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[ahenry75]

Hello my friends.  Typical "Long time reader, first time poster" kinda thing here, and would really appreciate any help that can be had.

I have a Lewitt 840 that has a major white noise issue, making the mic unusable for recording vocals (link to recording is below).

A few details:
- I picked up for $200 on Reverb, knowing that it had a noise issue, but I don't know the history of the mic, nor is it under warranty any more.
- When I got it, I discovered that the tube that was in it was broken (the getter was clouded) so I replaced it with a known good 12ax7wa from EHX(not the best tube by any means but good enough for testing I would think)
- When I replaced the tube, I noticed that the capsule had finger prints on both sides (sigh), and that a screw inside is missing, so I am clearly not the first person to open it up.
- I tested with both a Focusrite ISA one as well as directly into my interface (PreSonus 2626) - the sound was the same on both.
- The white noise is not present at all if the mic is not plugged into the power supply.  There is a very slight power hum with the gain on the preamp is maxed out, but nothing that would ever be heard on a recording.
- I have changed electrical outlets (used two different circuits in fact) and mic cables, no change.  I am not able to try a different cable from the power supply to the mic itself as I don't have another 11 pin XLR cable.
- The noise is lessened by the -10/-20db pads on the power supply.
- There are no obvious loose components in either the power supply or the mic, nor are there any obvious signs of damage, over heating, etc.

Link to sounds sample here:
https://soundcloud.com/ahenry75/lewitt-840-white-noise/s-0uNF8fxwCGh?si=b02408936c664c3fb5e778d7ecf3af9a&utm

Signal chain:
Lewitt 840 - Mogami Cable - Presonus 8686 - Dell XPS9520 running Reaper 6.81

I would value any opinions/advice that is to be had.  I have worked on many tube amps (for guitar), have build several mics from kits, am comfortable with a soldering iron and multimeter, AND, finally mature enough to ask for advice before starting in with the destructive stuff.

Thanks so much!
Andy

[klaus]

(...) so I replaced it with a known good 12ax7wa from EHX

I would not be satisfied with this type of initial troubleshooting. And I would not move on to any other suspected causes of the excessive noise until the tube was truly verified as quiet:
How did you determine that the 12AX7 you installed was microphone-grade? Did you try another high-quality 12AX7? New Sensor's Russian 12AX7 are not exactly at the top of the heap.

[ahenry75]

Thank you for your reply Klaus, I truly appreciate it.

I initially had this tube in another tube mic that I sold a few years ago.  I have tried it in several guitar amps (I know it is not the same thing) and it is dead quiet in that application. 

I have now tried 3 other tubes, all with nearly identical amounts of white noise: PSVANE ECC83, TAD ECC83 Selected, TAD 7025 E83CC.  The two TAD tubes were taken from a dead quiet guitar amp, so I have high confidence in their quality.  Again, I know that none of these are top tier tubes for Microphones, but this is what I have around my shop, and since the symptoms are essentially unchanged, I hope this is a good enough diagnostic of the tube stage.  The mic is a pain to disassemble and clearly not designed for easy tube swapping by us civilians.

Thanks again.

[Kai]

White noise is very bright sounding - is this what you hear?
White noise is typical for electronic parts and tubes.

Else, if the noise is darker, bass-rich and not very constant I‘d suspect contamination, typically e.g. on the capsule.

Try to replace the capsule with either a known good one (if you have one handy) or simply with a non-polarized capacitor ca. 60 pF / 60 V.

[ahenry75]

Thanks Kai,

Here is the sample of the sound - https://soundcloud.com/ahenry75/lewitt-840-white-noise/s-0uNF8fxwCGh?si=b02408936c664c3fb5e778d7ecf3af9a&utm

I figure the recording does a better job giving an example than my words can.

I don't have a spare capsule handy, but I probably have a capacitor that will work.  would a 50pf/50v work in a pinch?

- Andy

[klaus]

If it's not the tube, the next step would be testing the supply voltages: first, heater voltage, then B+ voltage.

This can be conveniently done by a competent microphone technician without taking the mic apart, by measuring inside the power supply (figure about a 0.4VDC drop of the heater voltage by the time it reaches the mic).

