KMV254c: Gefell MV691 to KM54c modification.

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Made a pair of these for @andYz00m . We powder coated them hot orange! They turned out really great! The schaal modified binder connector is really a nice touch too.
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I also made a cool badge with Frank at frontpanels.de. Im ordering some of those this week so they will be added soon :)

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andYz00m said:
These mics are fantastic! The kit and build are great and @SparkleBear did an awesome job.

Highly recommend if any of you are interested.

The photo of them being used was cymbal swell overdubs for an ambient brooding surf track. Sounded super cool
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Thanks bro! It was really fun building them for you and the colors made it extra fun! I had no idea how many powder coat choices there would be!

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Can you share some of the drum stems if you have a chance? I really do like how the m70 + tube head amp handles the metallic sounds!
 
Nice work, @SparkleBear! I’ve just happened upon on this thread. It changed my mind about selling my MV692 pair as this looks like fun.

Yes, same here as @elhorst - I’d be interested in this as well. I have many parts stocked already.

What are your impressions of the various capsules with the new tube circuit? Have you retained a stock MV69X for comparison to the original?
 
Unfortunately, this mod won’t work on the mv692 as easily as the internal mechanicals of the 692 are plastic. It’s certainly do-able but this board was designed for the metal rails of the mv691 and 690. The capsules and body tube are the same. I think the connectors are the same too. Bv130 transformer is the same too… so with a bit of fudging and carving the plastic to get the board to fit I think you could do it… albeit easier with the earlier models.

I do have some stock originals but have not done any comparisons yet. Years ago I used the fet bersions and they were fine. These tube models are really amazing to my ears. All the capsules play well with the Uber and I find them to actually be something unique and useful in my locker.

M71 ldc m7, m70 sdc nickel cardioid, m94 nickel card sdc with high peak, m93 Omni gold pvc.. they all sound great. I really like the m71 as a vocal mic on this mic.

If peeps are interested in this project I will do a final revision with the last changes I made after rev2 and make it available to the community :)
 
Ok! I made some updated boards, my friends. This circuit is a slight modification to the Neumann km54c. I modified the value for the plate resistor from 250k to 100k for the alternate tube choice. I also changed the voltage divider from 40v out to 60v out. The most recent revision has trimmers for nfb and cathode bypass resistor. There are 3 configurable jumpers: enable/disable NFB, connect calibration line direct to gnd or through 200r to ground, another for bypassing the trimmer for cathode incase you just want to use a fixed resistor.

The mod will use the stock gefell bv130 transformer but could also use a t13 or equivalent ratio transformer.. the solder holes are big enough to use small zip ties to secure a bobbin transformer to the pcb. And the pcb is set up for the Soviet 6s6b or the 5703 tube…

I chose a nice black color for the new revision and chose gold enig finish for the pcb. It’s going to be beautiful… ordered 20 pieces so for those of you who want some let me know! Granted i have no problems building on this board, they will be available to those of you who are interested :)

I’ll do a little step by step build thread for the mod using this next build process.

All of the capsules that work with the stock mv69x work perfectly with this design. This includes the sdc m70 cardioid, m94 distant cardioid, m93 Omni, ldc m71 cardioid and um70 multi pattern ldc. As I understand, the m94 cardioid capsules are the closest element to the original kk54 of the km54. The m70 is a bit darker than the m94… they all sound great

Excited to share this project with y’all.
 
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this circuit can also work in the Gefell PM750 microphone,
the capsule must be converted to M94 by Siegfried Thiersch,
or an M94 is used,
so you get a tube handheld microphone :cool:
 
SparkleBear said:
I chose a nice black color for the new revision and chose gold enig finish for the pcb. It’s going to be beautiful… ordered 20 pieces so for those of you who want some let me know! Granted i have no problems building on this board, they will be available to those of you who are interested :)
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Great news, @SparkleBear! They sound like they’re going to be great. Please let me/us know how much/per board and how to pay. I’m new to all of this.

