U87 (Revision A) in BM800 microphone donor body

[mihi_fuchs]

I started DIY three years ago right before all the lock down action started and as it provided quite some nice distraction I had quite some projects going. In the dropbox link below you can find my attempt on transferring a U87 based layout to the BM-800 donor body.
The donor body is particularly cheap and does the job well in my opinion.

In the link below you can find the BOM, a detailed building guide and some sound samples recording OH and some vocals with the mic. The sound samples are unprocessed unless otherwise stated (the vocals are also presented once with EQ). Both instruments were recorded by the U87-based clone in the BM800 body, with the Dachmann K87 capsule.

Here is the link:
https://www.dropbox.com/sh/70et2o4atzujc6x/AAAJNY-4vQnjCFEDbcRLeHWla?dl=0


I had to split the curcuit part into two because of the limited space. I used a Dachmann K87 with non-isolated backplates and a MAIKU K87 capsule with isolated backplates (both worked fine), a 3u audio GZT-87 transformer (ratio 9.5 : 1) and a 3D printed capsule holder. Most parts were obtained from mouser, the styroflex capacitors were purchased from reichelt.at and budgetronics (for details see the BOM in the dropbox folder).

Last but not least the project has been highly influenced by members of this forum (e.g. matador, khron, poctop, chunger, and others) - thanks for all of their posts, ideas and solutions that they have shared over the times.
The DC-DC converter in the current version (1.7) is based on Open source project: Little oscillator PCBs for LDCs shared on Osh Park with adoptions made (Schoeps type oscillator).

As there has been already a discussion about the BM-800 donor body maybe the sound samples prove helpful to some forum members.

As for the previous post I'll make some of the spare PCBs available on the black market for anyone interested

Thanks again for all the nice hours I spent browsing through this forum,
all the best
mike

 

[mihi_fuchs]

In case anybody is interested, I put the remaining PCBs on the black market.

here is the link:
https://groupdiy.com/threads/pcbs-for-u87-in-bm800-microphone-donor-body.78373/
here is another link to leftover Styros for the built - maybe somebody is interested:
https://groupdiy.com/threads/left-o...ne-builts-mostly-630v-capacitors-in-eu.79796/
I'm also open to exchanging them against other projects you may have elaborated - just drop me a note if you're interested.

Cheers
Mike

 

[mihi_fuchs]

As one of the new members pointed out, I forgot to describe the switches [polar pattern (Card., Fig. 8, Omni), PAD and Low Cut function]. These can be simply jumpered on the board. Additionally one can use the pins for jumpering to wire the PCB to a real sitch by a connector (I never tried that myself). More details can be found in the new Building Guide in the dropbox folder (Version 1_1).

Apologies as I forgot to include it right away.

Cheers Michael

 

[mihi_fuchs]

I figured out that there have been two mistakes in the BOM - First C6 is there two times. The right value is the one with 0.47 µF.
Second, R4 is missing in the BOM - it is a 560 ohm, 0.25 W, 1% resistor.
Sorry for any inconvenience that may have caused!

I corrected the BOM. The file BOM v2 has these mistakes already corrected.

Cheers Michael

 

[TwentyTrees]

Hi Michael,

This looks like a really great project! I've got an old M-Audio Luna that I don't use, and wonder if your U87 boards might fit - I couldn't find the PCB dimensions anywhere in your documentation, are you able to share them?

Thanks!
Andy

 

[mihi_fuchs]

Hi Andy,

sure no problem. Find the dimensions in the attachment. They are the same as for the U47FET built.
If you need any further information or dimensions just let me know.

Cheers
Michael

 

[soliloqueen]

Dachmann K87 with non-isolated backplates

How? What changes did you make to the board to allow a non-isolated backplate to be used?

 

[mihi_fuchs]

Hi soliloqueen,
There's no difference. The design polarizes the diaphragm. So the polar pattern is simply selected by the polarization of the rear diaphragm (+60, 0 or -60V). Front stays constant on +60V and the backplates are 0V. Therefore it does not matter if they are isolated or non-isolated. For isolated backplates you can find two connections on the board. Nevertheless they are electronically connected. But the same is true for the original design (see schematic of Rev A and fig. b below).
Cheers
Michael

 

[doqmemory]

@mihi_fuchs do you have schematics for your version of the circuit?

