Official C12 Clone - Build and Support Thread

[weiss]

Hi weiss,

Before you change you tube or start cleaning the PCB  (again):
I have built two C12-clones (and three U47-clones). I have listened to your audio files and it does not sound like a bad ground connection or a dirty PCB to me. Please check one thing first:
- With tube michrophones today we are using grounded outlets (and ONLY use grounded outlets).
- Since we have two pieces, the PSU for the microphone and the computer/mixer/soundcard, we are grounding the microphone at two places at the same time. This can cause ground loops.
- Make sure that you have both the PSU and the computer/mixer/soundcard connected to the same grounded outlet.

If you don't, you will have a ground loop. Sometimes a really bad hum, somtimes just a small hum in the background.

If you have a bad ground connection in you mic, it will give you a very loud hum (50Hz). If you have a dirty PCB, you will hear popping sounds or noise that sounds like "the wind blowing in the trees" (it comes and goes). The hum in your mic is steady and at the same level, so  I suspect that you have a grouding problem to the mains.

Been there, done that.

- Stefan

Hey Stefan! After doing some more testing i think i come to the exact same result like you do. Indeed, a bad ground connection would cause much louder hum. It must be because of the audio environment i'm using, i try to find out what causes the ground loop.
But what about the noise? Is that normal? I can live with the humming as i will cut below 120 Hz either way but the noise is really hearable during recordings.

 

[Matador]

Hallo weiss,

Try to narrow down where the noise is coming from.  I posted this several times in the thread, but the basic steps are:

1) First make sure you have a low impedance ground:  with everything powered off, check the resistance between ground on the PSU PCB and all ground points in the mike body, including all parts of the outside shell.  You should have less than 0.1 ohms everywhere in the mike
2)  Make a small length of wire with two small alligator clips, one on each end
3) Clip one end to ground on the PCB
4) Starting at the 'output' hard ground different points in the circuit:  then measure noise
a) the output of the coupling cap (where it enters the transformer) - it should be dead silent
b) the grid of the tube - this should only output the self noise of the tube
c) the capsule connection - this should only be the noise of the tube + HiZ components

You should be able to see the noise contribution of each stage this way.

 

[stelin]

I agree to everything Matador wrote. Fix all electrical problems first. But:

Even after you eliminate all electrical and grounding problems in your tube microphone, it will still have a higher level of noise than a modern FET-microphone.

The best modern FET-microphones have a noise level between 5 and 8 dBA.  A good tube microphone have a noise level of somewhere between 14 and 20 dBA.

In the olden days, tube microphones were used for everything (the FET-transistor had not been invented yet).  The recording equipment also added a lot of noise. As long as the microphone was quieter than the tape recorder no one complained.
Today, we don’t use tube microphones because they are quiet. We use them because we like the sound.

It’s all about the sound pressure level. If we use a tube mic for recording a soft spoken voice or an orchestra at a distance, the noise level will be too high. If we are recording a singer at close range or use them as overheads for a drum kit, the noise level will be ok.

We all want our tube mics to be quieter. When all electrical problems are fixed, we can sometimes reduce the noise level a couple of db’s more by finding an extra quiet tube. But to do that, it’s usually necessary to test five to ten tubes or more. They must first be burnt in (at least 48 hours) and then measured for noise. If you are lucky, you will find a tube that reduces hum and hiss a couple of db’s more.

 

[micaddict]

The best modern FET-microphones have a noise level between 5 and 8 dBA.  A good tube microphone have a noise level of somewhere between 14 and 20 dBA.

http://www.brauner-microphones.de/en/products/valvet_x/

 

[stelin]

I should have written "The best multi pattern tube mics". 
In a pure cardioid mic, where you don't connect the back membrane, you will have a noise level about 5 dba below that of a multi pattern mic.
But, even so, a noise figure of 9 dba should not be taken too seriously. That is a bit on the low side, even for a pure cardioid tube microphone. Try it you self. Measure the noise. Report your findings. 

Best regards
- Stefan

 

[micaddict]

I hear what you're saying and I basically agree with you. 

But the Valvet X really is a very quiet tube mic. I've used two, one of which I owned. I did not measure (dBa), but mine was very quiet indeed. And the other one was even quieter. As in black. Scary. Quieter in fact than any FET condenser I own.
Yes, pure cardioid helps (although for some reason the multipattern version specs only 2 dB noisier). A relatively modern, subminiature tube helps, too. And/but yes, despite careful selecting, they do vary a little from one to the other.

BTW, I have a preamp that is very quiet all the way up to 70 dB.
And I take 'em full throttle when I do a test. 

 

[Matador]

We all want our tube mics to be quieter. When all electrical problems are fixed, we can sometimes reduce the noise level a couple of db’s more by finding an extra quiet tube. But to do that, it’s usually necessary to test five to ten tubes or more. They must first be burnt in (at least 48 hours) and then measured for noise. If you are lucky, you will find a tube that reduces hum and hiss a couple of db’s more.

Exactly.  Wiring up a tube with gain, then grounding the grid is a time-tested way of evaluating self-noise and microphonics.  I've seen a few schematics where a Fender first stage is used, followed by a low-noise op-amp wired for 40dB of gain.  You can then monitor the test rig directly via your monitoring system to listen for the characteristics of the tube.

