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

[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.