RODE NT2 J.W. MOD - schematic attached

[dhammer]

Thanks for the tips!  I get what you describe with the foil and will give it a try.  I'll report back on the voltages when I get the parts in place.  8)

 

[dhammer]

OK – I added the bypass caps and wired the boards back together.

I'm getting 42.34v at the Cap B point.  I'm going to leave that for now.

I put the mic back together – It works!!!  Basically......  but I get a problem that sounds close to what In76d had:  Omni is much lower output than Cardiod.  I wouldn't say it sounds thinner but it is lower by about 12db.  And the pad doesn't work.  So I've done something wrong in the changes I made to the wiring off the capsule.  We'll have to try to figure that out over the weekend.

In Card, the mic sounds very nice and smooth.  And no caps exploded when I gave it phantom.  ;D

 

[ricardo]

....  but I get a problem that sounds close to what In76d had:  Omni is much lower output than Cardiod.  I wouldn't say it sounds thinner but it is lower by about 12db.

I've asked this before without getting a satisfactory answer.

Anyone with an UNMODIFIED NT2 tell us if the original had omni sounding quieter than cardioid?  Someone with the actual item please rather than half remembered stuff 
________________

Also does anyone have a RODE supplied schematic?  ie clearly marked as a RODE document with document control etc.

IMHO, it is unlikely the schematics posted here are by RODE.  Too unprofessional.  They might be by JW.

 

[dhammer]

Before I made the modifications, the Omni side did not have a lower output.  I used it on a session last week so it's a solid report.  I used it inside a kick and had to engage the pad. 

 

[dhammer]

It looks like the schemo In76d made his mod-version drawing from was originally drawn in 2004 by a guy on this board.  See this post:

http://groupdiy.com/index.php?topic=1494.0

The original drawing that guy did is attached.

 

[dhammer]

Do some of the answers given in this post point to the solution to the problem with the mod design here:

http://groupdiy.com/index.php?topic=48859.0

Ricardo, in your reply (#4), you say "The amp must be connected to the centre backplate so it sees both diaphragms.  If this is at 0V, you need to keep one diaphragm at 55V and vary the other over +/-55V"

Zap says the same thing in his reply #13.

If this applies, the changes to the un-modded scheme would be to simply:

Tie the Cap A wire to the backplate
Remove C15 & C21
(and remove R33 to disable the high-pass)

Is this correct?  Are there any problems with doing this? 

 

[ricardo]

http://groupdiy.com/index.php?topic=48859.0

Ricardo, in your reply (#4), you say "The amp must be connected to the centre backplate so it sees both diaphragms.  If this is at 0V, you need to keep one diaphragm at 55V and vary the other over +/-55V"

Zap says the same thing in his reply #13.

If this applies, the changes to the un-modded scheme would be to simply:

Tie the Cap A wire to the backplate
Remove C15 & C21
(and remove R33 to disable the high-pass)

Is this correct?  Are there any problems with doing this?

Yes.  The scheme in that thread works well.

The schematics posted in THIS thread are very poor and the original circuit isn't too hot either.  It's likely there are errors and everything is difficult to see.

But to convert a dual diaphragm LDC from working cardioid  to working omni, you only have to connect the front & back diaphragms together.  This can be to the 55V supply (or directly as in U47)

Looking at the original & modified schematics, it is possible (very unclear) that the modified switch just disconnects the backplate.  This would give a lot less output.  Just my guess.

 

[dhammer]

Thanks for confirming the alternate scheme.  I'll give it a try and report back.

I contacted Rode and got a very fast and friendly note back offering to work on the mic but refusing to share the real schematic.  Understandable.

Ricardo, can you share your thoughts on the problems with the original circuit?  It would sure help a guy trying to get beyond mod-by-numbers & into a real understanding of what's going on.

 

[ricardo]

Ricardo, can you share your thoughts on the problems with the original circuit?  It would sure help a guy trying to get beyond mod-by-numbers & into a real understanding of what's going on.

I'm not a fan of JW's work.  His original NT2 circuit is naive and the best that can be said of his mods & improvements is that they mostly don't degrade the performance.

If your mike works and gives good sound, I would leave it alone.

I have some inside knowledge of what Rode think about this.  Their latest NT2, NT1 etc is designed in house and good.

