Official C12 Clone - Build and Support Thread

[canidoit]

Are there any instructions for those that use 240 volts for the power supply for the C12 clone?

Thanks

 

[chunger]

Are there any instructions for those that use 240 volts for the power supply for the C12 clone?

Thanks

Yes. . . there's a little red switch at the back of the PSU box.  Set it to 220V.  ;D  You should be good to go!

That's the beauty of re-using the stock power transformer and mains wiring.

 

[canidoit]

Thanks Chunger.

Just curious, how does it compare sonically to an original C12??

Also how would you describe the sound and what records best on it.

Have you tried different tubes on it yet?

When do you think you will have all the stock to build a complete C12. I can see you are sold out on the body.

Thanks

 

[chunger]

Thanks Chunger.

Just curious, how does it compare sonically to an original C12??

Also how would you describe the sound and what records best on it.

Have you tried different tubes on it yet?

When do you think you will have all the stock to build a complete C12. I can see you are sold out on the body.

Thanks

I guess much would depend on which original C12 as there were many iterations and variances between individual microphones.  Tim explained to me that he has many original capsules, and they all sound different.  I do not own an original C12, so I have not had the opportunity to do head to head tests.  I have, however, located a pristine original C12 local to me and am trying to arrange a rental so I can test.  I am, however waiting a little while for the prototypes we have running in the studio to break in and loosen up a little bit before spending money on the rental for a test session.

This microphone in it's recommended configuration with the original GE tube, Tim's reproduction capsule, proper headbasket mesh specification (albeit with a larger diameter tube than original), and original spec transformer must land  at least in the general ball-park of the microphone it seeks to emulate.

Many kits have shipped now and I hope someone who also owns an original C12 might be able to chime in.

I have not yet tried different tubes in the circuit.  Only the Electro Harmonix 6072a and the NOS GE 6072A.

So far, the mic has worked well on drum overheads, female vocals, and acoustic guitars.  It has a good dose of air and top end, a bit of scoop in the mids. . . we find it not particularly forgiving of "artifacts" in vocal application.  It would work very well on smoother, polished voices, but can accentuate problems if the voice is a bit grittier.  The top can be a bit forward.  The microphone also does not seem to be as forgiving of a problematic room as some of the other microphones we use.  Off-axis, it seems to pick up a lot.  I hope to hear the microphone on piano and cello soon and report back.

This is a very different animal than the Neumann microphones we have used in our little home studio up until now.  I cannot say we have "figured it out" quite yet, but time and incremental optimizations will get us where we need to go with this mic.

I am actively working on another very large import batch of microphones from China.  I was hoping stock would hold out longer because I am doing development on all of the 3 models I carry to improve the features, improve quality, and lower prices.  Extensive development work particularly on 2 of the models is holding up the process a bit right now, but in the long run, I will be able to offer a much better product for DIY at a better price.  I am ordering a large number of HT-11A microphones to replenish stock. . . 100 units in the next shipment and that should last a while.  Stay tuned to this thread for updates as they come in.

 

[micaddict]

Thanks for the update.

I have not yet tried different tubes in the circuit.  Only the Electro Harmonix 6072a and the NOS GE 6072A.

Same type, but different at the same time. Despite the breaking in period, is there anything you could say about the experienced difference in tone between those two? If you did already, please forgive.

 

[Matador]

I got 118.5 VDC with the stock capsules with all three C-12s.
Thought that would change with Tim's capsules
Looks like it doesn't. Does it 1.5 vdc really matter?

The capsule has no effect on the B+, bias, or polarization voltages.


The tube itself sets it's own idling point:  there is a negative feedback that happens when fiddling with R4 (the B+ adjust resistor).  Decreasing the resistance causes the voltage to increase, which causes current through the tube to increase, which causes more drop across R1, R2, R4, which lowers B+ back.  This is not a regulated circuit which would tend to keep the voltage constant regardless of the current (all other things being equal).

