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Negative value on phantom power supply ok?
Old 18th September 2012
  #1
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celticrogues's Avatar
 
🎧 15 years
Negative value on phantom power supply ok?

Hi all- hopefully this isn't too stupid of a question:

I've been trying to use a Neumann TLM170 with my Millennia preamp, but whenever I turn phantom on all I get is a loud, rhythmic popping noise. I thought it sounded like it could be a phantom power issue.

When I hooked up a multimeter to the output, the phantom power was reading -49 V

The TLM170 works fine on other preamps like my BLA Auteur, and when I hook the multimeter up to that it reads +54 V.

Could the negative reading on the Millennia be what's causing this?

Other mics like a Sennheiser 416 and Sanken Cs3e work fine on the Millennia.

Thanks!
-Mike
Old 18th September 2012
  #2
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celticrogues's Avatar
 
🎧 15 years
Alright heres some more info after farther testing...

I assume the negative value is ok since the built in pres on my Yamaha 01V96 also show a negative phantom power value, and the TLM170 works fine in those.

I did discover that the TLM170 works fine in the Millennia if the -10dB pad is ON.

So if the mic's pad is off (no attenuation), I get popping and no signal, if the pad is on, the mic seems fine.

The pad only seems to affect things in the Millennia though, not other pres.

Do any of you wonderful people have a thought about what the issue could be here?

Thanks!
-Mike
Old 18th September 2012
  #3
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Richard Crowley's Avatar
 
🎧 10 years
NO, phantom power is POSITIVE relative to ground. It seems inconceivable that ANY commercial equipment would be designed or manufactured that puts out NEGATIVE phantom voltage. Sounds like you have your meter connected backwards. Negative voltage would FRY most microphones.
Old 18th September 2012
  #4
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Lotus 7's Avatar
As rcrowley said, it's inconceivable that a commercially produced mic input could have its phantom power polarity reversed, especially a Millennia.

It's likely you have your meter leads reversed or are mis-reading the XLR connector pins (1) and (3). Both pins (2) and (3) should be positive in respect to pin (1), and the voltage should read between 44 and 52 volts (on a properly calibrated meter) with no mic connected. See attached drawing.

Why your mic doesn't work correctly with the attenuator off is another issue.

Don't have a TLM170 schematic handy, but most LDC multi-pattern mics use a capacitive voltage divider at the input to attenuate the mic signal before the impedance converter circuit. They usually switch-in a small shunt capacitor to load the capsule and lower the signal level altthough other methods are also used. It's difficult to imagine how switching in the cap would make it work and removing it would cause problems. The Millennia preamps do have frequency response that is extended to very low frequencies, plus they are fully DC coupled except at the very input, so it may be a case of some very low frequency noise or oscillation coming from the TLM170 that is driving the Millennia into saturation that is ignored by your other mic pres. However, without a schematic of the TLM, all of the above is just a guess.

Have you tried turning the gain on the Millennia to minimum?
Attached Thumbnails
Negative value on phantom power supply ok?-xlr-phantom-power.jpg  
Old 25th September 2012
  #5
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celticrogues's Avatar
 
🎧 15 years
Thanks guys... I definitely did have the meter leads reversed. All readings are positive now...

So any thoughts on what could be causing this? The problem is there even with the gain at minimum and isn't affected by the gain setting at all. To add more mystery to this I have two other Millennia HV3C preamps at work, and my TLM170 has no problems with either of them - pad on or off. I had thought this might mean there is a problem with the AC power at home? Does that make sense? I have it running through a Monster power conditioner which is reading between 124 and 125 volts, and telling me the grounding is ok. I had this same problem on location too although I don't remember what the power conditioner said then.

Millennia was kind enough to replace the preamp when I called them about this issue and the replacement had the exact same issue, so the AC power is the only thing I can think of... Any thoughts?

-Mike
Old 25th September 2012
  #6
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JohnRoberts's Avatar
 
🎧 10 years
Check mic cable.. intermittent pin 1 connection can wreak havoc with phantom power.

JR
Old 26th September 2012
  #7
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Lotus 7's Avatar
Extremely unlikely that it has anything to do with the AC power line. Millennia preamps have highly isolated power supplies. As JohnRoberts said, it's possibly a bad mic cable or connector. Could even be the connector on the mic itself.

The really strange issue is that you say it works OK when the pad is on. That likely indicates a "front-end" problem in the mic since the pad usually adds attenuation by reducing the signal directly at the mic impedance converter input. Possibly could be a broken solder joint on the mic PC board and moving the pad switch affects that.

To check that the connections are all OK and that phantom power is properly getting where it should be going, you can do the following:

1. With nothing connected to the Millennia preamp, measure and write down the DC voltage between pin (2) and pin (1) on the Millennia preamp input female XLR connector. Pin (2) should be positive

2. Remove the outer shell from the female XLR connector at the mic end of your mic cable.

3. Connect the (open) female XLR to your TLM-170 and the male end to the Millennia preamp.

4. Positively identify the pins by number inside the open shell of the connector that is plugged into the mic.

5. Turn on phantom power. Be careful not to short anything.

6. Connect the negative lead of your DVM to pin (1) of the female XLR that is on the mic.

7. Connect the positive lead of the DVM to pin (2) of the female XLR that is on the mic. Record the voltage reading.

8. Connect the positive lead of the DVM to pin (3) of the female XLR that is on the mic. Record the voltage reading.

9. Subtract the voltage reading in step (7) from the voltage reading in step (1) and record that value.

10. Subtract the voltage reading in step (8) from the voltage reading in step (1) and record that value.

The voltage reading in step one should be between 44 and 52 volts. 52 volts is the maximum rated operating voltage for a TLM-170 mic.

The voltage difference recorded in steps (9) and (10) should be 10.2 volts.

In other words, the voltage at pins (2) and (3) should be 10.2 volts lower than the open-circuit voltage measured in step (1) or something like 34 to 42 volts. The readings in steps (7) and (8) should be almost identical.

Repeat all the above with the signal pad switch activated. If the voltage differences are correct (10.2 volts) the connections are solid, the phantom power voltage and the mic current draw are correct and you probably have a mic with an internal circuit problem.

The difference between your two locations may be related to humidity differences. A capacitor mic capsule that's going bad can be triggered into "breakdown" by small capsule polarization voltage differences as well as humidity differences. Even placing a mic in a hot car can affect it by drying it out (for a while). I've seen mics that appear to generally work OK but which can generate a loud breakdown "pop" every few hours. A real PITA to isolate.

Report back what you find.
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