AKG C12A Inspired Build

[justinheronmusic]

Hello, everyone! I am interested in making a pair of AKG C12A with a B (-20db pad) option. If we can get this up and running then I will be happy to get the PCB's printed, I can even take a stab at the layout if need be, but it would be my first time doing a mic; someone who has mic PCB layout experience would be great to have in my corner. I am hoping to build these fairly inexpensively and with ease; so I was hoping to focus my expenses on the capsule and utilize an existing donor mic body available from Chunger and easily obtainable capsules; though I am open to ideas & contributions on that matter.

A few questions before I get started.

- Is the body/headbasket of this mic integral to the sound?

- Is there already existing options for the AKG N12A power supply?

- Microphone Output Transformer Options?

- Is there any unobtanium in this build, or any reason for me not to pursue this?

- Is anyone else interested in building one and joining me in this endeavor?

 

[Khron]

Well, first you might want to decide on a donor body. At least to some extent, yes, the headbasket will affect some of the response, in the mid-to-high range - the shape as well as the size. To what extent, I couldn't tell you for sure, but opinions are divided.

I don't have the schematics at hand, but I think something like the cheapie t.Bone tube mic supplies should do fine (although I'd recap them preemptively).

 

[Ricardus]

This is interesting. I've always been interested in that mic.

 

[Spencerleehorton]

I'm pretty sure I detailed quite a lot of what i did in my build of a pair.
I use mine as drum overheads and they are very natural sounding.
Pretty busy with other stuff but will try and help where i can, pretty sure googling the c12a pcbs will bring up some pictures and you can just copy them.
It's not a complicated pcb, seem to remember the tubes were a bit hit and miss but i got there in the end.

 

[justinheronmusic]

Well, first you might want to decide on a donor body.

I am thinking Chunger's GT-2B, but there is a nice looking U87ish looking body from this website at a fairly decent price, shipping doesn't seem too bad but it's an epacket from China with an estimate of 8-30 days.

https://www.dhgate.com/product/diy-mircophone-body-for-wired-karaoke-handheld/467493452.html

Thoughts?

 

[RuudNL]

Keep in mind that most 'CK12' capsules on the market are nothing else than edge terminated K67 capsules, that don't sound anything like a 'real' C12. A real CK12 capsule is a lot more difficult to make and (thus) has a higher price.

(By the way: I have seen the Chinese bodies you mentioned; the quality isn't very good.)

 

[justinheronmusic]

I'm pretty sure I detailed quite a lot of what i did in my build of a pair.
I use mine as drum overheads and they are very natural sounding.
Pretty busy with other stuff but will try and help where i can, pretty sure googling the c12a pcbs will bring up some pictures and you can just copy them.
It's not a complicated pcb, seem to remember the tubes were a bit hit and miss but i got there in the end.

Just a few questions:

- Did you use solid or wet tantulum for the 15uf 75v?

- Do you have a resource for the 7586 socket footprint? I would like to build one in Eagle if it's not already available.

I am aware of the CK12 situation. Hopefully we can get these built with the capsule being the most expensive aspect. Does anyone know if Maiku's capsule is good? I would like to support Maiku if I can. I used his K47 in my M49 build and it's insanely good. Otherwise perhaps this may be of use as a mid-level option:

https://www.fullcompass.com/prod/521125-akg-2072z00090-silver-condenser-capsule-for-c414eb

Of course Tim's CT12 is still an option, yeah?

Can we use a solid 15uf 75v Tantulum for the output capacitor? Is it possible to use electrolytic?

The next question is the tube socket. A quick search brought me limited sources for the 7586 tube socket. However, there seems to be an equivalent on ebay from Russia with long leads; would it be possible to use these as a substitute if tube sockets become unobtainium?

https://www.ebay.com/itm/2x-NOS-6S51N-V-RCA-7586-USSR-military-NUVISTOR-TRIOD-tubes/263074725784?hash=item3d4079bf98:g:GpUAAOSw4CFY8Cu1

At the moment, there is a source for sockets on ebay, though most likely not sustainable. I picked up 2 of them this morning.
EDIT: The seller has 116 in stock as of 5/30/2020

https://www.ebay.com/itm/NOS-Vintage-5-Pin-Cinch-NUVISTOR-Vacuum-Tube-Socket-Qty-Avail/124136849222?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have the Matador C12 PSU here, will this work as the PSU for this build?

 

[Spencerleehorton]

Do not use electrolytic caps, you need good tantalum ones.
I use the microphone parts capsules and they sound more than adequate.
The transformer is pretty important, 10:1 I think I used if memory serves.
These are nice and small and work well

https://www.ebay.com/itm/111683319855

The actual headbasket is quite important, I made my bodies from brass, wasn't all that difficult, but I did try in a normal style head basket and I didn't like the sound.

I got the socket off ebay and most of the other bits, you'll just have to be resourceful and do a lot of searching.

Good luck.

 

[Gus]

A little off topic

Why do you want to build a C12a ?

