THAT 1512 mic pre - phantom power issue

[ripgtr]

I built a mic pre based on the data sheet of the THAT 1512 -
http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf
I followed Figuare 5, with the phantom power.

To power it, I used something like this -
http://sound.westhost.com/project05.htm
I wanted to have the transformer out of the box and had a 16V ac transformer.

I hook up the pre amp power, works fine. Everything is good, sounds good.

I then added a phantom power curcuit. I used a voltage tripler, 3 diodes and 3 caps. I ran it into an LM317, hooked it up just like the Fairchild data sheet. Pretty standard lm317. I check all the voltages, everything reads good. I then hook up the power going to the 6k resistors, and that is when I have a problem.


First, there is a buzz, and then the sound from the hooked up mic goes out. I have tried with both a sm58 and a condensor, same thing. I undo the power and after a couple seconds the volume comes back up. It almost seems like the 22u phantom filtering caps are loading up. I checked the polarity and it matches the THAT datasheet.

I have checked the wiring, at least a half dozen times. I opened up a box I have that has a center tap transformer, used some alligator jumper cables to power it, thinking it may be the wallwart type ps, but no change.

So, I am missing something here. I have looked at a lot of schematics, this seems pretty standard..
sorry I don't have a schematic to put up. I could draw one up by hand and scan, I suppose, but it is not much different than this -
http://www.forsselltech.com/downloads/schematics/MikePrePS1.PDF

So, what am I missing?

 

[JohnRoberts]

Sounds like maybe your inputs are not symmetrical...  Pins 2 and 3 should look identical to mics and outside world...

If mic is one legged it won't cancel noise on phantom supply and could make mic unhappy if phantom power runs through mic.

Check that 6.8K Resistors to phantom are only DC path at pins 2 and 3 of XLR input.

JR

 

[pucho812]

make sure the 6.8K are closely matched

 

[ripgtr]

Thanks guys.

I did match the 6k8 resistors. Both were 6.78 if I remember - I will check with the meter in the morning. I got 8 of them and tested until I found 2 that were really close.

On being symmetrical, both legs should be, but I may have something not right in there. I need to go over it bit by bit. I did it on a perf board, so I could have made a mistake somewhere. I will check for any other DC at the mic. Maybe I have some DC slipping in from a sloppy solder joint from the diodes or something.

Ok, thanks, that gives me some things to check over the weekend.

 

[Tekay]

I had a similare problem years ago and I just ended up feeding the voltage trippler from the other 16V AC, after that it worked, never understod why ???

Have you checked the voltage feeding the LM317T? Termal shutdown? I have a 180 ohm / 2w between the trippler and regulator in two of our products.