shielded inductors for an rf circuit mic??

[seavote]

hi, im planning on building a copy of the sennheiser mkh30 a few projects from now.but im learning more about these circuits in the meantime. i had heard that these mics should be well sheilded. should i sheild the inside of the entire mic body? also should i use sheilded inductors in the circuit? or do both?any ideas are greatly appreciated.im a newbie so the simpler the explanations the better.

 

[gyraf]

My advice would be to start with something simpler than the RF-type microphones - those are frightfully hard to get right unless you have a background in (and tools/test equipment for) radio frequency electronics.

After all, I think there is a reason why so few companies besides Senn has used this type of technology.

Jakob E.

 

[bcarso]

Agree with Jakob. And when RF mics don't work right, they are terrible!

However, you may learn a lot attempting. I wouldn't worry about shielded inductors, more about layout details. Shielded L's have generally rather low Q which can be debilitating.

 

[seavote]

hi and thanks for your input. i understand what you are saying. maybe if i explain how this project presented itself to me you will understand why i am determined to follow through with it. first i am working on 2 guitar effects pedals right now. i bought a marshall mxl 2001(e-bay $50 US) to do a royer mod on and i am planning a g7(thx gyraf) with a peluso capsule and a nos telefunken ef86 i aquired cheaply. this rf mic will not be my first mic. so by the time i'm ready for it hopefully ill have learned enough to be "ready for it". i bid on and won 2 capsules on ebay(sennheiser made in germany)dirt cheap and figured i couldn't go wrong. it was only later that i learned they were capsules for the sennheiser mkh30 rf microphone and i could not use them for other types of mics. so i began to see what i could do about building the mkh30. i have a schematic,pcb and layout diagram. ive been able to locate where to aquire most of the components,although a few are proving to difficult to buy in small quantities(a particular ntc thermistor),or to get at all(the 2 transformers(i'll try sennheiser). worst case i can buy the complete chasis from sennheiser and make my own housing. i'd like to make my own pcb and make the mic myself if possible. the diodes are bat85(a schottkybarrier diode) and some zener diodes.should any schottky barrier and zener diode(of the correct value) do. i know which caps are electrolytic,their direction,and which are not ,but , i dont know their composition. will this greatly effect the sound of the mic. i have the values for all the inductors. will their function determine what type of inductors i should be getting or will any inductor of the correct value do the job?any help is appreciated.

 

[bcarso]

The schottky should be as close as possible---but the part mentioned should be readily available. Don't know the circuit but zeners of similar power dissipation rating should be o.k.

The inductors should be as close as possible to the originals. From the values and knowing the approximate physical dimensions of the original mic you can probably guess at what they are (air core or ferrite core etc.).

 

[Svart]

the diodes are bat85

Good luck.

I used these in a video system. Semi easy to get the do35 part, impossible to get the MELF part. BAT54 is a good alternative for SMD, good luck finding that part also. They are heat sensitive also.. don't ask how I know it's a bad memory.

Schottky barrier are strange, a coworker found some replacements with perfectly matching specs.. I forget the part number but they didn't work. Don't know for sure what he found as the problem but I had enough BAT54 to fill his needs.


:green:

 

[Guest]

[quote author="seavote"]hi, im planning on building a copy of the sennheiser mkh30 a few projects from now.but im learning more about these circuits in the meantime. i had heard that these mics should be well sheilded. [/quote]
I agree with coleagues, that building of copy of MKH30 is full of
troubles. It is Hibbing s RF circuit.
Its princip is combination of phase detector {griese mic} and
two tuned circuit mic {early Schoeps} It is hard to understand
because all lies in construction of RF transformer which are rather coupled circuits than pure transformer. Make analysis of schemo which I had send here some month ago...
Only way to understand is to DIY both - Griese and Schoeps.
Then crosses it.
Try to make copy of van der Ziel s mic with modern devices.
Much more simpler to understand than Hibbing s and without secrets.
Yes, shielding. RF circuit in all RF microphones is parametric amplifier.
In all parametric amps, external RF field varies core permeability - it adds with capsule changes and causes hum. Hibbings circuit is symetric. Then some amount of noise {if it is in homogenous field} is cancelled. It is kind of astatised parametric amp. Then MKH30 {and also van der Ziel s} is much less susceptible to LF magnetic field than Griese design { i.e. MKH405.} But one layer of permalloy foil inside mic body may work.
And use very quality RF coils. Caps from siemens K1 works well.
Caps have small leak, you can not shield it {by the way, I can not imagine, how {and why} to shield cap core inside mic.
And you must have Q-meter, oscilloscope and RF generator to
make serious work. To DIY high - Q coil is sometime veird.

And have patiency. It is years of work to develop good parametric
amplifier {or RF mic}

Good luck,

xvlk