U87 (Revision A) in BM800 microphone donor body

[Khron]

4. I have isolated the rails with electrical tape, and I used m1.6 Screws to attach the PCB's without touching anything.

Why would you NOT want the ground pads of the PCB connected to the mic chassis?

 

[RBeTT]

Why would you NOT want the ground pads of the PCB connected to the mic chassis?

The holes of the rails to attach the PCB don't align with the PCB, so I use screws from one side to another [18mm] with nuts and washers which press the PCB against the rail to secure them. Since it isn't properly secure via the holes and might touch other components, I decided to isolate it from the rails and use the ground PCB hole to connect via pin1 [ground].

Isn't that a good idea?

 

[Khron]

The holes of the rails to attach the PCB don't align with the PCB

Are those side-rails cast, or sheet metal? If they're sheet, bend them so the holes line up with the PCB's.

 

[RuudNL]

Usually the chassis of these cheap Chinese microphones are made of die-cast metal.

 

[mihi_fuchs]

Hi @mihi_fuchs

Sorry for the late reply, it has been crazy weeks...but I am finally back to answering properly your very kind reply


I'm a bit confused about how to do it. Will I use the studio monitors to produce the sine wave generator? If so, do I have to open the mic every time to adjust the trimmer and do the test again?

I have confirmed that the PAD and transformer wiring are where they should be. We both used a 3u Audio transformer, too, so I just followed the pictures.

The capsule it's new and right off the box. Since it's backplate independent, I have attached 2 black wiring accordingly to the building guide.

I don't know how to answer you. Can you be specific about where and what should I look for? I'm still a bit fresh on this new world...Sorry!


INFO | QUESTIONS

1. The mic body it's conical and not cylinder, I can feel a bit of no being centered with the bottom screw, can that be a problem? Perhaps if the tall WIMA components touch it? ;

2. Since I didn't know how to calibrate properly, I adjusted to 60v and 10.5v on RV24 and RV10, respectively.

3. Via BOM, it comes 2x as Turrets, which I don't know why. On my first U87 build, I had a turret on J3 [front capsule wiring] like the @Wordsushi had done it, which helped a lot with the hissing noise. Should I keep it or not?

4. I have isolated the rails with electrical tape, and I used m1.6 Screws to attach the PCB's without touching anything.

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Here are the build pics, Sound spectrum via EQ and the sound recording I made in my last test.

20Nov24 Sound Test | AUDIO SAMPLE [Dropbox]

View attachment 140328

View attachment 140321View attachment 140322
View attachment 140323View attachment 140324
View attachment 140325


---

As always I appreciate feedback, time, and your patience.

Best Regards,

RB

Hi Ruben,

hope everything is fine?

Sorry to hear you still have issues. Components touching the rails may be a very good explanation. However, one thing caught my eye, when looking at the pics:
If I get your wiring right, you used the "Messeingang" for grounding your basket? If that's right, you have used the wrong pad. So if you have another ground connection from the head basket to GND, you maybe simply short R4. Maybe just solder the wire in the pic below to the square pad?

Best regards,
Michael



P.S.: I just double checked - the numbering on the PCB regarding the Messeingang is wrong. 1 is the square pad, 2 the round pad. My apologies. Was not your mistake but my wrong numbering. Apologies for that.

 

[RBeTT]

Hi Ruben,

hope everything is fine?

Sorry to hear you still have issues. Components touching the rails may be a very good explanation. However, one thing caught my eye, when looking at the pics:
If I get your wiring right, you used the "Messeingang" for grounding your basket? If that's right, you have used the wrong pad. So if you have another ground connection from the head basket to GND, you maybe simply short R4. Maybe just solder the wire in the pic below to the square pad?

Best regards,
Michael

View attachment 140357

P.S.: I just double checked - the numbering on the PCB regarding the Messeingang is wrong. 1 is the square pad, 2 the round pad. My apologies. Was not your mistake but my wrong numbering. Apologies for that.

