Green mic pre Buzz and phantom power hum

[Rob Flinn]

You should be using a TL783 for the 48v rail (i.e the reg in the top left corner in Whoops pic), The LM317 is a bad choice, if it gets shorted it will blow.    The TL783 can handle a lot more voltage across it.

 

[Khron]

... Or an LM317HV (good for 60v input).

You should be using a TL783 for the 48v rail (i.e the reg in the top left corner in Whoops pic), The LM317 is a bad choice, if it gets shorted it will blow.    The TL783 can handle a lot more voltage across it.

 

[amaurythewarrior]

Excuse my ignorance, but how do I measure AC voltage on the +48v output? between ground and +48v as well?

First measure DC voltage on phantom +48V output,  if it's more than 48V, do not connect any condenser microphone.
then  measure AC voltage on the same output (set your voltmeter to AC,  2V or 200mV ),
check the DC phantom voltage on XLR, pins  2 and 3 when phantom is on,
check the DC and AC voltage between pins 2 and 3 on input XLR,
report back.

 

[amaurythewarrior]

I think I might need a new multimeter, mine only does voltage and resistance... and DC mA

Print the PSU schematic, follow it and measure and check each component one by one, highlight on the schematic all the components that measured correctly or are good.
Check all component values, sometimes people swap a 220R resistor for a 220K, or a 10uf cap for a 100uf cap or vice-versa, re-check that. Measure all diodes with the multimeter.

Check the PCB traces for solder joints between adjacent tracks or pads, check for colder joints also

Measure the voltages like Moamps said and also measure all the voltages that are in the layout provided by PeterC,
write your values underneath the written ones and show us all.

 

[Khron]

Would you care to show us a photo of your multimeter?

I think I might need a new multimeter, mine only does voltage and resistance... and DC mA

 

[Rob Flinn]

... Or an LM317HV (good for 60v input).

Maybe, but you could quite easily be regulating from more than 60v to 48v in a Phantom circuit.  That is why I prefer the TL783.

 

[Whoops]

You should be using a TL783 for the 48v rail (i.e the reg in the top left corner in Whoops pic), The LM317 is a bad choice, if it gets shorted it will blow.    The TL783 can handle a lot more voltage across it.

I never used a TL783 before, I will try that out in the future, thanks Rob

 

[Whoops]

I think I might need a new multimeter, mine only does voltage and resistance... and DC mA

You can measure and test a lot of things even with limited modes

 

[amaurythewarrior]

Sure, that's the one
https://www.bkprecision.com/products/discontinued/117B-analog-pocket-meter.html

Would you care to show us a photo of your multimeter?

 

[Khron]

"Measures AC voltage"

Wonder what the "50 / 200 / 500 ACV" might be for... :

Sure, that's the one
https://www.bkprecision.com/products/discontinued/117B-analog-pocket-meter.html

 

[peterc]

Check the values of the resistors next to the Phantom LM 317. They should be 220ohms and 8k2.

Check the LM317 for shorts. On the ohms range, measure from pin to pin to make sure none of them is short circuited. Also check for shorts to ground from each pin of the 317.

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.

If the output shorts to ground with the PSU on, then the 317 will fail.

 

[amaurythewarrior]

Ah, no, you misunderstood me.

My point is +48v is direct right? I understand you measure it between ground and "+", but since you're asking me to measure AC I'm a bit confused. It's probably basic knowledge, that I lack. I have already measured AC, but I'm worried about doing something wrong if I don't do it properly.

"Measures AC voltage"

Wonder what the "50 / 200 / 500 ACV" might be for... :

 

[amaurythewarrior]

Thank you peter, noted, I'll do that too.

Check the values of the resistors next to the Phantom LM 317. They should be 220ohms and 8k2.

Check the LM317 for shorts. On the ohms range, measure from pin to pin to make sure none of them is short circuited. Also check for shorts to ground from each pin of the 317.

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.

If the output shorts to ground with the PSU on, then the 317 will fail.

 

[Khron]

I'm quite sure the point of advising you to measure the AC on the phantom power rail was to see if and how much ripple voltage (ie. AC) is present. You were complaining about him there as well, weren't you?

Ah, no, you misunderstood me.

My point is +48v is direct right? I understand you measure it between ground and "+", but since you're asking me to measure AC I'm a bit confused. It's probably basic knowledge, that I lack. I have already measured AC, but I'm worried about doing something wrong if I don't do it properly.

 

[amaurythewarrior]

Ah, I see, but just to be clear, this is to be measured between the same points (ground and positive?).
Complaining about "him"? AC ripple you mean? I'll be honest with you I first read about ripple recently, and I can't say I'm clear on what it is, other than it being something you don't want.

EDIT: okay, after reading more on the topic I think I found the answer and get the point.

I'm quite sure the point of advising you to measure the AC on the phantom power rail was to see if and how much ripple voltage (ie. AC) is present. You were complaining about him there as well, weren't you?

 

[Khron]

Slight typo there, my phone corrected "hum" into "him"

https://sound-au.com/power-supplies.htm

Ah, I see, but just to be clear, this is to be measured between the same points (ground and positive?).
Complaining about "him"? AC ripple you mean? I'll be honest with you I first read about ripple recently, and I can't say I'm clear on what it is, other than it being something you don't want.

EDIT: okay, after reading more on the topic I think I found the answer and get the point.

 

[amaurythewarrior]

Ah yes, I get a LOT of hum when phantom is on
https://soundcloud.com/amaurythewarrior/humbuzz

When phantom is off, I have a bit of noise, but not hum. (you can hear both in the link)

 

[Khron]

... Hence the thought of measuring the AC ripple on the phantom power rail.

 

[amaurythewarrior]

Does it make any sense that I read something absurdly high? 140V (I tried all AC ranges, 50 was too small, and 250 and 500v return the same result?)

EDIT: And I should I haven't forgotten the other measurements, which I'll do.

... Hence the thought of measuring the AC ripple on the phantom power rail.

 

[moamps]

Excuse my ignorance, but how do I measure AC voltage on the +48v output? between ground and +48v as well?

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.