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.

[Audiorepair]

If this mic were unbalanced, you would see pin 3 of the XLR connected to ground.

No microphone sold today would EVER be unbalanced, particularly a large diaphragm condenser as discussed here.

[TimFox]

See the drawing in reply 1.  Pin 3 goes to ground through 50 ohms and 47 uF, while pin 2 is driven through 50 ohms and 47 uF from a single-ended amplifier output.  A balanced output would require a push-pull amplifier output or an output transformer.  That drawing purports to be of an NT1-A.
As wired, it can drive a balanced input.

[Audiorepair]

The XLR output is connected to the mic pre by 2 electrolytic capacitors.

Pins 2 and 3 are floating and balanced with regards to signal output.

[floobydust]

The NT1-A (not OP's NT2-A) has a single-ended output, there is one push-pull output stage feeding pin 2+ while pin 3- is near AC ground, nothing in the mic is driving it with signal.
Some mics are cheap, others are very small with no room for a lot of electronics and compromises are made.

My point is when looking at differential mic output, best to treat each polarity individually. I think OP has a diff. scope probe in the pics and at 1mV would not be useful. One open cap can make it appear as low amplitude but the amplifier has full swing. Or a mic with only one hot output looks broken but it's the design like the NT1-A.

For schematics of the NT2-A, I haven't seen any.

[Audiorepair]

OK,

the point I am trying to get across here is you don't need a fully equipped test lab to diagnose a dodgy mic.


ALL you need is a pair of ears.


Obviously the temptation here is to impress people with ones  knowledge, rather than try and solve the problem.
I'm as guilty of that as others here.

[Cerebus]

OK,

the point I am trying to get across here is you don't need a fully equipped test lab to diagnose a dodgy mic.

Depends if you just want to be able to say "this is dodgy" or be able to say "this is dodgy, R5 and R7 are out of spec, and C4 is toast" and thus be able to repair it. As it is, I happen to know that Zucca is not short of test equipment.

[Cerebus]

See the drawing in reply 1.  Pin 3 goes to ground through 50 ohms and 47 uF, while pin 2 is driven through 50 ohms and 47 uF from a single-ended amplifier output.  A balanced output would require a push-pull amplifier output or an output transformer.  That drawing purports to be of an NT1-A.
As wired, it can drive a balanced input.

"Balanced" means that signals see the same impedances on both conductors (so that any external pick up affects both conductors equally and can be removed by a differential input further down the line), "differentially driven" means that those "balanced conductors" are driven with signals of equal magnitude but opposite sign.

This was all much easier back when people put proper transformers in microphones and desk inputs.

[Zucca]

Thank you so much guys.

The scope at 1mV (Keithley Msox3104T) signals is picking up too much noise, it is not helpful.
With or without the differential probe I can't even trigger the signal. The entire scope screen is painted with something....

Today I remeasured only between Pin2 and Pin3 in the XLR with a Keithley DMM7510 range 100mV (just to be precise).
I generated the phantom power with a Keithley 2400 (48V, 10mA compliance) and two 6.8KOhm.
I took the best gear available in my lab.

Results:

Keythley 2400 shows a 3.9mA flowing.... (just to confirm)
Keithley DMM7510
VDC = 18mV (little DC offset, it should be fine)
VAC@room noise no talking into the mic = 2.8mV
VAC@talking into the mic = 3.1mV - 4.5mV (when I was loud or with impulse sound like letter "p" )

That's enough test for the mic/source side. Now that I know the mic works somehow, I will check the Focusrite.
In the next days create a few mV differential signal and feed the XLR into the Focusrite.

Since both XLR inputs with the mic are not working, it could be the XLR diff amp in the focusrite 2i2 is busted (48VDC spike fed into the Focus rite by the charged mic??? your guess is good as mine.... ).

To choose the right rabbit hole, I need now some tests on the Focusrite too.

@Audiorepair: I can barely hear my voice when the volume is cranked up to 95%. the voice appear clean and not distorted.
Since both inputs on the Focusrite had the same problem I tipped first on the mic amp stage...

Thanks for all the discussion and siggestions!

