Official C12 Clone - Build and Support Thread

tonycamp · May 23, 2013

Hi Fella's,

I have an interesting clip for you guys, my C12 head basket seems to have a pretty nasty bell resonance to it, so i did what i recommended, DUCT SEAL!!

Listen to the clip, and judge for yourself, I could imagine if you were to use the C12 head basket untreated, in a high SPL situation, the ring could be quite problematic, the duct seal helps a lot, but i'm not sure if it's enough??

https://soundcloud.com/tonycamp/c12-duct-seal

let me know what y'all think, and i hope this helps

thanx
T

Edit; I mean this as no slight to chungs head basket, it's a really nice piece!, and for all i know, the resonant thing is part of the C12 sound(i've never used a real one), not to mention, the mic sounds friggin beauty gotta love it!!, seriously stellar!

sr1200 · May 23, 2013

Did you "break in" the mesh before you applied the sealant to it? Im trying to find that go2 glue mentioned by hellfire above, noone in my neck of the woods seems to carry it though. Gonna try home depot tomorrow. Most of my issue was taken care of just by loosening up the mesh manually... id only want to further damp it for things like high spl since you can almost kinda hear it still but its so feint on loud vocals it really doesn't bother me anymore.

tonycamp · May 23, 2013

sr1200 said:
Did you "break in" the mesh before you applied the sealant to it? Im trying to find that go2 glue mentioned by hellfire above, noone in my neck of the woods seems to carry it though. Gonna try home depot tomorrow. Most of my issue was taken care of just by loosening up the mesh manually... id only want to further damp it for things like high spl since you can almost kinda hear it still but its so feint on loud vocals it really doesn't bother me anymore.

I did break in the mesh, i gave it a few real good squeezes, and worked the crackling out, I'm worried about the ring on overheads, cause i play loud as fuck!lol!, thats about it though. But again, the mic sounds really great!

baadc0de · May 23, 2013

Yeah, good idea. I've had this running through my head too, so it will be the first thing to check!

tonycamp said:
baadc0de said:

Matador said:

Can we quantify exactly what "low output" means? How many DBFS is it lower than what you are expecting?

Click to expand...

Another build I just did seems to be ~ 20dB louder. I'll test at the studio soon to be extra sure. From my measurements, the voltages seem to be OK and since tube swaps are not working, I guess it's either the capsule or transformer. I can probably inject signal on both and see if the gain drop still occurs. If it does, it should be the transformer, if not, the capsule. Let me know if you guys have any other suggestions.

Click to expand...

Double check your bridge on the back plates??

Ericbazaar · May 23, 2013

Hi,
I have build this Mic for a few Weeks. Last Week came Tim's Capsule to me. I have Problems with the Polarity. When i solder the right Capsule-Cables to the right Capsule-Connection-Pins, also Rear-Bias to RB, Front-Bias to FB, Front-Capsule to FC, Rear-Capsule to RC, is the Polarity twisted and the Mic have lower Output. I set the Polarity to Cardioid, the Cardioid is on the Back-Side of the Mic and have low Output. Set the Polarity to Omni, is the Mic quiet. When I set the Polarity to Figure 8, is a Cardioid on Back-Side again but with High Output. I don't understand this. The Mic has not Multipattern yet, no Figure 8 and Omni. I have swapped the Cables RC to FC, RB to FB and back, no change. Does anyone know a solution?

Thanks

PS: I have reinstalled the RK 12 Capsule, same Problem! Damn, what is it! And now installed the CT 12 again, all very careful, right Cables to right Pins. The Microphone Multipattern works, but the Output Level is very low! Any Ideas?

Tim Campbell · May 23, 2013

The only problems that be capsule related would be a bad connection, wrong connection, damage or collapsing.
Take off the headbasket and make sure the membranes aren't collapsing.

Ericbazaar it sounds like you've got a short or a missing connection. Check your voltages and look for solder bridges. Make sure the solder tags on the capsule are snug (don't overtighten them).

Matador · May 23, 2013

There are only a few things I can think of related to low output:

1) Capsule polarization voltage is too low. I don't remember the equation off-hand, but the lower the polarization potential across the membranes the lower the sensitivity. If you've confirmed B+ is around 120V (and it doesn't have to be exact), then confirm that the voltage dividier between R12 and R13 is giving 1/2 of whatever you measured for B+. If this is good, you might try also replacing R14 (although I would think it would be rare for there to be a problem with R14). You can't measure the FB side because your meter will clobber this voltage (unless you have a very expensive 10G probe system).

2) Tube gain is too low: seems like a remote possibility. The tube gm would have to be WAY off for this to be the problem. But you should confirm that the plate is idling somewhere near 60V-80V by measuring right at the tube socket (pin1 or pin6 depending on which side you are using). I would also confirm the heater is set correctly by again measuring right at the tube pins (see prior posts in this thread for details). The tube should be pulling right around 0.5mA with a 100K plate resistor (R17) and the negative bias at -1V. Also confirm the negative bias by measuring right where it enters the mike at the P4 node (right before R11).

