MK7 - tube mic project

[Jonkan]

Ok, thanks!

Does the 2k2 resistor needed for the self bias have to be 1w also, or is 0.6w ok? I think I might have some 0.6w metal film resistors lying around somewhere.

/J

 

[ioaudio]

0.6w is ok.

 

[Jonkan]

that did the trick! Quiet as a mouse with self bias method.

Any ideas on what could make the stock psu work with the original bias? how much more capacitance would i need?

/J

 

[ioaudio]

probably 10000µf. adjust a higher voltage from the lm317 and insert another resistor dropping 1 - 1.5volt to form another RC stage.
play around with the values, record the hum with constant gain for reference.

 

[Jonkan]

ok, ill try that. 2x4700uf alone didnt do the trick atleast, already tried that.


I just realised that something is wrong with the mic that i tried the selfbias with. The frontside of the capsule doesnt work at all in any patterns. Gonna try another capsule and check if that helps (hope not, since the capsules are so relatively expensive).

Strange...Ill have to doublecheck everything.

/J

 

[Jonkan]

So I switched the capsule around, and the other side is working so its not the capsule. Good news i guess.

It seems that only the backside of the mic amp is working correctly. The side that the pattern switch supplies voltage to.

The frontside is totally dead.

Any ideas on where to start looking? I have triple checked everything, and cant find any obvious faults. I did it exactly like the self bias layout Ioaudio posted.

The psu voltages are correct into the mic. I get between 50-60v to the capsule backplate before the 100m resistor depending on how high the mains voltage is at the time.

/J

 

[ioaudio]

quadruple - check all connections surrounding the capsule.
post detailed pics, but probably you´ll find the (wiring?) error soon.

 

[Jonkan]

Heres a pic i just took. I had to remove the capsule to get a good look at the wiring.

I cant find anything wrong, can you spot anything?

/J

 

[ioaudio]

only the backside is working? and it changes with the pattern switch?

 

[Jonkan]

Yes. When pattern is 120 and 60v backside of capsule works. When 0v both sides are quiet.

The other mics with the non selfbias method works as expected except for hum with the psu.

/J

 

[Jonkan]

Meet the family. Well, the family pre incident. I had an accident and managed to trash or atleast severely harm one of the membranes. Dale who made the capsules, has agreed to help me reskin it, so it will be allright in the end I think. Yes i know, im clumsy.

Im building two for a friend, and two for myself.

Nice pic anyway i think.

Now if i could just get the psu up to spec, so that i dont have to use the self bias method...

Btw, I solved the problem with the mic that wasnt working correctly. It was the 10n styroflex cap that was bad. Good thing i found a couple of extras in my drawer.

Cheers,
Jonas

 

[rodabod]

It was the 10n styroflex cap that was bad. Good thing i found a couple of extras in my drawer.

Just read through this thread again and was about to suggest this. Styros don't like too much heat and can easily fail to a short. Glad you got it sorted - looking good.

 

[djn111]

Wauw that looks super, what body did you use ?

Also if there is a rerun of the kit/pcb transformer I am in for two .

 

[Jonkan]

Thats the suggested thomann body, the sct700. It works ok, nothing special though.

/J

 

[Jonkan]

I dont know if my mains voltage is fluctuating or what, but now i only have 2.6v between lm317t input and output when loaded with mic. Same in both psu:s i have left.

Is this correct thinking? The heater rectifier diodes now drop 2x0.7v=1.4v. If i change the heater rectifier to schottkytypes which drop around ~0.4v each, ill only drop 0.8v instead, giving me 1.4-0.8=0.6v more into the lm317t...?

That would put me at 2.6v+0.6v=3.2v, which is a little more than the 3v mimimum the lm317t wants to see between input and output to properly regulate.

That could be the cause of the hum, right?

Is 1n5818 a good choice?

Im also going to order another couple of 10000uf/10v caps to try if that would help.

/Jonas

 

[JoleFIN]

Has anyone contacted T.Bone (or was it this manuf?) and asked if they could sell the SCT700 bodies "as repair parts"?

 

[Jonkan]

So im still struggling to get the fixed bias working.

This is what ive tried so far:

I changed the heater rectifier diodes to schottky types to get the voltage differential on the lm317 to be more than 3v.

I added 2x10000uf caps after the RC network on the heater rail. I couldnt really add more RC network here because im only having 3.2v now between input and output on lm317t regulator.

I have tried changing and moving the ground connections around in both ends of the cable, psu, and mic. I can get the hum to be a bit lower that way, but its still very much there.

Should i just give up and use the self bias instead? Im all out of ideas now.

Any help greatly appreciated!
/J

 

[ioaudio]

time for a better power transformer - or self bias.

 

[Jonkan]

Ok, thanks for responding.

Have you compared the different bias methods? If not, what are the theoretical differences between the two different methods? To my ears, the self bias method sounds just fine, but I havent really been able to compare it to the fixed bias because of the hum so im very curious to what im actually missing out on.

Any ideas where one could find a suitable transformer btw?

Thanks!
/J

 

[ioaudio]

ah - did you try to seperate the electrolyts? especially seperate the last caps  before the output, since they pick up hum.

most (all?) new microphones are self biasing, andreas grosser said "it´s done this way nowadays" but there is a difference - especially when the source is loud.