Official C12 Clone - Build and Support Thread

Link to latest M251E conversion schematic

Cat 5 did one of the cleanest conversion builds I have ever seen using this.  Hopefully he posts up pictures!

I was planning on making a couple of changes to how I did the layout to make it easier to assemble but I will post a couple of pics of the last one since it really was one of the best sounding mics I've ever heard.  This version has the advantage of variable pattern switching, as opposed to the original's 3 fixed patterns.  nothing sounds even close to an old GE JG6072 or GL6072.  You don't need 5 stars, but 50's and 60's tubes sound much better (IMO) than even the green label GE tubes from the 80s.

1 - You can eliminate the red wire from the output cap to the plate connection and just use a jumper to the turret labeled PLATE.  I had already done the wire like the older version required, and so left it.

2 - If you cut the trace between the top right turret labeled (RC) and the rear mounted turret below it, and then jumper that rear mounted turret to the other side (turret opposite of RC - other side of r16) you can mount the rear 5000pf styroflex cap in it's usual position instead of running the long lead to the opposite side like my pics illustrate.  Functionally it's the same, but will be simpler, cleaner, and may help with microphonics of that capacitor.

Detail of rear - I laid it out to look cool, so you can obviously do the layout however you like.

Hi,

I'm in the process of building OPR's Point to point C12.  In the BOM version 1.3 and schematic version 3 are listed the values of:

R1, R2 = 90.9Kohms
R9, R10 = 392ohms  and 1Kohms
R6 = 470Kohms
C1, C2 = 100uF 250V

Then it is mentioned that the BOM and schematic has been updated (I can't see this anywhere on the sheets) and suggests the following values should be used:

R1, R2 = 68.1K
R9, R10 = 221R
R6 = 499K
C1, C2 = 47uF 350V

My question is: do I need to use these "updated" parts or am I fine using what's listed on the BOM and schematic for this P-T-P C12 build of OPR? 
Cheers!
Dennis

I have started to build this too according to Category 5's schematic. I am not sure on the R14. On the schematic, the R14 is a new 8Mohm resistor but I actually have an R14 labeled on my pcb for 250Mohm. I am not sure shall I put that 8Mohm there, or the schematics R14 should be somewhere else,?

This 8MOhm should be the one which connects the 5000pf Styroflex and the FC? (front diaphragm) or this is the R14 should connect the BP? (8Mohm)

Studiogearlover said:

I have started to build this too according to Category 5's schematic. I am not sure on the R14. On the schematic, the R14 is a new 8Mohm resistor but I actually have an R14 labeled on my pcb for 250Mohm. I am not sure shall I put that 8Mohm there, or the schematics R14 should be somewhere else,?

This 8MOhm should be the one which connects the 5000pf Styroflex and the FC? (front diaphragm) or this is the R14 should connect the BP? (8Mohm)

Click to expand...

Follow the schematic I posted above (in reply #2301):  the PCB was meant for a C12 circuit configuration, so you are essentially "hacking" the PCB to accommodate the 251 circuit.  This also means that some part values will not match what is on the silkscreen as well.

Studiogearlover said:

I have started to build this too according to Category 5's schematic. I am not sure on the R14. On the schematic, the R14 is a new 8Mohm resistor but I actually have an R14 labeled on my pcb for 250Mohm. I am not sure shall I put that 8Mohm there, or the schematics R14 should be somewhere else,?

This 8MOhm should be the one which connects the 5000pf Styroflex and the FC? (front diaphragm) or this is the R14 should connect the BP? (8Mohm)

Click to expand...

The schem I emailed you is the same as Matador's schematic posted above.  It is the only one that works properly with the remote pattern switching, and even though there are some part numbers that are labeled differently on the PCB it shouldn't be too tricky to figure out what's what.  Do you have the newer version PCB?

I started troubleshooting this hum this morning! (just came back from a month tour)

I did the following:
Power up the psu with microphone attached, waiting for a bit, then power off the psu while listening.
The audio stays for a bit, but the hum noise disappeared immediately. If I'm right, that means that there is either ripple in B+ or heater.
Then I did another test, as suggested by Matador: while the microphone is on for a minute, I disconnected the heater wire in the PSU. The audio stays for a bit, but the noise seems to disappear. That would mean my noise source is in the heater circuit.
I will try to ground the heater pin of the tube directly to the microphone chassis ground, as I suspect it would improve things...

The change of grounding in the microphone didn't change the noise.
I connected the Heater Psu of another Psu I've made for a previous C12, and the noise disappeared!
I I guess I've found the issue!!! ;D ;D ;D

This could either be a faulty regulator or capacitor right?
I've used LT317AT from linear technologies instead of the 512-LM317T could it be this?
I have a spare one, but does it need to be exactly the one listed?

Or maybe it is the capacitors...

What do you think?

Thanks

cariocaman85 said:

The change of grounding in the microphone didn't change the noise.
I connected the Heater Psu of another Psu I've made for a previous C12, and the noise disappeared!
I I guess I've found the issue!!! ;D ;D ;D

This could either be a faulty regulator or capacitor right?
I've used LT317AT from linear technologies instead of the 512-LM317T could it be this?
I have a spare one, but does it need to be exactly the one listed?

