Rode NTV noise issue

[RuudNL]

I am having a problem with a Rode NTV microphone.
The microphone works, but the self noise is much higher than expected. (I think 10 dB more than I would expect.)
The previous owner had this problem already, and changed the original tube (ECC81) for an ECC801S, but this did not change anything.
I have checked everything inside the microphone, exchanged capacitors and even the capsule, without any improvement.
The strange thing is that the voltages are all on the low side. (Even without the tube in place)
130 Volt = 108 Volt
142,6 Volt = 120 Volt
300 Volt = 255 Volt
Inside the power supply are two zeners, one has a voltage of  ca. 150 V over it, the other ca. 100 V.
This might explain the 255 V.
I can not find a schematic diagram of the microphone and/or the power supply.
Has somebody else encountered this problem before?
If so, I am curious to hear what the problem was.

 

[Whoops]

Hello Ruud,
I've used that microphone before and didn't notice a self noise higher than other mics.
A friend of mine has it I can try to measure voltages next time I visit his studio. Don't know when that will be possible at the moment.

Have you tried to contact Rode?
they might give you some support, maybe it's already a know fault in that specific power supply revision.

hope you can get if fixed

regards

 

[Bowie]

I have one of these and it's a pretty low noise mic.  Let me know specifically what you want tested and I'll check it with my meter.

 

[Matt Nolan]

I think in one of this series of EEV Blog videos about mics with a chap ex of Rode there is a discussion on the (rough) schematic - https://www.youtube.com/watch?v=_rTAqO7xz8U&list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw

 

[RuudNL]

Let me know specifically what you want tested and I'll check it with my meter.

Great! Inside the microphone there are 3 voltages marked on the PCB
They are:

130 V (I measure 108 V)
142,6 V (I measure 120 V)
300 V (I measure 255 V)

The 12,6 V ( 142,6 - 130) is probably the voltage for the filament(s) of the tube ( 2 x 6.3 V in series)

It would be good to know if the voltages I measure are within the tolerance.
But if you measure voltages that are spot on, something is wrong with this one!

 

[Gus]

NTV not NTK?

Check the cable connections. 
I had a NTV. I will look for the schematic I drew. 
How many tubes did you try?

 

[RuudNL]

It would be great to see a schematic diagram of the NTV!
The owner changed the (stock) ECC81 for a ECC801S, but there was absolutely no change in self noise.
I tried an other ECC81, but with that one the noise level was much higher than with the previous ones, so I don't suspect the tube.
(So two tubes were 'good', one was 'bad'.)
I am especially interested in the values of the zeners in the power supply.
I measure 150 V over ZD2 and 100 V over ZD1. That would give me roughly (supposed that they are connected in series) 250 V.
On the PCB inside the microphone however, there is a point marked '300V'.  But I don't measure 300V at any point of the microphone cable connector. The highest voltage I have is 255 V.  (-15%)

 

[Whoops]

It would be great to see a schematic diagram of the NTV!

Maybe Rode tech support supplies schematics for repairs

 

[RuudNL]

I have mailed Rode today, I hope that they can help!

 

[Bowie]

Let me know specifically what you want tested and I'll check it with my meter.

Great! Inside the microphone there are 3 voltages marked on the PCB
They are:

130 V (I measure 108 V)
142,6 V (I measure 120 V)
300 V (I measure 255 V)

The 12,6 V ( 142,6 - 130) is probably the voltage for the filament(s) of the tube ( 2 x 6.3 V in series)

It would be good to know if the voltages I measure are within the tolerance.
But if you measure voltages that are spot on, something is wrong with this one!

  That's odd because my mic lists 3 voltages, which are different than the ones you listed, but your voltages seem in-line with what are marked on my PCB.
100v (mine is 100)
112.6 (mine is 112.6)
On the other side of the transformer;
+230v (mine is 237)

If your PCB is mis-marked, then your mic might be ok according to my readings and board markings.  They didn't produce all that many NTVs so it's unusual to see two types unless one was for the European market.

  What type of noise are you hearing?  I've tested hundreds, maybe thousands of ECC81, ECC801s, etc s and it's not unusual for them to have too much noise for a mic.  The NTV boosts the highs a bit so tube hiss could easily be amplified. 

 

[RuudNL]

Interesting, those voltages.
It may be that this microphone is from an early series and that they changed the voltages later.
The owner changed the tube from an ECC81 to a ECC801S because the noise level was high, but this did not give any improvement.
Of course it could be possible that the noise of the replacement tube was exactly the same as the previous one...
The noise sounds like an even 'white noise', no irregularities in the noise pattern.
But compared to other microphones, the self noise is high, I suppose 10 dB higher than expected.
The owner brought the microphone already to Mafico, the official service point for Rode in the Netherlands, but (as he says) 'they couldn't find anything that was wrong with the microphone'.
Today I received an answer fron Rode Australia, but they don't recognise this as a common problem.

I hope that Gus can find the schematic of the microphone (and maybe the power supply).

 

[Gus]

Found some notes
112.6VDC
100VDC
230VDC

I did not trace the power supply

I would keep trying  different tubes.  There have been times for example with 6AU6s that I had to try a number before I found a low noise one.

Have you substituted a clean low noise capacitor for the capsule and tested the noise?

 

[RuudNL]

Thanks a lot Gus!
So it seems my voltages are more or less right, even a bit higher than yours.
I could be that the markings on the PCB are from 'an early series', and that they changed  the voltages later?
(Because forum member Bowie also reported lower voltages.)
And yes, I have tested the circuit with a fixed capacitor (and with an other capsule) and the noise level is still a bit (too) high.
The problem is that I don't have any reliable ECC81's at hand here, so I will have to order them.
I have a lot of ECC82 and 83's, but they are different from the 81.

 

[RuudNL]

This morning I received from Rode Australia the schematic diagram of the Rode NTV and its power supply.
I am not sure if I am allowed to share this information (or upload it to the technical documents), but if somebody needs this,  please send me a message!

 

[Whoops]

This morning I received from Rode Australia the schematic diagram of the Rode NTV and its power supply.
I am not sure if I am allowed to share this information (or upload it to the technical documents), but if somebody needs this,  please send me a message!

Well done Rode!