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

[Zucca]

Yes, but that is with the gain control turned up to the max, something that shouldn't be happening, and would be very noisy.

The 100 - 200mV input level is a reasonable level to test the mic pre with, when the gain can be set at a more sensible level, say around 10 O'clock as it should be in the real world.
This should give a good output signal and should be a good test of the mic pre.  It's just a ballpark setting.

It doesn't make sense to test the mic pre with a tiny signal at full gain, in an attempt to mimic a possibly faulty mic.

Mic output is max 4mVRMS, it should not happen.
Focusrite with gain almost full with 7mVRMS input, it makes sense.

For now:
--> Mic is not OK, Focusrite is OK

I will confirm this hypothesis this weekend.
@Audiorepair: I hope the capsule is shot so I can upgrade it to RK-47, but I still do not think it is the case.

[Cerebus]

It doesn't make sense to test the mic pre with a tiny signal at full gain, in an attempt to mimic a possibly faulty mic.

He isn't trying to "mimic a possibly faulty mic", he's measuring (roughly) the gain of the preamp to the point where it internally measures the signal (the clipping light being a very crude voltmeter) -41 dBm/-43 dBV inout => -6 to 0 dBFS output at maximum gain.

The Focusrite specifications are, erm, rough. It specifies a maximum input at +9 dBV at minimum gain (presumably for 0dBFS output) and a gain range of 56dB, from which one can extract +9dBV - 56dB => -47dBV input for 0dbFS at maximum gain. That seems to tally roughly with what Zucca is getting.

[Audiorepair]

A pair of headphones plugged into the Focusrite would tell you in one second what is actually going on with it.

 

[Zucca]

Will do the headphone (unfortunately I do not have any large over the hear ones, hope it works too) test and the breathed on the capsule test just to confirm one more time the mic is bad and the Focusrite is good.
Probably in 6 hours or so....

[Zucca]

Tests done. Everything as expected, no surprise.

Headphones test passed no problem. At 50% gain I could get the green led turning on when I was speaking into the headphones connected to the Focusrite.

Breath on the capsule, listen to yourself in the attached file.

Sorry I should have posted a mp3 way before.... the problem is so obvious....
The output of the mic is too damn low... remember it is a NT2A, it can't be right.

[Audiorepair]

The capsule may be ok, but that isn't what I meant by a breath test.

You need to try and coat the capsule with condensation from your breath, like breathing on a window to steam it up, or on a pair of glasses to clean them.

So leave the gain up high, open your mouth wide, and gently breathe on the capsule, then listen for any noises/changes in volume.
It is normally extremely obvious when the capsule is bad using this test.


And do this on both sides of the capsule, front and back.

[Zucca]

Will do sorry for doing it wrong.

Where should I flip the directional switch for the test?

[Audiorepair]

I'm guessing English isn't your first language, there is a subtle difference between "breathing" on the capsule and "blowing" on the capsule.
Not  your fault.


Fig 8 should be the test.
Both capsules should then sound similar.

But, and this is quite surprising when you try it, if you talk into the mic wearing headphones, you will get different bass from each side as you move closer and further away from the capsule, since the capsules are wired out of phase with each other in fig 8.

So you will get different cancellations between what your ears are hearing and the bass you hear booming inside your own skull.
One side will null the bass around, say. 40cm from your mouth, the other side won't.  (can't remember the exact distance)

[Zucca]

Got it.

say. 40cm from your mouth, the other side won't.  (can't remember the exact distance)

Well... @40cm with my mic it is almost impossible to pickup anything.
This time I will leave the gain at 100% and do further tests varying the distance mic-mouth.

[Audiorepair]

Just breathe on the capsule and listen for bad noises, you don't have to speak.

[Cerebus]

I'm guessing English isn't your first language, there is a subtle difference between "breathing" on the capsule and "blowing" on the capsule.
Not  your fault.


Fig 8 should be the test.
Both capsules should then sound similar.

But, and this is quite surprising when you try it, if you talk into the mic wearing headphones, you will get different bass from each side as you move closer and further away from the capsule, since the capsules are wired out of phase with each other in fig 8.

So you will get different cancellations between what your ears are hearing and the bass you hear booming inside your own skull.
One side will null the bass around, say. 40cm from your mouth, the other side won't.  (can't remember the exact distance)

Nothing to do with phase delay, everything to do with the well known and well understood bass proximity effect which is all about near and far field effects - basically bass is in the near field and follows the inverse square law, high frequencies are already in the far field and don't follow inverse square law effects. These are the result of the source being small wavelength wise at low frequencies (and thus a point source) and increasing large in terms of wavelength as frequency increases where eventually it would become a plane source at high enough frequencies. This affects any directional mic as the mic's front to back distance is significant in near field amplitude terms, but not in far field amplitude terms.

