MK47 PCB tube mic kit - build thread

[zayance]

well, touche..

the A+ and A- leads were reversed.. I somehow assumed that the righ hand one was A+ but it is actually A-.. though that didn't fix much. The level is still very very low and I seem to have a grounding issue. The buzz/hum is mostly 60hz harmonics and a lot of them go away if I touch the binder connector and the pattern switch.

Is the power supply (I used zayance's PCB version) supposed to connect to ground at the 0V output? I have it setup that way, so pin1 from 3-pin XLR, pin1 from 7-pin XLR and the 0V from power supply are all going to ground and at the other end of the cable, that's going to the B- connection. The PCB inside the mic isn't connected to the body with anything metal.. is that a problem?

Common practice, if i'm correct, is to connect the Mic body to your GND also, and Star Ground in the PSU, like the G7.
BUT iirc, ioaudios PCB has one side as GND (B-), so depends on how you attached it (plastic zip or metal) then the connection should do if metal of course....

 

[grantlack]

like tony said, definitely make sure your mic body is well-grounded.  if you didn't connect the grounding tab of the binder connector to the shield, that might be sufficient.  if not, make sure whatever pin you used for ground in the cable is solidly grounded inside the chassis.  easiest with all that unsolderable aluminum is to just attach a decent gauge wire to the cup of your ground pin and slip the stripped end between two of the rail mounting screws.  tighten the hell out of it and voila.

 

[baadc0de]

Well, the signal is getting up after a few hours of burning in, so I guess the mic is working better and better..

that said, it's still very very noisy..

I have zero ohms between the body and the mains ground, also between any panel mounted switch and ground, between either the 3-pin XLR pin1 and ground or the 7-pin XLR pin1 and ground. XLR shield tabs are connected to grounds.

EDIT: what's really funny is that if I take a separate wire and connect it to a PSU panel on/off switch, the other side goes to the binder connector shield - nothing happens, or maybe the noise goes up a few dB.. however, if I touch both of these with my fingers (!) the noise starts to subside after about half a second and goes down in level quite a bit.. help?? This is waay beyond my grasp / understanding.

 

[baadc0de]

There had to be an explicit wire going from the body (the support profile) to the B- spot. Then the mic is quiet (not really really quiet, but quiet enough). Huzzah.

 

[baadc0de]

I knew it was too good to be true. While the ground noise went away, the popping, swooshing and whatever else this mic is capable of is back in spades. It goes up and down in intensity, volume, at random. I recorded such great takes with it in the morning and now in the evening it's playing up with me. I'm so so mad because it works so great when it does work!

 

[zayance]

I knew it was too good to be true. While the ground noise went away, the popping, swooshing and whatever else this mic is capable of is back in spades. It goes up and down in intensity, volume, at random. I recorded such great takes with it in the morning and now in the evening it's playing up with me. I'm so so mad because it works so great when it does work!

Flux cleaned the board? Any spare tubes?

 

[0dbfs]

I just finished two of these, and two mk7's over the weekend (all these mic-parts had been sitting around for a while so I turned up the DIY-DIAL to 11 and knocked them out)...

There was GND noise which went away when I lifted the cable shield on the mic end and connected shield to chassis on the PSU end. I have the B- / GND connections from the PCB's wired to pin7 & 6 on the binder... No special connection from the EQU47 body to PCB (just the rails touching the GND area on the PCB because of how I mounted it)...

Out of four mic's I have only one wich had pops/swooshing (two mic's using the supplied 6028's, and the two mk7's using pf86's).... It didn't go away on it's own and followed the tubes when swapped.

I tried additional troubleshooting to rule out other issues. Reheated all Hi-Z nodes, cleaned, cleaned, cleaned, swapped the 10nf (backplate to gnd) styro, rejuvinated the tube pins with a small hobby file, overheated & underheated the tubes for hours and even days on end in some cases... I did all of those troubleshooting items after discovering the noises and while waiting for additional 6028's to arrive (100x @ $89.- so they should last a while). 6028's arrived Saturday. I swapped the first two in from the carton and they are great now as of late last night!

Just brought the mic's into the studio today to start using them on sessions in earnest. Have sessions booked all week starting tomorrow so we'll see how the mic's hold up. I'm sure they'll be fine but have not yet connected them up in the live room. I am expecting that it will take a little while to really get to know these new friends well. I am looking forward to it very much.

A/B'd against an MXL 2001 at home in the shop ($20 doner-mic for another project) with my voice and there was absolutely no comparison. The 2001 was a cheap ebay purchase so no telling what it's condition really is but I didn't have any other mic at home to compare so....

Sounded so good I literally fell to my knees and wept tears of joy. Very real tears. This was before any A/B test with the mxl.

I am planning to take the final PSU readings (in their semi-permanent installed locations) and adjust accordingly since they are unregulated. I have found that all four mic's are a bit low on the B+ and heater voltages when supplied with 120V (almost exact) from the wall so may need to adjust some PSU resistors.

Actually, I ended up initially installing the 680 Ohm resistor in the Mk47 PSU and swapping it out with the 330 to get into the 105V range... 680 ohms value there was going to max 101V maybe. With the 330 installed I get between about 101V and 112V when I have 120 on the PSU primary.

