Official C12 Clone - Build and Support Thread

[Matador]

I'm having a hard time parsing what exactly is wrong:  what isn't working as you expect?

For what it's worth, you need to tie PSU 0V to the same potential as the chassis if you want the metal shield of the body to act as a ... well .. shield.  In other words, I can't forsee any situation where opening the ground switch gives better performance.

I also don't understand how you are hearing hum when the PSU is off....

 

[cariocaman85]

Thank you matador for your answer.

My point with the lifting was to provide shielding to the PSU case, the cable and the body (they're always connect to ground earth), and then to choose if I want to connect this ground to 0v electrical of the circuit.
So with these connected I have only one connection between earth shield and 0v, inside the PSU.

I thought that I could lift it to prevent ground loop when the PSU is connected to a grounded preamp, as the 0v would be connected to ground via the preamp anyway, but maybe I'm wrong.

What isn't working as expected is this:
With the ground lift out of the equation (0v and ground connected in the PSU chassis), I still have hum while listening to the microphone.

That's why I'm thinking my grounding scheme is wrong.

The thing is, microphone body, grill, cable and psu case are shielded, but maybe not perfectly, as the connection between shield in cable and the microphone body is only mechanical, done with the strain relief contacting with the microphone metal body. How can I check that? Continuity test looks good.

The 0V in the microphone only connects to ground after traveling all along the cable and reach the inside of the PSU. So It carries the current that is coming back from the heater, right? Do you thing that hard grounding pin 9 directly in the microphone body, instead of merging it with the 0v all along the cable would improve the hum?
Or is it way better to connect the 0v to the ground inside the Microphone as well anyway, for all the circuit (not just the heater)?

Thanks

 

[cariocaman85]

So (removing the ground lift of the equation for now), the grounding scheme would be like this.
What do you think?

Thanks!

 

[Matador]

The heater return is DC, and it's heavily filtered:  it's unlikely to be causing your problem.

It's easy to test:  repurpose your ground lift switch to the heater + line, and after it has been running for a few minutes, cut the switch.  This will deactivate the heater, but the tube will still work for several seconds before the heater cools down.  If the hum disappears during this time you know where to start looking.

Ground lift isn't going to be doing what you think it will, because the transformer secondary (in the microphone) is already floating on the way out to the preamp.  The microphone would have a be adding a TON of noise onto the cable shield to make it past the CMRR of the preamp.

What frequency is the hum?  Is it 60Hz or 120Hz (or 50/100 if you are in the EU).  I'm guessing it will be 60Hz (or 50Hz).  How loud is the hum? Some noise is inevitable with tube microphones.

 

[Category 5]

If the shield of the cable isn't tied to ground it won't act as a shield at all.  If you have a long cable it could be picking up EMI from somewhere nearby in the studio.  I recommend connecting the shield to ground on the PSU end, so any stray EMI will be led there.

Also make absolutely sure you have continuity with the headbasket, body, and bell, and that they have a path to ground.

 

[cariocaman85]

Thanks for your replies,

The shield of the cable is tied to the ground earth, and to the chassis of both PSU and Microphone body.
I have continuity with the headbasket, body and bell until PSU chassis and ground earth pin of the shukko plug.

The switch is just there to link all these earth grounds to 0V of the PSU inside the PSU chassis.

But leaving that switch aside (0V and ground earth are connected in one only point in the PSU), I still get hum in the microphone.
Please note that when I open this link (switch lifted) the hum is getting very very loud and louder than the audio signal.

I will look at the heater disconnecting tip as soon as I get home, just to check if there is any influence.
I will also look at the nature of the hum (50hz vs 100hz). I supposed that if it is 50hz, it is then a grounding issue, and if it is 100hz, ripple from the PSU, right?

The thing is, why there is so much hum (like a lot!) when I disconnect 0v from the ground? The shield is still operational as it is still tied to all of the microphone's body, cable shield, PSU chassis and earth plug... In theory the microphone should still work right?

