Finished AKG C12 clone pics

There's a mountain of different variations of these, so i though i should report one succesful way of doing this for the ones who are lost in the jungle.

So, basically i used Alctron HST11A housing with one inner layer of grill mesh removed, leaving two layers in. Works perfectly.

The circuit is one half of the C24. With few added caps. PSU is Alctron tuned to give B+ 120v. I might pimp this later, but it works as is. Transformer is AMI/TAB T14. The tube is EH 6072A but i'll replace it with NOS GE 6072A later just to compare. I'll attach the (very badly drawn) schematic with details.

And finally the most important thing (as the capsule is about 80% of the condencer mic, generally): Tim Campbell CT-12. It sounds A M A Z I N G L Y good. Seriously, you do not want to hear any chinese clones after you've heard that capsule. I could not believe the diffrence would be THAT big. I used a chinese CK12 clone as a test capsule in the process but when I added Tim's capsule the sound got so much more detail, presence, thickness, 3D-ness... simply worth every penny...and more. It would take a seriously crappy circuit to destroy that sound.

Oh, and it was fun. I learned a lot, as i didnt know almost anything about building mics when i started. Still dont know much, but i have only one thing to say, gentlemen, USE YOUR EARS.

the capsule

The innards

Great job! But there is a mistake in your schematic. C8 should go to the other side of R2, otherwise you would have no signal. You obviously built it right .

If I were to build a C12-like microphone, I would try both triodes in parallel for lower noise (half the anode resistor, half the cathode resistor, double the cathode bypass capacitor).

volker said:

Great job! But there is a mistake in your schematic. C8 should go to the other side of R2, otherwise you would have no signal. You obviously built it right .

If I were to build a C12-like microphone, I would try both triodes in parallel for lower noise (half the anode resistor, half the cathode resistor, double the cathode bypass capacitor).

Click to expand...

Thanks Volker!

You're of course right! I draw it wrong, like you said C8 is connected between pin 1 of the tube and R2. But i'm too lazy to re-draw it.

If the both triodes would be in parallel wouldn't it also half the output impedance of the tube...?

The Corrected schematic!

Thanks Volker!

[quote author=telefunk]If the both triodes would be in parallel wouldn't it also half the output impedance of the tube...?
[/quote]

Yes, that's right. Might have to adjust the output capacitor and/or the cathode bypass capacitor to get near the identical bass response. But the differences might be low enough in the frequency range that you don't care. Difficult to tell without the gory details on the output transformer. I'd give it a try if it's not too much of a hassle and your psu is capable of double the B+ and heater current.

Few thoughts i should add: Usually taking some of the grille mesh out sounds sibilant and bad. But this time it worked really good because there was simply too many layers of it.

What works and what doesn't is a result of many things: the capsule, head basket dimensions, mesh hole diameter etc. But it's good to have a great reference mic that you can trust, and then go by ear.

The only reason i build it to a chinese housing was because it was cheap and i got the PSU, wire's etc with it. Of course, something like FLEA's C12 mic kit would be amazing, it would have cost thousands more, and that would have made the whole project impossible for me at the time. And regarding the mic as an instrument i think there would not have been much difference in actual sound.

If you're on a budget, put all you got into a good capsule and transformer and take great care listening the how head basket affects the sound and you'll get far. That's my take.

volker said:

[quote author=telefunk]If the both triodes would be in parallel wouldn't it also half the output impedance of the tube...?

Click to expand...

Yes, that's right. Might have to adjust the output capacitor and/or the cathode bypass capacitor to get near the identical bass response. But the differences might be low enough in the frequency range that you don't care. Difficult to tell without the gory details on the output transformer. I'd give it a try if it's not too much of a hassle and your psu is capable of double the B+ and heater current.
[/quote]

I think i'll keep this one as it is now because it sounds really good now. But maybe i'll try it in my next project.

Also, at parallel'ed triodes, you get double the grid leakage current, which may be very unwanted.

I think there's a good reason why the classic just left one side of the tube unused.

Jakob E.

This is a nice looking microphone!  I think you have hit on the essence of the C12.  How did you paint the body with the finish you have achieved?

David

This is great! I want to do something very similar. I'm sure everything you say about the Campbell capsules is true.