A slightly higher chance is that either C13 or C10 is cooked (doesn't take much with styrene's). There is a procedure for checking these in the previous posts half a dozen or so pages back.

3) Transformer primary and secondary reversed: disconnect the primary at the bottom of the board and confirm you are getting 800-1K ohms across the primary. If the reading is less than 100 ohms it's backwards and it would explain the low output.

The best way to confirm everything is working post-capsule is to disconnect the capsule FB connection (which goes to the coupling cap C13), and inject a 1VPP signal right into the grid at pin2 (or pin7 if you are using the other half of the tube) and then measure the output at the XLR connector. A 1VPP signal should go straight through and appear somewhere in the 1V to 1.5V range as measure between the two output pins (5 and 6 I think). The tube gain is about 25dB, and the transformer reduces this by about 20dB so the total gain through the circuit should be about 5dB.

baadc0de · May 23, 2013

Unfortunately, after triple checking about tying the backplates together, the result is the same. The 1st mic I did has about ~20dB less gain. The transformer is correctly wired (remeasured in situ) and it is using the same PSU and cable. Voltages arrive fine to the mic. I'm beginning to suspect the capsule rather strongly. It's the same make / model as the other one.

Ericbazaar · May 23, 2013

Tim Campbell said:
The only problems that be capsule related would be a bad connection, wrong connection, damage or collapsing.
Take off the headbasket and make sure the membranes aren't collapsing.

Ericbazaar it sounds like you've got a short or a missing connection. Check your voltages and look for solder bridges. Make sure the solder tags on the capsule are snug (don't overtighten them).

How do I see a broken capsule? I used 30awg silver teflon wire from capsule to pin connectors and i think are snug enough

Matador said:
There are only a few things I can think of related to low output:

1) Capsule polarization voltage is too low. I don't remember the equation off-hand, but the lower the polarization potential across the membranes the lower the sensitivity. If you've confirmed B+ is around 120V (and it doesn't have to be exact), then confirm that the voltage dividier between R12 and R13 is giving 1/2 of whatever you measured for B+. If this is good, you might try also replacing R14 (although I would think it would be rare for there to be a problem with R14). You can't measure the FB side because your meter will clobber this voltage (unless you have a very expensive 10G probe system).

2) Tube gain is too low: seems like a remote possibility. The tube gm would have to be WAY off for this to be the problem. But you should confirm that the plate is idling somewhere near 60V-80V by measuring right at the tube socket (pin1 or pin6 depending on which side you are using). I would also confirm the heater is set correctly by again measuring right at the tube pins (see prior posts in this thread for details). The tube should be pulling right around 0.5mA with a 100K plate resistor (R17) and the negative bias at -1V. Also confirm the negative bias by measuring right where it enters the mike at the P4 node (right before R11).

A slightly higher chance is that either C13 or C10 is cooked (doesn't take much with styrene's). There is a procedure for checking these in the previous posts half a dozen or so pages back.

3) Transformer primary and secondary reversed: disconnect the primary at the bottom of the board and confirm you are getting 800-1K ohms across the primary. If the reading is less than 100 ohms it's backwards and it would explain the low output.

The best way to confirm everything is working post-capsule is to disconnect the capsule FB connection (which goes to the coupling cap C13), and inject a 1VPP signal right into the grid at pin2 (or pin7 if you are using the other half of the tube) and then measure the output at the XLR connector. A 1VPP signal should go straight through and appear somewhere in the 1V to 1.5V range as measure between the two output pins (5 and 6 I think). The tube gain is about 25dB, and the transformer reduces this by about 20dB so the total gain through the circuit should be about 5dB.

I have all measured, the B+ is 120v on PSU, on Plate 67v, heater is 6,3v and Bias is -1v. the measure between R12 and R13 is 59v. I dont understand the measuring method of c13 or c10 :-[Sorry I'm still a beginner. When I exchange cable FC and RC, also RC cable to FC pin and other around, one side of the capsule works with good output. Only when the switch is Figure 8, on Omni is the Mic quiet. transformer is installed properly, 930ohm to PCB and 20ohm to XLR. Is it about the polystyrene caps?

chunger · May 23, 2013

due to the recent issues folks have been having with the single layer headbasket ringing, I played around with it again last night and have a few observations.

In general, the dual layer basket is MUCH more sturdy than the single layer. It is amazing how much more rigid the assembly becomes with the thinner mesh inside all soldered together. I am not an acoustics expert, but I believe the presence of the 2nd layer pushes the resonant frequency of the basket down significantly and the overall stiffness of the assembly makes it resonate less in general.

There is probably good reason most production microphones use 2 or 3 layer mesh setups or more robust hoop structures to support the single layer meshing. That being said, the main mechanical weak point that I found in the current single layer basket is around the perimeter where the basket meets the round base assembly. This is only soldered in 2 locations at the hoop attachment points. For my 2nd batch, I will discuss with the factory at least spot solder all around the perimeter on the single layer baskets. For now, it seems folks have been successful using a number of adhesives in the critical areas 90 degrees from the hoop attachment points to mitigate the basket ringing. FWIW, the ringing here in the untreated headbasket seems on par with the ringing I heard in the stock GT-2B microphone and has not proven problematic for me in practical use yet in studio. But, I can readily induce the problem by physically tapping on the body.

