U87 (Revision A) in BM800 microphone donor body

[Potti18]

Finally got mine finished today

You had to do some adjustments on that SC400 body or the PCB fit in perfectly? Starting mine very soon and can get one for like 10€.

 

[mihi_fuchs]

Hi @mihi_fuchs



Yes, I will do.




I have checked the U87 Building Guide and for my understanding, to refine RV10 for the best sweet spot is to generate a tone via J4. How can I do that without special equipment?

If not, which cheapest kind of machine should I be looking for?

Or preferable a portable digital tone generator or portable digital oscilloscope will do the trick for this and bullet proof for future projects?

A low distortion sine wave generator. Maybe you can use your sound card for that too. I never tried. But you can give it a go. Just be careful and start very low in volume.

I used GND on Capsule/body which i took a cable from XLR pin1 and J9, is that correct?

that's perfectly fine. Values look good too.

Good idea, let's see if I can figure it out.

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| QUESTIONS |

1. I must give a lot of gain to have a reasonable level of sound, is this normal?

Nope, make sure you have the PAD jumper in the right position. Check with guide, there is a description for it.
Additionally make sure you have put the Transformer wires in the right connection holes. If you changed the direction of the transformer that can also result in low signals.

2. When I activate phantom power and start giving gain, I hear some crackling which goes away after a few seconds ON. Is this anything with the transformer? Does it need to break in on order to have normal results?

No, a crackling noise is fine or the moment switching it on. But not for too long. The capsule is fine, is it?
The transformer has little to do with the phantom. It is more the DCDC converter. Voltages around the Transistor on the DCDC board are stable, are they?

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Always very helpful. Thank you so much @mihi_fuchs

Best Regards,

RB

 

[RBeTT]

Here you can find some good info about soldering
If i were in the US I'd try the Jli capsule you can find it here TSC-2
Never tried out for my build as I'm from Italy and shipping makes it inconvenient, but they're not so expensive so worth a try in my opinion if you are in the US.

About the cleaning process, I didn't wash the pcb as you see in the video you mentioned, I've just used some Isoprophyl alchool on a towel and a toothbrush, or cotton fioc for the HiZ junction, and the mic is incredibly silent.

One thing I receltly implemented are a pair of 22nF caps at the XLR, as I was having some RF noise.. now it works perfectly.

@antocu ,I'm kind of having the same problem with RF noise...which part specifically [on Mouser] did you use as 22nF caps specs for the XLR?

Thanks

 

[antocu]

@antocu ,I'm kind of having the same problem with RF noise...which part specifically [on Mouser] did you use as 22nF caps specs for the XLR?

Thanks

Hi, i think I had these caps K223K15X7RF5UL2

 

[mrgrooves666]

What kind of noise you're getting? I have built three of these, and two have sort of a hiss noise, more of a fizzy kind, not smooth, and kind of over the normal noise floor, but its only present with certain preamp combinations, and only noticeable at very high gain settings, otherwise not noticeable under normal circumstances. I think it might be RF interference too, since it was mentioned somewhere on this thread, mihi omitted some inductors that seem to act as RFI filters too. Boards are clean, and so is hiz section too, and mics sound wonderful. I'm going to give a try with the RFI OPA filter too, thanks for the tip....

 

[MicMaven]

This is a live video recorded with fuchs' u87. . . . This is the 8th single by Korean singer-songwriter Jo Jin-young.

What a treat. Lovely voice, pretty song, and very relaxing and pleasant. I do not notice the microphone, so it must be OK. A moment of respite and escape - thank you. James

 

[RBeTT]

What kind of noise you're getting? I have built three of these, and two have sort of a hiss noise, more of a fizzy kind, not smooth, and kind of over the normal noise floor, but its only present with certain preamp combinations, and only noticeable at very high gain settings, otherwise not noticeable under normal circumstances. I think it might be RF interference too, since it was mentioned somewhere on this thread, mihi omitted some inductors that seem to act as RFI filters too. Boards are clean, and so is hiz section too, and mics sound wonderful. I'm going to give a try with the RFI OPA filter too, thanks for the tip....

I'm still working on them. I need to set the gain very high, and I have a bit too much floor noise, too. I will do what Mihi said and see how things work out. I'll keep you posted.

 

[jp8]

Hi, i think I had these caps K223K15X7RF5UL2

These are X7R caps, generally frowned upon when used in the audio signal path for the distortion they add and their microphonic behavior. Admittedly, I'm not sure whether that will be audible. For RF purposes, they are at least much better than film caps.

In many mic designs, you'll find these caps as RFI filter. The 22nF value originates from an era without cell phones and in many cases prove to be ineffective against cell phone interference, which is the most common source of RFI nowadays. Even when these caps are mounted on the XLR's. I've done RFI tests on various mics with various 22nF locations. It was a hit or miss whether it worked or not. And combined with the capacitive load of long XLR cables, it could also cause early top-end fall-off.

The KM84 has a similar RFI susceptibility. I was able to effectively suppress the cell phone RFI issue, but only by adding a SMT component RFI filter, combined with a XLR insert with short pin 1 to chassis connection (Being short is important here. Literally every mm of wire between RFI filter caps and chassis ground reduces the effectiveness. That's also the reason wired components are less effective than SMT parts.) It's the same XLR insert as used in the Takstar CM-63, which has very effective RFI suppression.

I've copied the RFI filter to a small PCB that should be mounted on the XLR insert. See attached picture. I've tested it in several LDCs and works quite effectively when tested with an RF jammer between 500MHz and 1.2GHz. Will make the board available through PCBway in due time.

See also here: Post in thread 'Is there a way to reduce cell phone interference in DIY KM-84?' https://groupdiy.com/threads/is-the...-interference-in-diy-km-84.86707/post-1160686

Jan