AKG C12A Inspired Build

[justinheronmusic]

I've also redrawn pcb and will tidy up, it's a printable single layer and I think it will be much smaller than what I did before.
It certainly looks a better layout!!

I am a bit of a neat freak when it comes to DIY for sure. Progress pics as it unfolds please!

Also, I received the txf from Advanced Audio; 2.25:1. The leads are short ineed, but are usable. We'll see how it all turns out.

What capsule are you using?

 

[Spencerleehorton]

I've got RK-12 matched pair from microphone parts in mine.
I didnt upload pic first of all as it was just a pencil drawing, I will do a neat version today.

 

[Spencerleehorton]

hi,

Ive attached the first draft, it measures 45mm x 30mm.

I know the sizes for the caps is wrong, so will look into bigger pad spacings.
the 15uf defo needs more spacing, the 1000pf should be alright along with the 5000pf as you can stand then up rather than layed down flat.
Also the valve i've not put on there as i feel the besy way of getting it in there is with the socket on leads so you can lay it down rather than stand it up.
size wise its about half what im using at the moment!!!

updated

 

[Spencerleehorton]

the only pitty that i can see at the moment is the price for the 200Meg reisistors, mouser do the omite ones for £3.13 1% but you'll need x3 and then theres the shipping which will be at least £16, so your up in the £30 area all ready!! just for 3 resistors!!!

 

[Khron]

1) Is the exact value really THAT critical? Surely 220meg would do just as fine, and itcs an E6 value, and thus potentially cheaper.

2) And don't Mouser do free shipping for orders over 50-60 quid / euros? Good excuse to stock up on other parts for projects :

the only pitty that i can see at the moment is the price for the 200Meg reisistors, mouser do the omite ones for £3.13 1% but you'll need x3 and then theres the shipping which will be at least £16, so your up in the £30 area all ready!! just for 3 resistors!!!

 

[Spencerleehorton]

I was just trying to point out that mouser can be expensive if you just order the resistors,  I normally use farnell or a few other smaller companies.

 

[Spencerleehorton]

Well I've wound the first txf and specs wise its came out ok, bit high on the primary dcr but the inductance more than makes it up.
Looked at the pcb and noticed one of the ohmite 200meg resistors had snapped in the middle!!!
Gonna have to replace that!!!
Will rewire and check everything then power up again.

 

[Spencerleehorton]

Have redone one of my c12a with other txf and I think the output and signal is much improved, I might rewind the txf and do an humbucking version so to remove all hum.
Managed to get all 1800 turns on sec and 980 on primary,  my inductance meter stops at about 20H and this txf is over 20H and dcr is a bit high.
So I'll calculate the turns a bit less to bring down dcr and do two windings for sec and pri.
Also I've noticed there isn't a body gnd!! Best put that on there!!
I'm glad I've revisited these as I think I can improve them.

 

[justinheronmusic]

That's great news, Spence. I am happy to hear that you're making progress. Are you winding your txfrmr by hand or do you have a machine?

I had a lot of gusto toward this build, and I still do, but I am going to take a break from it for a bit while I finish up my PM670. I have been sitting on that build for 4 years now, and it's time that I have something to show for the money I have put into it.

I will post some photos of the PCB when it arrives just to keep things interesting, order the parts in late July, then I will be revisiting this build in August with full force.

 

[Spencerleehorton]

Ive had a few more ideas about making the body, so im going to print a few pcbs and make another pair as do love these for room mics.
I have wound my txf with a hand winder, you can pick them up pretty cheap on ebay, along with all the copper wire you want.
80% nickel lams and bobbins are also available but very expensive for what they are and large shipping prices from korea.
I will speak to my local company and see if i can get anymore for cheaper.

 

[Spencerleehorton]

Also been reading up and you can alter the circuit to provide more output gain, taking signal from plate rather than cathode follower circuit.
Will try and come up with a schematic, anyone else like to pitch in with what to change/leave out?

 

[Spencerleehorton]

Have wound another txf and much closer, gonna rewind this akg dual bobbin 5:1 I have to 2:1 and put that back in.
Getting buzz on my first txf version so either dodgy lead or patch bay trouble!!
Or I need to reverse wind a primary winding?
Somethings not right today but I'm on nights shifts so brain goes a bit gooey!!!

 

[volker]

Also been reading up and you can alter the circuit to provide more output gain, taking signal from plate rather than cathode follower circuit.

What would be the point? The cathode follower is the defining feature of the C12A, might as well build a C12 then. The low ratio transformer wouldn't fit an anode output either.

 

[RuudNL]

I've got RK-12 matched pair from microphone parts in mine.

Keep in mind that the RK-12 has little to do with a real CK12 capsule.
If you want to recreate the C12A, you may want to upgrade to a capsule that is close(r) to the original.

 

[Spencerleehorton]

Yes I agree, I already have a c12 so dont think it's worth making another.
I'd be more interested in making something more c414 style but last time I looked  I got stuck on the DC-DC converter!!
Any help with getting this sorted would be helpful!!

 

[Spencerleehorton]

Apparently the c414 EB P48 didnt have a DC-DC converter?
I cant seem to find a schematic?
Perhaps I could build this version?

Found it in the technical docs section!!

Will look into it.

 

[Spencerleehorton]

Well it seems doable,  just I've got no idea what the choke values are for DR1 and DR2 and the winding spec on the txf as it has a feedback winding, the feedback winding on other txf I've done is normally only a small winding?
Probably best to start a new thread on this as I'm unsure how many people exactly have actually even completed the c414 EB P48 build?

 

[Khron]

Since that thing is there strictly to provide the capsule bias voltage, does it NEED to be the same circuit as in the C414? What's wrong with the Schoeps type oscillator, or a 40106-based one?

Well it seems doable,  just I've got no idea what the choke values are for DR1 and DR2 and the winding spec on the txf as it has a feedback winding, the feedback winding on other txf I've done is normally only a small winding?
Probably best to start a new thread on this as I'm unsure how many people exactly have actually even completed the c414 EB P48 build?

 

[Spencerleehorton]

Could you post a pic of what you mean please dude?
Let me do a new thread

 

[Spencerleehorton]

I re wound both txf to 2:1 and reverse wound the 2nd primary to hum cancel and my two c12a sound amazing now!!