[ahenry75]

Attached is a picture of the circuit board behind the jack for the mic cable.

With the mic plugged in an power on, here are the voltages for each pin (measured 3 times to be sure):
"+200v" = 193.0v
"-10db" = .02v
"+TR" = 0v
"-6db" = 10.45v
"-TR" = 0v
"-TU" = 0v
"+TU" = 0V
"-20db" = .02v
"0-120v" = 33.03V  (each measurement starts at approximately 43v and then decreases to stabilize at 33.03v)
"+12v" = 11.66
"Grd" = 0v

[Kai]

Here is the sample of the sound - https://soundcloud.com/ahenry75/lewitt-840-white-noise/s-0uNF8fxwCGh?si=b02408936c664c3fb5e778d7ecf3af9a&utm

I don't have a spare capsule handy, but I probably have a capacitor that will work.  would a 50pf/50v work in a pinch?

Should do.

Do you need to bring up preamp’s mic gain unusual high, compared to a “normal”, working mic? Then it’s probably just the mic capsule or it‘s polarization voltage.
The recording sounds like the capsule’s diaphragm is collapsed to the backplate, thus giving very low level and sharp sounding output.

The noise sounds OK, like typical amplifier noise.

[Kai]

I’d like to comment on the measured voltages:

"+200v" = 193.0v - OK
"-10db" = .02v - Probably a remote switching voltage stearing a relay, try the -10dB setting on the PSU and re-measure
"+TR" = 0v - ?
"-6db" = 10.45v  - same like -10dB, but -6dB switch engaged.
"-TR" = 0v - ?
"-TU" = 0v - ?
"+TU" = 0V - ?
"-20db" = .02v - same like -10dB
"0-120v" = 33.03V  (each measurement starts at approximately 43v and then decreases to stabilize at 33.03v) - Bias, behaves OK
"+12v" = 11.66 - OK
"Grd" = 0v - Common ground reference, OK

The T#-designated voltages are questionable, maybe not ground referenced.
Try to measure +TR vs - Tr and +TU vs -TU

[ahenry75]

@Kai, you are a genius!! (and I am a dope for not having thought of that)! 

I plugged in another mic (Condenser) and it is MUCH louder at the same gain settings.  And the white noise is actually about the same at that amplification level.  Brilliant!  Given the couple of missing/cracked pieces in the mic, and the finger prints on the capsule, I can imagine that someone grabbed the capsule when trying to change the tube, or something link that. 

I will source a new capsule and try that.

Wish I could buy you a cup of coffee to thank you!

Should do.

Do you need to bring up preamp’s mic gain unusual high, compared to a “normal”, working mic? Then it’s probably just the mic capsule or it‘s polarization voltage.
The recording sounds like the capsule’s diaphragm is collapsed to the backplate, thus giving very low level and sharp sounding output.

The noise sounds OK, like typical amplifier noise.

[klaus]

This would not be the first time I have been accused to being a killjoy. But I am strictly going from many years of experience (with a decent probability of being wrong this time): I don't believe that a contaminated capsule has the noise profile you shared.

I think it is a polarization voltage supply issue or some other mic amp issue, rather than a defective capsule.

I will patiently await your final analysis.

[ahenry75]

Thanks Klaus, and no worries about being a killjoy, hahaha.

I reached out to Matt McGlynn at Roswell audio/Microphone parts and he is also skeptical about a collapsed capsule.  (Which is somewhat funny, since he sells good quality replacement capsules).

The one thing for sure is that the output of the capsule is much lower than on my other condenser mics (apologies for not having tested that to begin with), so it seems that the white noise was just the noise of the mic, but amplified to the extreme to get signal from the capsule.

Any advice on how to measure the polarization voltage supply?  For that matter, I can't guarantee that the capsule is even wired in correctly since the mic needs to be almost completely disassembled to change the tube and that puts a lot of strain on the wires to the capsule.

You guys are awesome to try to help me with this.

