MK7 - tube mic project

[Gus]

Ian

   Did the microphone hum when it was stock?

  Did you check the ground wiring?  Hum can be due to a ground wiring issue.

  Check the current draw of the B+.  Measure the voltage drop across the B+ resistors in the RC filters and use Ohms law.

  143 is close to 150 Zeners are not always exact things.

  To test the zeners measure the voltage across them without a microphone connected this is when the max current is in the zeners.

  What are the voltage reading at the Caps in the B+ RC filters?

EDIT is the 110VAC 220VAC input voltage selector switch in the correct setting?

  You could be at about 1/2 the transformer output voltage and still work but have a large amount to hum from the AC ripple.  The transformers I measured in china tube supply were 200VAC and about 10VAC IIRC.

1.4 X 100(1/2 of 200) with a full wave bridge = about 140 DC
5 x 1.4 = about 7VDC

US power can run at 124 or so in some places so the output could be higher.

 

[pH]

I'll offer my voltages. I also swapped the 2.2R on the mic for 1R. Brought the H+ up, didn't seem to increase the hum.

117vac coming out of the wall.
Secondaries: 196vac and 10.3vac
261v out of the rectifier/221v/182v/142v B+/ 128v after 20k on mic
72v in between zeners

Ground wiring is as it was with stock psu circuit. Mic transformer grounded via solder tab.

Mic didn't seem to hum noticeably when it was stock.

-Paul

 

[imo]

Max and Gus,
Thanks for the replies. My voltages are pretty in line with PH's (again, we are using the same mic, same components, etc (outside of capsule))
I should have clarified, but indeed the 1r subbed for the 2.2r is in the mic circuit, not the power supply, i left that intact.
and to corroborate again with PH, the ground is a star ground that was stock in the mic
Ian

 

[imo]

I measured my voltages unloaded,  and they are pretty close to PH's:
AC into rectifier: 193, 10
Dc out: 258, 221(after 1st 20k), 182 (after 2nd 20k), 145 (at top of zeners)
the zeners are dropping 72.8 and 71.8v DC
current unloaded is @1.85mA through the filter

 

[ioaudio]

yes, voltages are a a bit low. B+ can be simply cured, add a 20K in parallel to the first 20k resistor (to make it 10k)
to confirm that the heater voltage creates the hum (and i think so), try adding another 10000µF and see if it lowers the hum.

 

[imo]

Max,
I actually tried subbing in smaller resistors thinking that the zener was being starved, with the PT maybe not having enough juice, but the zeners were fine, so subbing in a 20k in parallel does the same thing, voltage maintains at the same level. I was looking for a couple of LED's or something to try to get it up a few more volts
I don't have any caps near the 10,000U size, will have to order some more, so i'll have to wait on that, but the suggestion hit on a question i had:
What are the apparent weak spots of a power supply like this for those of you with experience in mic building? Is there typically this amount of filtering on a heater circuit? 2 4700U and 3 10,000U? How much would a tube power supply differ in the actual audio signal, or for that matter the choke based power supply that Max posted?
Ian

 

[ioaudio]

i dont really understand your question, but:

this design needs very low ripple. for now you can try the alternative self biasing to get a hum free mic. most "modern" tube mics work selfbiasing and do not need the extended ripple rejection.
the reason for the huge capacitors is the low voltage drop. thats why i suggest finding a pwr transformer with higher output rails.

the zeners might be out of spec, as gus sugested. try removing them and measure the voltage, or replace them.

-max

 

[Gus]

Ian

You can add a 4.7VDC one in series to the two in the supply if you need to be 150VDC.

Did you install the super caps in the microphone?

Is pin one grounded at the 3 pin XLR in the power supply?

Can you list the heater supply voltages at the nodes Max has marked in the schematic at the 1 page of this thread?

What tube are you using, number and brand?

 

[imo]

Happy new years by the way! I'm off to play a show, back in the fold in 2009
so on to the mic, I have pin 1 grounded on the XLR, voltages unloaded are:
11.3, 8.6, 8.6. 8.6, 8.6.
The tube currently in the mic is an NEC 6267 that tested well. I've also used a National 6267 that is used well in my Gyraf mic
Supercaps are in the mic
I definitely understand getting a power supply with more power, but i would love to try to get this working with the current setup; trying to learn a bit more about the mic circuit in the process.
Thanks
Ian

 

[Gus]

Need loaded voltages.  The loaded voltage will help find the tube fil draw and voltages under load and if the regulator has a good voltage difference from in to out.  Also the voltage drop across the tube fil  will be useful

 

[ioaudio]

I definitely understand getting a power supply with more power, but i would love to try to get this working with the current setup;

the easiest thing is to add capacity to the heater rail. i meant to replace the pwr transformer only, not the whole psu. maybe even find a small additional 12vac 500ma transformer and use the original for B+.
you can use the alternative biasing for now if you plan to upgrade the psu later.