I did see the Uber PSU board won’t be available for the time being.

Did you powder coat the bodies yourself or farm it out? I wonder how affordable a system would be that gets the job done correctly?

Thanks for sharing this cool project!
 
erikrex said:
Have you retained a stock MV69X for comparison to the original?
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I do have a pair stock with the schaal power cables. I have not yet gotten to do a side by side comparison.. I look forward to that experiment.

erikrex said:
Great news, @SparkleBear! They sound like they’re going to be great. Please let me/us know how much/per board and how to pay. I’m new to all of this.
Click to expand...

I’ll have to figure out how much the board will be… stay tuned.

erikrex said:
I did see the Uber PSU board won’t be available for the time being.
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I have a fully regulated psu that is designed to work for one or a pair of tube mics. It’s designed for 6.3v but depending on the trimmer and adjustment resistor it can easily accommodate 4v. The form factor is designed for the Chinese alctron supply (same size as matadors). It’s really quiet! You can either get an edcor for $35 or just use the transformer that comes in the supply (sometimes they have acoustic hum). I could potentially hook those up with this board who need a psu also.

erikrex said:
Did you powder coat the bodies yourself or farm it out? I wonder how affordable a system would be that gets the job done correctly?

Thanks for sharing this cool project!
Click to expand...

Yeah, I just had a local powder coater process the bodies. So many color choices and really makes a sturdy finish. @andYz00m chose that rad orange… good choice. I do want to eventually do my own coating but I don’t have the space right now… eventually when I have my own workshop again :)

Definitely more affordable than a km54 pair with psu. If you build it yourself you’re probably looking at $300-600 per mv69x body, $200-500 per sdc capsule, $20-150 per cable (depending on connectors and cable choice), $100-400 for psu (depending on chassis and parts/design), $50 for powder coat, however much for a good case. PCB costs to be determined but will be in line with other offerings from other members… just gotta look into it :)

If you’re putting your own time for labor then that’s not an extra cost but if you commission a build to someone then it would be that cost for labor on top of the parts.

I am glad that I can keep my km254c in storage for just special occasions and use these gefell mods for my commercial work… brings me ease and joy.

Glad to be able to share this project.
 
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I made a new revision the PSU that could be acquired along side this project for any of those who are in need of it.

Features:
Well filtered Regulated Heater supply (set up for 3.8v - 6.4v)
Well filtered Regulated B+ supply with Mike Maida's high voltage regulation scheme
(Dual regulation allows for powering of 1 or 2 mics without needing recalibration. )
LED indicator for DC
Test points for H+ and B+
Isolated grounding scheme to reduce potential noise.
Pattern voltage filtering.

Designed to fit in the smaller alctron psu chassis. But can be used in any creative way... ;)

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Mouser BOM is about $145

Very happy how this supply turned out.
 
After consideration I think it would be good to share the Schematic too.. I'm sure there are some places for improvement but its been working well for me. It uses this similar scheme to regulate the HV. LV is just simple LM317. If there are any noticable needs for revision let me know, otherwise ill be printing another revision in the next week or so. :) And if anyone wants a pcb along with the KMV54 boards just let me know.

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Resources that helped me understand this design:
http://www.pmillett.com/HV_reg.htmlhttps://www.diyaudio.com/community/threads/21st-century-maida-regulator.209067/This was the most helpful though:

Cheers :)

Mic PCBs arriving tomorrow!! :)!!!
 
Thanks for posting the schematic!

I have some questions.

What is the idea behind the series connection of capacitors C52 and C56? Makes no sense to me atm.

Wouldn't it be better to position R50 between C50 and C54?

Wouldn't a TL783 be better suited for the HV job? saw the video ;-)

I need to take a closer look, some things I don't understand yet.

Cheers

Edit: One suggestion, the LM317 likes 100nF caps at the input and output in close proximity, this should help against possible oscillations

Edit2: thanks for the links, I will check this later. Interesting!
 
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