Unfortunately, I'm not sure if they re-furbish your mike, whether you get their biggest improvement .. their in-house capsule designed & built by Sydney virgins.

If you want to see how a Schoeps circuit should be done, use Zapnspark's GenChinaMod.pdf from his MicBuilders Files.  To incorporate a Cardioid/Omni switch, just short the Front & Back diaphragms together like U47.

But the switch is at the Hi Z end so is VERY critical.  Any contamination, poor wiring, leakage etc will result in yucky hum, hiss & scratchy, intermittent noise.
_________________

BTW, don't bother trying to measure the polarizing voltage.  You need an electrometer to do that .. a voltmeter with more than 10G input resistance.  For stuff like NT2, you only need to be worried if the diaphragm collapses against the backplate.

 

[tonycamp]

Ricardo, can you share your thoughts on the problems with the original circuit?  It would sure help a guy trying to get beyond mod-by-numbers & into a real understanding of what's going on.

I'm not a fan of JW's work.  His original NT2 circuit is naive and the best that can be said of his mods & improvements is that they mostly don't degrade the performance.

If your mike works and gives good sound, I would leave it alone.

I have some inside knowledge of what Rode think about this.  Their latest NT2, NT1 etc is designed in house and good.

Unfortunately, I'm not sure if they re-furbish your mike, whether you get their biggest improvement .. their in-house capsule designed & built by Sydney virgins.

If you want to see how a Schoeps circuit should be done, use Zapnspark's GenChinaMod.pdf from his MicBuilders Files.  To incorporate a Cardioid/Omni switch, just short the Front & Back diaphragms together like U47.

But the switch is at the Hi Z end so is VERY critical.  Any contamination, poor wiring, leakage etc will result in yucky hum, hiss & scratchy, intermittent noise.
_________________

BTW, don't bother trying to measure the polarizing voltage.  You need an electrometer to do that .. a voltmeter with more than 10G input resistance.  For stuff like NT2, you only need to be worried if the diaphragm collapses against the backplate.

Really? what have you had done by Jim that could back up your "Jim's naiveté" statement? I've had a few things done by the guy, and the results were incredible, namely a RODE NT2 lol, and a 32/8/2 Soundcraft Delta 200(yeah custom 8 buss), resulting in one of the best sounding boards i've ever heard..period.

other things he makes sound great, 414's, aphex 651's, lexicon reverb units, even the lowly dbx 166(original) end up sounding really great.

Jim lives a block away from me, i'll ask him if i can post a pic of the insides of the NT2 mods here? BTW, mine has erics k67 in it, and it's an 87 killer imv.

 

[micaddict]

Jim lives a block away from me, i'll ask him if i can post a pic of the insides of the NT2 mods here?

Tony, could you ask him to chime in here to share some thoughts?
Tell Jim that for guys like him this is a better place than Gearslutz. 



Henk

 

[Krumlee]

Interesting posts from everyone.

On the polarising voltage, I know a voltmeter isn't perfect for this sort of circuit. I tend to think that the readings that we're getting aren't far off the truth. I tried an alternative method by putting a spare 1G resistor in series with the multimeter probe and measured a current of 0.04uA through it.

I may try lowering the resistor to the oscillator board and playing around with the zener values just as an experiment.

 

[ricardo]

Really? what have you had done by Jim that could back up your "Jim's naiveté" statement? I've had a few things done by the guy, and the results were incredible, ... bla bla

I designed this and other stuff for Calrec in the early 1980s http://img.photobucket.com/albums/v379/SSLtech/Soundfields.jpg

If you think JW's stuff is incredible bla bla, just ignore everything I say 

But IMveryHO, I know a bit more about microphones than JW.  I was only answering dhammer's question so now I'll just shut up.

PS: IMnotsoHO, the Calrec Mk4 was the best mike of the last Millenium.

 

[tonycamp]

Really? what have you had done by Jim that could back up your "Jim's naiveté" statement? I've had a few things done by the guy, and the results were incredible, ... bla bla

I designed this and other stuff for Calrec in the early 1980s http://img.photobucket.com/albums/v379/SSLtech/Soundfields.jpg

If you think JW's stuff is incredible bla bla, just ignore everything I say 

But IMveryHO, I know a bit more about microphones than JW.  I was only answering dhammer's question so now I'll just shut up.