My cable shield ended up being connected at one end to get rid of "gnd-loop" noise. New tubes fixed the popping/swooshing. I also did some other fixes for the pops/swooshes but they didn't actually go away until I swapped the tubes.

Cheers,
jonathan

 

[0dbfs]

Wow. Every almost artist I put one of these up for immediately wants to buy one.
You know the story:
We talk about sourcing the parts literally from around the globe... The fact that certain parts are pretty "custom" or at a minimum special-order and in many cases are literally handcrafted. Waiting and delivery periods and/or delays...

Anticipation builds....

And we are not disappointed. No we are not disappointed. Quite the opposite.

I generally work with fairly high profile talent. A good combination of musical and spoken vocal talent as well as significant instrumental talent.

When they "do their thing" and put the headphones on it's all over. Work day is over. We're immediately in the zone. It's just smiles for miles.

Thanks Max! These are awesome and that's an understatement.

Merry xmas and happy holidays!

Best,
jonathan

 

[JamesW]

Need opinions of mounting the 1k5 resistor 'heater' from MK47 kit.


Thanks

Jason

Here is a pic of my mounting I've used it twice now worked great

 

[Marc Duchesne]

Hi everyone, in reference to popping noise that would not disappear after a couple days, I experienced the same problem and it went away when I replaced the 1.5Kohms filament resistor to a 10W with heat sink mounted in the botom part as JamesW did above , thanks for the photo. I searched for this one quite a lot and got the tip from a post I read in the forum can't remember where, anyways, thank's to this person. An unstable filament current that was... So first thing let the tube settle for a couple days, than replace the resistor if symptoms persist.  Hope this helps anyone...

 

[etiefenthaler]

Hi

want to show some pictures. I built two mk47 this year. I love them.

Max - thanks a lot.

Erich

 

[etiefenthaler]

and another picture

 

[kante1603]

Wunderschön Erich-echt jetzt!

Udo

 

[lonepariah]

Hi, I am transformer shopping for my MK47 PSU. I am located in the US, so it needs to be 115v. I am looking for suggestions, what are you guys using?

Will something like the link below work, or is there a better/cheaper alternative? Thanks.

http://www.alliedelec.com/search/productdetail.aspx?SKU=70218047

Also, what is being used for patten selection switch?

 

[0dbfs]

Antek AN-05T200 50VA Toroidal worked nicely here.

Cheers,
Jonathan

 

[0dbfs]

I've got one mk47 installed in a broadcast studio as an announcer mic. This mic sounds very nice and performed exceptionally in other environments. This environment must have a high RF level for whatever reason.

In any case, the mic is picking up a radio station pretty far down in the noise floor. Audible if you listen closely but dwarfed by any source audio. I intend to try some different grounding schemes but this is what I have implemented at the moment:

- MK47-PCB-GND connection is connected to a conductor on the cable (happens to be the larger heater return conductor on the 7 conductor gotham cable). This is connected in the PSU.
- Mic body (EQU47) is grounded via the support rails touching/mounted to the MK47 PCB.
- Tube-mic-cable shield is connected on the mic end and disconnected on the PSU end (there was hum with it connected on the PSU end).

I'll try some different things to fix this and report back any findings but am interested in anyone else's experiences with picking up RF and solutions that worked.

Cheers,
jb


EDIT:
This reference has some good info on RFI issues:
http://audiosystemsgroup.com/RFI-Ham.pdf
Still need to test and confirm the exact source of the issue in this case as well as the solution.

 

[hop.sing]

- Tube-mic-cable shield is connected on the mic end and disconnected on the PSU end (there was hum with it connected on the PSU end).

That is probably the reason for picking up RF. In my experience you have to connect both ends of the shield to eliminate rf issues with a microphone.
And if you connect ground and shield on both (mic end and psu end) connectors, there should be no reason for hum, since there is no ground loop. But you still have to watch the Power supply, I always had good luck with my star ground at the mic cable connector in the PSU, so housing, cable shield and main ground meet there. It has been the quietest solution for me.
Good luck, I hope it helps, those things can be really troublesome.

 

[david-p]

I think you have a grounding problem.  Reconnect the screen at the PS end of the cable and seek a different solution to the hum.  I have put a longer screw at the bottom of one of the struts of my EQU47 body, with a nut and solder tag, to which my circuit, and the ground from the power supply are referenced.

David

 

[wave]

Antek AN-05T200 50VA Toroidal worked nicely here.

Cheers,
Jonathan

Jonathan,

How did you wire up this transformer? Did you make your own PCB for the PSU?

 

[0dbfs]

Here is a picture of the PSU before I wired it up. I just made the perfboard look like the schematic. With this Antek transformer I initially wired with the 680 ohm resistor and the voltage was too low so I just paralleled the 330 ohm over that one and it was fine.

I wired the primaries in parallel for 115V with the line-voltage switched through the power switch then tied back and capped the unused secondary taps. I just used the tap for 200V to feed in to 0V and 200V on the bridge rectifier.

I'll try to get some better photo's next time I open them up.

Cheers,
jonathan