MK47 PCB tube mic kit - build thread

Picked up my P/S today and tried the mic for the first time.  It sounds VERY good.  I've used a lot of high end mics and this is right up there with the best that I've tried.  I went with the Thiersch Blue Line PVC M7 and love the way it sounds.  Lush, warm, and quiet.  Love it.  Thank you Max, for helping me realize a dream I really wasn't sure I could attain.

Kante1603,
Thanks for the photo of the Perf PSB.
If you could show the bottom of the board, It would help in building one.

cool stuff, thanks for your kind words crackerzot!

RE: PSU

Before you plug something to the mains, be sure that it complies to safety.
If you're unsure how to do it, or if it is safe - better get help from someone with experience.
The resistors get HOT - mount them securely, leave some space to the capacitors.
In use, always connect the mic to the psu before power-on.

Thanks for the tips, Max.  I'm glad you mentioned the resistors getting HOT, because my friend who built the P/S for me based on your schematic with and noted that they get really hot and wanted me to keep an eye on the P/S to make sure the case wasn't getting too hot to the touch.  The case is warm, certainly, but not too hot to leave my hand on it indefinitely.  I've had the P/S connected to the mic and have had it turned on overnight to let the capacitor and tubes burn-in. 

ioaudio said:

In use, always connect the mic to the psu before power-on.

Click to expand...

I didn't do that one time, won't happen again, since I got a nice shock from the mic body.

Quick question, I'm noticing a very noisy crackling sound while switching the polarity relay.  It's not a huge deal, because when the pattern switches the noise is gone.  It's just during the transition for half a second.  Is this normal operation for the relay? 

Winetree said:

If you could show the bottom of the board, It would help in building one.

Click to expand...

Shure,but as max(ioaudio) said-do not do it on your own when you are not familiar with mains voltages.This is really dangerous and can kill you (no place for joking here!!!).
Here the bottom side,I used nearly all very thick legs of the resistors and massive silver plated wire to form the traces.
With a good amount fo solder it gives an overall stability.



Have set it up for testing and left it on for about 2 hours.
As Max said the resistors get hot so take care!Mine are mounted in upright position not only to save space but to give them "air" for a good heat dissipation.This works good when they´re standing free and not touching other parts.Also the pcb will be mounted in a modushop-case which is well ventilated and will have enough space.
Here´s a pic of the working psu-I´m still waiting for my frontpanels but will post when it´s finished.



Good luck with your build,be carefull and get some help-nothing to feel ashamed here-your health is worth more than anything in the world.....

Best,

Udo.

crackerzot said:

Quick question, I'm noticing a very noisy crackling sound while switching the polarity relay.  It's not a huge deal, because when the pattern switches the noise is gone.  It's just during the transition for half a second.  Is this normal operation for the relay? 

Click to expand...

I have no idea of the mk47, but every other mic I own makes a terrible popping noise while switching patterns, so turn down the monitors while doing that, it is perfectly normal.

crackerzot said:

Quick question, I'm noticing a very noisy crackling sound while switching the polarity relay.  It's not a huge deal, because when the pattern switches the noise is gone.  It's just during the transition for half a second.  Is this normal operation for the relay? 

Click to expand...

Hello crackerzot,

yes,i think it´s a normal behaviour because all of our condenser mics (e.g.U87s etc.) do so.We are used to turn down the monitor volume first and then switch.

Best,

Udo.

Hah! I finally found out what was wrong with my mic. A bad 10nF cap. My fault though. On close inspection I had melted part of it with the iron soldering something else in place. Replaced with a wima and it works. Yay!

Max, do you have any more of those 10nF polystyrenes or could you tell me where to get one of the same brand? I want it to be close to my other MK47 because that one sounds perfect.

