Rode NT2a KM84 mod

[LevinGuitar]

I had a NT2a with a slightly less output. The main voltages seemed to be good so instead of looking for the real problem, I thought how can I do it sound better than the good one I have.
With the big help of a member who traced the schematic here, I found that the main amp zone was feed with stable 23.6-24v voltage, and since the main signal comes from the backplate (that will keep working 3 patterns), there was little to mod for the next circuit.

Convert 24v into 21v via 10k and a 4.7uf cap, and feed the drain of the new fet (2N3819) through 47k biasing it to 10v. For the gate I could desolder imput capacitor and use original Gohm resistor but I chose to leave the original board as it is so the mod could be reversible (desoldering only the backplate wire).

The smd board is a bit less mod friendly but there are enough ground pins from the switches where is easy to solder components to ground, and there are also a hole with 24v to place the 10k resistor. So the only two "floating" components are the output cap and the trafo, will find a way to secure them.

Since the 75v polarization makes a huge gain with KM84 circuit, I played a bit with the sensitiviy cap and finally tried to lower the polarization, as Piotr suggested somewhere in this forum, jumpering the last caps (in both polarities) that brought me to 65v. You will tell it does not make difference, but I clearly hear it, not by far but still more relaxed, natural sound. Maybe just more harmonics?

So how do it sound in the end? Similar but with a bit more lows and open top end, the transformer size play an important part here. The original circuit make it a bit more midrangy and less transparent/natural for my ears.

Is it worth to try? If your microphone sounds right, and you are not confident of the mod, not. Also the only switch that will keep working with the new circuit is the polar pattern!


Not sure if the audio attachments work, lately I can't open audio files in the forum.

 

[kingkorg]

Not sure what Piotr meant by removing those caps for lowering the voltage, i believe more elegant way would be to use lower zener voltage before the ic.

 

[LevinGuitar]

Not removing, jumpering

 

[Khron]

Not sure what Piotr meant by removing those caps for lowering the voltage, i believe more elegant way would be to use lower zener voltage before the ic.

... Or a higher series resistor just before the zener.

 

[LevinGuitar]

I don't want to play with smd's or have to buy them, if the jumper works, I'm happy

In case of the Behringer B1, yes, I have found a way to achieve higher voltage changing the resistor and the 7.2v zener, but there was one more zener before the resistor that wasn't in the schematic, that got me crazy before I found it. I will update that thread soon.

 

[LevinGuitar]

With a bigger core transformer like this UTM 587 (I have no 547 for now) , it start to feel like a different microphone, still same character but way nicer.
Now will try to fit a real BV8 copy into the NT1000 body, since it have more space than the NT2A

 

[ZilaMike]

With a bigger core transformer like this UTM 587 (I have no 547 for now) , it start to feel like a different microphone, still same character but way nicer.
Now will try to fit a real BV8 copy into the NT1000 body, since it have more space than the NT2A

Hey , i would like to do the same thing on my NT2-a , adding a carnhill or cinemag transformer, because the mic on it`s own is pretty clean, and that would help it have the character that i would like to hear on it .
My only problem is that i have no idea where to solder the wires....could anyone help me a bit ?

 

[Khron]

My only problem is that i have no idea where to solder the wires....could anyone help me a bit ?

Decide what transformer you want, so we can then see a datasheet, and then we can point you in the right direction re: the wiring.

 

[LevinGuitar]

Hey , i would like to do the same thing on my NT2-a , adding a carnhill or cinemag transformer, because the mic on it`s own is pretty clean, and that would help it have the character that i would like to hear on it .
My only problem is that i have no idea where to solder the wires....could anyone help me a bit ?

I would suggest you to try first on a cheap schoeps like microphone (with a clear schematic to design your mod first), so there will be less probability to damage a nice microphone in a bad way. Also, you need a quite big core transformer to make this mod worth.
Also, this way you'll see if the mod is worth or not for you, since the change will be similar.

 

[kingkorg]

Hey , i would like to do the same thing on my NT2-a , adding a carnhill or cinemag transformer, because the mic on it`s own is pretty clean, and that would help it have the character that i would like to hear on it .
My only problem is that i have no idea where to solder the wires....could anyone help me a bit ?

No point in doing that. The mic is voltage follower which means the signal swing is pretty low throughout the circuit. To get any coloration you would need higher level to excite the transformer. For example tube mics transformers can see up to 120v peak to peak at primary, which is then lowered to something a preamp can take. In this process you get color. NT2 output voltage is in milivolts for the most part, maybe couple of volts when pushed.

 

[LevinGuitar]

I don't find this kind of mod to add any coloration (I would say even less) the main (and small) difference is a bigger low end and slightly less nasty top end.

Btw, next time I would use a polyester 4.7uf 50v cap for the fet source, since they are cheap, small and perform better

 

[ZilaMike]

I don't find this kind of mod to add any coloration (I would say even less) the main (and small) difference is a bigger low end and slightly less nasty top end.

Btw, next time I would use a polyester 4.7uf 50v cap for the fet source, since they are cheap, small and perform better

The extra bottom heft and less pronounced high end is exactly what i`m looking for, also it helps with the transients a bit
This one seems small enough to fit inside the body : T8 Transformer - Tab-Funkenwerk

 

[Khron]

This one seems small enough to fit inside the body : T8 Transformer - Tab-Funkenwerk

Just be aware then, that you'll also be getting a 7x reduction in level (-15dB).

 

[ZilaMike]

These are the mics that i`m curently using, both the sE and Warm Audio have transformers inside, and they are definetly audible in the circuit
But i personally prefer the response of the Rode, i just wished it was transformer coupled and i would ditch the other two for main vocals .
This one is inside the Wa87R2 : https://cinemag.biz/mic_output/PDF/CM-13113.pdf

 

[ZilaMike]

Just be aware then, that you'll also be getting a 7x reduction in level (-15dB).

Yeah, i`m just starting to learn about all of this, it seems like a 1:1 ratio would be better with less to worry about ?

 

[LevinGuitar]

1:1 trafo to drive a high gain fet?

With a reduced polarization voltage, and a higher ratio trafo than the native one for the circuit, I even had to reduce the gain by adding a small drain-gate cap to match the impedance of that fet

 

[Khron]

1:1 trafo to drive a high gain fet?

With a reduced polarization voltage, and a higher ratio trafo than the native one for the circuit, I even had to reduce the gain by adding a small drain-gate cap to match the impedance of that fet

Eeeeeexcuse me? Could there be some language barrier at work here, or..? What circuit are you talking about, even?

 

[LevinGuitar]

In the first post, I went into a KM84 (as the thread title says too) kind circuit using a 2N3819 fet

 

[Khron]

In the first post, I went into a KM84 (as the thread title says too) kind circuit using a 2N3819 fet

Ok, but @ZilaMike was talking about something COMPLETELY different.

Hey , i would like to do the same thing on my NT2-a , adding a carnhill or cinemag transformer,

So there's no "1:1 trafo to drive a high gain fet?" - why would you put the signal from the capsule through a transformer, before it meets the JFET anyway?

 

[LevinGuitar]

"doing the same thing", as he says, involves changing the amp circuit into a new one, as I understand. But I insist to not recommend that mods for people who don't know exactly what they are doing.