initial 12v to 13v or so, in order to get 48v in the XLR pins. Do you think the CD4049 could manage this ?

Oswaldo.

GeorgK · 16th December 2012, 03:53 PM

The 4049 is designed so handle 18 Volts, so this should be no problem. If you want to experiment just add one or two serial forward biased Si Diodes to the ground lead of the 7812, each rising its output voltage by about 0,6 Volts. But I doubt it makes too much difference for the microphone. In a resistor fed configuration like this the nominal 48 Volts are only true with no load connected and drop as the mic's preamp draws current. The mics can handle this.

7th December 2012, 10:32 PM	#1
oswcall diyAudio Member Join Date: Aug 2011	48v Phantom Power Supply Questions Hi friends, I have a Rolls PB23 Phantom Power Adapter, there are 2 versions, the second is made since 2004, I think. I have the old Version 1. Here The Schematics: Version 1: http://i.imgur.com/LdOcp.png Version 2: http://i.imgur.com/nrabx.png Questions: 1. Since I have the Version 1, I'd like to know which is the function of R8 and R9 (in Version 2, inside the red circle), because these resistors are absent in Version 1. 2. In Version 1, when I Select 12v, then I measure the voltage in the XLR jack, and is 12v. I guess is 12v because comes directly from the 7812. But when I Select 48v, the voltage measured in the XLR jack is only 40v... it's possible to change a resistor to get 48v? or at least close? Which one? I hope you can understand me. Thank you in advance and sorry for my english. Oswaldo.

7th December 2012, 11:10 PM	#3
Simon B diyAudio Member Join Date: Dec 2011 Location: North-East England	Hi Oswaldo, R8 and R9 in the second version are there to provide a path to take the DC leakage currents coming through the two electrolytic capacitors, C3 and C4 to ground. Electroytic caps are always at least a little bit leaky, and if there's a difference in how leaky the two caps in the pair are, that would put a dc voltage into the mixer, which might cause problems. They would also help stop a dc level remaining on the outputs if feeding into a mixer with capacitor input coupling and no dc discharge path. If you want to add these to your unit, just for peace of mind - standard 0.6W, metal film 100k types should be fine - check resistance with a meter before connecting. The voltage multiplier circuit shown for version 1 will never reach 48v when run from a 12v supply. It's nominally a 4x multiplier, but a small voltage is dropped across each of the diodes in the chain. Because 48v phantom power is quite 'saggy', ie, when you plug an actual mic into it, the voltage drops a lot anyway (because each leg is fed through a 6k8 resistor), I wouldn't worry about it unless you have to - I don't think it's going to cause you problems. Particularly, leave R3 and R4 alone, they should be a closely matched pair. __________________ Engineer: One who can do for 10 shillings what any fool can do for 10 pounds.

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7th December 2012, 11:03 PM	#2
JMFahey diyAudio Member Join Date: Mar 2009 Location: Buenos Aires - Argentina	1) those resistors are "anti-pop" by providing a DC path to ground for C3 and C4. You can easily add them. 2) the multiplier design in theory should reach 48V but real world parts have normal losses and rob a little from that. By mental calculation it should have around 44/45V so having around 40 is still within parameters. Don't worry, it will work perfectly To have exact 48V it should have 1 extra multiplier stage (reaching roughly 50/52V) and then regulate it down to 48V. In practice it's not worth it.

16th December 2012, 12:53 AM	#4
oswcall diyAudio Member Join Date: Aug 2011	Thank you guys, i'll put those resistors. I'm thinking to replace the 7812 with a LM317 (with its resistors) to rise the initial 12v to 13v or so, in order to get 48v in the XLR pins. Do you think the CD4049 could manage this ? Oswaldo.

16th December 2012, 03:53 PM	#5
GeorgK diyAudio Member Join Date: Jan 2005 Location: Vienna	The 4049 is designed so handle 18 Volts, so this should be no problem. If you want to experiment just add one or two serial forward biased Si Diodes to the ground lead of the 7812, each rising its output voltage by about 0,6 Volts. But I doubt it makes too much difference for the microphone. In a resistor fed configuration like this the nominal 48 Volts are only true with no load connected and drop as the mic's preamp draws current. The mics can handle this.