How can I use these cheap "RK87" capsules?

janahdfrazier

New member
I recently purchased two of these.

It looks like a u87 capsule, but it is a cardioid, single sided diaphragm with only two leads.

I had hopes of popping these into a couple of MXL V63M microphones I had laying around, and I knew I had would have to contend with an extended HF response, but what I was not prepared for was a huge drop in output. The output of these "rk87" capsules is much lower than the stock rk47 found in the V63M, so much so that the S/N ratio makes them unusable on anything but a very loud source. Despite this, they do sound *much* better than the stock MXL capsule, so I am inclined to find a way to make them work.

Is it normal for an rk87 capsule to have a lower sensitivity than an rk47 style? Are these capsules defective? Is it possible to modify the V63M to accommodate this rk87 style capsule, maybe with a quieter FET? If it IS possible to increase the S/N ratio, is there also a HF attenuation mod that can be made without an entire revamp of the schoeps circuit? So many questions!
 
I'd post this also in the DIY section and I'd also query over at GroupDIY. I think "RK87" is the brand name of "microphone-parts.com" I don't know if he's actually trademarked it, so these aren't actually "RK87" or even RK67 (the 87 has isolated backplates and four terminals.)

I have used some similar "knock off" K67 capsules. FYI The stock capsule in that microphone is a 32mm K67 clone, which also has the same HF rise. A couple things you could try:

-Make sure your phantom power is actually 48 volts.

-You could also try the "direct connect mod" which eliminates the coupling capacitor between the capsule and FET. I've done that on several of the generic China/Schoeps mics. See this thread: https://groupdiy.com/threads/mxl-2006-generic-schoeps-mod-question-direct-connect-of-capsule.48859/

You could also swap out the FET and/or make sure it's biased properly, but that's a little more complicated.
 
Thanks for recommending GroupDIY and providing the link to your thread! I have a lot of soldering to do today, so I will give these ideas a try and report back..
 
Thanks for recommending GroupDIY and providing the link to your thread! I have a lot of soldering to do today, so I will give these ideas a try and report back..

Did you see that other link for the "direct connect mod?" You could also use those and make an "Alice" mic from scratch, get a cheap NW or BM800 for like $15 off eBay, and then get one of these: https://hlpimpedalice.wordpress.com/

Link to the PCB is in the article as is a BOM from Mouser. It includes an HF EQ section for these types of capsules. Caveat, I haven't built one yet, but I have the older board and the parts, just need to get the mic body and the time.
 
Yep, I went with the direct connect mod as described in your thread, but no joy. No perceptible difference in the S/N or output. I did a comparison between my stock V63m, and there is a significant sensitivity discrepancy; ~15dB or so. But once again, the new capsule still sounds better. Really, the HF issue is not a problem for me; it's easy enough to throw on a high shelf. The S/N problem is the big hurdle, here. Fixing it and keeping most of the stock components/PCB intact is my goal, but it just might not be possible.

The Alice mic looks really cool! I'd love to do this build, but it seems like the PCB is no longer available. The capsule is identical (in appearance, anyway) to mine. With the schematics available I suppose it might be possible to breadboard it, but for $10.. I'd take the PCB if I'm able to find it.

Also, definitely getting 48v, as I just rebuilt my console's PSU and it has WAY more than enough juice, and all other mics are working just fine.
 
Yep, I went with the direct connect mod as described in your thread, but no joy. No perceptible difference in the S/N or output. I did a comparison between my stock V63m, and there is a significant sensitivity discrepancy; ~15dB or so. But once again, the new capsule still sounds better. Really, the HF issue is not a problem for me; it's easy enough to throw on a high shelf. The S/N problem is the big hurdle, here. Fixing it and keeping most of the stock components/PCB intact is my goal, but it just might not be possible.

The Alice mic looks really cool! I'd love to do this build, but it seems like the PCB is no longer available. The capsule is identical (in appearance, anyway) to mine. With the schematics available I suppose it might be possible to breadboard it, but for $10.. I'd take the PCB if I'm able to find it.

Also, definitely getting 48v, as I just rebuilt my console's PSU and it has WAY more than enough juice, and all other mics are working just fine.
That's odd. Did you try swapping the capsule leads? That solved an issue I was having on the same mic (with the K47 knock off.) Also, make sure the connections at the board are very clean. You may want to clean the teflon standoff with 99% isopropyl and resolder, or even open air it to the FET like the pimped alice does.

The pimped alice guy usually has boards available, I'd try again in a couple weeks, or contact the guy on Ebay. I bought one of the earlier versions from him (without the DC-DC converter) and I'm going to point to point that part of it because I want to use the same kind of capsule you're using.

There is a lot of troubleshooting tips on GroupDIY. Search there first before asking. Also, I can't remember how you test the voltage at the capsule. I don't think its a straightforward as seems and there's guidance there as well.
 
Well, my fears were confirmed; the capsule I was using was defective. If you remember from my previous post, I mentioned I ordered two of these capsules. So, I swapped them out and sure enough, one of them produces an unusably low output signal. Very unfortunate! I've contacted the seller to see if a replacement can be sent, so fingers crossed. I like my mics in pairs!

I was really excited to do this super quick "shoot out". The modded V63M has the capacitor bypass mod, and C3 and C4 replaced with 1uf EPCOS film caps. This is probably responsible for the extended LF response. Mics are about 18' from the source, aimed at the 12th fret.

I think for this instrument/passage, I prefer the modded mic, as it sounds closer to what I'm hearing when I'm playing the instrument. I will eventually do an entire track with keys, bass, drums, guitar and perhaps vocals to really put it through its paces and determine if it truly has been "improved".

Keep in mind this is a $30 mod!
 

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  • Stock V63M.mp3
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