 

[imo]

The loaded voltages are:
11.3v after rectifier, 8.6v at the 4700U, 7.9v st the 1st 10,000U, and then 7.4 to the mic.
From there i altered the 2.2r in the mic to 1r, which got the cathode and heater voltages pretty close. The heater is right at 5.7, and the cathode is at 1.33v

 

[Gus]

11.3 -8.6 = 2.7  I just looked at the specs for the 317 in a NS data book it specs >3VDC and <40VDC

So what I would try is a reducing the 2.2 ohm to 1ohm in the power supply so you have reg out then  3.3 ohm 10,000, 1 ohm 10,000 then to the microphone

Yes this will reduce the RC filtering but it might work just fine.

5.7 VDC from heater pin to heater pin?

7.4VDC then the 1 ohm then 7.03VDC measured to ground? .37VDC across the 1ohm?  .37/1 =370ma fil current.  If this is true you can use the current draw at the heater voltage to calculate the after the reg Rs of the RCs values that will total up to give a > 3VDC drop at the regulator.

 

[imo]

that would definitely make sense with the lm317. I will try to see if i can make the reg happy.
The voltages were 5.7v between the heater, pins 4 and 5, then 1.3v between cathode and ground
It would be nice to make this work with the current heater/cathode pattern, but if i hit a true dead end i will try the other bias

 

[pH]

So here's my update:

I put all values back to stock on the mic itself and changed some resistors in the psu. Lowering R1 to 10k didn't change the B+ after the zeners, stuck around 142v. I changed the 2.2R resistor in the heater supply to 1ohm as per Gus' suggestion and it got the voltages up nicely but didn't eliminate the hum.

Finally, I changed the cathode 2.2k to ground, pins 2,4,7 to ground and the hum is gone.

So now I'm curious what are the advantages/disadvantages to self biasing vs. biasing off the H+? It seems like there might be a touch less gain now but my methods are less than scientific...

-paul

 

[angryman]

Hi there ioaudio,

I am a real newby to this forum and to building circuits , however I am more than capable of good soldering work and am extremely keen to build my own mics and preamps. Your build here is the easiest i have found on this site to follow, but I have a few questions if you have the time to answer them it would be much appreciated.

I live in Australia and our power scheme is 240V, is your PSU able to be adapted for Aussie Standards?

Would i be able to to source most of the parts easily, if so where from?

Is the Mic housing from the German Site what you would recommend?

I would want to use the best parts for this Mic so what would you recommend in terms of tubes and capsules.

Would I be able to source the PCBs from you if and when they are available.

My next door neighbour and good friend is a machinist that owns his own workshop and CNC machines and all sorts of lathes and he has said to me that he would be able to pretty much make anything I wanted so Maybe I could get him to do a run of housings, fittings and such to make a custom housing for this mic if you or anyone else here is interested. Could be a nice way to get an exactly sized housing and PSU's even!

Anyway Happy New Year and Thanks in advance.

Kind Regards
Patrick Noonan

 

[ioaudio]

hi patrick,

welcome!

all DIY projects here can be build for the varying mains voltages. just specify the primary of your power transformer accordingly.
maybe start with an easy project which uses cheaper parts - you will learn where to source standard parts, learn about voltage requirements for capacitors etc.

making your own mic housing is a good thing - you will have your very own mic, recognizable also from the outside 
the thomann mic body is ok, nothing special. not very pretty as-is.

paul,

yes, gain is a bit lower. you can additionally bypass the 2,2k with a capacitor (220µF/16V, positive to cathode) and get the missing gain back.
dont forget to adjust the heater (always measure between the pins on the tube)

gus,

i tried giving the lm317 more to eat - but it didn't improve the ripple rejection.

 

[Jim50hertz]

My next door neighbour and good friend is a machinist that owns his own workshop and CNC machines and all sorts of lathes and he has said to me that he would be able to pretty much make anything I wanted so Maybe I could get him to do a run of housings, fittings and such to make a custom housing for this mic if you or anyone else here is interested. Could be a nice way to get an exactly sized housing and PSU's even!

Hi Patrick,

Have you seen this thread? 
http://www.groupdiy.com/index.php?topic=30742.0
It's somewhat related (It spun off from the MK7 project at least initially) and your friend's skills will likely find a lot of interest...

 

[Jeremy.Starseed]

I'd like to buy 3 kits, if and when they become available.

How's it looking, right now? Have I reason to be enthusiastic?

 

[Gus]

Max

    What I often do is use a 1 ohm after the diode bridge and before the first filter cap.  Think of it as moving one of the RCs to the front.  This cuts down the current in the diodes if using  large values or very low ESR caps.  This has helped a lot with DC heater supplies that I had low level noise "tics"