Why not take the good from other cathode followers(CF) microphones and build a version of a CF microphone? You can use what ever body, transformer and capsule and learn about CF circuits from experiments

OR

I don't understand why people build tube based follower microphones instead of solid state source followers. An adjusted Sony C38B or AKG 414 EB or something else might be more fun.

 

[Spencerleehorton]

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!!!

 

[RuudNL]

These are nice and small and work well

https://www.ebay.com/itm/111683319855

I can confirm this. I used one in my 'U67' recreation and was surprised by the good low frequency response, even though the core is small.

 

[justinheronmusic]

- So the body still undecided based on Spencer's recommendation. I have a friend who is into 3D printing who may be able to replicate the plastic body of the original, but that wouldn't include the head basket, and I don't have any dimensions of the body or headbasket whatsoever. Chunger's GT-2B can be retrofitted with Advanced Audio's Cm49 grill - I have tested this on my M49 build with success. Would anyone like to weigh in? Does anyone know of another head basket that will fit?

- I found a data sheet for the 7586 which appears to have measurements for the footprint, but I am having a hard time making sense of it. Would anyone be up for helping me decode it? I would be happy to make an Eagle package.

http://n.manet.nu/tubedoc/RCA_Nuvistor_data.pdf
(Document page 10, Adobe page 34/97)

 

[Gus]

Maybe my post was misunderstood.

I did not say not to build a microphone you can do what you want. And I did post adjusted.

I do not consider a clone to be a clone unless it uses almost all the same important parts. Capsule, grill, output transformer and tube specs( does not have to be the same tube number but one with the same important specs).
So building the circuit with out the rest of the exact or close to the same parts is not a microphone clone IMO.

Nothing wrong with building a CF microphone. I built one based off an AKG C60, C61 circuit like fragment  I changed the biasing for the tube I used. I like how the capsule had the charge applied.
To me with all the information available on the web now why not pick the best from each circuit, grill etc and build a microphone. You do need to be careful with the information on the web.

Also look closer at the stock supply.

 

[justinheronmusic]

Also look closer at the stock supply.

Are you referring to the 3 & 5 pin connectors on the N12A?
Will Matador's C12 PSU PCB work for this AKG C12A build?
Can we make this a 7-pin connector microphone?

There are no wrong opinions here, Gus. I can jump this ship and help sail a solid state C12A ship, but I can not captain that ship. Leading that project is beyond my limits; If I have a schematic and dimensions for a PCB, I can source components, I can do a layout; I can not design the circuit. If you have ideas on how to make the C12A a solid state circuit, or utilize a different tube for the C12A, then I am not only willing to listen, but I will help with that. Especially if it will save us all money.

What I personally want out of this project is to invoke the spirit of the C12A as that is the sound that I am after.

 

[Gus]

I read in your post you are going to use a Nuvistor tube?

Invoke the spirit is cool

You can build it with a k67 edge term to start then buy one of the capsules built like a real CK12 later

get a good transformer and you should be good

The AKG supply is set up a little different in the heater section

You could use an adjusted China supply or Matadors PCB however, I have not used a Nuvistor so I an not sure how it heats up and if it sensitive to the rate of current change at start up in the heater supply.

Maybe someone knows and can share real life use of a Nuvistor

 

[RuudNL]

I have been using NOS nuvistors lately and to my surprise they have a very low self noise, even lower than the best EF86/EF806.
What I found a bit strange is that they take longer to heat up, anyway longer than a regular tube.
As a safety measure, I use a resistor in series with the filament. Since the nuvistor acts as a PTC, the series resistor limits the current when the filament is still cold. Not really sure if this is necessary.
I used some Russian ones, with wires instead of pins. (I would have to look up the type number.)

Edit: The nuvistors I used were: 6C51H-B.

 

[Icantthinkofaname]

I'd say if you want an affordable capsule to be used, go with the Beesneez CK12. It's like $280 USD or so (I know it's priced similarly to the Peluso CEK12, except one is high quality and one is basically a rebadged K67 that's significantly brighter than the other edge terminated K67s).

I'm using rebadged and edge terminated K67 capsules to describe  Chinese "CK12" capsules. For $30, I like the ones iSK makes. For $280, get something better (so don't buy a more expensive capsule that sounds the same or worse).

No experience with the Maiku compared to a proper CK12. I can tell you it's noticeably smoother than a K67 ( compared in a Behringer B2 Pro), but that's about it. It'll certainly be smoother than the CEK12 as well. Peluso should just sell his P414 body and electronics without the capsule. Better yet just the body, though I've heard some bad stories about the quality of his mics' machining. Easily stripped threads and the like. I can't tell you whether the Maiku is better than an RK12, as we've seen from a lot of "machined in China, skinned elsewhere" capsules they do perform very similarly to the Chinese capsules. My guess is they don't adjust anything on the capsule and just skin it using similar spec'd mylar so it ends up more or less the same.

Now IMO, if we're ranking CK12 capsules, for me it's CT12>HK12>Beesneez CK12>Maiku CK12≥RK12 (even Telefunken uses the RK12 in their cheap crap)

I'm with Gus, a C414EB build would be great. I'd actually love to do this one, not too keen on a C12A.