Hi Michael!

Well a bit in the dark and frustrated, but I will figure it out eventually.

Here goes the new wiring:





I have checked the clearance from the rails which was fine from any soldering poking the electrical tape. Anyway, I grounded the rails and the head basket with a direct wire and to the PCB ground as recommended on the other hole, and it got worse. Perhaps the clearance from the mic body to the components isn't enough. Truth to be told, I still don't know how to calibrate as you recommended via audio interface, But was hoping to some sort of not having this hiss issue...

At this point, I've already surpassed the 200-hour mark on this project...I will move this build to a BM800 mic body, see what happens and go from there.

Please advise me on the following, should I fully ground the Bm800 mic body as well as I did on this Aurycle one?

By the way, the BOM comes with twice the turrets. Does this is intended to be used in the HiZ connection and in J3 for the front capsule diaphragm wire, or for any other reason?

Thank you for your time and patience in responding...

Best Regards,
Ruben

 

[jp8]

Anyway, I grounded the rails and the head basket with a direct wire and to the PCB ground as recommended on the other hole, and it got worse

Not surprising it gets worse if the headbasket is grounded to the PCB ground. The grille is part of the Faraday cage, protecting it against hum and RFI interference and should be a closed, uninterrupted metal construction. Assuming the grille has insufficient contact to the body, i.e. non-circumferential or a high impedance contact, the wire connected to the PCB ground sends RF signals picked by the grille through the circuit, causing RF interference. The grille mesh should circumferentially contact the mic body and not be connected to anything else.

The best way to resolve RF interference is to: a) ensure the whole mic is a perfect Farady cage and b) have an RF Common Mode filter at the XLR input as I posted before, or use the Neutrik NC3FXX-EMC on your mic cable, preferably paired with an XLR cable with proper RF shielding.

Jan

 

[RBeTT]

Not surprising it gets worse if the headbasket is grounded to the PCB ground. The grille is part of the Faraday cage, protecting it against hum and RFI interference and should be a closed, uninterrupted metal construction. Assuming the grille has insufficient contact to the body, i.e. non-circumferential or a high impedance contact, the wire connected to the PCB ground sends RF signals picked by the grille through the circuit, causing RF interference. The grille mesh should circumferentially contact the mic body and not be connected to anything else.

The best way to resolve RF interference is to: a) ensure the whole mic is a perfect Farady cage and b) have an RF Common Mode filter at the XLR input as I posted before, or use the Neutrik NC3FXX-EMC on your mic cable, preferably paired with an XLR cable with proper RF shielding.

Jan

Hi Jan,
Thanks for responding and well explaining! Understood!

1. So, should we keep the ground connection from the rails to the grill and get ground from XLR directly into the PCB? Because it's a hole in the PCB for it...That's why I'm confused...

2. When you say RF common mode filters you mean the RF caps on the XLR PIN2 and PIN3 into the PIN1? I used neutrik but mostly I'm using sommer cable Galileo which is double shielding, which helps a ton.

RB

 

[Khron]

1. So, should we keep the ground connection from the rails to the grill and get ground from XLR directly into the PCB? Because it's a hole in the PCB for it...That's why I'm confused...

Why would you NOT want the ground pads of the PCB connected to the mic chassis?

Are those side-rails cast, or sheet metal? If they're sheet, bend them so the holes line up with the PCB's.

 

[jp8]

Hi Jan,
Thanks for responding and well explaining! Understood!

You're welcome!

1. So, should we keep the ground connection from the rails to the grill and get ground from XLR directly into the PCB? Because it's a hole in the PCB for it...That's why I'm confused...

Understand your confusion. It can be quite challenging to get RF interference suppression and proper grounding right. Even "experts" often disagree about the best approach. In this 13-page thread, it was finally fixed using that RFI filter from my previous post in this thread. That filter has a ferrite bead in the ground between XLR pin 1 and the circuit ground. With that filter, the circuit ground should not be tied to the chassis ground as it would bypass the bead.