As I understand it, nickel plate while difficult to solder is not impossible. I will experiment with dremeling away the little tabs inside the headbasket and manually soldering around the perimeter of the headbasket. This should be the most robust solution as long as the finish is not damaged. Fortunately, I have a wonky B-stock headbasket on hand that I can experiment with.

The dual layer headbasket seems solid and does not suffer from this particular problem.

sr1200 · May 23, 2013

Whats the chances of not plating the mesh in the next batch?

tonycamp · May 23, 2013

perhaps inquire about aluminum on the next batch? takes plating excellently, shouldn't ring near as much? who cares if it adds $20 to the bill?

Tim Campbell · May 23, 2013

Ericbazaar, if the capsule isn't collapsed and the connections are good it can't have a lower output than when I sent it to you. try installing the original caplsule and see how that works. In my experience low output is usually a poor connection, a solder bridge or a wrong value resistor.

Ericbazaar · May 23, 2013

Tim Campbell said:
Ericbazaar, if the capsule isn't collapsed and the connections are good it can't have a lower output then when I sent it to you. try installing the original caplsule and see how that works. In my experience low output is usually a poor connection, a solder bridge or a wrong value resistor.

I have reinstalled the RK12 Capsule again. Exact the same Problem. I have all checked three times, i don't know what it is. Can I destroy the capsule when I change FC and RC Cables? Change RC and FC works one side of the capsule, switch on Omni the Output is quiet, switch on Cardioid the Output is ok, switch on Figure 8 the Output is high. I hope the capsule is not broken :-[

Tim Campbell · May 23, 2013

Eric, do you mean you have the same problem with the RK12 installed? If so then it's not a capsule problem. I still believe you have a missing connection or short in one of the cables leading to the capsule. You could ruin the capsule by getting hot flux or solder (even a microscopic bit) on the membrane. This is why I include covers for the membranes. Don't solder/unsolder at the capsule, desolder it's wires at the pcb.

Ericbazaar · May 23, 2013

Tim Campbell said:
Eric, do you mean you have the same problem with the RK12 installed? If so then it's not a capsule problem. I still believe you have a missing connection or short in one of the cables leading to the capsule. You could ruin the capsule by getting hot flux or solder (even a microscopic bit) on the membrane. This is why I include covers for the membranes. Don't solder/unsolder at the capsule, desolder it's wires at the pcb.

Yes Tim, same problem with RK 12! What is the principle of a double membrane capsule? one side have static voltage of +60volts, the other side is switched from -60 to 0v to +60. http://www.sengpielaudio.com/DoppelmembranKondensatormikrofon.pdf can this also be a problem? and yes i will checked all cables and xlr.

Ericbazaar · May 23, 2013

Tim Campbell said:
This is why I include covers for the membranes. Don't solder/unsolder at the capsule, desolder it's wires at the pcb.

I have solder the cables with cover and I have made a small bag over it. I have the capsule viewed through a magnifying glass and did not detect any damage.

Matador · May 23, 2013

Ericbazaar said:
Tim Campbell said:

Eric, do you mean you have the same problem with the RK12 installed? If so then it's not a capsule problem. I still believe you have a missing connection or short in one of the cables leading to the capsule. You could ruin the capsule by getting hot flux or solder (even a microscopic bit) on the membrane. This is why I include covers for the membranes. Don't solder/unsolder at the capsule, desolder it's wires at the pcb.

Click to expand...

Yes Tim, same problem with RK 12! What is the principle of a double membrane capsule? one side have static voltage of +60volts, the other side is switched from -60 to 0v to +60. http://www.sengpielaudio.com/DoppelmembranKondensatormikrofon.pdf can this also be a problem? and yes i will checked all cables and xlr.

Not quite: the voltages are relative. The FC sits at a fixed ground potential (0V). The backplate(s) sit at +60V. The rear diaphragm goes from ground (omni) to +60V (cardioid), to +120V (figure 8). When the back is at 120V, the backplate(s) can be though of as sitting at ground (really +60V), then the rear capsule is +60V from that (120V), and the front diaphragm is at -60V (at ground).

Ericbazaar · May 23, 2013

Matador said:
Not quite: the voltages are relative. The FC sits at a fixed ground potential (0V). The backplate(s) sit at +60V. The rear diaphragm goes from ground (omni) to +60V (cardioid), to +120V (figure 8). When the back is at 120V, the backplate(s) can be though of as sitting at ground (really +60V), then the rear capsule is +60V from that (120V), and the front diaphragm is at -60V (at ground).

Ah ok i understand…almost… :

…now I still do not know where the problem is :-\. it would be easy it would be boring. and when the micro works, I'll love it twice…

HellfireStudios · May 23, 2013

You could take a technique from Neumann, and use solder paste in the perimeter of the base ring to make a complete solder joint between the two pieces. It would be easy enough, and probably cheap too. Just an idea...

-James-

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