Possible to mod a K870 to fit a vintage U87 DIY build?

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Zebra_PD

Well-known member
Joined
Nov 7, 2017
Messages
196
Location
Umea, Sweden
Hi all!
I'm about to embark on my first real DIY journey and build a U87 clone for use in our studio :)

So here comes real newbie question so hope you guys have patience with me.

I guess that the only real difference between a K87 and a k67/k870 capsule is that the K87 has isolated backplate and the other one not right? K87 has the two back electrodes insulated against each other. This was necessary for electrical reasons, as no DC/DC converter was available at the time according to a post I found on the Neumann forum. Mechanically (spacings etc.) and acoustically they are identical? This also means that there's a 6db difference in headroom between them?

So here comes my real questions :)

I think I read somewhere that it would be possible to modify a u870 capsule to work for a vintage U87 build by adding a forth cabel?
How exactly would one do this (if it's possible and/or "right" way of doing it)?

The reason I'm asking is that looking around the web the price of k870s is a lot cheaper then vintage K87. And even if I probebly end up going with a good clone capsule for my first build I still like the option swap for a "real" Neumann capsule and/or try to build a true vintage U87 replica in future.

 
Without separating the two "halves" of the capsule and replacing the (uber-thin) brass shim with a same-thickness isolating one, i don't see how that could be possible. Laws of physics and stuff, y'know? :)
 
haha well of course it would not be easy ;)

Im still figuring stuff out so thx for replying to what I know understand was a stupid question :) 
 
"The only stupid questions are the ones that remain unasked" ;)

That being said, it's technically quite easy, it's just 4 screws holding the two halves together. The tricky part is getting the right-thickness insulating shim, with all the screw holes in it, etc.

If you take a peek over on my blog, i made a double-diaphragm franken-K67 capsule out of two AKG Perception 200 single-sided capsules, a while ago ;D You can see that shim i'm talking about there.
 
It's easy peesy !

I can send you the 53um shim and you can then just replace it.

It may take some readjusting once you get the four screws in.

Be sure to check for impedance with each screw so you can find the culprit if one makes a connection.

All I need is your address!

Cheers
 
Thank you both so much!

I haven't bought the original K87 capsule yet but if/when I'll do i most def will send you a PM Ben!

At the moment I have only bought a body from Murdock here on the forum. I'm still trying to figure out how "adventurous" I'll end up being for my first ever build. My dream would be to get as close as possible to a "real" Neumann  using the PCB and BOM in the vintage U87 build thread and try to paint it as close as the original and put on a badge etc to make a sort of replica. But the 140+ page thread and all the unknown difficult technical terms and numbers intimidate  me A LOT. 

So perhaps I should be "smart" instead and order a less complex kit with all I need from one of the suppliers in the white market and build a more complex mic when begin to find one’s feet?

Soo still trying to figure that out haha :)
 
Jut a quick note, if you want to save the time, I do have a few original hanging around that I would sell.

let me know.

thanks.
 
Zebra_PD said:
Thank you both so much!

I haven't bought the original K87 capsule yet but if/when I'll do i most def will send you a PM Ben!

At the moment I have only bought a body from Murdock here on the forum. I'm still trying to figure out how "adventurous" I'll end up being for my first ever build. My dream would be to get as close as possible to a "real" Neumann  using the PCB and BOM in the vintage U87 build thread and try to paint it as close as the original and put on a badge etc to make a sort of replica. But the 140+ page thread and all the unknown difficult technical terms and numbers intimidate  me A LOT. 

So perhaps I should be "smart" instead and order a less complex kit with all I need from one of the suppliers in the white market and build a more complex mic when begin to find one’s feet?

Soo still trying to figure that out haha :)

Here's my advice - you're never going to build a Neumann.  If you want a real vintage U87, buy one.

I can't afford the real mics, otherwise I would buy them.  I'm interested in mics I've read about, seen pictures of, and "know" how to use.  For instance, U47 is amazing on male vocals, bass instruments, room mic, etc.  U87 sounds good on everything and great on a few things.  Bruce Springsteen and Tom Petty like to use ELA M251 for vocals.  U67 Drum overheads, vocals, guitars, etc.  On and On.

