Phantom Powered Tube Microphone (Frankentube) DONE!

[kingkorg]

I hope Zapnspark won't be mad at me for reviving this guy again, but here it is. IT'S ALIVEEEE!!!

And it sounds awesome, love it! Right now there's a Transound electret inside, but i am thinking about upgrading to something fancier.  Maybe even some of AKG's LDC electret capsules, like in c3000.

I am missing a couple of tantalums here, i'll put them in as soon as they arrive. They are for filtering i guess, but it works great even without them.

Next comes tube roling for least microphonic one.

Thanks to Zapnspark for not removing his guts pictures on  Photobucket, otherwise I wouldn't have been able to do it. Not too much hassle with SMD stuff either.

 

[kingkorg]

And the other side:

 

[kingkorg]

Oktava capsule fitted nicely, i polarized it at about 30v just like stock Oktavas. Nice output and surprisingly quiet. Just a 1G resistor from pin 7 of the IC. And added capsule coupling capacitor.

Love the steampunk look of the capsule, i am considering modding the body to create that kind of vibe.

 

[craigmorris74]

That looks cool.  How does the Oktava capsule sound with the tube circuit?

 

[pucho812]

Got a schematic

 

[kingkorg]

Here it is:


http://www.sdiy.org/oid/zapnspark/FrankenTubeUpd.pdf

And Zapnspark's images help a lot with layout:

http://s114.photobucket.com/user/zapnspark/media/FrankenTube-B.jpg.html?sort=3&o=135

http://s114.photobucket.com/user/zapnspark/media/FrankenTube-A-1.jpg.html?sort=3&o=134

http://s114.photobucket.com/user/zapnspark/media/FrankenTube2-9.jpg.html?sort=3&o=118

 

[kingkorg]

That looks cool.  How does the Oktava capsule sound with the tube circuit?

Low end sounds huge compared to stock circuit, i am even considering taming it a bit.  This tube creates wonderful vibe in the mids, but it's hard to explain.

I managed to kill the microphonics. One should cut tube wires short and use flexible wire (like AudioTechnica did with 3060), they don't transfer vibrations. I used cotton bandage under tube and a tape over it(temporary solution) to hold it in place. The tube doesn't emit heat btw.

 

[Khron]

Would it not have been slightly easier to use something like ICL7660 (or whatever version / clone you wish), widely available in DIP8 packages?

I wouldn't be surprised if Zapnspark went with that SMD LM2665 part just to save on board space or something(?).

 

[analogguru]

Anybody knows why audio-technica dropped their patent for their AT3060 (which is very similar to the circuit used here) ?:
http://www.freepatentsonline.com/7295675.html

(At least they didn't use an opamp in the signal path of a "tube"-microphone.)

And anybody knows which grid current/(resistor) is necessary for the 6418 to ensure proper bias ?

 

[Icantthinkofaname]

I'm guessing it's single pattern only since the only Oktava capsules I've seen available (from Oktava directly) that aren't K67 like ones are the ones used in the MK-319 and obviously if it's phantom powered you don't have the tube mic PSU with the pattern dial. Either way, nice mic. I definitely hear where you're coming from saying the capsule has too much low end, that's the only reason I haven't picked up a 319 from eBay yet.

 

[kingkorg]

Yupp, it's cardioid only. However there shouldn't be a problem with too much low end with stock oktava. I believe it's my mic that has better low end extension compared to stock, has a bit of 30-50hz rumble.

 

[Icantthinkofaname]

So does the tube not produce a lot of heat because it's getting a lot less voltage through phantom power, or is it that specific tube doesn't get hot? How long does it take to warm up enough to use? Haven't used a tube mic yet, looking at getting the Apex 460 and putting a quality C12 style capsule in it along with some other mods, though a phantom powered tube mic sounds cool.

 

[kingkorg]

Nope, this tube gets exactly perfect voltage, it's designed to wotk at lower voltages. It's nothing like starved plate design, it works at it's full potential. That's what's fun about it. There are just a couple models made this way, one of them being Gefell UM900 which uses slightly diferent tube.

However, since it has tiny filament it takes 1.2v and uses just 10ma, so it never gets hot.

I wouldn't compare this mic to anything like c12. Whole different animal.

This is very, very affordable way to get a tube mic, and practice smd soldering. Audio Technica uses transformer instead of IC. I just didn't want to invest too much in it.

Frankentube got very popular at some point, but very few people managed to pull it off because of smd, and lacking layout i believe.

 

[Khron]

Frankentube got very popular at some point, but very few people managed to pull it off because of smd, and lacking layout i believe.

Would it not have been slightly easier to use something like ICL7660 (or whatever version / clone you wish), widely available in DIP8 packages?

I wouldn't be surprised if Zapnspark went with that SMD LM2665 part just to save on board space or something(?).

 

[kingkorg]

Are they interchangeable?

There is plenty of space, i even ended up using pretty large electrolytics instead of tantalums. And this is st51 body. Even though smd, they have to be placed on these adapter boards that take same amount of space as ICL7660.

 

[Khron]

That's precisely why i was asking ;D

See the bottom of page 9: https://www.intersil.com/content/dam/intersil/documents/an05/an051.pdf

 

[analogguru]

ICL7660 is very sensitive to overvoltage and especially in the franken-opamp-circuit I would not use it.

And it uses a very low switching frequency of only 10kHz !!!  I don't think that anyone wants to have this in a microphone.

I would omit the ugly opamp and use a transformer (e.g. from an el cheapo DI-Box) and one SMPS-downconverter which is specified for 52V input voltage, min output voltage 1V and has a switching frequency of 1250 kHz - ready-made modules available:

 

[analogguru]

And yes, these modules even fit into an el cheapo pencil mic:

 

[kingkorg]

Great stuff!

 

[Khron]

Well, i'm pretty sure that's what that 10V zener is for, at the input of the dividing chain.

And there are other variants / derivatives of the ICL7660 that work at higher frequencies. Stuff like the TC7660H - 120khz high enough for ya? Under 1eu/pc one-off from Mouser, DIP package...

ICL7660 is very sensitive to overvoltage and especially in the franken-opamp-circuit I would not use it.

And it uses a very low switching frequency of only 10kHz !!!  I don't think that anyone wants to have this in a microphone.