I have a mic very similar.  It is a cathode follower however.  I found that tim's capsule made a massive improvement.  However after using it for a while now I found that the mic now has a pretty big bump around 270-280hz.  Any idea why that is or how it could be corrected. 

Even with the bump its still much better then previously.  And knowing that its there I can eq it out.  The capsule is clear and bright but not peaky like I have found many of the other capsules to be.

david-p said:

This is a nice looking microphone!  I think you have hit on the essence of the C12.  How did you paint the body with the finish you have achieved?

David

Click to expand...

Thanks David!

Actually, the paint job was a real PITA. Because i had to paint it twice as there was some grease left under in the first time 

So, I tried to cover some mistakes with another layer of paint. Mistake, as two layers of Hammer Laquer is the ugliest surface imaginable.

Well, it was Nitromors in and all over again. That first picture is taken before the catastrofic second layer.

Now I added a picture of the final paint. Now it's really good, as good as it can be with spray.

EDIT: Maybe i should add, that in order to get that hammer finish to form evenly throughout the tube shaped surface, i found that you have to stick that metal tube in to a cane of some type and keep rolling it slowly right after painting, until the paint starts to stick. As the best results for that type of paint would require a horizontal surface.

telefunk said:

I used a chinese CK12 clone as a test capsule in the process but when I added Tim's capsule the sound got so much more detail, presence, thickness, 3D-ness... simply worth every penny...and more. It would take a seriously crappy circuit to destroy that sound.

Click to expand...

Tele, This is awesome! Can you share which Chinese CK12 clone you tested with?

Thx,
jb

0dbfs said:

telefunk said:

I used a chinese CK12 clone as a test capsule in the process but when I added Tim's capsule the sound got so much more detail, presence, thickness, 3D-ness... simply worth every penny...and more. It would take a seriously crappy circuit to destroy that sound.

Click to expand...

Tele, This is awesome! Can you share which Chinese CK12 clone you tested with?

Thx,
jb

Click to expand...

jb,

my understanding is that there really isn't any chinese ck12 clones made at all. They are basically K67 capsules with edge terminated diaphragms. Those are the same chinese capsules all over the net, just with different labels on them but made in the same factory.

Also, those capsules are not bad at all, it's just that there some that are much better.

And also it's not fair to compare a 20€ chinese capsule to a hand-made european tradition masterpieces like Campbell's, Neumanns or MG's.

Just my 2cents...

gyraf said:

I think there's a good reason why the classic just left one side of the tube unused.

Jakob E.

Click to expand...

Thanks for the info Jakob,

That was what i rationalized and though i'd start with the original design.

Now the only thing separating C24 and C12 would be the fixed tube bias of the C12 coming from the PSU.

Would there be another way of achieving the fixed bias without supplying it from the PSU?

Hi telefunk,
Great build! I like that Hammer paint trend .

telefunk said:

Now the only thing separating C24 and C12 would be the fixed tube bias of the C12 coming from the PSU.

Would there be another way of achieving the fixed bias without supplying it from the PSU?

Click to expand...

There is simple way : add resistor in series with heater network like it was in U47 circuit (don't forget about cathode pin). Resistor should be R=Ubias/Ih, so about 3-5 Ohm.

Hah, you made my day.. This is exactly what I'm after I already bought the T4 transformers ages ago and will be getting HST11 mics soon. It's very nice to see pictures of how it's supposed to look like in the end.

bezen4uk said:

Hi telefunk,
Great build! I like that Hammer paint trend .

telefunk said:

Now the only thing separating C24 and C12 would be the fixed tube bias of the C12 coming from the PSU.

Would there be another way of achieving the fixed bias without supplying it from the PSU?

Click to expand...

There is simple way : add resistor in series with heater network like it was in U47 circuit (don't forget about cathode pin). Resistor should be R=Ubias/Ih, so about 3-5 Ohm.

Click to expand...

Interesting. I wonder how it will affect the sound... I remember reading that at least mr.Heyne is very fond of fixed bias.

Has anybody tried that U47-type of bias method in a C12 type of circuit?

btw, later when AKG designed the ELA M251 for Telefunken, they used similar tube bias than in C24. Not the U47 type. Wonder why, if the fixed bias would have been somehow better...