There is some unobtainium here, and that's a good C414 style body and grille. Perhaps reach out to Peluso or sE Electronics (sE T2 and 4400a) and see if there's any spare bodies from broken mics you can buy. Unless someone on here can machine the bodies.

That Full Compass capsule is a teflon one, so it'll be very different from what you have in mind. Very flat. The XLS capsule would be flat-ish too, with some boosts, and the XLII would probably be too bright, it sounds "spitty" to me (really bright, sibilant, with a high end sheen that also sounds veiled, sounds very musical, but I'd rather get 4 CAD M179s than a single C414 XLII).
So I think it's the RK12<Beesneez CK12<CT12 in terms of best value from cheapest to most expensive.

I'd say the MXL 990 or 770 with a sloped grille from Mic-Parts would be the best body, except for an actual C414 shaped body. I'm sure there are a ton of 990s on eBay since they're frequently the stupid deal on Musician's Friend. The Apex 415 can be found for cheap too, that's always an option (I've seen it for like $50 at pawn shops). The MXL 2010 is another way to go. They're discontinued I think, though they were going for sub $100 towards the end.

 

[justinheronmusic]

Thank you for your input on component selection. I will keep the capsule & body suggestions in mind when the time comes. Part of the idea behind this project is to make a PCB to mitigate unobtanium - personally I am ok with not having the original body design. I am undecided though - I was able to find some rough dimensions of the body that could be of use to someone who is handy with 3D printing - it may be possible to print the body of the microphone. Alas, the head basket and connector would still be of concern.

Regarding the transformer, I was able to find a thread where someone needed to replace the transformer and in the process found a data sheet for the original, and also sought out guidance from AMI. He stated that the original transformer was 1.8:1 ratio, but Spencer had mentioned 10:1 ratio.

What is the reason behind this? How are both ratios compatible with this circuit?

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

 

[Icantthinkofaname]

If you don't mind me asking, if you're not using am accurate recreation of the capsule, or a body in the style of the original, why specifically build a C12A?

It's probably easier just to do the "C12" builds that exist for the Apex 460, especially if you end up using a Maiku "CK12" or RK12.

Don't get me wrong, I like the classic AKG mics, but if you take liberties and start using other bodies and capsules, you're not really building a C12A/C414. It'll be something that's probably in the ballpark of what you want.

With the Chinese capsules (I included Maiku and Peluso here because the important parts are made in China), they're K67 capsules with edge termination so they follow the same rise that K67s do in omni and figure 8 patterns. Interestingly enough the ELA M 251 documentation shows a similar rise too. AKGs documentation for their older mics showed them to be fairly consistent sounding in omni and cardioid (how accurate that is I don't know), with only slight boosts. Tim's CT12 also has an omni rise peaking at about +5 dB in the ones used by Fox Audio Research for their FAR 12+ build, and a boost of +4 dB in cardioid. It's worth mentioning that Brian Fox says that he had the capsules he used tuned to their specs, so it may be different in a stock CT12. Both boosts are centered at about 10K. The RK12 will have a +5-6 dB boost at 10K in cardioid, and a +7-8 dB boost in omni. The RK12 I'm going by mics we know use that capsule (the Telefunken TF51, the Advanced Audio CM12, etc.)

3U Audio has an interesting capsule they use in their MKIV. It seems to have similar boosts to the CT12, but it's higher up, at like 12 KHz. The 3U Audio one will be a bit more expensive than the Maiku (B stock Maiku is $100 USD I believe, individual Warbler mic capsules from 3U are $150 for dual diaphragm versions).

Of course you can't necessarily trust the advertised responses of mics. They're smoothed like crazy. People on here, and a lot of engineers and technicians would probably prefer the unsmoothed or barely smoothed versions, but the average Joe (and probably a lot of project studio engineers and home recordists) finds the smoothed versions a lot more appealing, and I'm sure the marketing team likes that version too.

 

[Spencerleehorton]

Im gonna chime in here, I built a C12 and liked but to me it didnt make a very real sound, a good sound but not very natural.

I wanted a mic or mics which i could mic up a room and it would sound like that room.

The C12a came up as a mic that would do this time and time again.

Many studios using it for overheads or room mics or on toms or vocals.

Its quite simple to build the body and headbasket of the mic out of brass, cut out the main body pieces and braise them together to form a box, put a bottom on it then drill a hole for the socket.
for the headbasket, shaping some mesh for each side and a strip of brass down the middle then braise them together.
For the capsule mount i just used a bit of copper bent in a U shape then mount the capsule to it and it just slotted in.
The headbasket i then drilled and screwed it securly, then just getting the pcb in there along with the nuvista and transformer.
It was all pretty tight but I did two of the mics and they work wonderfully.
Didnt cost a bomb and i will get round to taking them apart and nickel plating them, but im using them all the time so i dont know when that will be!!

Just thought i'd speak up, and hope this helps.

My original plan was to make a c414 EB but there ended up being a few things that just didnt work out, the DC-DC converter for one!!!
I love to make a few C414 EB and make a few more bodies but pretty busy with everything else now!!

Good luck.

Spence