AKG C12A Inspired Build

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xeawr

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Spencerleehorton said:
I wanted a 414 before I built my c12a's so I demoed a few and hated them, also tried out the SE 4400a thinking it would be the one but didn't like any of the SE range and sent about 5k of mics back, at the time I ordered a big selection box of SE mics to try!!!
Which you could do then, don't know if they still do it?
It's all horses for courses Gus, one man's silence is another man's jet engine!!!


when (which year) was that with the sE Microphones?
 

justinheronmusic

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Khron said:
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).

Hey, sorry I am like this, but could you please clarify this for me? What I am hearing from you is that I need to build this into the PCB:

- 1 x Teflon post for the joint of R2 and C3
- 1 x Teflon post for the joint of C3 & R7

Then the secondary back plate lead, and the primary backplate lead will be soldered to the same post that C3 & R7 is on?
Below is the third draft of my PCB.
 

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Khron

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Condensed version - both legs of C3 should be on isolation pins, as do the respective ends of the 200M resistors connecting to it, and the tube's grid, and the respective lead coming from the capsule.
 

justinheronmusic

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Khron said:
Condensed version - both legs of C3 should be on isolation pins, as do the respective ends of the 200M resistors connecting to it, and the tube's grid, and the respective lead coming from the capsule.

Thank you so much. I will make the corrections in the schematic when I get home from work (first day back from quarantine!).

  The pins for the tube will be mounted on the back, and the rest of the pins will be top mounted. After I add footprints for the isolation pins, how is everything looking for the 4th draft?  Any other issues that need tobe addressed?
 

Khron

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Ok, allow me to reiterate:

- isolation pin 1 = tube grid, one end of C3, one end of R2
- isolation pin 2 = other end of C3, one end of R7, backplate wire(s)

"Footprints" are unnecessary, you don't want traces to connect to them, solder pads around them, or plated through-holes (in case you're planning to get these manufactured). Use the plain holes in the "holes.lbr" Eagle library; pick whatever diameter suits the size.

justinheronmusic said:
The pins for the tube will be mounted on the back, and the rest of the pins will be top mounted. After I add footprints for the isolation pins, how is everything looking for the 4th draft?  Any other issues that need tobe addressed?
 

Icantthinkofaname

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Oh wow they look really good with the nickel plating.

I kind of liked liked the brass look too, I always thought it looked kind of steam punkish since it looked a bit rough (but really good for a handmade body).
 

justinheronmusic

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I've reached out to AMI, Cinemag, and Crimson. So far Crimson is the only one willing to actually follow through with the 1.84:1 transformer build.

I did find this 2.25:1 on AA website. I know that this isn't the exact windings, but will this work for the AKG C12A?

https://advancedaudio.ca/collections/diy/products/bv225
 

RuudNL

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justinheronmusic said:
I did find this 2.25:1 on AA website. I know that this isn't the exact windings, but will this work for the AKG C12A?
https://advancedaudio.ca/collections/diy/products/bv225

Are these pulled from another microphone?
(Short red wire, looks like the other wires have been soldered before.)
 

justinheronmusic

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RuudNL said:
Are these pulled from another microphone?
(Short red wire, looks like the other wires have been soldered before.)

Ha! Good eye. I guess we're about to find out, as I just placed an order for one  :eek: It looks like the going rate for a custom wound transformer is $200USD + shipping. I ordered the BV225 & the headbasket for the GT2B for just over half that shipped from AA. I will have time to cork sniff transformers later. Crimson seemed super down with the project, but would prefer to do it under a higher quantity situation. Could be a cool group buy situation if my PCB gains traction (doubtful); the same goes for AMI.

I love Spencer's idea of winding one home brew style, but I wouldn't know where to start.
 

Gus

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A little off topic but might be fun to try and compare to the tube version in this thread

I posted a simple cut and paste follower schematic in the linked thread
https://groupdiy.com/index.php?topic=75301.0
The follower has the JFET drain bootstrapped so the drain tracks the input
Biasing should be investigated
could try one of the China about 2:1 transformers in from the JFET, PNP microphone circuits
30VDC as shown
other followers could be used this one is from the EB
 

justinheronmusic

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Gus said:
A little off topic but might be fun to try and compare to the tube version in this thread

I posted a simple cut and paste follower schematic in the linked thread
https://groupdiy.com/index.php?topic=75301.0
The follower has the JFET drain bootstrapped so the drain tracks the input
Biasing should be investigated
could try one of the China about 2:1 transformers in from the JFET, PNP microphone circuits
30VDC as shown
other followers could be used this one is from the EB

An intriguing idea, for sure!

Just waiting on the PCB's that I got printed. I've included the modifications suggested by Khron. These will just be a test run.
 

Spencerleehorton

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Well now checking that U11 txf winding doc it does seem it needs a 2:1 step down txf rather than the 1:5 step up as I have mine co figured!!!?
I'll wind one txf and try and see how close I can get to the 17H, 100R Primary and the 2H, 60R secondary!!!
 

Spencerleehorton

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I'm going to check I have my 5:1 and 7:1 txf wired correctly as I thought I had them wired for step up!!
If I have got them wired for step down, as they should be then that will answer why the output levels are low and the fact that they both have different output levels.
Day off tomorrow so will also wind a 2:1 and see how that sounds
 
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