C12a mod to remove cathode follower?

[Spencerleehorton]

Assumption is the mother of all f-ups (in this case, assuming the cable waa intact)

Have i won the internet yet? :O

today youve won

 

[Spencerleehorton]

the one very annoying thing is that i wound both mic transformers identically, although within the panic of trying to fix this hum i have resulted in rewinding one of the transformers to a 3:1 but didnt record what or how i wound the other one.
luck would have it that i still have the windings on a bobbin, so if i gingerly put them back, everything should be fine.
i did record the ohms and inductance and i have the other one to compare so its not a complete loss but still annoying!!
seems i love learning the hard way!!!

 

[rock soderstrom]

seems i love learning the hard way!!!

Learning through pain - the best way to remember new knowledge

Spence, I'd be interested to know how you build your transformers and what ingredients you use! If you have the time and energy, could you make a little photo thread about it?

 

[Spencerleehorton]

well i can give a quick run down here no problem, i need to write this down anyways:

so for these C12a it needs a small bobbin nickel lams, i chose these of ebay:

https://www.ebay.co.uk/itm/334340331691
you get two.

mobys U11 is 1.8:1 ratios so i thought id go either 2:1 or 3:1
pretty sure i used 0.1mm guage for PRI ans SEC
i have a hand winder with a little attachment screw which is able to hold this small bobbin.
start with the secondary as it going to be the most winds
sec = 1800 turns
tie off the wire and solder, make sure to do even layers applying a bit of tension so it all gets on the bobbin nice and neat.
i then test the winding and write down the DCR, in this case its 168R
then i cut some masking tape and wrap just enough round that winding so i can do the next winding.
next is PRI A = 200 turns, then tie off once done and solder, then test 22R and cover with tape.
Next is PRI B = 365 turns, then tie off once done and solder, then test 46R and cover with tape.
i then attach and solder any winding joins and then cover with tape and put in a mumetal case or i make a mumetal encolsure for it.

so my transformers for these c12a turned out like this
SEC = 168R inductance = 64H
PRI in series = 68R inductance = 6.3H

and thats how i roll, or rather wind!!

 

[rock soderstrom]

Thanks a lot! Very informative!

I think I'll wind my first transformer myself in the new year!

 

[rock soderstrom]

i have a hand winder with a little attachment screw which is able to hold this small bobbin.

DIY or bought?

 

[Spencerleehorton]

This is what I’ve got

https://www.ebay.co.uk/itm/27226017...hLm__RtThm&var=&widget_ver=artemis&media=COPY

Just mount it on a bit of wood and I made two posts with a bit of metal through it to mount the enammaled wire on

 

[Biula]

That's about as sacrilegious around these parts as putting a Schoeps FET circuit in a U47 . . .

I agree, C12a reason to exist IS it's cathode follower topology... have you heard it on a loud source (e.g: drum overhead...), with it' snappy, infinite headroom response? Only C12a and Sony C37 can do this, why butcher a fine engineered vintage mic? Il you want a plate loaded mic you have tons of (not so expensive) different models...

 

[rock soderstrom]

I agree, C12a reason to exist IS it's cathode follower topology... have you heard it on a loud source (e.g: drum overhead...), with it' snappy, infinite headroom response? Only C12a and Sony C37 can do this, why butcher a fine engineered vintage mic? Il you want a plate loaded mic you have tons of (not so expensive) different models...

Did you read the whole thread or just the title?

 

[Matt Nolan]

or i make a mumetal encolsure for it.

Hey Spencer. Are you annealing your enclosure after you've built it? What construction methods are you using? From what I understand, cutting, bending, even hard knocks can reduce the magnetic field screening properties of mumetal, but they can be restored with appropriate heat treatment.

Perhaps folks with first-hand knowledge of this could chime in.

 

[ccaudle]

Are you annealing your enclosure after you've built it?

Don't forget that after any bending you should anneal mumetal in a pure hydrogen atmosphere at 1121˚C for 4 hours.
Typical DIY tools.

 

[rock soderstrom]

in a pure hydrogen atmosphere at 1121˚C for 4 hours.

Wow!

Typical DIY tools.

no doubt

 

[rock soderstrom]

By the way, how can I identify mu metal?
Are there clear indicators for a layperson without special equipment?

Here are two examples: