Groupbuy capsules on BM700/800 microphones. Opinions and suggestions appreciated

Hi everyone, I'm trying to figure out what to do with the K47s and CK12s I recently got from the groupbuy.
I bought 2 of each and 2 BM700 + 2 BM800, as donor bodies for my projects.
For the CK12s I'd like to make a set of C12/C414 style mics. I only used a couple of C414-TL2 (which I liked a lot), so I don't really know on which model should I take inspiration.
For the K47s I was planning on making one with valve and the other FET?
For both I'd like to make patterns and pad switches.
I know the schematic of the BM700/800 is not compatible with LDC, as it is developed for using it with electret capsules. And I also know BM700/800 chinese mics are not pro, but that's what I could get now.
I'm newbie on microphones engineering, I have experiences on electronics though, so I don't need explanations on the components but just clarification on the various schematics for mics.
Ok, finally the questions:
- which schematics should I use for the projects I'd like to achieve?
- is it worth modifying the pcb of these mics for any of my projects or should I just go for new PCBs?
- will you choose for making a set for the K47s or is it ok for one valve and one fet?
- will the valve style fit in these bodies?
- which model of mic body will you choose for the CK12s and which for the K47s?
Sorry fot the dumb questions, I bought everything in a GAS moment with lots of ideas, but now I don't really know what to do with them. I've read many posts talking of so many projects, schematics, pcbs and I feel a little confused now, so any opinions, suggestions or explanations will be very appreciated.
Thanks
Fabio

Well, it is and it isn't compatible

http://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html

The Chinese 'BM-700' microphones can't be compared with the original ISK BM-700!
http://www.micsdirect.com/iskbm700.htm
The ISK microphones use a real condenser capsule and the construction of the body is a lot better!
In fact, the ISK's aren't bad at all.

The 'good' BM-700 microphones are sold under different brand names: 'McCrypt' (Conrad) and 'Pronomic' (Kirstein)

Thanks Khron, I read that article, but I wasn't sure if it is worth modifying the circuit or was a better approach if I just make or buy new PCBs for new circuits.
Sorry RuudNL, they're the cheapest mics available, chinese BM700/800, not ISKs.
My bad, I didn't specified that.

Don't expect that chinese k12 sound like brass or teflon CK12.
For Fet (for both capsules) i recommend KM84 circuit, if you need PCB, you can order from Dan "Poctop".
Circuit is easy as hell so you can even build it on perfboard.
Another one is based on schoeps topology but to be honest more profitable is to buy used mxl like 2006, 990, 770 etc. and mod only few components and change the capsule.
For tube circuits there's many options, also you don't need any PCB, perfboard, turretboard or any PTP option is great.
I can recommend my projects
This one is truly simple, and it's easy to make PSU, you don't need really high voltages:
http://groupdiy.com/index.php?topic=62601.0

This one is little bit more complicated but sounds really good:
http://groupdiy.com/index.php?topic=62423.0

With all mentioned options there's no problem to use pad and hpf if you really need it in all of it.

I think the best thing you can do is forget the original PCB (intended for use with an electret capsule) and make two new PCB's; one with the DC/DC converter for the polarisation voltage and one with the audio circuit, as used in the 'good' BM-700's.
If you want a 'serious' microphone, you will need a DC/DC converter anyway.

RuudNL said:

If you want a 'serious' microphone, you will need a DC/DC converter anyway.

Click to expand...

Ok, am going to put C414EB P48 and U87 into the trash

Ruud, if you'll check the link i suggested above, you'll see that "even" the stock circuit can be made to work pretty well, with minimal modification

RuudNL said:

I think the best thing you can do is forget the original PCB (intended for use with an electret capsule) and make two new PCB's; one with the DC/DC converter for the polarisation voltage and one with the audio circuit, as used in the 'good' BM-700's.
If you want a 'serious' microphone, you will need a DC/DC converter anyway.

Click to expand...

Some time ago Camilo Silva published this PCB with the Scott Dorsey schematic from Recording Magazine:

http://groupdiy.com/index.php?topic=37258.0

He is very accessible and no doubt will share his design with you if asked.


Regards!

HL

Technically, the stock circuit in these BM800's is a slightly altered Schoeps circuit, actually.

For further details & circuit analysis: http://audioimprov.com/AudioImprov/Mics/Entries/2014/12/18_Another_Chinese_mic_Circuit.html

Khron said:

Ruud, if you'll check the link i suggested above, you'll see that "even" the stock circuit can be made to work pretty well, with minimal modification

Click to expand...

Yes, it can be done, but you would have to add a DC/DC converter anyway.
It that case I would prefer to also make a new PCB for the audio part, instead of modifying the existing board. Much neater!

Thanks for the replies, guys.
First of all I don't expect this capsules to sound like any other,  I know that. Just trying to get the best I can, out of them.
Thanks for the tube projects, I'm gonna give a try at the first one, probably. It looks very simple, so why not?!
May I ask links for the FET ones, KM84 and schoeps? I've found many with different values, so I don't know which one to use. As you say it's so simple I can even make them on perfboard, I may try both and then choose.
Thanks Khron for the article.

For In76d, when you've done with your C414EB P48 and U87, just let me know which trash bin do you use  ;D

Rasta3 said:

May I ask links for the FET ones, KM84 and schoeps? I've found many with different values, so I don't know which one to use. As you say it's so simple I can even make them on perfboard, I may try both and then choose.