[Cerebus]

Dumb question. You have made sure that the inputs aren't on the 'line' gain setting, haven't you? 

[Zucca]

The question is legit and not too dumb according to my standards, yes I made sure the led INST was not on.
Just because I could, I tested both inputs with INST on and off: no Joy.

If the Focusrite failed on me I will return it, it should still be possible.

I am now thinking a dbx286s it is slightly more expensive but offer the proper interface to the RodeNT2-A and more nice features...

To digitize, I will go (un?)balanced in the Mainboard mic input... it should be good enough.

[Audiorepair]

If you don't have a dynamic mic to test the focusrite, you could test it using a pair of headphones as a microphone.
Just connect the sleeve into pin2 and either tip or ring into pin3, then talk into the headphone diaphragms.

A microphone and a speaker are kind of almost the same thing.


There's a famous trick where recording engineers use a Yamaha NS10 bass driver as a microphone in front of the kick drum.
Apparently lots of sub comes out.

[mansaxel]

There's a famous trick where recording engineers use a Yamaha NS10 bass driver as a microphone in front of the kick drum.
Apparently lots of sub comes out.

Finally some use for the NS10. Piece of shit, never was anything else.

OTOH, the day I can lay my hands on a working pair of NS1000, I will be a very happy man.

Computer speakers at previous job:

http://vvv.besserwisser.org/Public/Bilder/we-support-loudness.jpeg

(I can't be arsed to turn on TLS, so you'll have to copy and paste the link. Sorry. )

[Zucca]

Tonight test, it's Focusrite input stage now....
Here the little circuit I made for the test:



I was playing with the sig gen amplitude to see how and when the Focusrite input was saturating/clipping.

I measured this between pin 2 and 3 in the XLR on my trusty DMMM7510:



when the Focusrite led input was on the edge between yellow and red (clipping edge). Note the preamp input pot almost at 100%.



No wonder I have to set the pot  preamp input to 100% to hear something from the mic.

Details:

- both inputs same story
- INST mode on/off and or Phantom on/off did not change anything.
- 1KHz tone was clear enough to my hears

Tomorrow I will do the test with the headphone if necessary.

Now I have to do my homework to check the XLR balance signal specs.

[floobydust]

That test setup would have a problem if the sig gen is mains-powered, its GND is connected to earth ground (like a scope) and the Focusrite GND (XLR pin 1) connected to earth-ground through the USB if it's connected to a PC.
It would just short one leg i.e. pin 3 would see no signal but pin 2 would, for 1/2 the amplitude. But you're still getting nothing much for gain in the mic preamps.

Double check your XLR cables are wired correctly. If phantom power is on, and you plug in a cable with pin 2 or 3 shorted to GND, most mixers can't handle it. The input cap discharges into the op-amp, like 47uF at 48V around 54mJ and poof. It costs money to add big clamp diodes lol and TVS has high non-linear capacitance and is not preferred.

[mansaxel]

That test setup would have a problem if the sig gen is mains-powered, its GND is connected to earth ground (like a scope) and the Focusrite GND (XLR pin 1) connected to earth-ground through the USB if it's connected to a PC.
It would just short one leg i.e. pin 3 would see no signal but pin 2 would, for 1/2 the amplitude. But you're still getting nothing much for gain in the mic preamps.

Double check your XLR cables are wired correctly. If phantom power is on, and you plug in a cable with pin 2 or 3 shorted to GND, most mixers can't handle it. The input cap discharges into the op-amp, like 47uF at 48V around 54mJ and poof. It costs money to add big clamp diodes lol and TVS has high non-linear capacitance and is not preferred.

The Focusrite might. A more traditional input would shrug.    (borrowed pic, so disregard the red arrow and text.)



Oh, and by the way, Zucca as far as I can tell tested the cables quite early in the process.

Another thought:

I'd try playing a sine wave through a speaker close to the microphone (perhaps through headphones put directly on it) and look in the signal chain for traces of a the wave. A full signal chain tracing exercise. Anything before the first buffer amp will be very jittery with probe loading et c, so that must be considered.