The difference in distance from source to one side of the mike to the other is going to be about 1 - 2 cm, with the speed of sound in air being 340 metres per second, that's not going to be enough to cause a significant phase difference at bass frequencies. If we say bass is < 100 Hz, then it's at wavelengths of 3.4m or more - far too long for there to be frequency specific cancellation effects moving over distances measured in a few 10s of centimetres. For a 100Hz source wave that 1-2 centimetres represents an insignificant phase shift of 1-2º.

Phase differences only matter at much higher frequencies, which is why mic polar patterns are nice big smooth lobes at low frequency and have gnarly hill and valley polar responses at high frequencies as all sorts of desired and undecided phase cancellation effects make themselves felt. It's also why directional microphones are more directional the higher the frequency you go, and poorly directional at LF.

[Audiorepair]

This effect, which can be demonstrated if you have a fig 8 mic and some headphones, is that there is a small time delay between the bass transmitted by your voice, through your chest and head and to the ears, and the sound that comes out of your mouth, into the mic, and back to your ears via a pair of headphones.

That bass that comes from  your head can be demonstrated simply by putting your fingers in your ears and talking, it is really quite loud and bassy, but mostly we've learnt to totally ignore its presence unless we have a cold.

So with a fig 8 pattern, one capsule will be out of phase with the other, and there will be a time difference between the mic signal and the one that comes  from your head.
So both will cancel differently.
It is like putting a signal and a delayed version of that signal on 2 channels of a mixing desk, then mixing them together whilst flipping the polarity of one of them as you change the amount of delay between them.



Try it.  The front capsule has the well known bass proximity effect, the back capsule has a bass null some 30cm or whatever from the mic, they sound completely different, yet only the polarity has changed.



This only works with headphones, of course, a  recording of the mic itself will be far more normal, not what you are hearing on headphones at all.

[Audiorepair]

Ha,  this video explains this better than I could.

First he demonstrates the difference in polarity reversal alone using a mic and headphones.
Then goes on to demonstrate the effects of latency (delay).

The situation discussed in this thread is something like a 1ms delay, equating to the 30cm or so I mentioned.
The video concentrates on several mSecs of latency, but you can hear that the effect  just repeats in half cycles of a couple mSec or so.


So when you speak into the back capsule and draw it away from you, you are already dealing with a significant loss of bass compared with the front, and 1msec further delay at 30cm puts the bass loss at a null.



There will naturally be a bass proximity effect on both capsules, but the difference the polarity difference makes is quite astounding if you hear it for yourself.



https://youtu.be/APj0ZaMy4Xw


At 10.50 minutes he conducts a test of 1msec delay in and out of polarity, corresponding to the front capsule vs the back at 30cm.

[Zucca]

Here the breath test. Fig 8, front and back, 100% gain.

To me the capsule is okay.

Time to rev eng the mic and get the schematics.... 

[Audiorepair]

That is a perfect breath test.

The capsule is almost certainly ok.

[Audiorepair]

Zucca,

what is that black stuff that you said was a sharpie?

Try and clean it off and see if that helps. 
Use Isopropyl alchohol or flux remover if you have anything like that.

[Cerebus]

Woah! Unless it's something completely from left field that "black stuff' is probably potting compound of some sort and is on the high impedance part (read several G ) of the circuit and would be there to prevent surface leakage. If that's right then removing it would not be a good idea. Be certain what it is and why it is there before doing something.

[Audiorepair]

The coating could have failed, and be now shorting out the high impedance stuff.


Which would fit the symptoms.



It is not normal practice to apply any such type of coating to the PCB in large diaphragm condensers, maybe they have done so because this is a known point of failure, and have tried to fit a sticking plaster to fix a design problem.



Or, if Zucca didn't buy the mic brand new, this might be someone's attempt at a previous repair.

I've seen the insides of quite a few Rode mics, but have never seen any kind of black stuff smeared all over the PCB.