PS: IMnotsoHO, the Calrec Mk4 was the best mike of the last Millenium.

I guess i'll interpret your answer to my question of "have you ever used JW's stuff?" as, no bla bla  :

 

[tonycamp]

Jim lives a block away from me, i'll ask him if i can post a pic of the insides of the NT2 mods here?

Tony, could you ask him to chime in here to share some thoughts?
Tell Jim that for guys like him this is a better place than Gearslutz. 



Henk

Will do mr. addict!

 

[dhammer]

Geez I hope I didn't light the match that started a JW flamewar...  I love what Jim did for my mix desk & that's why I bought this mic a few years ago.  But I appreciate hearing from a mic guru like Ricardo.  It's allllll good so far as I can see...

I went through the mic and drew a new schemo.  I didn't double-check the resistor values, just pulled them from the older drawing.  I am pretty sure I have this right but will check it again tomorrow night.  I might have the orientations of the transistors and the diodes messed up – I'll have to look those up tomorrow.  I THINK it's a little more organized than the old drawing but I'm a newb so what do I know...  anyway, it was a good exercise for me & it might help clarify the circuit. 

Gonna have to get me one o' them Calrec mics!

 

[ricardo]

I went through the mic and drew a new schemo.  I didn't double-check the resistor values, just pulled them from the older drawing.  I am pretty sure I have this right but will check it again tomorrow night.  I might have the orientations of the transistors and the diodes messed up – I'll have to look those up tomorrow.  I THINK it's a little more organized than the old drawing but I'm a newb so what do I know...  anyway, it was a good exercise for me & it might help clarify the circuit.

Thanks for this dhammer.  This is a much clearer circuit but there are still some errors.

When I refer to Zapnspark's GenChinaMic.pdf as a reference, I will use italics.  When I refer to your NT2_Traced.pdf, I will use underlining.

• Your R4 variable should be connected as a potential divider like Zap's R7/R10
• Your Front diaphragm should always be connected to C21 NOT via the switch.  Your switch should simply connect the Back diaphragm to the Front diaphragm in Omni position.

You may have changed the switch connections when you modified the mike which would explain the soft Omni.

• In the oscillator, your Q18 collector should be connected to the junction of R18/D3
• Q18 base should be at the junction of R18/C28
• R18 should be much larger than 1k.  Maybe between 2M - 750k
• C28 should be maybe 6p8

Have a look at Zap's GenChinaMic.pdf to see how it should be.

Please remove the erroneous circuit and post a corrected one.  Don't want more faulty circuits on this forum.

 

[dhammer]

Ricardo, I can't thank you enough for the notes on my drawing.  I took it down and will make the corrections before reposting it.  Your earlier reference to Zap's drawing was what got me working on this schemo -- the organization of the original nt2 drawing made it too hard to compare to Zap's. 

Thanks again
David

 

[dhammer]

I've made the changes to the drawing but will wait to repost until I can double check the values of the resistors.

I am now able to clearly see the differences between the original circuit, the JW mod circuit, and Zapnspark's GenChinaMod.  The biggest difference seems to be that Zap's mod keeps polarization on the diaphragm but moves the signal output to the backplate, while JW moves polarization to the backplate while keeping the audio coming from the diaphragm.  Both methods allow for removal of the cap between the capsule and the JFET. 

What does one method offer over the other? 

I also see that Zap's drawings (reflecting the SP1) has a 6.2v Zener (D5) and 220u Electrolytic on the Oscillator board.  This seems to be a change JW mentions for the NT1 (quote from the 2nd post on this thread):

"I removed the 6.2V zener diode on the main audio pcb to provide a higher voltage to that oscillator pcb. I then added a 220 uf/25V cap after that 1.5k resistor and placed a 6.0~6.2 V zener across that cap to limit the voltage."

What does this accomplish? 

 

[dhammer]

Revised schematic attached with the corrections from Ricardo added and verified to match the pcbs.  Also, checked/corrected all component values.  This is the un-modified circuit.

One note:  the component numbers on the oscillator board restart at 1 (R1, C1, D1, L1).  I added "B" after each to distinguish them from the parts on the amp board.  Ricardo read the Bs as 8s.  Is there a convention to deal with this?

EDIT:  removed the attachment, posted revision to later reply