2. When you say RF common mode filters you mean the RF caps on the XLR PIN2 and PIN3 into the PIN1? I used neutrik but mostly I'm using sommer cable Galileo which is double shielding, which helps a ton.

See also my post #35 in the thread I linked to above. Those output caps may have worked well against your neighbor's ham radio station in the 1960s and 1970s, but not against current-era Cell Phone RFI. The long wires of through-hole, wired parts, even when mounted directly on the XLR, make these capacitors act more like inductors than capacitors at Cell Phone frequencies. Secondly, they may increase THD and IM significantly, especially with KM84/U87 style circuits running at low bias currents. I'd propose to remove them, or use something like 1nF C0G caps with the shortest leads possible. Another suggestion is to insert a 470R resistor between Drain and output capacitor. It is not as effective as an RFI filter on the XLR insert, but it worked noticeably against RFI in the KM84 designs that I made, without adding noticeable noise due to the transformer attenuation.

I can imagine the double shielding Galileo cable you mentioned helps a lot. from picking up RF noise. But it's effectiveness can be greatly improved with a non-standard Neutrik EMC plug (type NC3FXX-EMC). Expensive plugs, so you don't want to equip your whole studio with it if not needed. But you could try one and if the RFI issue is gone, it's obvious it enters through the XLR. If not, the next step is to look for "RF leaks" in the mic body and grille.

Jan

 

[mihi_fuchs]

Here goes the new wiring:

And what was the outcome? Did you measure now every part of the body agains GND in the closed state of the mic? They should be all close to 0 ohm. Could you report back?

If it is a shielding problem, I suppose you should get more problem if you make a phone call and put your phone next to the mic. Try that.

However, as you describe that you have to crank the gain of the preamp very much, I suppose it could be also the problem that you lose level somewhere inside the circuit or before. Do the supply voltages read 60V?

If you go back to the BM800 body you can omit all the extra grounding wiring.

The rf bead is a nice feature, but you have already RF filtering caps. And in all other builds there has been chassis to ground connection directly without any components in between (a lot of mics, commercial ones too) do that.

Can you post pictures of the body. And measure if the body itself (metal tube around the mic) makes connection to ground by 0 ohm.

Make sure the rail does not touch anything (you could lose level by that ... )

Try these things and in case you are not getting to the happy end, report the observations - then we can try to help further ...

Best regards,
Michael

 

[RBeTT]

@Khron

Unfortunately, it doesn't fit the holes, That's why it is so much hassle...

 

[RBeTT]

You're welcome!


Understand your confusion. It can be quite challenging to get RF interference suppression and proper grounding right. Even "experts" often disagree about the best approach. In this 13-page thread, it was finally fixed using that RFI filter from my previous post in this thread. That filter has a ferrite bead in the ground between XLR pin 1 and the circuit ground. With that filter, the circuit ground should not be tied to the chassis ground as it would bypass the bead.


See also my post #35 in the thread I linked to above. Those output caps may have worked well against your neighbor's ham radio station in the 1960s and 1970s, but not against current-era Cell Phone RFI. The long wires of through-hole, wired parts, even when mounted directly on the XLR, make these capacitors act more like inductors than capacitors at Cell Phone frequencies. Secondly, they may increase THD and IM significantly, especially with KM84/U87 style circuits running at low bias currents. I'd propose to remove them, or use something like 1nF C0G caps with the shortest leads possible. Another suggestion is to insert a 470R resistor between Drain and output capacitor. It is not as effective as an RFI filter on the XLR insert, but it worked noticeably against RFI in the KM84 designs that I made, without adding noticeable noise due to the transformer attenuation.

I can imagine the double shielding Galileo cable you mentioned helps a lot. from picking up RF noise. But it's effectiveness can be greatly improved with a non-standard Neutrik EMC plug (type NC3FXX-EMC). Expensive plugs, so you don't want to equip your whole studio with it if not needed. But you could try one and if the RFI issue is gone, it's obvious it enters through the XLR. If not, the next step is to look for "RF leaks" in the mic body and grille.

