Phantom Power Neumann pv76

[jakobz]

Hi there
I am about to install a couple of Neumann pv76 in a rack. I have a PSU for the 24v supply, but need to have a 48v supply for phantom power. The PSU is at around 38vdc before regulation. What would be the best approach here: Should i get a extra transformer for the 48v supply or should i look into some voltage doubling circuit?

 

[Khron]

Probably easiest to tap off a voltage doubler from the raw AC. Especially since you'd only need maybe 20mA, 30 absolute max.

 

[jakobz]

Thanks, i made it up on a breadboard and it seems to do the trick.

 

[jakobz]

This is causing me some 50hz hum issues. I have tapped the AC from the transformer, and made a voltage doubler and regulated to 48v with a lm317, at first it seems to work just fine but i am getting a 50hz hum. I have connected ground from the 24v psu to the ground on the 48v circuit, and it seems to be part of the problem. The Hum disappears when i disconnect the ground from the 48v part. But i suppose the 24v and 48v do need to have the same
reference?

 

[radardoug]

The way you have done that doubler is probably making a ground loop. You would be better to use another transformer.

 

[gyraf]

Yes, your PV76 and its phantom power need common reference

your problem is because the ground on one circuit is not compatible with ground on the other (probably D2 that shunts one end of trafo to gnd) - if you draw both circuits you'll clearly see why

you could possibly use two electrolytics to isolate the transformer and the original psu from the the input of your additional circuit, making it AC-coupled. Perhaps two 1000uF/35V or so?

/Jakob E.

 

[mjrippe]

Perhaps like this?

 

[jakobz]

Perhaps like this?

This makes sense, i will try that out. Thanks