AKG C12A Inspired Build

[justinheronmusic]

Just wondered where you got the info for the txf being 1.18:1 ratio?
I was told 4:1 or 5:1 ratio for CF mic?

I have a lot of respect for you and the work you've done, I just want to say that first and foremost; if you hadn't have built a few of these, I would not have had the confidence to challenge myself at doing my own builds. In my search to find the part number for the original transformer I came across the transformer data sheet, as well as a GDIY thread about transformer issues with the mic. In the thread the thread starter sought out guidance from AMI using the data sheet that he got from AKG. Thinking that it could have been misread, I sought out a second opinion from Cinemag, as well as a request for service. I posted the transformer data sheet earlier on in the thread, but I will attach it again for convenience. Below is the e-mail I got from David of Cinemag and the link to the GDIY thread.

NOTE: 1.18:1 is an error, actual turns ratio is 1.84:1

https://groupdiy.com/index.php?topic=63158.0

Hi Justin;

Thank you for your inquiry and for a good picture of the AKG winding sheet.

The turns ratio is 3.67:1+1.  Take a look at the 3rd column which shows how the different coils are hooked up, and next the 4th column which gives the number of turns.  The primary is A1+A2 in series, for a total of 900+900=1800 turns.  There are 2 secondaries, being A3/A4 and A5/A6.  These two secondaries are each hooked up in series with each section having 245 turns.  Hence, the number of turns is 245+245=490.  1800/490=3.67.  If the two secondary windings are hooked up in series the turns ratio becomes 1800/(490*2)=1.84:1.  AMI's information is correct.  Being a cathode-follower, this turns ratio makes sense.

The next problem becomes the proper lamination to use.  I did not spot that specification on the winding sheet.  However, my guess is that they used the same material as the T14/1, which is a metric size that we do not stock.  I probably could do it on 124DU laminations, which you can see in the mechanical drawing for the CM-13114.  However, it would not be exactly the same as the original even though it would be pretty close.

Kindest regards,
David Geren

 

[Spencerleehorton]

Interesting, I was going o  the idea that I had mine right!! Not so, I shall look into winding a 2:1 txf and see what difference it makes

 

[justinheronmusic]

Interesting, I was going on  the idea that I had mine right!! Not so, I shall look into winding a 2:1 txf and see what difference it makes

Feel free to post your findings!

 

[justinheronmusic]

Can anyone find any errors in this or any reason why this wouldn't work?

Also, the original used 1/2W resistors throughout, is this necessary in this circuit? Can I use a 1/4W anywhere to save space?

 

[Spencerleehorton]

I would concentrate on getting the psu going first, read my thread, took a while to get psu correct.
Schematic looks fine to me and I would just get the 8 x 1/2 watt resistors.

 

[justinheronmusic]

I would concentrate on getting the psu going first, read my thread, took a while to get psu correct.
Schematic looks fine to me and I would just get the 8 x 1/2 watt resistors.

Earlier in the thread it was mentioned that I could use Matadors Universal PSU. I have one here that is finished, just needs to be adjusted with a 50k load. The Heater is set to 5v.

What are your thoughts?

 

[Spencerleehorton]

As long as its adjustable should be fine  I'm not familiar with his universal psu so would have to take a look.
Nickel plated o e of my c12a today, looks lovely and shiny nickel now, almost chrome.
In one of them I have a 7:1 and the other is 5:1, if I can find these other small txf I had I will wind a couple of 2:1 and try them out in one and see what the output is like.
I think that's why I went with a higher ratio but thinking about it now it's a step down txf so that's why the level is lower than it should be on mine!! Oops!!!

 

[justinheronmusic]

As long as its adjustable should be fine  I'm not familiar with his universal psu so would have to take a look.
Nickel plated one of my c12a today, looks lovely and shiny nickel now, almost chrome.
In one of them I have a 7:1 and the other is 5:1, if I can find these other small txf I had I will wind a couple of 2:1 and try them out in one and see what the output is like.
I think that's why I went with a higher ratio but thinking about it now it's a step down txf so that's why the level is lower than it should be on mine!! Oops!!!

I would love to see what it looks like! I've never constructed for DIY stuff, only soldering/wiring, mostly because I live in a small apartment.

I have attached the MataChung PSU Schematic. If it works then I'm golden. If I have to design a PCB for the N12A then that is a can of worms I will definitely need help with, particularly with available/recommended components. Pictures of your finished PSU will help  me with layout.

Right off the bat, I feel like I will need to arrange the pinout to compensate for the Metador PSU. Should be easy, right? I just wouldn't connect anything to the bias?

I will heed your advice and stick with the 1/2W.