Also, to establish some references: What happens if you drive the Focusrite with 775mv AC RMS @1KHz betweeen pin 2 and 3? Will you get a usable signal (i.e. will the gain control let you get to -18dBFS and is it distortion-free) ?

[Audiorepair]

You should be looking at injecting 100 - 200mV rms to get a reasonable range using the gain control, 7mV is way too low.

[Zucca]

That test setup would have a problem if the sig gen is mains-powered, its GND is connected to earth ground (like a scope) and the Focusrite GND (XLR pin 1) connected to earth-ground through the USB if it's connected to a PC.

No.
I forgot to mention the Focusrite was powered by a laptop on batteries. The GND USB was floating in the breeze.

It costs money to add big clamp diodes lol and TVS has high non-linear capacitance and is not preferred.

That's my guess too.

[Zucca]

Also, to establish some references: What happens if you drive the Focusrite with 775mv AC RMS @1KHz betweeen pin 2 and 3? Will you get a usable signal (i.e. will the gain control let you get to -18dBFS and is it distortion-free) ?

You should be looking at injecting 100 - 200mV rms to get a reasonable range using the gain control, 7mV is way too low.

Guys my mic output is 4mVRMS when I scream in it like a monkey in the jungle.
If the Focusrite can not handle those low signals by spec, I have to return it.

I am more than sure that with 100-775mVRMS I will get a clean signal in the Focusrite, but I can test it.

Bare in mind in the first 5 minutes with the preamp at 50% I was getting a reasonable signal (LED was turning green) when I was talking normally. After dis- and re-connecting the mic with phanton on (moving it on the boom) then pufffff the volume/gain went into the drain and this is where I am since (in a rabbit hole).

[2N3055]

That microphone has sensitivity of -36.0dB re 1 Volt/Pascal (16.00mV @ 94 dB SPL) +/- 2 dB @ 1kHz
and Maximum Output Level   16.0mV (@ 1kHz, 1% THD into 1KΩ load).


100 mV is a Line level input voltage.. Mic preamp has to have few millivolt sensitivity. .....

[Zucca]

Doing my homeworks

Condenser mics such as the RODE NT1A, the Manley Reference Cardioid, or the AKG C414 XLII will have hotter outputs requiring drastically less amplification (less preamp gain) to achieve suitable signal levels, sometimes as little as 10dB–30dB of gain. The reason for this is that condenser mics have amplifiers built right into the mics (sometimes called head amps) that provide the voltage for the mic’s output.

Do we then all agree that my mic head amps is acting funny here?

[Audiorepair]

Have you breathed on the capsule yet?

[Zucca]

Will do, now I am sure there is something wrong with the mic.
This weekend I will test the mic on a mixer with phantom power to reproduce the problem, just to be 100% sure it is the mic.

After that I will reverse eng the mic board to get the schematics... it is not too hard (even if they masked some components with sharpie)

[Cerebus]

You should be looking at injecting 100 - 200mV rms to get a reasonable range using the gain control, 7mV is way too low.

I think you're missing the point.

I was playing with the sig gen amplitude to see how and when the Focusrite input was saturating/clipping.

I measured this between pin 2 and 3 in the XLR on my trusty DMMM7510:

when the Focusrite led input was on the edge between yellow and red (clipping edge). Note the preamp input pot almost at 100%.

So that ~7mV is a measurement of when the focusrite is "on the edge between yellow and red (clipping edge)". It's not an arbitrary chosen injection voltage.

[Cerebus]

The Focusrite might. A more traditional input would shrug.    (borrowed pic, so disregard the red arrow and text.)

Like I said earlier:

This was all much easier back when people put proper transformers in microphones and desk inputs.

[Audiorepair]

You should be looking at injecting 100 - 200mV rms to get a reasonable range using the gain control, 7mV is way too low.

I think you're missing the point.

I was playing with the sig gen amplitude to see how and when the Focusrite input was saturating/clipping.

I measured this between pin 2 and 3 in the XLR on my trusty DMMM7510:

when the Focusrite led input was on the edge between yellow and red (clipping edge). Note the preamp input pot almost at 100%.

So that ~7mV is a measurement of when the focusrite is "on the edge between yellow and red (clipping edge)". It's not an arbitrary chosen injection voltage.

