SOLVED: Help with a Rode NT2-A Mic - phantom power

[Zucca]

To the audio guys.

I just plugged my NT2A Rode to my Focusrite 2i2. I turned the 48V phantom PW and everything was fine.
I disconnected it and mounted on my mic stands and zippididuda I can not get it working again.
I can barely hear something when I crank the input volume to almost 95%. Something is wrong Jim.

Same thing if I connect the mic to input 2 (was on 1 before) on the Focusrite, so I assume it is something wrong with the mic.

The Focusrite appears to work fine.

Does disconnect a cond mic with phantom PW active kill it?

Quick check I see 47.4V on the output of the XLR between pin 1-2 and 1-3 when the mic is not connected.

When the mic is plugged in I measure  on the XLR 1-2 = 1-3 at about 34 volt.

I do not have too much experience with phantom power but is a cond mic sooo delicate??

Depending on the response I will then post more pictures and scope out the NT2-A board....

Thanks in advance for your help.



Somebody had fun with a black sharpie :-)?

[Zucca]

Interesting

This is his little brother NT1A



I did not know there were so many Hex Schmitt Triggers to crank up the voltage.... wooow

Even more interesting

[Zucca]

PS check



78.95V at 2
79.95V at 1

reference was ground at XLR... looks like the power supply is in business.

[Zucca]

Check with a infrared camera, nothing on the board get hotter than 30C in ambient T of about 25T. Looks legit to me.
Sorry I did not post a pic, but it would have been not soo interesting.

[Cerebus]

Does disconnect a cond mic with phantom PW active kill it?

In theory, possibly, but no mic manufacturer worth their salt would let a design out of the door where that happened - they would be out of business in no time.

Quick check I see 47.4V on the output of the XLR between pin 1-2 and 1-3 when the min is not connected.

When the mic is plugged is I measure  on the XLR 1-2 = 1-3 at about 34 volt.

The specs for P48 are:

• Supply voltage: 48V ±4V
• Supply current: 10mA max.
• Rated current: 7mA

The standard arrangement is to feed phantom power to hot and cold on the mic input via two 6800 resistors thusly:



So, we've got two 6800 resistors in parallel = 3400 and we're seeing a voltage drop across them of (47.4 - 34) = 13.4 volts which gives us a current draw of 4 mA.

So, the mixer appears to be meeting spec. and the microphone is pulling a reasonable current that is also within spec.

So, to a first order, it would seem that all the phantom power side of things is working to expectations.

I would say the next order of business would be to get the scope and sig gen out, inject a few mV ptp sine wave into the mixer port and check the channel gain is reasonable, and check the microphone output levels as it hits the XLR on the mixer input. (AC coupled, remember that there's +48V DC floating about.)

[Zucca]

Thank you Cerebus....

I would say the next order of business would be to get the scope and sig gen out, inject a few mV ptp sine wave into the mixer port and check the channel gain is reasonable

When I was touching to check powers with the DMM probes the XLR pin 2-3 I was hearing some bumps/noises in the speakers. This means nothing but I tend to think the mixer is good.

check the microphone output levels as it hits the XLR on the mixer input. (AC coupled, remember that there's +48V DC floating about.)

I would like to create on a bread board a phantom power circuit and then with a differential probe scope out pin 2-3. Problem is I do not know what are the signals level to expect.

NT2-A specs

Maximum Output Level   16.0mV (@ 1kHz, 1% THD into 1KΩ load)
Sensitivity   -36.0dB re 1 Volt/Pascal (16.00mV @ 94 dB SPL) +/- 2 dB @ 1kHz

so I assume I should see some 10mVpp signals...

It would be also interesting to desolder the entire board and "simulate" the capsule with some circuits and or sig gen... but I still have to do some homeworks

[Cerebus]

Yeah, a few mV rms is the expectation.

Be aware that some condenser mics, and I don't know what the Rode are like in this respect, can go to surprisingly high output levels - I've known self-powered condenser mics that you could plug into a line input and still get a useful signal. Useful if you're miking up someone who insists that their stack has to be turned up to '11' at all times. 

[mansaxel]

I concur with Cerebus.

I did build a box for testing/building P48 circuitry, which I call the Phantominator. It in essence is a transformer box with centre tap (a substitute for the matched 6k8r pair) exposed on the input side, and some bananas, and an XLR.

Try injecting a small signal, as has been suggested, and follow it backwards through the mike amp.

Looked like a lot of components, though. Is it multi-pattern?

[Cerebus]

I concur with Cerebus.

I did build a box for testing/building P48 circuitry, which I call the Phantominator [Fx: Ominous hum] . It in essence is a transformer box with centre tap (a substitute for the matched 6k8r pair) exposed on the input side, and some bananas, and an XLR.

Try injecting a small signal, as has been suggested, and follow it backwards through the mike amp.

Looked like a lot of components, though. Is it multi-pattern?

FTFY

[Audiorepair]

The mic amp input is the gate on the JFET JF1.

Try tapping the gate with a meter probe or something, if you get nasty noises out of the XLR then the mic amp is probably working, and the capsule is faulty.

It is far more common for the capsule to die than the mic electronics.

[Audiorepair]

Also, the capsule is basically 2 small capacitors, sometimes only one is in use.   So you could replace the capsule with 2 small capacitors then reassemble.

Then thump the body of the mic to see if you get any handling noise.
If you do, the electronics are probably OK and the capsule toast.


The problem with running the mic disassembled is there is no longer a faraday screen around everything, and you will get huge amounts of hum.