[Cerebus]

Normal practice in an electrometer (which one might use to measure the charge on a capacitor via voltage, which is exactly what a capacitor mic does) is to use guard rings, isolation slots and really, really clean PCBs. As a microphone is a comparatively dirty and uncontrolled environment compared to a laboratory instrument it might be quite a smart mitigation for contamination, like spittle entering from someone singing into it, or even breathing heavily into it, from getting onto the area of the PCB where it could cause problems. The downside is that it might itself degrade and be a harder problem to solve than a bit of PCB that just needed a proper, careful clean with the right materials.

I'd want to know what it was for sure and what condition it was in before messing with it - if it does look like potting compound when examined up close, a quick clean with dry IPA and a (low voltage, don't take the megger to it) conductance measurement to see if it's degraded would be a sensible first step.

Doug Ford, who designed a bunch of Rode mic internal amplifiers, is a mate of Dave's and might even be on here somewhere.

[Audiorepair]

Yes, that may be so, but I have never seen any such kind of conformal coating on a condenser mic PCB of any manufacture.


Someone put that there, and I bet it wasn't Doug.

[2N3055]

I was looking at the board and it seems very similar to that schematic of NT1A..

You can check if capacitive voltage multiplier works, if it oscillates and if it makes enough voltage.

You can also trace signal from XLR, back to those 47uF capacitors. Between transistor amplifier and capacitors  there are clamp diodes  in schematics. They could be damaged from voltage on XLR signal pins.
There is also 15 zener, and 15V power bus is powering amplifier. Check that too if you didn't already.
I couldn't see from the posts if you did check all of that. If you did, sorry for the obvious.

[Zucca]

Too late, I read all of this too late... I already removed the black stuff. IPA took that black matter away like butter.

Anyway let's start from the beginning. Sorry for the shitty pic quality, next time I use my Canon 5D.

Pics of the entire board front and back after desoldering the wires.




The capsule does not look clean, I do not know if I have to expect here a mirror finish...



Here the "sharpie" region after the cleaning (ups  )



A few observations:

1) Since it is a little bit too dirty, I think it is an old used mic (yes I got it from from ebay)
2) the sharpie region seems a reworked area, the solder joint reminds me some hand job not a reflowing oven.
3) in the sharpie region there is a area no filling ground plane, it could be for minimizing capacitance beetween tracks, or for minimize current leak and/or dirt contamination...
4) Few component are for me a mistery (sorry for my ignorance)
      - No. 1, 2, 3, 4 are strange capacitor? Audio stuff?
      - No 5, 6 have a black line in the middle, WTF? Nerver saw something like this before.

Anyway if current leakage is a deal why not use a guard ring? Ohhh maybe there is one... do you see it too?

After reversing rev the board I will probably clean it with my ultrasonic cleaner... do not know if the coating was necessary for the proper function.... I mean it would have been for sure not a mass production process in that way. It has to be some rework jazz.
My bet some guy fixed it and put the black stuff to protect his job.... or to tell me now "yeah in this area was me"

It will be a lot of fun this weekend digging into this interesting board.
Rode eng knows how to build a mic...

[Audiorepair]

Do not touch the gold surface of the capsule with anything, you will likely destroy it, that gold coating is just microns thick.

The breath test tells you there are no holes in the coating, which is what ends the life of a capsule.

This one is indeed pretty dirty, but it is not uncommon to see this and it still works fine, so be extremely careful with it.

[Audiorepair]

Before you go any further, I would reassemble and test the mic again, cleaning the PCB  might have fixed the problem.

Or at least you will know the problem isn't down to a dirty PCB.

[Cerebus]

Too late, I read all of this too late... I already removed the black stuff. IPA took that black matter away like butter.

Wasn't potting compound then, so probably better off than on.

4) Few component are for me a mistery (sorry for my ignorance)
      - No. 1, 2, 3, 4 are strange capacitor? Audio stuff?
      - No 5, 6 have a black line in the middle, WTF? Nerver saw something like this before.

Those just look like the alumina backside of resistors, and from where they are and how physically big they are, probably high value resistors.  High value resistors tend to be bigger to get a high breakdown voltage rating, most people don't want a 10 G resistor and then only put 10V or 100V across it, but those are huge which kind of militates against my thesis. Perhaps the only relatively cheap gigaohm resistors they could get were large ones?  If they are, there sure are a lot of them, and one tends to minimise the number needed if at all possible because compared to run of the mill 0603 and 0805 resistors they are bloody expensive. The jury is out on that one.

5 and 6 look like printed carbon resistors but I would not expect to see them here. On a ultra cheap remote control where you're already printing carbon for pads? Yes. On a ceramic hybrid circa 1980? Yes. But not here.

Sorry, not much help - lots of "looks like but ...".