-Andy

This would not be the first time I have been accused to being a killjoy. But I am strictly going from many years of experience (with a decent probability of being wrong this time): I don't believe that a contaminated capsule has the noise profile you shared.

I think it is a polarization voltage supply issue or some other mic amp issue, rather than a defective capsule.

I will patiently await your final analysis.

[Kai]

I noticed that the capsule had finger prints on both sides (sigh), and that a screw inside is missing, so I am clearly not the first person to open it up.

This sentence and mainly the very thin sound of the recording makes the capsule or the bias circuit a suspect.

The collapsing of the diaphragm can be examined visually, when applying the bias the diaphragm touches the backplate, which is obviously visible.

The one thing for sure is that the output of the capsule is much lower than on my other condenser mics (apologies for not having tested that to begin with)

I doubt you mean the capsule, just the whole mic - or did you transplant the capsule to a known good mic?

To estimate if the bias is correct, locate the biasing resistor.
Now, with a 10 MOhm DMM measure the voltage on both sides:
One side should read ca. 10 to 60 V, depending on extra filtering built in.
The other should read ca 5 to 20 times lower volts, as the voltage drops along the typical 50 to 200 MOhm biasing resistor.

Of course, without the whole schematic at hand, this is only a rough check, but better than nothing - if the voltages are as stated, bias could largely be excluded as a problem.
If not, it doesn‘t mean much.(? KH) it doesn‘t necessarily mean something is wrong, see below.
If there is a value printed on the resistor, the check would be more precise, higher value = lower voltage.

If the mic has a variable pattern, or a different biasing scheme, the whole thing becomes more complicated, bit over a layman‘s capability.


Maybe ask the manufacturer for a schematic?
Mic amp circuits are no rocket science, so no trade secret involved.
Would take me 10 min to draw it with the real thing at hand.

[ahenry75]

This sentence and mainly the very thin sound of the recording makes the capsule or the bias circuit a suspect.

The collapsing of the diaphragm can be examined visually, when applying the bias the diaphragm touches the backplate, which is obviously visible.

At the suggestion of another very helpful chap (Matt McGlynn), I powered on the mic without the windscreen attached, to see if there are any noticeable differences with the capsule, but there are none on either side. (I have pictures of both sides of the capsule with and without power, they look identical to me.)

I doubt you mean the capsule, just the whole mic - or did you transplant the capsule to a known good mic?

Sorry, you are correct, I meant the whole mic.  I do not have another capsule that will work with this mic.

To estimate if the bias is correct, locate the biasing resistor.
Now, with a 10 MOhm DMM measure the voltage on both sides:
One side should read ca. 10 to 60 V, depending on extra filtering built in.
The other should read ca 5 to 20 times lower volts, as the voltage drops along the typical 50 to 200 MOhm biasing resistor.

Where is the bias resistor located?  I come from the land of tube amps and guitar pedals, which don't have the micro components. Tracing the pcb for a mic is something fairly new to me.  Attaching pictures of both pcbs in the mic body.

If the mic has a variable pattern, or a different biasing scheme, the whole thing becomes more complicated, bit over a layman‘s capability.

Well, it IS a multi pattern mic.  There is no difference in the sound in any position (other than the polar patterns, which seem to be working as best as I can tell.)

Maybe ask the manufacturer for a schematic?
Mic amp circuits are no rocket science, so no trade secret involved.
Would take me 10 min to draw it with the real thing at hand.

I did ask them and they said no. I guess they think they have reinvented the microphone and don't want anyone to know how.  I've drawn up schematics myself as well, but am not used to dealing with components of this size.

I am very grateful for all the help, thank you!

[Kai]

OK, I wouldn’t call this a typical “tube mic”, but a mic “with tube”.
The high count of components would make drawing of a schematic a high effort.
You would also need to identify the 16-pin IC (probably a logic IC) and other unlabeled components.

If you are familiar with guitar amps I’d suggest another approach:
Let’s check the amp by feeding a signal of ca. 50-100 mV through a 50pF cap.

Then see if it comes out correctly.
Spec’ed 23 mV/Pa let’s expect about 6 dB of gain.

If it does, the amp is OK and the capsule is the culprit.