Phantom power overheating

[Bomper]

Hey guys

A while back I made some API312 inspired preamps which work well and sound great. For some reason I didn't  come to making the phantom power PSU and used them only on dynamic mics. A few weeks ago I made the PSU and started using it. When connecting all the gear and starting everything up the regulator on the phantom PSU overheated and was burned.
Eventually I discovered that one of my output cables was an XLR to mono jack. So pin 3 was connected to 0 V and case ground. When removing this cable I didn't have any problems any more and everything worked fine.
Problem is I don't really understand what's going on here. Seems to me that the 48V can't really get to the outputs...
Any suggestions?

Thanks, Thomas

 

[gswan]

48V does not go to the outputs. It goes to the inputs.
It is connected via the two 6k8 resistors to the input connector J1.

 

[Bomper]

I know. That's what I don't understand. When I connect the output of the preamp in an unbalanced manner the 48 V PSU output gets shorted... 

Thomas

 

[gswan]

Can you measure 48V on the output connector? If not, then shorting it won't short the 48V supply.
When you short the output as you have described, what happens to the 48V rail (and the other rails)? Perform some measurements to see what is going on.

 

[ruffrecords]

Normally, shorting  the phantom power at the XLR does no harm because of the 6K8 series resistors. Even if both sides are shorted the current drain will only be 14mA which should not really cause your regulator to burn out. I would suspect the 6K8 resistors - you didn't fit 680 ohms by accident did you?

Cheers

Ian

 

[Bomper]

All components are correct, I double checked. The units work perfectly with phantom power, I recorded with them just last night. It's only when I connect the output of the preamp to the input of my soundcard (motu 24io) with an XLR to mono-jack (pin 3 connected to pin 1) that I have this problem. If I use balanced cabling all the way I don't have this problem. That's what's so strange.
I'll have to further investigate tonight (if I find the time...)

Regards, Thomas

 

[ruffrecords]

So if I understand this right, the phantom reg burns when the preamp OUTPUT is connected  with an XLR to mono jack (pin3 strapped to pin 1). Does it do this even if you do not plug the jack into anything? SOunds a bit like the output does not like having pin 3 connected to ground.

Cheers

Ian