MK7 - tube mic project

[yosh]

My apologies if this has already been covered. But, I believe I solved a hum problem in a tube mic I built a while back by putting a 10 ohm resistor between the power supply ground and the chassis/signal ground. Perhaps something like that might help here? Maybe the resistor wouldn't even have to go in that specific place. The idea would be (assuming its a ground loop- which I guess maybe its not) to find where the loop is happening and to stop the flow of that current.

-Yosh

 

[yosh]

I forgot to ask: Is the next round of MK7 kits still on the way? I was too late on the last one and I'd love to get in on the next round. Thanks!

-Yosh

 

[ioaudio]

the pcbs finally arrived, but i have to finish a commercial prototype project i´m working on before i get to the mk7 sets. i will announce it here and have a thread at the black white market.
-max

 

[djn111]

Uhh, patience is so hard sometimes 

 

[Jonkan]

I had a breakthrough! I got one of the mics to be hum free!!!!

I have no idea why its working now, but what i did was the following:

I lifted the tube mic cable shield on the psu side.
I added more capacitance to the heater rail output after the first two 10000uf caps. The positive side goes to the output of the heater, and the negative side goes to the shield of the tube mic cable.

If i connect it to star ground or ground on the pcb I get hum. This way i dont.

Can someone explain what is going on here?

Thanks!
/Jonas

 

[yosh]

My guess would be that you have a ground loop somewhere. I haven't built these mics and I don't know how the T-Bone mic cable is wired so I'm not sure where it might be, but you might try drawing out the paths of the grounds to help you find it. Don't forget to include the body of the mic and the body of the psu. Sketching this out has helped me locate problems in the past. Good luck!

-Yosh

 

[Jonkan]

Almost there. When i keep the 7 pin connector in psu outside of the box so that it doesnt touch chassis, mic is working 100% and sounds great.

When i attach the input connector to the chassis, i get a small groundloop.

So either i have to isolate the connector somehow from chassis, or find a way to break the loop.

Or third option, i could always try to move my star ground point to the input xlr instead and see if that helps.

Ill let you guys know what worked for me as soon as im done, so that maybe someone can save a couple of hours of experimenting when building these bad boys.

/J

 

[Jonkan]

I finally got it working.

This is what i had to do:

Move ground connection for the 2 10000uf caps to the ground point instead of pcb groundplane.

Replace diodes in heater rectifier to schottky ones.

Move the shield in mic cable on psu side to pin1 to isolate it from chassis. When shield had connection to chassis through the xlr housing, i had hum.

Connect pin1 on both 7 and 3 pin xlr (i also had to get rid of stock connection between these connectors).

Connect these pin1 to star ground point.

I chose my star ground point to be the same point where the IEC safety ground is connected.

I hope this can help somebody if they are having hum problems with these mics and are using the thomann mic body.

/J

 

[ioaudio]

congrats jonas, and thanks for describing what lead to your solution.

-max

 

[Jonkan]

I hope that someone can benefit from it and save some time when building these.

Now i have two mics that i built for my friend that has selfbias, and two for me that uses fixed bias. I will try to compare these and see which i like the best as soon as I can get my friend to bring over his mics.

Maybe even post some clips. Im guessing im going to have to convert my friends mics to fixed bias, but who knows...

What id like to try also now is to find some PIO caps that can fit in the mic, and see how that sounds. Any ideas where you can buy them?

/J

 

[Davo]

Great, glad you got you're mic hum-free!!

You can buy PIO's in the UK:
http://www.hificollective.co.uk/components/capacitors.html

 

[Jonkan]

Even though ive only tried these for a couple of minutes now while my friend sang and played some acoustic guitar for me I can tell that these sound great!

Its a whole nother sound compared to for instance my g7 which is the only other tube mic i have. Even though im using the same type of capsule from dale, the mk7 sounds alot smoother and "older", and a bit meatier in the lowmids. Very nice midrange. The g7 sounds a bit more "open" and clear/neutral to my ears. This mic has alot of character, which is nice, since most of my other mics are more neutral.

My friend was very impressed how good his voice sounded, because it is very hard to record most of the time with other mics. It tends to sound raspy in the 4-8k area when closemiced, but almost no sign of it with the mk7.

A bit more noisier than the g7, but nothing that bothers me much.

This is with dales m7 capsule, and polypropylene 0.33uf output cap. Gonna try some 0.33uf PIO caps soon (thats the biggest i could find locally that fits).

I hope that it will even further accentuate the smoky, velvet soundqualities this mic already excibits to my ears. It sounds very "expensive" already though, so im very happy!

First chance when i have recorded something more serious, ill post clips, promise.

/J

 

[ioaudio]

hey thats cool, thanks for your feedback. you should get rid of the noise with a battery-quality heater supply.
-max

 

[Jonkan]

Youre welcome. Thanks for sharing an awsome design!

Heres a quick recording i did of a friend in my livingroom with two mk7 mics in blumlein directly into my motu traveler preamps (modded though).

Not a perfect recording by any means, but you get the idea.

Setup was 2xmk7 with dales m7 capsules, in fig8/blumlein about a foot from the performer. The guitar might be a little low in the mix, but what are you going to do when its sounds that way live, and you want to pickup the whole performance using two mics

No eq, compression, or anything else.

Oh yeah, theres also another friend in the background playing pump organ.

http://www.gearslutz.com/board/attachments/geekslutz-forum/125136d1245153692-quick-test-my-diy-mk7-tube-mics-mk7test-blumlein.mp3

Enjoy!
/J

 

[MagnetoSound]

Wow, great tone!

Slightly noisy, but not enough to bother me. That little bit when the organ gets louder, I thought the sounds fit together really well.

Those capsules are impressive.

 

[Jonkan]

Sweet, some russian NOS PIO caps just went through my mailbox. Gonna install them later today. I guess i have to reinvite my friend to test them out again.

I heard these take a while to form, improve soundwise, get better, or whatever its called.

Is there any recommended practise how to "burn them in", so to speak. Or can i just install them and then leave the mics on for a couple of hours/days (or how long it would take)?

I might try a couple of different tubes aswell, and more capacitance on the heater rail and see if i can get the noise down a bit.

I really enjoy building mics, its so fun!
/J

 

[synthi]

Jonkan, are the mics in the recording fixed or self bias?

 

[bradb]

how did you find PIO that were small enough to fit inside these bodies?

 

[Jonkan]

They are fixed bias.

I found a supplier here in sweden that had a couple of 0.33uf caps that will fit without too much trouble. They cant be mounted flush to the pcb, but have to be flush to the metalwork/body struts. About 43mm long, but since the inner diameter of the body tube is around 50mm, i think it will be ok.

they only have 6 capacitors left though, so if anybody wants to get them i bought them at "Dahlberg Audio Design" in sweden.

/J

 

[Jonkan]

A bit tight, but it fits!

/J