Rode NTK question re Gus mods

[tinn]

Dear Metako,

you are right, but these effects can't be seen without the topology of the circuit.

My rough estimation according to the drawing of GUS:
(i see there an interesting schizzophrenic Transistor Q5 trying to
be on two positions the same time) :

Cap B is for smoothing the negative power
for the grid and capsule polarization. life is easy for this cap, high impedances, DC.
(As well I have the feeling there should be one more resistor to ground).
CAP A (.1uF) righses the impedance of the input reference point, microphonics
might there be an issue, a big ceramic cap on long leads won't be a good idea.
bigger influence at lowest frequencies.

The 15p integrator cap is not so easy to judge, but it is inside the feedback loop
and it's distortion should be 10000 times under the rest of nonlinearities.

220uF are paralleled by 15 Ohms of the LED, no issue.
The circuit might even work without it. affects the bias in the output.
The impedance on this point is around 100kOhm (load/ beta of Emitter followers.)

C1 has a good deal of polarization Voltage, cause the star point of the emitter followers is below ground level.
The impedance of this cap is Z=100 @34Hz. For a better sleep bypass it with a film cap or replace it.

C2 does not see a polarization voltage, which is not ideal, but otherwise not a big issue.
This cap affects the sound of the unwanted noise from outside , which should be cancelled out
by your balanced input.

Impedance on the output pins should be matched for noise suppression. This does not mean same Resistor values(100Ohm)
but nearly identical cap values, cause pin 2 sees the Impedance of the emitter follower in series with C1 and 100Ohms.

This circuit has a high amount of internal feedback but:
The biggest influence has the parasitic capacitance from plate to grid. This will
be summed to the input and will determine the frequency response and sound the most.
1.5pf versus 50pF (capsule)  . . 3% feedback of the Plate signal to the input signal.
Plate signal is not output signal.

You can take off the capsule, feed a signal to the input with a cap of 47 pF and measure the signal
at the plate. and check how high  the level of the plate signal is. Stage Q1 might have a gain of 2, so
it might be that the plate signal is lower than the input signal (less influence on the input though)
It is possible to change the gain of stage Q1 if you like to modify around.



As mentioned in the thread before: parasitic effects caused by the dimensions of the cap can have substantial
influence on the circuit. As well there is in most film caps an outer foil connected to a marked lead.
This is often overseen.

Most of the sound is caused by the capsule design.
Most annoying for me would be the uneven response of the capsules, where the rode seems to be fairly good.

In any case would say, that  minimal tweaking of the EQ in your channel should do much more
than cap swapping. Changing the valve might have a bigger influence, but not as much
as in a single valve circuit with substantial gain like the Neumann U47 or KM54.
________________________________

Non-electrical thoughts:

I think I underrated the desire of the cap-swappers to have a personalized tool or mechanical/electrical friend.
This gear, spit out of the factory in anonymous mass quantities is definitively  lacking some kind of aura.
So better degrade it with some critical soldering attempts, let a fly, which is living in your creative space
puuh on the diaphragm and here it is:
The mic , which you can believe in.


all the best, Tito

 

[ricardo]

http://www.cliftonlaboratories.com/capacitor_voltage_change.htm

A very good link there Tinn, although it seems to support the idea of replacing electros with film caps due to inherent instability of electros doesn't it?

Actually what it shows is that NPO ceramics are at least as good and usually better than polystyrenes & silver Mica.  Scott Wurcer will confirm this in his latest of his Linear Audio series on low noise mike amps.

 

[metako]

Dear Metako,

you are right, but these effects can't be seen without the topology of the circuit.

My rough estimation according to the drawing of GUS:
(i see there an interesting schizzophrenic Transistor Q5 trying to
be on two positions the same time) :

Cap B is for smoothing the negative power
for the grid and capsule polarization. life is easy for this cap, high impedances, DC.
(As well I have the feeling there should be one more resistor to ground).
CAP A (.1uF) righses the impedance of the input reference point, microphonics
might there be an issue, a big ceramic cap on long leads won't be a good idea.
bigger influence at lowest frequencies.

The 15p integrator cap is not so easy to judge, but it is inside the feedback loop
and it's distortion should be 10000 times under the rest of nonlinearities.

220uF are paralleled by 15 Ohms of the LED, no issue.
The circuit might even work without it. affects the bias in the output.
The impedance on this point is around 100kOhm (load/ beta of Emitter followers.)

C1 has a good deal of polarization Voltage, cause the star point of the emitter followers is below ground level.
The impedance of this cap is Z=100 @34Hz. For a better sleep bypass it with a film cap or replace it.

C2 does not see a polarization voltage, which is not ideal, but otherwise not a big issue.
This cap affects the sound of the unwanted noise from outside , which should be cancelled out
by your balanced input.

Impedance on the output pins should be matched for noise suppression. This does not mean same Resistor values(100Ohm)
but nearly identical cap values, cause pin 2 sees the Impedance of the emitter follower in series with C1 and 100Ohms.

