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

[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.