 

[Spencerleehorton]

Yeah you'll need to do polar switch ladder it looks like and you good to go.
Mine sits at about 113v for B+ and 5.05v for H+.
I remember the 63.5v off the tube being important I think on the cathode?
I'm a bit vague here but I seem to remember I had only 65v tants and it was all too close for comfort so I changed them for 75v.

 

[Spencerleehorton]

Looking at the winding sheet I might be able to wind a small nickel core version on a dual bobbin.
Just need to see if I can get about 2500 turns on there so I can get roughly the right amount of inductance and get close to DCR.
Looking again at the txf it is a step up rather than a step down as its 2 x 50 ohm in series going to a 200 ohm secondary so my txf should give me slightly higher output but be biased at a different point which will effect tone.
Mine do sound really good as they are but I'm interested in how they would sound with as close as I can to the real spec.
It looks to me also on the dual bobbin txf the each other winding will be essential reverse wound when put on a single bobbin with dual chamber.
Phase wise this will change things so also be good to hear.
Will nickel plate other body tomorrow and finish up my other mic bodies then start testing these again and see if I can get better results

 

[justinheronmusic]

Thanks for the update, Spence. Looking forward to hearing your experience. Attached is my first draft for the PCB. Dimensions are for the GT-2B body; I know, I know. It's not ideal, but it will get me close if I order the head basket from Advanced Audio. Plus I will have plenty of space to work with, which will be nice. The original is pretty tightly packed. Let me know if you have any feedback.

 

[Khron]

Two layers, for so few components? Why not move the H+ & 10 pads to one edge of the board (say, top right for 10 and bottom right for H+)? The other two traces don't need to be on a separate layer either. It's not like you need all that solid groundplane for massive currents anyway... Do you?

R1/2/3 can be connected with a straight trace, no need for that J-shape there.

I know the C12A schematic appears to have four wires coming off the backplate, but i'm 99.8% sure they're (supposed to be) connected together (ie. SB and PB on your board).

Also, you'll probably want two teflon standoff pins for two points in the circuit - the tube grid (and one end of R2 and C3 respectively), as well as the backplate (SB / PB, the other end of C3 and one end of R7).

 

[RuudNL]

Personally I don't like high impedances (200 M.ohm) connected to (long) traces on a PCB, especially if there is a groundplane very close to those traces... This is asking for problems!

 

[justinheronmusic]

Two layers, for so few components? Why not move the H+ & 10 pads to one edge of the board (say, top right for 10 and bottom right for H+)? The other two traces don't need to be on a separate layer either. It's not like you need all that solid groundplane for massive currents anyway... Do you?

R1/2/3 can be connected with a straight trace, no need for that J-shape there.

I know the C12A schematic appears to have four wires coming off the backplate, but i'm 99.8% sure they're (supposed to be) connected together (ie. SB and PB on your board).

Also, you'll probably want two teflon standoff pins for two points in the circuit - the tube grid (and one end of R2 and C3 respectively), as well as the backplate (SB / PB, the other end of C3 and one end of R7).

I used a ground plane to make it easier to do the layout. I can probably get rid of the ground plane and use the second layer for ground, would that be better?

So if I go back into the schematic and remove one of the back plate pads and consolidate them to one; solder a teflon pin into the pad, and tie both the back plate leads together and solder them to the teflon pin, that will be correct?

So on the schematic it would be C3>Teflon Pin>R7, is that right?

I am a little confused about how to implement the other pin you mentioned. Also, what do the teflon pins do?

Here is what I have so far based on your feedback. I placed the back plate pin under R2.

 

[Khron]

1) Increase the "isolate" parameter on the groundplane.

2) There's no reason for the backplate pin to be in the middle of the board, is there?

3) https://microphone-parts.com/pages/insulate-high-impedance-connections

In the first photo in the link, i've often seen the coupling capacitor (the 1nF C3 in this case) being soldered to those two isolating pins, on the back of the board (given the space constraints in many JFET condensers). The black-and-white rectangular components are 1Gohm resistors there (200M in your circuit).

 

[Spencerleehorton]

Crikey!! Let me do a photo of my pcb!!
Just copy that, no need for dual layer.

 

[Gus]

look at the heater supply

 

[justinheronmusic]

look at the heater supply

Gus, can you please elaborate? What would you like me to see? Please keep in mind that I am still learning all of this, so certain things may not be obvious to me.

 

[Khron]

My guess would be, that wee little trace might have a noticeable voltage drop at the 135mA-ish that the heater requires / draws. But i could be wrong...

Gus, can you please elaborate? What would you like me to see? Please keep in mind that I am still learning all of this, so certain things may not be obvious to me.

 

[Spencerleehorton]

Here's the bottom pcb