Green mic pre Buzz and phantom power hum

[amaurythewarrior]

No problem, but I did measure that and get 140V. Surely I'm doing something wrong?

I apologize for not being clearer, just put the instrument in the AC measurement position and watch what you will get at the DC output + 48V. The AC voltage that is actually hum is or can be superimposed on a DC phantom + 48V voltage.

 

[moamps]

No problem, but I did measure that and get 140V. Surely I'm doing something wrong?

No doubt. What's the secondary voltage (AC) of the power transformer you are using?

 

[amaurythewarrior]

2x15v secondaries.
https://www.mouser.fr/datasheet/2/410/VPT30_830-781532.pdf

I'm measuring with my black lead on the gnd and red on the +48 rail, multimeter on ACV (tried 500, 250, and 50) at the output of the PSU

No doubt. What's the secondary voltage (AC) of the power transformer you are using?

 

[radardoug]

You need to study and learn how to use a meter. At this rate I wouldn't be surprised if you kill yourself.

 

[moamps]

2x15v secondaries. I'm measuring with my black lead on the gnd and red on the +48 rail, multimeter on ACV (tried 500, 250, and 50) at the output of the PSU

Ok, maybe your simple measuring instrument is a little confused and doesn't display the values correctly. That part of the circuit is the voltage multiplier which multiplies the input voltage to a value dependent on the number of degrees of the multiplier. The optimal value of the multiplied voltage at the input to the regulator IC is about 60V DC. You need to measure the input and output voltage of the regulator IC, and if they are not 60V and 48V DC, then that IC is probably faulty. In any case, TL 783 should be used in that position instead of 317. The hum that appears is most likely caused by a malfunction of that IC, but the problem can also be in the incorrectly made grounding. You need to check if pin 1 of the XLR input connector is connected to the GND output of the power supply. The resistance between these two points should be 0 ohms.

 

[amaurythewarrior]

I think you've put your finger on something. The LM317 in and out voltage are identical. I measured the other regulators to make sure I was doing it right, and get correct measurements (going from 21 to 15, -21 to -15)

I might have a spare LM317 which I'll try (I know I'll put the other one next time I order) and remove the 4th diode as peter said. But that seems to be the source for the incorrect 55v.
The grounding between pin1 on input XLR and board is correct.

Ok, maybe your simple measuring instrument is a little confused and doesn't display the values correctly. That part of the circuit is the voltage multiplier which multiplies the input voltage to a value dependent on the number of degrees of the multiplier. The optimal value of the multiplied voltage at the input to the regulator IC is about 60V DC. You need to measure the input and output voltage of the regulator IC, and if they are not 60V and 48V DC, then that IC is probably faulty. In any case, TL 783 should be used in that position instead of 317. The hum that appears is most likely caused by a malfunction of that IC, but the problem can also be in the incorrectly made grounding. You need to check if pin 1 of the XLR input connector is connected to the GND output of the power supply. The resistance between these two points should be 0 ohms.

 

[Rob Flinn]

As I said before use a tl783 instead of an lm317.  This could well be why you have a blown lm 317

 

[amaurythewarrior]

Even if just for a short period of time?

As I said before use a tl783 instead of an lm317.  This could well be why you have a blown lm 317

 

[Rob Flinn]

Even if just for a short period of time?

It will be ok for test purposes.  But if it blows again you will potentially be applying the unregulated phantom voltage to your mic.  But it's up to you.

 

[Khron]

That may well have already happened several times, so...  :-\

It will be ok for test purposes.  But if it blows again you will potentially be applying the unregulated phantom voltage to your mic.  But it's up to you.

 

[amaurythewarrior]

So that's not the first time, because I've tried it with condensers before... I suppose they're protected to some extent? But that indeed doesn't sound like a good idea, at least if I do it, I'll do it with cheap t.bone and not the brauner or AKG...

But I'm looking at the spec sheet and it says max output is 37, max input 40...
So why choose this regulator in the first place? How does it regulate the voltage when it's higher than what it is supposed to receive?

It will be ok for test purposes.  But if it blows again you will potentially be applying the unregulated phantom voltage to your mic.  But it's up to you.

 

[moamps]

...But I'm looking at the spec sheet and it says max output is 37, max input 40...
So why choose this regulator in the first place? How does it regulate the voltage when it's higher than what it is supposed to receive?

LM317 can be used with certain precautions, those values you mentioned apply to differential voltages.
I do not recommend this IC for phantom powering at all, but if you are going to try it again, add two more diodes as shown below to protect it.

 

[Whoops]

But I'm looking at the spec sheet and it says max output is 37, max input 40...

maximum output is 36v difference between input and output.
PeterC already explained that

LM317 works very well for this application and it's commonly used

The LM 317 is good about 36v difference between input and output, so if the tripler (input to the 317) is at 63v, then the 317 will be OK with 48v at the output.

 

[Rob Flinn]

maximum output is 36v difference between input and output.
PeterC already explained that

LM317 works very well for this application and it's commonly used

Sorry, disagee with you there.  if someone is using the mic pre wired into a patchbay it is quite easy to short the mic pre input if someone plugs in/out of the patchbay while the phantom is on.  In the case you talk about that would put 63v across the 317. 

 

[Rob Flinn]

LM317 can be used with certain precautions, those values you mentioned apply to differential voltages.
I do not recommend this IC for phantom powering at all, but if you are going to try it again, add two more diodes as shown below to protect it.

This is very good advice.  Add the 2 diodes.  If you don't have them there already that could be another reason for a blown LM317. 

 

[amaurythewarrior]

Well, I replaced the LM317 and removed the 4th diode (wire instead of it, not simply remove - just in case that wasn't clear).
I still get 55v in, and out. The resistors around it seem good... Is there any possible explanation to this?

(over the diodes I get slightly lower voltages than on the PCB doc, so I suppose over 3 diodes it's not surprising that it only brings it to 55 but should that be a concern?)

 

[Rob Flinn]

Well, I replaced the LM317 and removed the 4th diode (wire instead of it, not simply remove - just in case that wasn't clear).
I still get 55v in, and out. The resistors around it seem good... Is there any possible explanation to this?

(over the diodes I get slightly lower voltages than on the PCB doc, so I suppose over 3 diodes it's not surprising that it only brings it to 55 but should that be a concern?)

Maybe post a close up pic of your pcb board.

 

[amaurythewarrior]

Here it is
https://imgur.com/a/HOTTqCr

I'm secretely hoping you"ll tell me there's someone very basic and obvious I missed, but I've checked it a few times already...
I'm getting close to just take the whole thing apart and rebuild it.

Maybe post a close up pic of your pcb board.

 

[moamps]

Well, I replaced the LM317 and removed the 4th diode (wire instead of it, not simply remove - just in case that wasn't clear).

Put that diode back on, it's protecting diode, but for sure remove that wire, you shorted in and out right now, so it's normal that the voltages are equal.  Why you didn't placed protecting diodes I suggested, BTW?

 

[amaurythewarrior]

I don't think I have any spare 1n4004 at the moment (but I'll double check)
But I removed the 4th because peterc said it could cause the voltage to be too high, so now I'm really confused.

Put that diode back on, it's protecting diode, but for sure remove that wire, you shorted in and out right now, so it's normal that the voltages are equal.  Why you didn't placed protecting diodes I suggested, BTW?