I want to build a stereo U48 type mic

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Nollman

Well-known member
Joined
Mar 16, 2018
Messages
64
I have a DIY C24 clone that sounds fantastic and a Royer SF24 but I am looking for a different flavor and I think the Oliver Archut U47 with the EF800 tube would do the trick. Two patterns (Cardioid/Figure 8) would work but if I can squeeze three patterns out of it that would be great. It's going to be two mins in the same tube so the EF800 saves a bit of room;  I can fit two side by each. A mini DPTT on-off-on would join the capsule diaphragms with and without the 105V back diaphragm voltage. Here's a schematic I scratched out. I am new to this and a bit over-ambitious so I have no idea if this will actually work. Any technical help would be appreciated.
 

Attachments

  • U48 Hybrid.jpg
    U48 Hybrid.jpg
    30.2 KB · Views: 115
Hello Nollman,

welcome to the forum ;)

Not really sure what you have in mind with "stereo U48 type mic", since your schematic is for casual "mono" mic.
Do you want double this circuit in one body with two capsules?

Few notes for the circuit:

100M instead of 1G (in all positions) is much better choise.

Capacitor on your switch - 1nF is much better option than 10nF.

Currently you have three patterns - cardioid, omni, supercardioid.

Backplate polarisation voltage is 63V, your back diaphragm polarisation voltage is 105V - it's supercardio not fig.8.
You have two options for it.
Change voltage divider for double 3M like in U48 - little bit lower sensitivity for all patterns.
Then you will get ca. 53V on backplate.
Or better option, make additional cable connection, connector 6pin or 7pin,  and make voltage node with 126V direct from PSU for fig. 8 pattern,


 
That's just the kind of info I needed, thanks for the help. You are correct that this is one half of the two circuits that would be in the microphone.  I plan on buying a dual U47 power supply from Telefunken Elektroakoustik, rather than trying to design and build one. The backplate voltage may not be adjustable to 126 volts, I will have to wait and see there but at least I have options. I was concerned that the backplate voltage circuit would somehow interfere with the other patterns even when disconnected from the amplifier circuit (make some kind of filter). 
 
I would build PSU and definately make it custom for this project ;)
Anyway i just spotted one error on your schematic, back diaphragm polarisation voltage should be from the opposite site of switch capacitor - otherwise you will burn the tube.
Nope, everything is fine, this is one of the typical options for multipattern ;)
 
It occurred to me that the Telefunken power supply will not have 6.3V filament voltage so building the power supply myself might be the only option. Another related question, can the 0V leg from the filament just be tied to ground or is that some proprietary thing that Oliver's power supply used for the filament voltage circuit. A C24 power supply will give me the 6.3V filament and 120V back diaphragm voltage but the main feed will be 120V not 105V requiring a lot of resistor value changes and resulting math...
 
These PSU are simple as hell, definately not a rocket science ;)

Anyway your switch connection is wrong, look on U67 schematic and connect it same way. SP3T is enough ;)
 
Here is the revised schematic, thanks for the suggestions.
 

Attachments

  • U48 Hybrid 3 pattern.jpg
    U48 Hybrid 3 pattern.jpg
    28.8 KB · Views: 85
The cutter is a 3/16" double fluted HSS end mill in my Shopsmith mill/lathe.  The first try did't work out so well. The machine has a bit too much slop in it and gouges the corners when you try to change direction.  I am going to re=think the process. :(
 
I got a clean frame for the headgrille, next we add some mesh.
 

Attachments

  • Stereo Headgrille.jpg
    Stereo Headgrille.jpg
    51.5 KB · Views: 80
Great! Keep going ;)

For the schematic, now looks definately better.
Think about extra voltage node for figure of eight - worth go higher with cardioid polarisation voltage like in U47.
Also you can consider not using 30k resistor and 1uF filtering cap. Instead you could use 150k-200k potentiometer and set most optimal point for plate voltage.
 
60 volt polarization would be great if I could keep the number of wires to a minimum as there will be two mics in the same body. I can share B+ , filament and ground for both halves but need at least four wires for audio output. Can I just tie the "0 volt" leg of the tube filament to ground and save a wire or two in the cable? I guess Oliver's power supply had a  system for the filament voltage that needed it's own 0 volt leg. The cable I have is 8 conductors and a shield. I have been looking into building a power supply but haven't found any kits that address a stereo mic specifically. I guess stereo tube mics are a rare thing in the DIY community, my luck. It would be nice to know some more of the math around the resistors and why Oliver went with the 1Gig resistors  in the original design over the 100M. The more I learn, the more questions I have, LOL. Again, thanks for the help.
 
Nollman said:
I got a clean frame for the headgrille, next we add some mesh.

So how did you get a clean frame for the headgrille as per the last photo? Would be interested to know your technique. I have had to make my mic bodies by careful cutting with an angle grinder and thin cutting disk, and lots of careful filing to clean them up. Easy to cut too far if I am not super careful! Filing is time consuming!! Fortunately they have turned out well. Hardest ones to do was when I used stainless steel handrail for bodies. Would not do that again - too hard to work and a bit heavy. At least very durable!
Regards,
zephyrmic
 
Here's a pic of one brass mic body I made - one of the first mics I made.
zephyrmic
 

Attachments

  • Mic Body.jpg
    Mic Body.jpg
    19.3 KB · Views: 48
And here are a couple of completed mics.
zephyrmic
 

Attachments

  • 2 Completed Mics.jpg
    2 Completed Mics.jpg
    95.3 KB · Views: 67
Here is a closeup of the stainless mic body and grille - all handmade in my workshop. Not stereo, but to give an idea of what I have been able to do.
zephyrmic
 

Attachments

  • SS Mic 2 New Mesh Grille.jpg
    SS Mic 2 New Mesh Grille.jpg
    136.3 KB · Views: 63

Latest posts

Back
Top