Also, this is another question concerning the power supply. Where is the most susceptible part of the power supply to RF interference? I'm getting some of that. Moving the power supply around helps, but I'd like to try and shield the right part if I can. I might have to just go for a total p.s. rebuild after reading a couple of the most recent posts. I have my electrolytics pretty close together. Might want to get them a little more breathing room. By the way, mine is built into a standard military ammo box. Box is closed and I'm still getting unwanted interference. I'm using shielded wire where the audio wires go from the multi conductor cable to the 3 pin XLR connector on their final path to victory.

However, the connections from the p.s. pcb to the pattern switch are not shielded. I imagine this would be the next place to look? For the multi (5) conductor cable, I'm using Canare star quad. It's about 20 ft. long. I don't think this is the issue as that cable has a very nice shielding braid. Part of the reason I chose to use it.

And, 1 more question. . . I'm using a 110V AC neon bulb at the switch rather than the LED in the schematic. This would negate the need for the 100K 1 watt resistor right?

I suspect the connection of the cable shielding is the culprit. Do you have it connected to the ground pin and the housing on the mic side and the power supply side?
Just connecting it to one side might not be enough, as I learned the hard way.
Best Tobias

Here's a pic of my kit during burn-in yesterday. 

Hey Tobias,
Yes, the cable shield is connected to ground on both sides. Actually, that's the only way for the mic ground to reach the earth ground because I'm only using 5 conductor cable (including shield). Perhaps I should have a separate ground wire (requiring that I use a different tube mic cable?)

I am a total newbie and could use some guidance.  I have built my mic and everything is great except that it is noisy.  When in cardioid there is an audible hiss that is constant.  The noise is also there in omni only a lot quieter.  Has anybody experienced this?

In cardioid the backplate seems to be on.  When speaking into the rear of the mic the sound is very bass heavy but just as present as speaking into the front.  The front has clear high end.

I am using the Beez Neez capsule and Tab-Funkenwerk body.  The power supply was designed and built by a good friend of mine.  He designed a regulated supply.  

Even with these issues I can tell that this mic is going to be amazing.

-Andy

JW said:

Hey Tobias,
Yes, the cable shield is connected to ground on both sides. Actually, that's the only way for the mic ground to reach the earth ground because I'm only using 5 conductor cable (including shield). Perhaps I should have a separate ground wire (requiring that I use a different tube mic cable?)

Click to expand...

It is uncommon to use the shield as the only ground connection on a microphone, but since you would tie the shield and the ground conductor on both ends of the cable together anyway, I guess it should work, but I maybe wrong, I never tried it.
To test the issue, you could temporarily disconnect the pattern switch conductor and reuse it as a ground connection and see if things change for the better.
It is really important that the connector housings on both sides of the cable are soldered to the shield as well.

JW - important - if you ommit the LED connect the 100k resistor from B+ to ground so it works as a "bleeder".
the bleeder drains the capacitors when no load is connected.
I'll send you a replacement 10nF, pls email me a reminder.
Use professional mic cable.

Andy,
how long did you burn in the tubes?
Can you try another capsule?

Hi Max,

The tubes have burnt in for 50ish hours.  I don't have another capsule that I can try.

Thanks,
Andy

I've got a small problem and wonder anyone might have a handle on what the problem is.

When the mic is in omni position, the noise floor is dead quiet.  But when I switch to cardioid pattern, I am hearing a very faint buzz, buried pretty deep in the background, which is why I didn't notice it right away, but it's there.  Any idea what might be causing this?  Again, very quiet in omni, but I hear a low level buzz the second I flip it to cardioid.  I wish it were the other way 'round, because I will be using the mic in cardioid pattern for the most part.

Thanks in advance.

That is the same problem that I am having. In mine the sound is more of a hiss or wind sound, not a buzz.

Let me know if you figure anything out. I'll do the same.

Andy

I've ordered some new tubes to try.  That's where I'm going to start.  I was in error, also.  The noise is still there in omni, but since the level is lowered by going omni, it's just harder to hear.  It's almost inconsequential, this noise, but... not quite...  I've burned in my tubes about 24-26 hours now.  I'm using a Thiersch PVC M7 capsule.  For the most part this thing sounds heavenly...