AKG C12 A / B Version Build

[Spencerleehorton]

Hi Guys,

Im going to attempt to build the AKG C12A but also going to include the -20db pad switch as per the B version.
The are a couple of components i wanted to verify first and check on the psu side as well.
I will get together a BOM which i will update when i know the parts are.
So far i am starting to plan out how to build the body, or something that will be similar.
I have tracked down the schematic, Valve and have a couple of AKG transformers which are in need of some repair.
if anyone has any info on this i would greatly appreciate it.

regards

Spence.

 

[Matt Nolan]

AEG Moscow do fantastic re-wind work on those little T14 style AKG transformers. I don't know if they do it as a service for third parties though, but you could ask.

 

[tubestation]

Hello,
The c12a/b is a Cathodefollower to Transformer coupled Mic. Because of the already low Impedance Cathodeoutput You can use a lower Ratio Outputtranny like 1:4 or 1:5 . A  Rewind of a existing AkG Core would be a good Thing  to get the Caracter of the Original.
Greatings ,
Lothar

 

[Spencerleehorton]

Hi,

I've started to order some of the bits, will attempt to start planning the body.
This build is just so I have some decent Tom Mics!

Regards

Spence.

 

[Spencerleehorton]

Hi,

this is the schematic for anyone who's interested.

BOM

Resistors
560R
4.7k
27k
200M x 3
470K x 2

Capacitors
1000uf/125v Styrene
5000pf/250v Ceramic
15uf/75v Tantalum

Transformer
1:4 or 1:5 200ohm

Valve
Nuvistor 7586
5 pin Nuvistor socket

Edge terminated Capsule

PSU

Transformers
240 - 20v – H+ Supply (200mA) delivers 5v
240v – 155v – B+ Supply (5mA) delivers 120v
1N4007 x 8

Capacitors
47uf/250v x4
1000uf/50v x 2
1uf/125v
330pf -10db or 1500pf for -20db pad


Resistors
100R 10W pot
1k 0.5W
2.2k 0.5W
2.3k
5.6k
2 x 15k
6.8k
330k 0.5W

Other
T OC 26 PNP transistor
12v Zener
Lorlin 12 way switch
3 way switch (-20db)
Mock up body and circuit board for C12 A/B
Case for PSU
IEC socket
Power switch DPDT
Pilot light or LED (diode and 4.7k)
3 pin XLR Socket



H+ psu wise i may use a slightly different arrangement.

regards

Spence.

 

[Matt Nolan]

OC26 is a basic PNP BJT. OAZ is indeed a Zener. Simple constant current supply for the heater. Set it up while measuring the voltage right at the heater pins on the Nuvistor, once it is well warmed up.

 

[Matt Nolan]

OC26 is a basic PNP BJT. OAZ is indeed a Zener. Simple constant current supply for the heater. Set it up while measuring the voltage right at the heater pins on the Nuvistor, once it is well warmed up.

Figure out the approx trimmer potentiometer setting in advance using Ohm's Law. R = (Vzener - 0.6) / 125mA. I think they're underheating the valve to around 5.7V - others here may know better?

 

[Spencerleehorton]

Is anyone able to explain the pad config and what should be used, as the A version was -12db and the B version -20db.

Regards

Spence.

 

[tubestation]

Hello,
Thank s for sharing Your Work and Findings.
The Voltagedivider for the Capsule Bias are done with two 0,47 M (470 k Ohm ) Resistors. In Your BOM You  write 470 M. You probably made a Typo.
The Paddesign  of this Mic looks like Something very different. It s  done  after  the Outputtransformer, or is the Inductor Part of the Transformer.? Even if it s sitting in the Poersupply ? i mean not of the Transformercore  directly, there is no magnetic coupling through the Cable but Part of the Transformer Circuit ?  Looks like more Turns give a bigger Damping,a  lower Ratio of Primary to Secondary, should give a bigger Output, strange, but I  probably  get something wrong.  Or is it a reactive or conductive Pad ?
I hope Somebody can explain this Design. If You have Amp or Capsule Overload this Kind of Pad won t help ? OK Capsuleoverload is not very Common for Soundpressure below 170 dB .
Maybee it s not  nessesary  to lower the Capsuleoutput , because the Cathodefollower with its Gain below 1 can t  Overload the Amp  so easy ? I think this Pad is to lower the Output of the Mic for not overloading the next Stage, the Micpreamp or the Inputtransformer of this Micpreamp. I think this Pad could be obmitted or if Your are after the Sound of this Pad for the autentic Sound You need to find out what Inductor is used and maybee You have to DIY . Is it a Metallamination or Powdercore, what Al Value, howmany  Windings for  x Henrys  etc....
Greatings
Lothar

 

[Matt Nolan]

Sorry, I thought I posted about this yesterday. Something must have gone wrong.