 

[Matador]

I suggest starting with a baseline which is stock:

1 - all microphone 0V signals tie to the wire that sends 0V from the PSU
2 - all shielded parts tie to the shell of the cable
3 - cable shield strapped to the  PSU chassis
4 - 0V and chassis tied together in the PSU

From the microphones point of view, there is only a single 0V signal, and the heater terminates into it.

 

[jasonallenh]

If anyone needs the justradios/erse cap, I've got a couple sets available in the black market. I had a lot left over because I had to meet the minimum quantities :-\

 

[cariocaman85]

Thank you for your replies,

Yes I believe that pin9 is supposed to be grounded, it is the return from the heater, right?

1 - all microphone 0V signals tie to the wire that sends 0V from the PSU

When you say this, do you mean that there should be continuity within all the 0v points in the microphone, or that all the 0V points in the microphones should be hard wired to a same single point (in the XLR 0v pin for example) following the star wiring scheme method?

Thanks

 

[Matador]

Thank you for your replies,

Yes I believe that pin9 is supposed to be grounded, it is the return from the heater, right?

When you say this, do you mean that there should be continuity within all the 0v points in the microphone, or that all the 0V points in the microphones should be hard wired to a same single point (in the XLR 0v pin for example) following the star wiring scheme method?

Thanks

Yes, the heater return is the same as 0V.

And yes, there should be continuity amongst all 0V nodes.  Don't worry about star grounding.  The main point is that we want overall impedance of the 0V net to be minimized, which is why we have a ground plane where all 0V nodes eventually end up.

 

[Studiogearlover]

Hi Guys,

I know this is an old thread, but maybe someone is still interested to build a DIY AKG C12 clone.

I am about to make one clone, ordered the PCB already, however, I have a tube microphone PSU already...7 pin one, but different to the one the guys who sold the mic PCB recommend...

I will change the pins which are fine, however, I am thinking will the recommended resistors do the job with my PSU to get the right voltages?

My PSU has two separate pins with 200V which I am planning to connect it to the B+ and the Polarizing pin to feed the capsule.

The rest is fine for the tube but I still need to figure it out how am I going to get the BIAS -1.1V...my PSU has two pins of GDN, so I may be able to get this done through one of the pin connected to the bias.

I am getting a bit confused on the resistors in question...

the two 30 MOm's and the two 250MOhm's the mic PCB manufacturer recommends.

I am only going to use the front panel of the capsule, as my PSU has no polarity switch but I am OK with that, I only want cardioid as the patterns.

Now....I am feeding the mic with 200V 2.5mAmps on both polarizing and B+...I can not figue it out the 30MOhm resistor would do the job here with the 5000pF cap??  and one of the 250MOhm resistor? 

Maybe I was doing the math wrong but to obtain a 70V for the capsule out of 200 V wouldn't you need only an 18KOhm resistor?

Maybe I am  completely wrong here...

Also...would be a possiblilty to change the value of the 30MOhm resistor to 40MOhms? (i have a few great ones within this range)

If anyone out there still have some thoughts on this, I would really appreciate a response..

Cheers Guys!!

 

[Matador]

Which kit are you referring to?

 

[Studiogearlover]

Hi Matador,

The c12 clone ones...this is my first ever project doing this and please consider me as a beginner  I don't have the full kit, just the PCB and trying to understand the recommended resistors and the voltage going through from my PSU. I have attached the PSU and pin wiring, I understand by looking at your wiring requirement on the pictures that needs to be changed which I will do so.

With regards the resistors in question, the required resistors 2x of 30MOhms next to the 5000pF cap and the other ones 250mOhns next to the 1000pF cap.  .....IF the values are different lets say I have a 40mOhms cap not 30MOhms...will this change any required voltages?  Also, I have read the on the threads that I would need to change the 100KOhm res down to 47kOhms if I would use an ECC81 tube with it. Am I correct?

My PSU doesn't have a polarization switch, so I was thinking to use only a cardioid pattern, so my apsule would have only one frond side pick up, no back side.

Should I leave in this case the two pins on the mic PCB not connected and only the front and side of the capsule should be connected?

The voltages also a bit of a concern..if I apply 200V to B+, and the 200V to the polarization pin, would I have the required voltages with the resistores mentioned above?

Apologies for the amount of questions, I just want this to make it right.

