MK47 PCB tube mic kit - build thread

[kante1603]

Awww 680 grams. She's very cute.

And how have you named her?

Hi,
not decided yet because I don´t know if it is male or female maybe "BLU47"?

Udo.

 

[Winetree]

Looking for a 7-pin chassis mount connector for Grand Master PSU box. Needs to fit the XLR
pre-drilled hole. Any part numbers?

 

[Songguy]

Hi all,
If someone could put together a psu kit would be happy purchase one. I'm not fully confident building from schematics alone. If someone wants to stuff a psu board I may be interested in purchasing that as well.

Cheers,
Chris

 

[Denyle Guitars]

Has anyone experimented with different sized mesh? Do you have a favorite or can someone take measurements? I don't have a mic here to measure. I'm putting together a raw materials order for a mic body and the range of wire size/holes per inch is making my head spin. I know the head basket design is a whole different science, I'm just looking for a good starting point. It'll be approx 77mm outside diameter, if that matters.

 

[idylldon]

I just finished my power supply and it was right at 105 when I fired it up.  There wasn't enough adjustment on the pot (I used a 1K), so I had to use a 1K 5W resistor in place of the 330/670.  That put the adjustment area almost in the center of the pot's travel.  I used the Triad toroid and the choke, and I measure about 3mV of ripple at the output, which seems pretty good.  Since I'm using a found case for the power supply, I now have to disassemble it and give it a nice paint job.  Once that's done and I get my cable and connectors, I'll hook up the mic and give 'er a go.

So far, so good.

BTW, the mesh is discussed in great length in the thread where Skylar is working out the details for his mic body. 

Here are the measurements:

The screening is as follows (from brabb):

front layer, 0.25cm square
finer middle layer, 0.05cm
inside layer 0.2cm

There might be more discussion about this further along in the thread.

http://www.groupdiy.com/index.php?topic=30742.0

Cheers,
--
Don

 

[Winetree]

Thanks to Bobine and all his help.
Got my power supply Almost finished. Fired it up and getting 106vdc with pot all the way.
Will replace a 1K with a 1.5K to try and center pot. Just need the 7 pin connectors and some cable.
Wanna hear the mic BAD.

 

[Winetree]

PSU BOARD

 

[crackerzot]

So is the 15-20 second warm-up time before audio that I'm getting about what's expected with this mic and P/S combo?

 

[idylldon]

So is the 15-20 second warm-up time before audio that I'm getting about what's expected with this mic and P/S combo?

Yes.  It takes a bit for the heaters to get the cathodes up to temperature.

Well, I finished my power supply and all the voltages are fine but the fuse blows after about an hour.  It seems thermal related and it happens even when I leave the case open for extra cooling.  Has anyone else experienced this? 

Cheers,
--
Don

 

[ioaudio]

which fuse do you use? is the transformer getting hot?

 

[idylldon]

which fuse do you use? is the transformer getting hot?

.125 amp slow blow and the transformer is running pretty cool.  The 5-watt resistors are getting way too hot, though, so I think they're up against their design limit using this transformer and the voltage it's putting out.  I'm going to test the tubes to make sure there isn't a problem there since it seems folks have had some issues with them.  I'll most likely have to get some higher-wattage resistors to cure the problem for good.  

Cheers,
--
Don

 

[ioaudio]

as per request: feeler for the Mk47 PSU http://www.groupdiy.com/index.php?topic=44296.0

 

[ioaudio]

which fuse do you use? is the transformer getting hot?

.125 amp slow blow

..which is appropriate for 230 VAC countries - you need to double this value for the US.

 

[idylldon]

which fuse do you use? is the transformer getting hot?

.125 amp slow blow

..which is appropriate for 230 VAC countries - you need to double this value for the US.

Aha!  I just saw someone posted that value and I wasn't paying attention to the 120/230 voltage difference.  I'll try a 1/4-amp slow blow.  Thanks for the clarification.

Cheers,
--
Don

 

[JW]

Despite what Udo said, I think the RF issue I'm having is related to the Canare Star Quad cable I'm using, unless I'm grounding things wrong? Can somebody comment on my grounding scheme?

I have the mic's ground (B-, which is also connected to the mic housing) connected to the Star Quad shield, which connects to pin 3 of my 5 pin XLR on the power supply side.

Pin 1 of the output XLR is also connected to this spot. Connected there as well, of course, is the ground from the power supply pcb. The p.s. pcb ground is connected from the spot that Max's schematic says to the chassis.  This spot on the chassis is where the earth ground is connected as well.