 

[soliloqueen]

Hi soliloqueen,
There's no difference. The design polarizes the diaphragm. So the polar pattern is simply selected by the polarization of the rear diaphragm (+60, 0 or -60V). Front stays constant on +60V and the backplates are 0V. Therefore it does not matter if they are isolated or non-isolated. For isolated backplates you can find two connections on the board. Nevertheless they are electronically connected. But the same is true for the original design (see schematic of Rev A and fig. b below).View attachment 89148
Cheers
Michael

The original u87 doesn't polarize the capsule this way. It uses a circuit where it uses the 48 volt phantom power directly and it doesn't support non-isolated back plates because of the way it accomplishes switching the polar patterns IIRC. That's why I was asking how you accomplished this. Do you have a different method for providing the polarization? Did you integrate the oscillator sub board project etc

 

[mihi_fuchs]

@mihi_fuchs do you have schematics for your version of the circuit?

Hi, of course, schematics are in your mailbox ;-) ...

 

[mihi_fuchs]

The original u87 doesn't polarize the capsule this way. It uses a circuit where it uses the 48 volt phantom power directly and it doesn't support non-isolated back plates because of the way it accomplishes switching the polar patterns IIRC. That's why I was asking how you accomplished this. Do you have a different method for providing the polarization? Did you integrate the oscillator sub board project etc

Exactly - from Rev. A onwards the capsule connection changes to the grounded capsule assembly. The older version of the U87 requires - as you say - the non-isolated back plates.
And exactly for a second time - the polarization voltage is increased by an oscillator circuit similar to other projects and other circuits also used frequently by other designs here (I also used the same approach for the schoeps type circuit I posted here: Dual Output Microphone: 2x Schoeps Circuit + K87 capsule BM-800 Donor Body)).
That increases the polarzation voltage to ca. 60V (I don't remember the exact value anymore, but it should be given in the assembly guide - if not contact me again - I can measure it for you very easily) ...

Hope that could help,
Best regards
Michael

 

[Khron]

And you missed soliloqueen's question from the end of his post

 

[mihi_fuchs]

And you missed soliloqueen's question from the end of his post

Oh, I'm very sorry - I thought I answered everything. But maybe the schematics can help. If you mean, what I changed about the schematics: the audiopath is very close if not identical (I think I omitted the inductors near the XLR ...) to the U87Ai schematics.
The polarization voltage is coming from the oszillation circuit (see schematics). But others have used the same design for their clones (for the Schoeps type circuit that supply of voltages is all over the designs herein) and for the U87 I think the Mic and Mod design uses the same setup.
For the limited space in the donor body, that was fitting very well ...

Hope I was able to answer your questions, otherwise just help me out and let me know what I missed ...

Cheers
Michael

 

[soliloqueen]

Oh, I'm very sorry - I thought I answered everything. But maybe the schematics can help. If you mean, what I changed about the schematics: the audiopath is very close if not identical (I think I omitted the inductors near the XLR ...) to the U87Ai schematics.
The polarization voltage is coming from the oszillation circuit (see schematics). But others have used the same design for their clones (for the Schoeps type circuit that supply of voltages is all over the designs herein) and for the U87 I think the Mic and Mod design uses the same setup.
For the limited space in the donor body, that was fitting very well ...

Hope I was able to answer your questions, otherwise just help me out and let me know what I missed ...

Cheers
Michael

Oh, i see where the confusion happened. Around here we tend to call out whether or not it's based on the A version pretty explicitly, because people think they perform differently. If you say u87, people will assume you mean earlier revisions.

 

[mihi_fuchs]

Oh, i see where the confusion happened. Around here we tend to call out whether or not it's based on the A version pretty explicitly, because people think they perform differently. If you say u87, people will assume you mean earlier revisions.

Hi Soliloqueen,

Yeah, I see the problem - sorry, that was unintentionally - I just adopted the thread title. I just recently started to post here - sorry for the confusion.

Thanks for the hint ,
Cheers
Michael

 

[doqmemory]

the audiopath is very close if not identical (I think I omitted the inductors near the XLR ...) to the U87Ai schematics.

What is the purpose of those two inductors (Dr 1/Dr 2) in the U87/U87i circuit?

 

[Khron]

Assuming you mean that "connected" pair of inductors in the U87A / Ai (-i is the designation for the XLR connector version) they're the heart of the oscillator that ends up creating the +/-60V for biasing the capsule.

 

[doqmemory]

Assuming you mean that "connected" pair of inductors in the U87A / Ai (-i is the designation for the XLR connector version) they're the heart of the oscillator that ends up creating the +/-60V for biasing the capsule.

Not the ones in the oscillator, but the ones connected to audio + and - of the XLR 3-pin connector. These are labeled Dr 101 and Dr 102 on the U87Ai schematics, Dr 1 and Dr 2 on the U87/U87i schematics.

 

[Khron]

Anti-RFI (Radio-Frequency Interference) filters, in conjunction with the small capacitors connected to them.