This circuit has a high amount of internal feedback but:
The biggest influence has the parasitic capacitance from plate to grid. This will
be summed to the input and will determine the frequency response and sound the most.
1.5pf versus 50pF (capsule)  . . 3% feedback of the Plate signal to the input signal.
Plate signal is not output signal.

You can take off the capsule, feed a signal to the input with a cap of 47 pF and measure the signal
at the plate. and check how high  the level of the plate signal is. Stage Q1 might have a gain of 2, so
it might be that the plate signal is lower than the input signal (less influence on the input though)
It is possible to change the gain of stage Q1 if you like to modify around.



As mentioned in the thread before: parasitic effects caused by the dimensions of the cap can have substantial
influence on the circuit. As well there is in most film caps an outer foil connected to a marked lead.
This is often overseen.

Most of the sound is caused by the capsule design.
Most annoying for me would be the uneven response of the capsules, where the rode seems to be fairly good.

In any case would say, that  minimal tweaking of the EQ in your channel should do much more
than cap swapping. Changing the valve might have a bigger influence, but not as much
as in a single valve circuit with substantial gain like the Neumann U47 or KM54.
________________________________

Non-electrical thoughts:

I think I underrated the desire of the cap-swappers to have a personalized tool or mechanical/electrical friend.
This gear, spit out of the factory in anonymous mass quantities is definitively  lacking some kind of aura.
So better degrade it with some critical soldering attempts, let a fly, which is living in your creative space
puuh on the diaphragm and here it is:
The mic , which you can believe in.


all the best, Tito

So if I am understanding you correctly you believe only C1, the tube and the capsule are effecting the sound of this mic. Thanks for your analysis of the circuit. If the new tube I am waiting on is not enough then I will have a look at C1.

 

[tinn]

Yes, I think the tube can change the sound a bit.
but not substantially.

Maybe you are looking for a more linear mic, try the NT5.
If you don't trust the EQ in your mixer put a blanket over it.
I have seen fancy windscreens for treble smoothing (500$ including wooden stand)
Maybe you need a different directional pattern: change the mic.

If you like the physical size of your mic, try to install a small diaphragm inside the NTK.

I would not change any caps. The top end is not affected with a different brand of C1.
Furthermore I have not seen any cap modification with a proven result or a only halfway acceptable A/B comparison.
An  A/B test with two mics, which have been checked to sound the same before the modding, might be nice.
Haven't seen it yet.

As well it is possible to test the mic electronics with a "artificial
capsule" when you feed the test signal to it with the parameters of the capsule.
In most cases a voltage divider and a cap equal to the capsule will do it.
This way you can measure differences much smaller (way more than 10 times) than you can hear.
This would be much better than a new recording with the modded mic 3 days later.

Interesting for checking and training your abilities to hear distortion:
http://www.klippel.de/listeningtest/lt/

This old paper of SCHOEPS might be interesting too, maybe you can put it in a translator,
physics and emotions about mics, schoeps chapter 14, tubes are mentioned at the end:
http://www.schoeps.de/de/downloads/papers

Back to the mod: the stray capacitance of the "big" styro at the input may have some influence.
You can leave the old cap at it's place and solder a piece of copper foil to one of it's legs.
Which leg?: depends where the mod-guru of your choice likes to put the outer foil
of the cap (In most cases this has been overlooked anyway).

In short: I have not seen any sense in cap swapping yet.


Tito

You can't change the frequency dependent directional pattern of the mic by these mods,
as well they won't compensate  the mountains and valleys in the response above 3k in a large diaphragm design.
If a sharp "S" at 8 kHz is sitting on a high peak of a 200$ mic(not the NTK) . . . you hear it very clear.



More about capacitors:
http://sound.westhost.com/articles/capacitors.htm (easy to read)

for low frequency the following might be ok:
http://www.waynekirkwood.com/Images/pdf/Cyril_Bateman/Bateman_Notes_Cap_Sound_5.pdf

things are a bit much simplified  at Cyril Bateman,  ok for audio but not in the nanosecond area:
a funny response to the simplifications by Ivor Catt:
http://www.electromagnetism.demon.co.uk/2644.htm

 

[metako]

Thanks again Tinn. Will take me a while to read through all those links and comprehend them. I am basically happy with the NTK but just looking to improve it a little if it can be done. I know that no one mic suits all situations.

 

[shimon]

capsule changes have a bigger impact, it seems.

 

[LevinGuitar]

It's an old thread but don't want to start a new one. I tried a EF86 in my NTK and do like the change. The stock socket can be rewired but it's a bit tricky since there are traces under the socket. Also adding a small cap a la M49 helps a bit with the top end.

I tried higher anode resistor but not sure if I liked it, so came back to the stock one 43k.

 

[LevinGuitar]

Just realized that the NTK can fit the headbasket from NT2a (but no at reverse, as you can see). The NT2a headbasket matches the two front screws, but none of the rear ones, anyway it seems pretty solid and can be useful if someone want to try a different headbasket acoustic on his NTK/K2.

The NT1000 headbasket fits even better and matches all 4 screws.