BTW I have not bought the capsule yet as I would need some rassurance on the above that everything will work in order with the PSU before I spend a bit of money on the capsule.

Thanks Guys, Thanks Mataor!

 

[Category 5]

@studiogearlover.  I highly recommend you pick up the corresponding PSU and circuit board that goes with this project.  You need 120V B+ coming from the PSU, plus 6.3V heater for the tube and you also need bias voltage for the tube in the C12.  The recommended PSU has dual secondaries off the power transformer to accommodate these voltages properly, a regulator for the heater and the pattern switch.  I'm not saying yours couldn't be made to work, but it sounds like it's not terribly suited for this project and would be prohibitively difficult to get it working correctly considering this is your first build.

The C12 project here was put together so that most builders can find success easily, but trying to make do with the stuff you have on hand may throw a wrench in that.

Chunger has all of this stuff on his site ridiculously cheap, and IMO it would be wise to invest in the rest of the parts to get your mic up and running.

Just my .02

Good luck with your build!

 

[Studiogearlover]

@Category 5

Thanks a lot for your input and I totally agree with that...its just that I have this great PSU and not using it really...I might combine the two, this one and start building the other one....I could actually install a trimmer resistor(s) in this PSU just like on the one which I am supposed to build to get the right voltages.

Having looked at the building schematic, I thought at the test of the PSU section with the 18K resistor plugged into B+ and Ground is the one who decreases the voltages to 120V just like the microphone will when its plugged in...? Or am I wrong?

I haven't started the project yet, but here is loads of info and interesting to read different opinions in terms of the components and sound.

Thanks guys and would love to hear interesting stories with DIY projects.... Loving it!

 

[Category 5]

If you look at the PSU here, http://musicalsparks.com/images/c12/c12_schematics_v3.pdf the 91K resistors and the 100K pot following the rectifier bridge drop the B+ to the desired voltage.  Not sure about the PSU you are using, but if you follow the output of the bridge you'll likely find similar resistors in your PSU.  These would be the values you need to adjust.

Since your PSU looks like it has 6V heater already the 251 would probably be a better project for you, as the tube in that circuit does not require a bias voltage from the PSU.  You'd be closer to a working setup already.

Careful though, man.  These PSUs put out some painful DC.  I know from experience.  It's easy to get careless and "get bit."  I totally don't recommend any of this for a first project.

 

[Studiogearlover]

Thanks for that man!

I was thinking to divide the 6V pin as I have two ground pins on this PSU and use one of the two ground for the bias voltage installing a trimmer resistor from the divided 6V lead, other way around and turn it into -1.1V?

Or would this only work in my mind in theory  ;D

I will get this PSU soon and will check the voltages...but i believe this was built for a U67 clone which required about 200V voltage right into the capsule...but could be wrong

 

[Studiogearlover]

If you look at the PSU here, http://musicalsparks.com/images/c12/c12_schematics_v3.pdf the 91K resistors and the 100K pot following the rectifier bridge drop the B+ to the desired voltage.  Not sure about the PSU you are using, but if you follow the output of the bridge you'll likely find similar resistors in your PSU.  These would be the values you need to adjust.

Since your PSU looks like it has 6V heater already the 251 would probably be a better project for you, as the tube in that circuit does not require a bias voltage from the PSU.  You'd be closer to a working setup already.

Careful though, man.  These PSUs put out some painful DC.  I know from experience.  It's easy to get careless and "get bit."  I totally don't recommend any of this for a first project.

Hey

Thanks for this info. Finally I have received the PSU and it is providing me all the requirements for this mic to work besides the -1.1V bias. I will take your advise Category 5, to make a 251 clone instead of the C12. Do you know anyone or any forum  about how to switch the c12 mic into a 251 "ish" mic?  I read a few comments about this but no straight forward answers really...

appreciate your help man

 

[Category 5]

PM me your email and i can send you the info if Matador doesn't get to it first.  I recently did a build of a 251 on this platform for someone and it sounded stellar.

 

[micaddict]

I recently did a build of a 251 on this platform for someone and it sounded stellar.

A little more, puhlease.