It is not a pad, it's an HPF. On the N12A you can switch between "no" roll-off, -7dB at 50Hz or -12dB at 50Hz. It is an inductor across the output which forms an LR HPF with the pre-amp input impedance. Strange way to do it, and somewhat pre-amp dependent, but there you go.

If you want to pad the mic (why?) then look at something like the C414EB circuit on the gate of the FET. You could do a similar thing at the grid of the Nuvistor. Or just have switchable caps to parallel across the capsule, something like 330pF for -10dB and 1n5 for -20dB if my maths is right.

Cheers,
Matt.

 

[Matt Nolan]

Look at the N61A (for the C61) for a more overt version of this HPF which uses series caps and shunt inductance.

 

[Spencerleehorton]

Hi,

Right I get it now, yes that is the HPF on the schematic, the C12B version had the -20db pad included to improve the use of this mic with higher dynamic source material, made the mic much better apparently?
Having never used this mic I don't know but would like to know what you guys think of it in use in the studio?

Regards

Spence.

 

[tubestation]

Hello
Matt, Thank You for clearing it up ! Strange Way to implement a HPF . Ins t the Cutoff Frequency now related to the Inputimpedance of the Preamp whitch may differ ? In the Time the Mic was developed 600 Ohm or Even less was the Standart. Today it s more like 1,5/2,5 k Ohm., no ? I think this kind of HPF  Isn t the best Idea  for Today.
Spence, i only heard this Mic once in a Studio when  my Drums were recorded. They used them for my Toms. The Sound was great, but when I remember correctly there was a Problem with the big Crosstalk. Was difficult to gate because of this. Long Time ago. Today gateting is not so  Popular any more.
I think with the use of a decent CK 12 Copy You could build a nice Mic with a lot  of Aplications. Just another nice Flavour.
If I  had a original  brass CK 12, i probably would build a c12/ ela M 251 with it.
I was reading that People mod  there C 12 a to a Plate Out  Circuit .
Greatings and happy Building.
Lothar

 

[Matt Nolan]

It seems odd to me that a cathode follower mic would have headroom issues but, reading around, apparently the C12A did, hence the 20dB pad switch on the C12B. I can't find a schematic, but it sounds like it was exactly the same as the C12A just with a switch and a capacitor for the pad.

The C12Bs had the top part of the mic body made from plastic with conductive paint on the inside. This is one design element you perhaps shouldn't reproduce!

My guess is that this would be a good mic for saxes and drum overheads, where the higher THD and IMD of a plate loaded valve mic can make things sound a little nasty. It should excel in the same scenarios as a Sony C37a.

 

[Spencerleehorton]

Hi,

Really good information we are getting here on this.
So are you saying that if built this C12B version would still have crosstalk issues? Are you saying crosstalk is like phase issues? If I'm honest I don't exactly know how your using this as a phrase.
I have built a ELAM 251 and it really excels as a absolutely fantastic vocal mic, I haven't tried it on anything else yet to be honest as it's just so much better than any U47 variant I have.
The whole reason for me investigating this C12A or B version is I just wanted a really good set of Mics for toms, I have some dynamic Mics for toms but everywhere I read that these are the bees knees!!
I have a C12 as well, it's a chunger pcb one sounds good but again it's no where near the 251.
I have just test the couple of akg transformers I have and one checks out at 20ohm/750ohm pretty sure this is a 5:1 and the other can be configured to either 5:1 or 10:1, so these should be fine for this couple of Mics.

Regards

Spence.

 

[Matt Nolan]

Yeah, can we define "crosstalk" in this case?

Actual electrical crosstalk / pick-up of other signals?
Bleed / spill from other instruments due to a not tight enough polar pattern?