Similar to this:

https://www.youtube.com/watch?v=tx0G_ObVwn4

 

[stelin]

Very interesting video, Matador.

I use a microphone and replace the capsule with a 70-75 pF cap. Then I record each tube at high (and constant) gain to a track in Pro Tools. I also tap the mic body with a pencil to find tubes with more microphonics.  Some tubes have very ugly microphonics and they go to the paper basket at once.

When I have recorded all tubes I listen to the tracks and grade them. Sometimes two tubes have the same amplitude of noise, but one has a “darker” noise. That tube is better than a tube with a brighter noise. That is one thing you can’t see on a scope.

My findings is that if I have about ten tubes, I don’t often find a tube that is very much more quit than the other tubes, but I can sort out the ones with high microphonics, loud noise level and the ones that pop and click.

That said, I would be very interested in a tube tester as the one in the video.  I have an old 5 MHz scope. Could that be a project that would interest you? I know there are lots of people out there searching for that extra quiet tube.

- Stefan

 

[Matador]

That said, I would be very interested in a tube tester as the one in the video.  I have an old 5 MHz scope. Could that be a project that would interest you? I know there are lots of people out there searching for that extra quiet tube.

- Stefan

Are you asking for a project, or offering one?  ;D

 

[stelin]

Asking. I have no clue how to make  yellow circuit boards. 

 

[Patrick from Davis]

Circuit boards in sonically superior yellow are few and far between.

 

[Carl Cress]

Matador, I'm to the point where I'm installing a Tim Campbell CT12 capsule and the labeling on the v1.4 PCB and the schematic provided at the beginning of this thread are dissimilar. Please correct me if I'm wrong. I can read schematics to a point and I believe this to be true, I'm just nervous about screwing it up at the finish and I'm doubting myself.

Front Capsule = RC
Front Backplate = BP
Rear Capsule = Ground
Rear Backplate = R14 (after R12 and 13)

 

[ln76d]

I'm not Matador, but hope my post help

Ok, if schemtic description means:
FC - front capsule
FB - front backplate
RB - rear backplate
Then it's wrong...

Here's how it should be:

Front diaphragm to ground.
Front backplate to R15
Rear backplate to R14
Rear diaphragm to R16

 

[Carl Cress]

I'm glad I asked. Not sure why there's conflicting acronyms on the pcbs and schematic? Now that I look at the schematics with your information it makes sense. Thank you.

 

[Matador]

Everything ln76d said was correct.   V1.4 was supposed to be "obvious", but apparently it wasn't.

I still need to upload the lastest schematic and board plots, but I lost my user login to my web server and haven't got around to resetting it yet.

 

[Carl Cress]

The schematic is clear. I was unsure of which was the front of the capsule and with the acronym FC (front capsule, front chamber?) being the back of the capsule it added to the my confusion. Just having the front identified solved my query. Great kit in all other aspects and I can't wait to hear it in use.

 

[weiss]

Okay time to say thanks for your help even if I didn't get the chance to follow your advice. The funny thing is, I powered up my microphone another time and guess what.. the noise disappeared  Maybe it was because the case didn't completely close with the heaeadbasket. I removed it, put it back on and fixed the screw a little bit tighter than before so maybe that's what caused the noise..

 

[Matador]

The grounding/shielding on the stock Chinese body is really a compromise between ease of assembly and good shielding practice.  In the ideal sense, the microphone body should only be an extension of the microphone cable shield, which is an extension of the PSU chassis, and should not intermingle with the local circuit 0V in the microphone at all.  However minimizing the impedance to ground is also a design goal, which can be a challenge given the stock parts: sometimes, the lesser of two evils ends up performing better in practice.

Ideally, the setup should be like this:

[list type=decimal]
[*]The 7 pin cable shield should be separate from microphone 0V reference.  The microphone PCB should be electrically isolated from the mike body/internal metal parts
[*]The 0V reference in the mike should travel from the PSU, through the 7-pin cable, and connect directly to the 0V reference on the mike PCB
[*]In the PSU, the PSU 0V can be tied to the chassis to keep the potentials the same
[/list]

However this might be more effort than the benefit gained.

What you don't want is induced currents in the microphone body to flow through the PCB and modulate the PCB's 0V reference, as even tiny currents will be reflected on the astronomically high impedances as hum/noise.

 

[pundit]

This is still confusing as the v3 schematic indicates the front backplate connects to C13, not R15.
I think an updated schematic to match the V1.4A board would be helpful. 

As I am using Tim Campbell's CT12 capsule should I still bridge the backplate terminals together as this is not shown in the schematic?

Thanks.

I'm not Matador, but hope my post help

Ok, if schemtic description means:
FC - front capsule
FB - front backplate
RB - rear backplate
Then it's wrong...

Here's how it should be:

Front diaphragm to ground.
Front backplate to R15
Rear backplate to R14
Rear diaphragm to R16

 

[Alexandru marian]

I am curious about the very open type of mesh used on the original AKG mics.  Do you guys find it has more hum than a Neumann style triple layer mesh? 

I am asking because for my G7 I have a home made single layer head and a Neumann U47 copy.  The Neumann is silent, mine hums enough to make the mic unusable for any serious recording. But I find mine sounds sweeter, deeper, more 3D.  And it is not even as open as the c12.  Maybe there is something poor in what I did cobbling it together, and not the fault of the mesh density.

Thanks