Jan

Sir, Thank you very much for your valuable insight! This has been very humbling.

I will read the whole thread and learn from wise people like yourself. I will consider as well buying the XLR plugs for testing purposes. About the caps, at this point, I will move this build to the body initially intended to be built on. For testing purposes, If it is any mic body issues, they will be eliminated or at least attenuated which, unfortunately, I still have a long way to learn and figure out by myself, what and how to fix the main problems.

I think it's wise to learn better electronics before engaging more in very basic questions to experts like you and @mihi_fuchs .

I really appreciate your time, expertise, and patience with first builders like myself.

Thank you,

Best,
RB

 

[RBeTT]

And what was the outcome? Did you measure now every part of the body agains GND in the closed state of the mic? They should be all close to 0 ohm. Could you report back?

If it is a shielding problem, I suppose you should get more problem if you make a phone call and put your phone next to the mic. Try that.

However, as you describe that you have to crank the gain of the preamp very much, I suppose it could be also the problem that you lose level somewhere inside the circuit or before. Do the supply voltages read 60V?

If you go back to the BM800 body you can omit all the extra grounding wiring.

The rf bead is a nice feature, but you have already RF filtering caps. And in all other builds there has been chassis to ground connection directly without any components in between (a lot of mics, commercial ones too) do that.

Can you post pictures of the body. And measure if the body itself (metal tube around the mic) makes connection to ground by 0 ohm.

Make sure the rail does not touch anything (you could lose level by that ... )

Try these things and in case you are not getting to the happy end, report the observations - then we can try to help further ...

Best regards,
Michael

Hello Sir @mihi_fuchs

First and foremost, thank you so much for replying, and as always, very helpful.

Since I have 2 Bm800 mic bodies initially intended for this build, and since I am still very fresh in the electronics understanding, I will do exactly to the letter, trying to eliminate any kind of variations or unnecessary hassles, and for testing purposes, I will move this build to the Bm800 and see if everything works as it should. Like this, it's wiser than wiggling and complicating something that I don't have any knowledge of how to fix it.

I will keep this info for future reference, and I will keep you posted and report back as soon as I do it as well.

It has been very humbling to learn from all of you.

Thank you Sir,

Best,
RB

 

[Khron]

@Khron

Unfortunately, it doesn't fit the holes, That's why it is so much hassle...

Are those side-rails cast, or sheet metal? If they're sheet, bend them so the holes line up with the PCB's.

Would you mind reading all my questions and answering them already???????

 

[RBeTT]

Would you mind reading all my questions and answering them already???????

Hello Sir, My apologies.

I have read all of it...have you read as well all my posts and threads? I mentioned the body I was using...

It's side rails cast...as I said before, it doesn't align horizontally or vertically with the PCB holes. Even if it was possible to bend would be to far from it...

 

[Khron]

It's side rails cast

Ok, thank you, finally...

 

[Jarbcd]

Hey all,
Finally getting around to building mine, and I'm almost done!
The schematic and updated BOM calls for a 47uh inductor. When I ordered form Mouser, I used the BOM provided by Ostrol, which I should have checked for myself. Anyway, that BOM indicates a 68uh inductor.
If my understanding is correct, I believe that the increase in value here would mirror an increase in HF suppression, is that right? If so, how much trouble am I in if I move forward with the 68uh inductor? Should I just be patient and order the 47?


Edit: context
I'm only asking because I definitely don't want to lose TOO much top end. If the difference is minimal, I'll proceed, but if it will result in a noticeably darker sound, I'll just have to be patient

 

[Khron]

I'm only asking because I definitely don't want to lose TOO much top end.

If you happen to notice that (which is highly unlikely to begin with), just reduce the value of the high-frequency feedback capacitor..?