Click to expand...

http://audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html

First try KM84. Leave schoeps for later.

http://groupdiy.com/index.php?topic=61057.0

Use JB schematic for reference.

Buy cheap and tiny Neutrik NTE10/3 trafo for start - is worth.
After all you can always upgrade transformer if you will need.

If you don't want to deal with DC-DC converters, you can use the "DIYgenericNonElectret.pdf" schematic from our good friend, Zapnspark, available at the files section on the Yahoo Micbuilders Group.

Its a Schoeps style schematic, wich uses 35v polarization very similar to the Scott Dorsey's from a Recording Magazine Article.

On the other side, if you don't want to deal with desigining and building your own PCB, the easiest option is to use Poctop FET847 board, wich fits the BM800/BM700 body.  The bill of materials from mouser is also available.

Regards!

Thanks guys, I think I'll follow some of your suggestions.
I'll make 2 KM84 circuits for the 2 ck12 capsules in BM700 bodies.
For the K47s, I'll use the BM800 bodies, one with the first tube project In76d linked to me and for the other one I'll just try to modify the stock "shoeps-style" circuit.
It won't be brief, but once I have updates or doubts, you'll hear news from me.
Thanks again 

Hello, does anybody has 2x capsules (12 or 47 style) to sell me?
Thanks, regards
Enrico

Hi, I finally ordered the PCBs and components for the Fet847s.
I went with Dan's PCBs and mouser BOM. The boards are almost done, now the questions are:
- how to bias fet without oscilloscope? Is there any other valid method?
- should I remove the inner mesh of the headbasket as I've seen it suggested somewhere?
- I've seen the C2 less and more bass option, obsiously I wanted the "neutral" bass, I went with the 4.7, could it be ok?
- what's the purpose of capsule test capacitor (500V 75pF)? Is it for biasing the fet?

I'm also trying to modify the stock circuits, for the other 2 mics, as suggested, following the audioimprov.com schematic.
The tube project is in stand by for the moment, just wanted to hear the mics working for now
Thanks guys for your support and also many thanks to this community, I didn't know where else to get the infos found on this great forum. I tried to follow as many suggestions I found, also I hope I did the High impedance connection the right way, it has been a bit confusing at start but hopefully all went good.

Rasta3 said:

- how to bias fet without oscilloscope? Is there any other valid method?

- what's the purpose of capsule test capacitor (500V 75pF)? Is it for biasing the fet?

Click to expand...

One way is to do a test signal BEFORE YOU CONNECT THE CAPSULE. You take a mic-level signal from your interface, for instance, and play it through a capacitor to the FET's grid, and then you can start around 11.5V and tweak +- based on how it sounds. You might want to push it a little so you can hear when it starts to distort, it might make it a little easier to hear where to bias it. Alternatively, you can just jam with the approximate ideal voltage. I am not convinced you or anyone else would notice the difference between a perfect bias done with an oscilliscope and a "that sounds about right" bias done with your ears. The last mic I built with variable bias I did that and it won a (non-scientific) blind shootout at the studio on 2 out of 3 of our voices against a pretty well-regarded u47 clone, so I must have done something right even if it's not repeatable.

I've seen the C2 less and more bass option, obsiously I wanted the "neutral" bass, I went with the 4.7, could it be ok?

Click to expand...

That capacitor forms a high-pass filter with the resistance in parallel with it (more or less). If you're recording some really deep bass (like if you stick this in front of your bass amp and play a 5-string, you will absolutely be able to tell the difference between the 4u7 and a 22uF. If you're recording voice or electric guitar, you will probably never notice the difference. In fact, you'd probably end up cutting what the 22uF would add in post production.

Also, note that the capacitor won't make up for any shortcomings in the capsule -- if your capsule is low-end shy, increase the source capacitor size might help a little but it can never BOOST the bass -- it only sets where it gets cut.

You can drive yourself nuts with the finer details, so my advice is to just decide between the two and live with it.

Hi Jon, thanks for the answers.
About the high pass filter cap, I just thought the same thing. I just wanted to be sure the 4u7 was the "stock"  value for that circuit. I didn't want either more or less bass, but I'm sure I wouldn't even notice the difference, if not in particular circumstances, as you said.
About the biasing method, I'm not sure I understood it completely.
What I understood....I have to test the complete circuit (with transformer and 75pF cap instead of the capsule), connect the circuit through a pre and turn the phantom power on, put the black probe of a multimeter to ground hole and the red probe to the drain hole near the fet, turn the variable resistor screw till I get 11,5V, is everything right until this point? Could I just leave it like this? I mean, I know it isn't a perfect bias, but at least is in the right working range, isn't it?
What I didn't undestood...where do I feed the audio signal into the circuit? And how to adjust it by ear? Am I assuming to understand an exact spot where it can be considered biased? Also, as I want to make a "stereo set", can I replicate the bias by ear for the two mics?
One last question, about the transformer, I took the tiny neutrik nte10/3 suggested from In76d. I attached the pic for schematic. Are the dots plus, minus, or is the same as long as I refer to them equally?
Still interested if anyone can suggest me if it's worth removing the inner mesh layer from the headbasket.
Thanks