One can lower the values of R1 and R2 to around 82K to bias the adjustment range to higher voltages.  The same works in reverse as well.

 

[Matador]

I agree with Chunger's assessment of the tone:  the sound is very "high-forward".  The C12 circuit has bandwidth well in excess of the range of hearing, so the high-end emphasis of the capsule (above 8K, with a peak in the 10K range) brings forward little details like mouth smacks, lip noises, string squeaks, etc.  You have to have your recording environment in order or else everything comes through loud and clear!

This is as good a time as any to bring up the ELA M251 tweaks:  if we take the 50V/0.6mA idling point as constant, this means the plate looks like about 83K resistance.  The plate resistor R17 (100K) sits at AC ground, which means that the output impedance of the tube stage looks like about 45K (100K || 83K).

We can form a low-pass filter from the output impedance by putting a cap from the plate to ground.  A 1,000pF cap from a 45K source impedance gives a -3dB point of about 4kHz, which means that the circuit response is down -9dB at 8kHz.  This is essentially an inverse of the high-frequency peak of the C12 type capsule.

It is very easy to add this cap to the C12 circuit: 

It sits "next" to the output coupling cap, and attaches down at the other end of the 500K resistor R13 (the bottom node is ground).  Lower cap values will move the filter up in frequency, so a 470pF will be -3dB down at about 8kHz.

My opinion on this:  without much of a Neumann-esque midrange focus, and taking the high-end out with a cap filter, then nothing is left to really "shine" through.  However if you doing a lot of acoustic instruments in sparse mixes (like perhaps a female singer and a guitar player), it might just be the ticket to getting a natural sound without having to do any additional processing after-the-fact.

 

[HellfireStudios]

I brought this idea up before in another thread, but maybe it can be answered now. Could the ELA M251 smoothing cap be implemented on a switch? I guess what I'm asking is would it hurt the mic to switch a smoothing cap in and out of the circuit while it is turned on? Thanks.

-James-

 

[micaddict]

My opinion on this:  without much of a Neumann-esque midrange focus, and taking the high-end out with a cap filter, then nothing is left to really "shine" through.

Good point. Don't throw away the strong points in an attempt to average everything out. Do I read you right there?
And yes, U47 and C12 are very complementary or yin and yang.
Still, the Ela M251 is very highly regarded indeed, perhaps even more so than the C12. Well, for vocals that is.

Anyway, interesting stuff. Keep it coming, please.

 

[JessJackson]

Great points,

Another option is to discus the component materials used.

Replacing the output coupling cap with a PIO (paper in oil) cap such as the original bosch caps used in this circuit. I have found to smoothen out the top end and take away some of the "brittle" sibilance.

Also I read a somewhere on klaus forum a while back that metal oxide resistors in the high impedance sections of the mic dont sound good and add to the "brittle" sterile top end. and carbon wirewound resistors used in the original design definitely contribute to the original sound of this mic.

These are not sourced with ease though and i've been through boxes and boxes of high ohmage resistors looking for the odd few. only to come across one or two of the correct values.

 

[Matador]

I mis-stated something about the 251 circuit above:  the stock plate cap is 100pF, not 1,000pF.  The 1,000pF value is my change, not Telefunken's.  The stock roll-off (with 100pF) happens above about 20kHz.

 

[JessJackson]

correct, but i think the frequency was slightly lower. The 251 (modded c12) was created with the sole purpose of broadcast use (de-emphisis) added as to not effect the pilot tone (19khz) and subcarrier frequency needed for stereo transmission.

 

[Matador]

Oh no! Is the silkscreen incorrect?

 

[dandeurloo]

A pair of these are in my future!

 

[drumminkiger]

I'm really curious about any option to switch between the C12 and 251 frequency response. It would be awesome if there was an option for a daughter board or something that would allow this. I dunno if im savy enough to create something like that myself though..