Or maybe it is the capacitors...

What do you think?

Thanks

Click to expand...

What exactly did you change about the heater? I'm having noise issues too..

For now I didn't change nothing in the supply.
I just used another PSU that I made previously (exact Matachung PSU schematic and components this time).
I used only the heater wire from this psu, the rest of the voltages (Bias/B+/0V) are still being fed by the new supply.

My guess is that there is an issue in the heater circuit on my last built. And as I swapped some components it might come from this.

Maybe there is another explanation. As I first tested the microphone, setting the heater voltage to 6,3V I couldn't get the proper voltage with the microphone plugged, it was to high (around 8,5V). But it was a mistake, the DDM pin was making contact between heater and B+... :-[
After I sorted this out, I had no trouble getting the good 6,3V.
But, in the process, this might have damaged the regulator, as it was shorting with B+.

What is your noise problem?

...talking about voltages...what should be the capsule voltage or the polarizing voltage for this? and which resistors are responsible for the capsule voltage? Thanks a lot guys!!

cariocaman85 said:

The change of grounding in the microphone didn't change the noise.
I connected the Heater Psu of another Psu I've made for a previous C12, and the noise disappeared!
I I guess I've found the issue!!! ;D ;D ;D

This could either be a faulty regulator or capacitor right?
I've used LT317AT from linear technologies instead of the 512-LM317T could it be this?
I have a spare one, but does it need to be exactly the one listed?

Or maybe it is the capacitors...

What do you think?

Thanks

Click to expand...

Hmm.  Do you have any way of observing the heater waveform (like a scope)?

Any 317 should drop right in and work properly.

Studiogearlover said:

...talking about voltages...what should be the capsule voltage or the polarizing voltage for this? and which resistors are responsible for the capsule voltage? Thanks a lot guys!!

Click to expand...

Capsule polarization comes from two places:  first, the 120V B+ is divided by 2 (via R12 and R13).  This fixed 60V is sent to the backplate through R14.

Second, the front diaphragm is fixed to 0V.  The rear diaphragm is set via the polarization supply (the switched resistor network on the PSU) and is sent up through the wire labeled P3 to the microphone, and filtered via R16 and C11.  If set P3 to 0V, you get omni, if set to 60V you get cardioid, and 120V gives figure 8.  Since there are 9 steps on the switch, you also get 6 intermediate patterns:  three between omni and cardioid, and another three between cardioid and figure 8.

Thanks Matador.

Hmmm...in this case as I am only using the front diaphragm and my PSU doesn't have a polarization switch (which is fine by me as I only want cardioid) does this mean I don't even need the P3, R16, C11 at all? And no need to connect the P3 anywhere?

I am trying to find out how to work with my old capsule which requires 70-100V polarization and it is the only cardioid.

Following what you are mentioned, I should leave the P3 out and keep the front capsule at 0V and feed the backplate in my case not with 60V but 100V (as the capsule require this) would technically work?

With this project, I have ended up building a microphone for my already built PSU and not a PSU and a mic together..

Cheers!!

Matador said:

Hmm.  Do you have any way of observing the heater waveform (like a scope)?

Any 317 should drop right in and work properly.

Click to expand...

Unfortunately I've got no scope for now to check this...

As I wrote later, maybe I fried it when shorting with B+... what do you think?

I am trying to find out how to work with my old capsule which requires 70-100V polarization and it is the only cardioid.

Click to expand...

The only edge terminated LDC that I know of that needs that high a voltage is a Lomo.  All others I can think of would collapse when breathed on if polarized to 100v

Hello Guys,

Thanks for your input on this. I did a test so far without the capsule on to see the voltages and I am a bit confused.

Following Matador's response earlier...I am measuring the correct voltages on B+ 120V, R12 and R12 are 60V each however, when this voltage os reaching the R14, it drops to 7V ish at the R14 which goes in to the BP means, there is no 60V's on the back plate at all?

But reading Matador's response, backplate should be 60V or whichever polarization is desired through the P3....?

question, will I get the extra polarization voltage from the P3 in addition to the R14 at all?

Cheers for any answers on this!

Matador said:

Capsule polarization comes from two places:  first, the 120V B+ is divided by 2 (via R12 and R13).  This fixed 60V is sent to the backplate through R14.

Second, the front diaphragm is fixed to 0V.  The rear diaphragm is set via the polarization supply (the switched resistor network on the PSU) and is sent up through the wire labeled P3 to the microphone, and filtered via R16 and C11.  If set P3 to 0V, you get omni, if set to 60V you get cardioid, and 120V gives figure 8.  Since there are 9 steps on the switch, you also get 6 intermediate patterns:  three between omni and cardioid, and another three between cardioid and figure 8.

Click to expand...

When you say you're measuring R14, are yo measuring at the R12/R13 junction? That's the last place you can measure you backplate voltage. If you measure on the capsule side of R14 your DVM will load the circuit, dropping the voltage.

Delta Sigma said:

When you say you're measuring R14, are yo measuring at the R12/R13 junction? That's the last place you can measure you backplate voltage. If you measure on the capsule side of R14 your DVM will load the circuit, dropping the voltage.

Click to expand...

The capsule side...but isnt should be 60V there? as this is needs for the capsule or am I confusing here sthing?