Strange advice for Rode NT1/NT2 mod
Pollo
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#1
7th February 2013
Old 7th February 2013
  #1
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Strange advice for Rode NT1/NT2 mod

I recently looked for possible modifications to the (original) Rode NT1 and NT2 microphones. I own both, but especially the NT2 I found rather harsh sounding. A common complaint.

I found the post below in which Jim Williams, the original designer of the mic, explains some improvements he found for the original design:


My Rode adventures

Quote:
I recently went through my original Rode NT2's I have here. I'm the original designer of that mic.

I found and fixed a couple of things. First, the polarization voltage was a bit high, like 70 volts. I removed the DC converter pcb and added a series 1500 ohm resistor to the + voltage input and added a large 330 uf cap with a 6.0 V zener clamp. That along with some fast recovery rectifiers set the polarization voltage right at 60 volts.

I then removed the 2 ferrite beads on the outputs and replaced them with a pair of Dale 22.1 ohm CMF60 resistors, that solves some loading problems.

Next, to cure the excessive k-87 capsule's 12k hz peaks, I changed the 470 pf roll-off caps and ended up with 3300 pf. That tamed the 12k peaks and gave the mic a very U-87'ish quality I'm enjoying.

Replacing the capsule is another option, one I fitted with a C-12 capsule.

Reading through it looked like a do-able hobby project. I am not extremely knowledgable in electronics, but I would say I have a reasonable basic understanding of the principles and I know how to solder.

Replacing the capacitors to filter out more high frequency was easy enough. The part about lowering the voltage was not detailed enough for my limited knowledge to make sense of, so I skipped that.
But the third thing he mentioned about replacing the ferrite beads with resistors seemed a no brainer. Who wants loading problems, right?
So I found the ferrite beads, replaced them with 22 Ohm resistors, put the mic back together, hooked it up to a pre-amp, engaged the phantom power, and.....Nothing. No sound. Mic completely dead.

I took the mic apart again and studied its innards in search of the fault. It didn't take me long to realize what was wrong and I don't think it takes a degree in electronics to see it. I was effectively shorting the 48 V input to ground with a 22 Ohm resistor.

This had me quite confused. Did I do something wrong? I looked over and over but couldn't find any error in what I had done. I did some research on ferrite beads on the good old internet. I had no idea what a ferrite bead was, nor what it was supposed to do. From the information I gathered it seemed perfectly alright to replace a ferrite bead with a resistor. So what was wrong? It obviously couldn't work this way.

Eventually I reached the only possible conclusion. What I had so nonchalantly removed from the circuit were not ferrite beads at all! They were in fact little capacitors. Proof of my theory was found when I removed the resistors and the mic came back to live.
Fortunately nothing was blown, probably mic pre-amps have some sort of protection against shorting the phantom voltage? Just guessing.

What remains is some confusion. I mean, sure these little surface mount components are hard to tell apart, but for the guy who designed it?
I hear you thinking that maybe I replaced the wrong part; I can assure you there is nothing else in there that anyone could hold for a ferrite bead.
The rest of the circuit is on a PCB with normal sized components that are easily recognizable.

So I guess my point is, if you're thinking about doing this mod to the NT1 or NT2: DON'T REPLACE THE 'FERRITE BEADS'!

Finally a question to the smart electronics wizards out there, should I put some small capacitors back in? If so, what capacitance to use?
I'm assuming their function here is to filter out high frequency noise from the balanced connection. They were put between pin 1 and 2, and between pin 1 and 3 of the XLR output connector.
BTW, as far as I can tell the mic is not more noisy than it was. In fact it is fairly quiet.
#2
7th February 2013
Old 7th February 2013
  #2
I have the original NT2 and there is no SMD components in mine as far I remember. Anyway, I swapped the capsule with K47 from microphone-parts.com and skipped the filtering, sounds nice now.
Pollo
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#3
7th February 2013
Old 7th February 2013
  #3
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I was talking about the original NT2 as well. The 'ferrite beads' are not easy to find. They are on the flip side of the PCB. There are two PCB's screwed with their backs to each other. One of them contains the pre-amp circuit and the beads are on the other side, close to the output connector.

How did you skip the filtering? Removed the caps? Is that possible?
#4
7th February 2013
Old 7th February 2013
  #4
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Here is the NT2 schematic

The capacitors are for filtering RF. They are C17& C18 they are 15nf or .015UF. I use ceramic plate capacitors for RF. You can also use multi layer ceramics or film capacitors.

Happy Modding!!







.
Pollo
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#5
7th February 2013
Old 7th February 2013
  #5
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Great! Thanks.

Apparently I was wrong. I DID replace the wrong components. Ouch.
Apologies to Jim Williams. Of course he knows what he is doing. Unlike me.
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