 

[Melodeath00]

I think there is a tiny bit more of a difference in the circuit of a C12 and 251 than just a capacitor. I'm not expert, but it's what I've been told.

 

[Matador]

I cannot speak to the differences between the capsule construction, and the electro-mechanical headbasket/mounting considerations, but just comparing schematic to schematic:

1) The 25x variants have the plate->ground 100pF cap, the C12 does not
2) The 25x variants use the capsule itself as the grid coupling element.  This means there is no coupling capacitor like in the C12
3) The polar-pattern switching is accomplished on the 25x in a similar fashion as a U87:  with isolated backplates and using 0V to 60V in different connections to the four capsule elements.  I'm not convinced the polarization design accounts for any of the possible sound differences.
4) The 25X contains a cathode bias bypass cap that is much higher in value than the C12 equivalent.  Many claim that the C12 is fixed bias but it isn't.  The 25x probably has higher gain over some portion of the passband due to this.

#1 and #4 are easily accommodated with a few BOM changes, and placing a few components directly on the tube board.  #2 is much more tricky, as we wouldn't be able to make use of the PSU polar pattern switch.  On the C12, one diaphragm sits at ground, the backplate sits at half the supply voltage (60V), and the other diaphragm goes from 0V to 120V (to go between omni and figure 8).  It may be possible to make the mike switchable between omni and cardioid and eliminate the grid cap but I'll have to give it some thought.

 

[HellfireStudios]

I cannot speak to the differences between the capsule construction, and the electro-mechanical headbasket/mounting considerations, but just comparing schematic to schematic:

1) The 25x variants have the plate->ground 100pF cap, the C12 does not
2) The 25x variants use the capsule itself as the grid coupling element.  This means there is no coupling capacitor like in the C12
3) The polar-pattern switching is accomplished on the 25x in a similar fashion as a U87:  with isolated backplates and using 0V to 60V in different connections to the four capsule elements.  I'm not convinced the polarization design accounts for any of the possible sound differences.
4) The 25X contains a cathode bias bypass cap that is much higher in value than the C12 equivalent.  Many claim that the C12 is fixed bias but it isn't.  The 25x probably has higher gain over some portion of the passband due to this.

#1 and #4 are easily accommodated with a few BOM changes, and placing a few components directly on the tube board.  #2 is much more tricky, as we wouldn't be able to make use of the PSU polar pattern switch.  On the C12, one diaphragm sits at ground, the backplate sits at half the supply voltage (60V), and the other diaphragm goes from 0V to 120V (to go between omni and figure 8).  It may be possible to make the mike switchable between omni and cardioid and eliminate the grid cap but I'll have to give it some thought.

Could an internal switch be used to select the 251/C12 plate-to-ground cap? A simple SPDT on-off-on switch could be used to accomodate three different flavors of high end. I like the idea of using a 100pF for the 251-ish sound, nothing for the C12 top, and a 500-1000pF for an even warmer/darker sound.

Would the switch have to handle a higher voltage and current than a normal mic switch (switching that 120V plate voltage is what concerns me)? Also, can they be switched while the mic is turned on without harm coming to it?

-James-

 

[Matador]

I don't see a problem with it:  most switches can be found in 120V flavors.  If its inside the body, one would have to unplug the mike to get at it in any case.

It's odd to think of a mike with a "high cut" switch, but with a C12 it might be just the ticket to a new sonic crayon.

 

[HellfireStudios]

I don't see a problem with it:  most switches can be found in 120V flavors.  If its inside the body, one would have to unplug the mike to get at it in any case.

It's odd to think of a mike with a "high cut" switch, but with a C12 it might be just the ticket to a new sonic crayon.

My original intent was to apply this capsule switching design into a full DIY mic that has a 251 variant as its circuit (no smoothing cap). Implementing the switch in the apex 460 type of mic of mic might require the models with a pad and low cut switch.

-James-