 

[tubestation]

Hallo,
Maybee i was missinderstood. In the Studio my Drums were recorded the Recordingroom had a very lively, open and undamphed  Acustic. The Mics were  placed in a close Mic Position. I don t know the selected Pattern  but probably it was  cardioid. We had a lot from the other Drums on  the Tommics with the C 12 a. The Sound of the Crosstalk was Great, so no Phaseissues.
I think the Mic is great for Toms and Overheads like  Matt  mentioned because of the low  THD of the Cathodefoller. If You use this Mic  on a loud Source like heavy played Toms, the integration of the Pad from the C12b may be a  good Idea.
The dc Resistance of the Transformerwindings doesn t say anything about  the Turnratio or the  ac Resistance  at different Frequencys. If You have a Induction Meter  set it to the lowest Testfrequency and  meassure the Induction of both Windings. If You have both Results You can calculate the Turnratio.
Take the Root of L1:L2.
Exampel: L1 = 100 H. L 2 =  1 H    100:1 = 100    Root of 100 = 10.    Ratio  is 10:1.
But that s not the whole Story. If You use a Induction Meter the meassuring Frequency is too high in most Cases.      The best Way how  to analyse a Transformer is the Way  CJ  explains here.

CJ :
you can check the OPT low freq pri inductance with a generator and ammeter,
use Ohms Law and Inductive Reactance formulae,  E/I=R and XL = 2 pi f L

apply 10 volts @ 10 hz  thru the current meter into the pri,

math goes like this, use 1 ma ac as example for current,


10 volts / .001 amps - ac = 10,000 ohms reactance,

divide 10,000 ohms reactance by 10 hz and again by 6.28 ,

so 10,000/10 = 1000, 1000/6.28 = 159 Henries,

let say you read 10 ma at 10 hz and 10 volts,

10 / .01 amps -ac =  1000 ohms reactance

1000 ohms  reactance / 10 hz = 100,  100/6.28 = 15.9 Henries,

i do not know what the pri Henries should be on the OPT, maybe Gus knows,

here is a chart for figuring out inductance using 10 volts at 10 Hz


ma  H
1  159.2
2  79.6
3  53.1
4  39.8
5  31.8
6  26.5
7  22.7
8  19.9
9  17.7
10  15.9
11  14.5
12  13.3
13  12.2
14  11.4
15  10.6
16  10.0
17  9.4
18  8.8
19  8.4
20  8.0
Thanks again to CJ  for explaining  this in a Perfect Way !!!
Greatings ,
Lothar

 

[Matt Nolan]

That's a great explanation for how to measure inductance, but I would recommend doing it at your local mains frequency i.e. 50 or 60Hz and adjust the calculations accordingly. Most AC ammeters are calibrated for mains frequency and may be wildly off at other frequencies. Even a true RMS full audio spectrum meter might be rolling off at 10Hz.

If you just want to know the trafo ratio, simply stick a small AC signal on the primary and measure what you get (ideally with an oscilloscope) on the secondary. Measure it unloaded or loaded with the expected impedance. Obviously, you'll get a higher figure unloaded, which will be closer to the actual turns ratio, but it might be more instructive to see what you get under load.

 

[tubestation]

Hello Matt,
Of course You are right to meassure at 50 Hz is better to get a good and correct Result with normal Meassuringequipment.  I think the 10 Hz were just an Example for an easy and descriptive  Understanding.
But If You already set up your Meassuring I would have a look at 30 Hz because it s audiorelated and at this Frequency Transformer get to there Limit and make funny Things. For a fast Ratioanalysis the Way You describle is a good and easy Way , but than it s good to know in what Ballpark Your Tranny is. If not You don t know If it s a 200:5000 Ohm. 1:5 or a 600: 15000 or a 2k : 50 k Ohm  1:5 Transformer. It helps to terminate the Output with the expected Resistance and have the right Sourceimpedance but some Trannys take a missmatch better than Others s  and some Trannys  work even quiet linear frequencywise with a heavy  Impedancemissmatch but with higher THD and Phaseissues.
Another Great CJ Statement : "for me a 2 Henries  winding represents a 200 Ohm Winding, 20 Henries a 2k Ohm and a 200 Henries a 20k Ohm Winding.  "
Greatings
Lothar

 

[Biasrocks]

The Mics were  placed in a close Mic Position. I don t know the selected Pattern  but probably it was  cardioid. We had a lot from the other Drums on  the Tommics with the C 12 a. The Sound of the Crosstalk was Great, so no Phaseissues.

C12a/b's and 414eb's are generally used and sound best in the hyper-cardiod position for close tom micing.

The leakage is most pleasant and not harsh in the least, not so true of the dynamic mics I've tried in those positions.

Mark