MK47 PCB tube mic kit - build thread

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Guys,

if the 1uf capacitor connected to the output xformer is changed, is it important that the voltage of this capacitor is close to 250V?

If the voltage is higher, does it act as an HPF?

Also, what is the ratio of the transformer in Mk47? Is it 6:1 or 12:1? What is the output impedance too?

Kind regards and thanks for any help

Michael de A
 
Finally got my build finished and the mic works beautifully. How do you guys find it compares to an actual U47? It seems to be more dependent on placement then most U47s I've used. The top end seems to drop off during vocal tracking when the singers shifts their heads even very slighty. 47s certainly aren't overly bright but the drop seems more drastic on these. (using the Dale M7 capsule by the way).
 
muffy1975 said:
Guys,

if the 1uf capacitor connected to the output xformer is changed, is it important that the voltage of this capacitor is close to 250V?

If the voltage is higher, does it act as an HPF?

The value of the C changes the bass response: 2µF will pass signal an octave lower than 1µF will.  The voltage rating can be higher, but not lower: this does not affect the frequency response, but rather the point at which the capacitor breaks down permanently, and it's value is determined by the high DC voltage on the anode.

David
 
LHS said:
Finally got my build finished and the mic works beautifully. How do you guys find it compares to an actual U47? It seems to be more dependent on placement then most U47s I've used. The top end seems to drop off during vocal tracking when the singers shifts their heads even very slighty. 47s certainly aren't overly bright but the drop seems more drastic on these. (using the Dale M7 capsule by the way).

My M7 (Chuck D) is darker than my Beez Neez K7.  Both are in MK47s and EQ bodies.  I haven't noticed a placement issue, though. I prefer the K7 for lead vocals...astounding presence but never shrill.  The M7 can be beautiful, too, in the right setting.
 
Guys, I have kind of a dumb question.  When building up my Cable for the tube mic to the PSU should I connect the shielding from the cable to the ground point on the Binder connection on my PSU?  I am assuming that at least one end of the shield should be connected to ground and it seems to make the most sense to do it at the PSU but I could be completely wrong.

Thanks as always.
 
dandeurloo said:
Guys, I have kind of a dumb question.  When building up my Cable for the tube mic to the PSU should I connect the shielding from the cable to the ground point on the Binder connection on my PSU?  I am assuming that at least one end of the shield should be connected to ground and it seems to make the most sense to do it at the PSU but I could be completely wrong.

Thanks as always.

Dan, I had my shield floating on the mic side with a conductor connected to mic-body and both shield and GND conductor connected to the star gnd point. That was fine until I was in a high RF environment where I got radio on the mic instead of mic on the radio.

So to fix it I ended up remaking my cables and connecting shield to star-gnd on the psu side and the GND/0V conductor (heater return) to the minus side of the last filter cap in the PSU. Quiet as a broken mic now :)

GND plane on the mic PCB is connected to mic-body through the body-rails.

Shield is connected to binder/chassis on both sides.

Cheers,
j

hmm.. I'll have to look in the PSU. I may not have a connection to star point from the shield on the PSU side but thought I did. The RF problem went away though.
 
so basically connect the shield on both sides to the ground on both sides.

My PSU has everything going back to a star ground point so this makes sense that it would all really just be connected to one point then. 
 
I believe that you want the body, the shield, and the PSU chassis all connected to the safety earth so if anything shorts no-one get's zapped. I did that by connecting shield to the binder connector on the little solder tab inside the binder on both sides.

Then, 0V on the mic PCB goes back via a conductor to the negative side of the last filter cap in the PSU. My 0V on the mic PCB is also connected to the mic-body through the screws that attach it to the rails. I'm thinking that creates a so-called gnd loop but it works well in practice over here. I would think that if you had some plastic or fiber washers you could have the PCB 0V float in the body and only have one path back to the last filter cap/0V.

The 0V side of the bridge rectifier in the PSU is also connected to the star gnd.

I also have pin1/shield of the XLR connected to star-gnd & XLR chassis-connector.

There is no more RF or ground hum. I did have RFI when I had the shield disconnected from the mic-end of the binder connector/chassis.

Does that help?

Cheers,
jb
 
So, I got the mic powered up and about ready to do an audio test with my crappy Chinese test capsule.  I figured if anything goes bad I would rather it happen to that capsule then my blue line.

Anyway, most all my voltages look good.  I am getting pretty low voltage however at the 2M/3M/100M point.  I get 28.6 volts.  Does that seem correct?  I thought it should be twice that.
 
ioaudio said:
should be approx. 60vdc - re-check resistor values?

OK, I will check.  It seems to sound fine.  I get a little wind noise and when I record the noise it to my DAW it shows up as a littel 120hz peak that comes and goes.  Maybe that is ripple?

Taking it apart again now to triple check resistor values.
 
The resistors are all correct.  I have read that it isn't possible to read the voltage at the capsule because of the high impedance.  Is this the same for measuring at the Node point of the 3M/2M/100M resistor?  Could it be the mic is fine?
 
Hi guys,
I 'm having an issue with my mics ( all of them )
the mics can work fine for hours or days, and suddenly that nois :
https://dl.dropbox.com/u/56011166/Mic%20noise.wav
any idea of what it can be ?
I've changed the PSU caps .....
wondered if this can be the 1K5 Resistor inside the mic , not being cooled well
in the PSU , I Also have a 1K5 Resistor that's heating much..... how can I cool it ?

Thanks Guys,

CHEERS
 
Those cracklings/sputterings Sounds like bad tubes, now if it's on all of them, then i guess that's bad luck?
Or tube socket bad solder joint maybe? Flux etc....
I don't think changing the caps would have done something here, well if these are brand new and within specs.
Try to get some new tubes and check it out...
 

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