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oswcall 7th December 2012 10:32 PM

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.

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.


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