Hey Y'all,
I'm excited about my KM54 build and wanted to share it with you all here. The plan was to build a pair of SDC tube mics. Years ago I was inspired by the idea of a tube SDC DIY project. I didn't see much info about it... After a lot of research and some conversations with Stephen of Cathedral Guitars, Dennis over at AMI, the kind folks at RMS audio and AusTex64 (thanks guys), i moved forward on the project build... It took over a year from conception to production but the day has come!
I put together a tube mic based on the KM54 circuit built into an old Gefell MV691 body with the m94 capsule, TAB t8 6:1 traffo, and raytheon 5703 sub mini tube. I didn't change any of the component values as per the neumann schematic except I substituted the 3.9k Ohm Cathode Resistor (Rk) so I could experiment with the cathode bias point (thanks to RMS audio for that tip). I set the pot to 3.9k for initial testing and listening and plan to take it from there.
I did choose to incorporate the NFB network just incase I wanted to tame the 8k peak of the m94 capsule.(IIRC m94 was intended as a distant cardioid pickup pattern so the peak was to compensate for HF loss at distance). I was thinking of adding a NFB bypass switch that would break the connection C3 (.01uf cap) has with ground on pin 6 at the PSU.
The MV691 + M94 combo is the perfect starting point for the KM54 build. The m94 capsule is supposed to be very similar or the same as the rolled nickel capsule of the KM54. I had to remove the dropping resistor from inside the capsule assembly to match the km54 circuit specs. Also the mv691 body has way more space than most SDC which is helpful to fit all of the components (thankfully because they barely fit as it is)...
The MV691 I found to be way better than the MV692 because it has metal rails and threads as compared to the similar mv692 which was all plastic inside. Not ideal for a DIY mod, originally I was studying my KM254s and was planning to do a point to point that was the same as the 254 but decided to just go with protoboard (uxcel 2cm x 8cm) that I ordered from amazon. This made it way easier to conceive and build the circuit, at least for my first go around as I've never done a project like this before.
The MV691's board is attached to the rails by solder via the little metal tabs coming off of the rails. This is the securing method as well as the grounding point for the mic circuit. I desoldered these points along with the capsule assembly wiring being careful to not damage the original board incase I want to revert back to FET 48v head amp. Anyway, I utilized the solder mounting method with my build, drilling holes in my protoboard and lining them up with the tabs of the mv691 rails. This is where I started.
For the tube I ended up using the 5703 as I heard good things about its similarities to the AC701. This is a 6.3v heated sub mini tube. But I got a bunch of other tubes while planning this mic; ec1000s, 5840s, EC92s, UC92s. I decided to use the 5703 because they were inexpensive and a good place to start. Maybe i'll swap tubes in the future or build the second mic with a different tube to compare their sound. We shall see... Does anyone have experience building around a 5703 tube that may want to impart their wisdom upon my build?
I have not built the power supply for the pair of mics yet, still researching the best way to go about that. I was planning to just build two of Dany's m49 pcb PSU into one chassis (skipping the pattern section of course). Right now I am just using my KM254 PSU from Peak Power (Bill Bradley) turned up to 6.3v to test the microphone out before building the PSUs.
Unless there is a better idea, i was just going to put the 4 traffos and two chokes necessary to power two mics with the m49 psu circuit... Thats a lot of space and weight. Does anyone have a better idea about building the PSUs for this mic pair? I'd love to be efficient with space and parts.
After a day of arranging components, soldering, wiring and staring at schematics for hours and hours i finally had a (hopefully) finished mic circuit. I fired it up with my peak power PSU and to my delight heard my voice through the new tube mic build! YAY! I definitely heard some white noise but knew better than to judge before a 72hr burn in. After the burn in the noise went away.
Anyway, the microphone sounds really nice! I have not messed with the pots for the NFB or cathode bias yet... That should be fun. Just finished burning in the tube yesterday. Made a recording of a Larrivee OMV3r acoustic guitar and it sounds beautiful to my ears. I will attach the recording I made. I have a pair of km254 that I will eventually compare against these DIY km54. I won’t be able to do a side by side until i build a PSU for the DIY pair. Stay tuned.
(link to recording of mic
KM54 DIY Sample.m4a )
Considerations and questions:
The backside of the head amp functions but is not beautiful... I will tackle this design challenge on the second mic build.
Should I cut ventilation holes for the sub-mini tube?
The circuit polarizes the capsule at 40v for the kk54. The MV system can also have other capsules screwed into the circuit but these are designed to run at 60v. How will this effect their performances should I decide to use different capsules with the microphone?
Another weak point of the build is the cheap and flimsy 7 pin XLR that RFT/Gefell utilized in the mv series of microphones. Wow, so light and untrustworthy as compared to a tuchel or binder system. The threading is large and cant be interchanged with a tuchel/binder part. I am considering my options for this problem. I learned that Oliver was using a metal lathe to modify the binder/tuchel chassis mount connectors by removing metal and tapping screws to match the internal rails of the mv691 so that we can use tuchel/binder cable connectors rather than the cheap unreliable RFT ones. I have yet to tackle that operation but perhaps in the near future.
Also, as mentioned above… I want to build the most efficient (cost and space) Dual PSU for the mic pair. Ideas?
If there is interest, I can document the build of the second one of the pair!
Well, Thanks for checking out my post. I am so grateful to this microphone community. Its been so wonderful reading, learning and sharing on these forums. I started getting into building mics only a few years ago and now its nearly all i think about when I’m not making music It makes my life better, so thank you all.
Cheers!
Best, Eric
Circuit front:
Circuit board back:
Capsule Side:
