Phantom power voltage

[davoux]

Thank you so much for your help. Here is what I did while I was waiting for response. I went through all my old resistors and found a 220ohm 2w resistor. So, Replace the 2.2K with it. I figured it would disipate some heat before the LM317 to help. When I did this, the volt drop was 4V across it, from 77 to 73v. Then when I set the voltage from the LM317 at 50V, I end with 49v going to the mic input section. Approx. 1v is dropped over the series resistor and I end up with 48v that will go to each 6k81 resistor. I let it run for about 10 min. and then felt the LM317. It is only slightly warm about the same as the LED. Using your equations, I calculate the LM317 is dissipating about 1.15w. So I will get a cheap heat sink and see if it holds up. Looks good for now.

Thanks so much, John

 

[AnalogPackrat]

Sounds like you've got it working, John. Good job. You're welcome for the help. I'm curious what project this is for, though. Be sure to post some pics when it's done.

Cheers,
A P

 

[davoux]

I'm doing a Pultec MB-1 dual channel mic preamp which was reverse engineered by Rafafreddy and the Jon Fleig did a great job of laying out. I've done the boards and wired the boards to a strip where I have all the tubes. However, I had ordered an enclosure 6 weeks earlier from Sescom that never, never came. So about a week ago, I ordered an enclosure from Par-Metal which I hoped would come today but the blizzard here and thoughout the upper midwest stopped that. Hopefully it comes Monday.

Until the enclosure comes, I can't finish a lot of the connections. I've probably got about annother 4 hours left to complete. It should be a very clean and slick setup pretty much due to Jon Fleig's design. This is my first build so we will see. I've had some experience troubleshooting guitar amps to prepare for this. Unfortunately, my Son called about an hour ago and said his Hiwatt amp just blew the main fuse when he switched on the standby and it smells of smoke. So, it looks like I'll get a little more experience.

Thanks so much again for your help. John

 

[Brian Roth]

Just my 2-cents-worth re. Phantom supplies....

The worst case situation is if a bad mic cable accidentally shorts XLR pins 1/2/3 all together. In that case, you end up with 3k4 Ohms (the 6k8 resistors will be in parallel) across 48V, which will draw approx. 14.1 mA.

Under this fault condition, each of the 6k8 resistors will be dissipating approx. 0.34 Watts, which will begin to scorch the typical 1/4 Watt resistors commonly used in this application.

Bri

 

[philkerg]

Sorry to come in late ....

Wouldn't it be the same result with only one short line ? 1-2 or 1-3
That means 48/6810 = 7mA & 48x0,007 = 0,336W in the resistor connected to the faulty point ....

Did I miss something ?

Regards,
Phil