I'm buying the best parts I can afford, which are focused on performance and not thrills.  So Chinese bodies, capsules from beezneez, poctop, maiku, Heisenberg, Campbell, and I like cinemag transformers.  I put in good electronic parts, use Neutrik connectors and good cables for the tube mics.

The mics I've built sound great and work in the applications I expected them to.  Does my U47 sound exactly like a real one?  I have no idea.  A real one costs about the same as my car, and I have $800 in my clone mic.  It sounds amazing on my voice, and actually surprised me on my acoustic guitar on a track I was working on the other day.  Works for me!

These mics are pretty simple circuits for the most part, but quite hard projects.  They aren't designed to be a paint by numbers "kit", and things could be a little clearer in the instructions at times (but I'm far too lazy to do anything about this!).  If you look at the schematics and think about it you probably can figure it out.

I don't really recommend starting with the neumann clones.  The FET847 PCB is a much easier build and sounds great.  It is somewhat like a cardiod u87 without the corrective EQ.  Using a K47 style capsule with this circuit turns out very nice.  I would recommend picking one of the artisan capsules and running with this project to start


 
Please bear in mind that it is not sufficient to replace the aluminium backplate spacer for an insulating one since the 4 metal screws will still connect the backplates electrically!
The original M1,4 screws on the K87 are half way insulated with a plastic sleeve. If you know someone with a precision lathe and some skill it´s not too difficult to make such screws. Just machine away the upper portion of the thread and cover with plastic sleeves. It´s advisable to use stainless steel M1,4 x 8mm screws - like the original. Brass is too weak after thread removal and you can still use the original screws to reverse the mod.
On the K87 one backplate is connected via a solder lug directly on one of those screws - see attached picture.

One more important detail: The K67/870 archieves the vent for atmospheric air pressure adaption by the use of a C-shaped backplate spacer - it´s not a full ring!
The K87 uses a round spacing ring and the backplate fitting is airtight. The necessary vent is a tiny hole in the rear diaphragm.

BTW, I just measured the alu backplate spacer of an original Neumann K67/870. It is precisely 50µm. I haven´t measured an original K87 spacer but since the capsule components are identical 50µm might be the best choice here as well. Varying spacer thickness affects frequency response and polar patterns and sensitivity. 


 

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MS Vienna said:
Please bear in mind that it is not sufficient to replace the aluminium backplate spacer for an insulating one since the 4 metal screws will still connect the backplates electrically!
The original M1,4 screws on the K87 are half way insulated with a plastic sleeve. If you know someone with a precision lathe and some skill it´s not too difficult to make such screws. Just machine away the upper portion of the thread and cover with plastic sleeves. It´s advisable to use stainless steel M1,4 x 8mm screws - like the original. Brass is too weak after thread removal and you can still use the original screws to reverse the mod.
On the K87 one backplate is connected via a solder lug directly on one of those screws - see attached picture.

One more important detail: The K67/870 archieves the vent for atmospheric air pressure adaption by the use of a C-shaped backplate spacer - it´s not a full ring!
The K87 uses a round spacing ring and the backplate fitting is airtight. The necessary vent is a tiny hole in the rear diaphragm.

BTW, I just measured the alu backplate spacer of an original Neumann K67/870. It is precisely 50µm. I haven´t measured an original K87 spacer but since the capsule components are identical 50µm might be the best choice here as well. Varying spacer thickness affects frequency response and polar patterns and sensitivity.

No need to machine away the thread, just use clear heat shrink for the first 1/2 of the thread and shrink it.  Also the Centre spacer ranges from 49-54um.  This is due to different measuring devices, film inconsistencies and batch consistency!  A um is a really small measurement and can be very hard to measure accurately.  Two different devices will give you two different readings on the very same piece of film, especially at the um level!

Btw, I'd really love to see the hole in the rear diaphragm!  Any pics!
 
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