Does this make sense?

I think the issue is the star quad because when I move the cable around and listen, the strength of the RF frequencies changes. (I'm moving the cable without moving the power supply or the mic.)

Also, a totally unrelated question: does anybody have a part number or link to some appropriately sized perf board from Mouser?

 

[idylldon]

Got it all dialed in now.  I decided to put some heat sinks on the resistors and move them off the board so they get as much cooling as possible.  I also put some venting in the case I'm using to house the PS to ensure a bit of airflow.

The mic sounds incredible, very balanced throughout the frequency range and there are no sibilance issues at all.  I think this will become my go-to vocal mic and I can't wait to put it through its paces on some sessions I have booked this week.

Thanks to everyone involved in this project!  You guys are freakin' amazing and I appreciate your efforts immensely.

Cheers,
--
Don

 

[drgrateful]

Also, a totally unrelated question: does anybody have a part number or link to some appropriately sized perf board from Mouser?

I am using this one: http://www2.mouser.com/ProductDetail/BusBoard-Prototype-Systems/PR2H3U/?qs=VEfmQw3KOavWaKpcqBqhXQ%3d%3d

It is larger than needed for this project, but fits perfectly in the Grand Master Audio PSU box. You may want to cut the perf board, depending on how you intend to put the PSU/Choke in the box.

I also have a question: for P1 (the 500R, 5W, pot in the schematics) I would like to use something like

http://www2.mouser.com/ProductDetail/CTS/026TB32R501B1A1/?qs=GxOUx7aO6ny%2fLYTjImZnNQ%3d%3d

or (this is 1K, but should do anyway):

http://www2.mouser.com/ProductDetail/Alpha-Taiwan/VW24F-10-195K-B1K/?qs=sGAEpiMZZMtxdMMi52izynF7ZLtMY%252bzpmAcDgBFEwt4%3d

However, this is the only part I am missing and I would prefer not to have to pay 20 euro of shipping fees (this is the Mouser shipping fee to Europe) just for this. Is anybody aware of a source for this part in Europe?

 

[Jim50hertz]

This one maybe?

http://www.tube-town.net/ttstore/product_info.php/info/p4385_Rheostat-Alpha-1-kOhm---5-Watt.html

 

[drgrateful]

This one maybe?

http://www.tube-town.net/ttstore/product_info.php/info/p4385_Rheostat-Alpha-1-kOhm---5-Watt.html

Thanks, that's just what I needed!

 

[JW]

Can someone theorize on why not to use Canare star quad cable? The 4 conductors are 22awg copper. And the shield has a very tight braid. What is the reasoning for needing an extra ground wire?

I haven´t heard of why not to use starquad-maybe I missed something here,but I´m too lazy to search at the moment.
I have heard somewhere arround here that somebody uses starquad.
Will give it a try on my build as I have a big spool of Gotham Audio Starquad cable.
I don´t see a reason for an extra ground wire as-looking at the schemos and on the pcb-the ground has direct connection to 0v and the microphone-housing via the mounting rails.
Compared this to my original SM69´s psu some minutes ago.The audio grounds,the psu´s 0v and the backplate(where the connectors go)-so the chassis- definetely connect to each other at one screw as to the mic-housing.
Still waiting for my frontpanels so I can´t tell if this works at the moment,but I´m pretty shure it will .
Someone here mounted the pcb with cable ties-this can cause problems if no ground-wire runs/connects to the mic body.

You've run all the current THROUGH the shield, which could be injected into your audio.

Yes,but this is dc current-if there´s no ripple then there´s nothing to inject to the audio.
Even if running a seperate wire for grounding:what to do with the cable screen?It must connect somewhere and this will be the chassis ground connected to 0v and so forth....if it doesn´t connect anywhere it is useless.
And something else.In a phantom powered mic the current runs through the shield too!

Best,

Udo.

yes, it's DC current, in that it's traveling from A to B, but at varying currents. The current noise is small, but so is the audio signal. I was just explaining the school of thought there, may not necessarily be a problem.

Still not sure if Star Quad is my problem. If it didn't cost like $75 to order new cables and connectors, I would just do it, but if I get 7 conductor cable and find out the star quad isn't the problem, that would be a bummer. Would running all the current through my cable's shield inject RF frequencies into the audio?

Does my grounding scheme (a couple posts back) seem okay? Did I explain it well enough?

Thanks for all the help everybody