[mansaxel]

Yeah, a few mV rms is the expectation.

Be aware that some condenser mics, and I don't know what the Rode are like in this respect, can go to surprisingly high output levels - I've known self-powered condenser mics that you could plug into a line input and still get a useful signal. Useful if you're miking up someone who insists that their stack has to be turned up to '11' at all times. 

I did end up in that situation couple weeks ago; the double bass player had a very nice rig with a small 12" amp that's got a balanced DI out. I had to move to the line input (my mixer has separate line and mic inputs, both on XLR) to get a decent level. Sound was fantastic, very true to form. The guitar was about the same, even with the 20dB pad on the mike (a Studio Projects B1) engaged. Sadly, I did not have the line input option, since I had to have P48. Max pad applied and minimum gain and it still was bleeping loud. Band was very happy, though.

[floobydust]

I would try another phantom-powered mic with the Focusrite, to make sure it doesn't have two dead channels now. Never a good idea to hot-plug, some mixers have have inadequate input protection and a bad/wrong cable tried out everywhere can kill the front-end op-amps on multiple channels.

For the mic's I usually start by measuring the JFET's voltages source and drain, to see if the capsule shorted, got wet (spit) etc. and the JFET and following circuitry is saturated.

[Audiorepair]

Yes, there is no point in testing anything until you can prove that the equipment you are testing it with is working right now.
It is a logical process from the ground up.

"It was working yesterday" doesn't count.

[Zucca]

Like water in the deserts.....

Thanks for all the comments especially to Audiorepair!

This weekend I wanted to start to play with my new Cisco 2960X but you guys deserve a follow up.
I will check output levels of mic with a scope and report back.

[Cerebus]

"It was working yesterday" doesn't count.

You haven't worked in modern software development, have you? 

[Audiorepair]

"It was working yesterday" doesn't count.

You haven't worked in modern software development, have you? 

My works database is something I cobbled together decades ago using Delphi 6 and is still the perfect solution for me, as I designed it to be so.
It is still just about hanging on via its fingernails in Windows 10, but one more Windows version will probably kill it totally.

 

[Cerebus]

You see how far we've come in modern software development? It's taken you years to get your software to the point where it's about to break. A modern 'full stack' developer can get their software to that "hanging on by its nails" state in just one sprint of an Agile development cycle.   

[Zucca]

checking if the HP 1142A can help the measurement of mV signals with 48VDC offset.



I do not like the results, better going directly AC couple in 1Mohm scope.

I was just curious.

I can't wait tomorrow to test the mic.

[Zucca]

Ok tested the mic. about 1mVAC (V3<->V2 XLR) when I am talking 1cm away into the capsule.

It seems too low for me.

Tomorrow I will post the details.

[Cerebus]

Maximum Output Level   16.0mV (@ 1kHz, 1% THD into 1KΩ load)
Sensitivity   -36.0dB re 1 Volt/Pascal (16.00mV @ 94 dB SPL) +/- 2 dB @ 1kHz

Assuming that your 1mV is RMS (That what the quoted sensitivity will be measured in):

16 mV : 1 mV -> -24 dB. If the mic is working within spec. we'd expect that to represent 70 dB SPL @ 1m. Depending on how quietly or loudly you were speaking that could still be within spec.

A rule of thumb is that normal conversational speech is 60 dB SPL @ 1m, but you were @ 1cm which, rough calculation handwaving about directivity, would be 20dB more - 80dB SPL @ 1m. At the quoted sensitivity figure that out to be about 3mV RMS. Our uncertainty figures are high so this is rather 'no mans land'. The mic output might be low, it might be normal, it all depends on how loud you are. Were you speaking in English  or in Italian (+15db ref English)? 

Sorry Zucca, we're going to have to send you out to be calibrated. I wonder where they'll put the cal. sticker?

If you're talking 1mV peak or peak-to-peak, then I'd side on this being just enough evidence that the mic output is on the low side, but not drastically low, not 'totally broken' low. As I've already hinted, too much uncertainty sloshing about here to be definitive.

[Audiorepair]

There is a simple test for a bad capsule that always works.

Breathe gently onto the capsule through the microphone grille.  A good capsule will tolerate this without any noises.
A bad capsule will either make lots of nasty noises, or just cut the mic off altogether.

A capsule that fails this test can still be used if it isn't noisy otherwise, but probably not for vocals any more.


In my experience, these types of microphones do not have faults where the level is lower than normal, without any accompanying noise or distortion, so measuring the voltage output is not really that helpful, other than to tell you the mic is working.
There are far too many variables involved to make any kind of decision on whether the level is correct or not, as Cerebus alluded to.

Ears and a pair of headphones on a mic amp are the way to go, and a sexy hot breath onto the diaphragm is a must.

Oo-err, missus.

[floobydust]

OP did you measure both outputs to GND, to see if they are active?
Most mikes have differential outputs, but the NT1 doesn't - it's just single-ended in the schematic, compared with MXL and original Schoeps style circuit. Sometimes one-leg dies or goes in-phase.

What about taking a sig gen at many kHz and putting the hot wire next to the capsule wiring, as a gimmick capacitor to inject a sine-wave and see what comes out?
OP says it's distorted that might test the amp circuitry.

[Audiorepair]

The NT1 has a balanced output.
All mics do.


Really, a pair of ears, and a pair of headphones is all you need.

And a quick breath on the capsule.

[TimFox]

The diagram for the NT1-A at the top of the thread clearly shows a single-ended output, with the other “phase” bypassed to ground through an impedance equal to that seen by the driven phase.