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.

[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 ...".

[Zucca]

Not a bad idea to retest the board without the black matter....

No I did and I will not touch the capsule. Maybe (I repeat maybe) with a little air pump I use for my son, gently squeeze some air parallel to the surface, but only after I get again a decent signal out from the mic.

Golden rule:
One and only one step, then retest and decide.

[Zucca]

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

No worries, even so I am learning and seee... it make sense to pay every month that stupid ISP, so I can talk with you all.

[Cerebus]

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.

If you do decide to try cleaning it at all be aware that there are often tiny pressure equalising capillary tubes hidden in the edges of these capsules - thin enough to present a high resistance to moving air but not to slow pressure changes. They may be too small to see. They are also easy to block with cleaning fluids or similar so the usual trick of gently spraying on a cleaning fluid or solvent is not a good idea. The word capillary should be a hint at how wrong that could go. If you can't bear to leave it alone then a new, clean, very soft make up brush like a blusher brush (ask the wife what that is) is a fair solution.

[Zucca]

No liquid, no brushes on capsule. Just gentle air if if if if big IF needed... and I do not plan to do it.

[Audiorepair]

You would be well pissed off if you tried to remove a bit of crud on the capsule using a blast of air, and it took a whole chunk of gold coating off the almost to the end of its life capsule rendering it useless.

Especially once you realised that all that crud did not affect the sound of the microphone in any appreciable way whatsoever.

[Bassman59]

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.

A bunch of years ago I built an amplifier for a Faraday cup, basically a piece of metal that is put in an ion beam and when it's struck you get a tiny current. To measure it you need an op-amp with crazy-low input leakage current, and for gain you need gigaohm resistors. About the only things I could find were tubular through-hole 1000V jobs. The circuit was built on Rogers duroid, too.

Anyway I'm not surprised that the load resistor for the capsule is big and unwieldy. It's basically the same idea.

[Cerebus]

Anyway I'm not surprised that the load resistor for the capsule is big and unwieldy. It's basically the same idea.

What makes me stop from just saying "Those are gigaohm order value resistors" is the number of them. (Counts board) Six of them. The usual places they're needed in a capacitor mic are (1) feeding the polarisation voltage to the capsule, (2) impedance converter bias (i.e. JFET gate bias resistor). Figure 8 mic, so two diaphragms, two JFETs => 4 high value resistors, what are the other two? Like I said before, people generally avoid designing in more than they absolutely need because they're chuffing expensive.

They don't have to be large and unwieldy nowadays, I've seen 50G in 0603 (400V working voltage rated) through to 3512 at 3500V from Stackpole. I wouldn't like to be the man who had to convince someone that a 0.35" long resistor met creepage and clearance for 3.5 kV.

Did you have the really fancy glass encapsulated type with the guard bands? They are even more wincingly expensive than the SMD flavour.

[Zucca]

Reassembled with clean board. Yes, after soldering everything back I cleaned up again with more IPA .

Unbelievable, everything works.

Tested fig 8, omni, front and back as all the other functions.
Listen to the attached file if you want to rejoy with me.
I do not bother with a frequency analysis. Now It sound like a NT2-A for me.
Still a little low output for my taste but now I can use it, I don't dig further in this tagic-comic hole.

TO ALL AUDIOPHOOL OUT THERE: IF YOU PAINT IT BLACK IT DOES NOT SOUND BETTER! IF IT DOES IT IS YOUR FUNNY BRAIN NOT THE IMPROVED CIRCUIT/SIGNAL!



Failure analysis:
Mic was stored somewhere for a long time and ended up in Ebay. Few minutes after powering up the mic, the 80VDC broke off the black no-sense dielectric and created a resistance path in the input stage.

Thanks to everybody who helped me!

[Bassman59]

TO ALL AUDIOPHOOL OUT THERE: IF YOU PAINT IT BLACK IT DOES NOT SOUND BETTER! IF IT DOES IT IS YOUR FUNNY BRAIN NOT THE IMPROVED CIRCUIT/SIGNAL!

The only things you should paint, black, are red doors.