DIY RF Condenser Mics

[rogs]

I have a feeling we should investigate the FM discriminator principle!

Where to start with the inductors though? ...

I think Baxandall's 'balanced' AM bridge might be worth  a look as well? - as suggested by Ricardo

BTW, I received my 5 samples of your prototype PCB.... 5 days from JLC in China - free postage - total cost ?.... $2 (£1.63) ..  amazing!

I don't yet have my body from Wgtcenter, so I wondered if I could fit one into a spare BM800 body, to try it out? ....
5 minutes with a file - and a bit of lateral thinking  - and it will fit (sort of!) (See attached)

Works well --- interesting to note that I needed to change some of the values from my stripboard prototype to optimise things....

 

[rogs]

Microphone engineering handbook(M Gayford) has a section on Rf mics , written by Manfred Hibbing  who seems to have been involved with Sennhieser , Its an awkward book to find, I have a ring bound copy I can easily scan if someone wants it  for reference .

Its about thirty pages , anyone know a smart way of compressing the scans into something more manageable so I could post it here ? 

Heres a link to PJB's chapter on mic amps and transformers,

http://leonaudio.com.au/microphone.engineering.handbook..chapter.8.pdf

I'll put that chapter on RF mics up later once I get it onto a memory stick .

Looks really interesting ... look forward to seeing the chapter on RF mics in due course....

 

[Khron]

What the..? Hey that's not fair! DHL shipping took about a week, and cost like 30$ Well, lesson learned then...

Re: fitting into BM800 body - some of the resistor & diode footprints could've been reduced to 7.5mm instead of 10, imho So the fit wouldn't have needed to be quite that snug.

No surprise about the value changes either - slightly different layout, different parasitic capacitances, etc...

BTW, I received my 5 samples of your prototype PCB.... 5 days from JLC in China - free postage - total cost ?.... $2 (£1.63) ..  amazing!

I don't yet have my body from Wgtcenter, so I wondered if I could fit one into a spare BM800 body, to try it out? ....
5 minutes with a file - and a bit of lateral thinking  - and it will fit (sort of!) (See attached)

Works well --- interesting to note that I needed to change some of the values from my stripboard prototype to optimise things....

 

[Gerard]

I don't yet have my body from Wgtcenter, so I wondered if I could fit one into a spare BM800 body, to try it out? ....
5 minutes with a file - and a bit of lateral thinking  - and it will fit (sort of!) (See attached)

Works well --- interesting to note that I needed to change some of the values from my stripboard prototype to optimise things....

That's worth knowing. I measured a BM800 I have against Ruud's first image of the PCB and thought the space in the BM800 was too short; but I'm still waiting for my PCBs (yes, amazing price); if they can be made fit in the BM800, maybe I can make more of this project.I've ordered multiples of the quantity of components, as I want to get back into building electronics DIY projects like this again.

 

[Tubetec]

That looks like a great job you did Rog , nice symmetrical layout , elegant simplicity .
What kinds of spl is it capable of handleing ?

Ive been aware of the original Baxandall article  for years,  but way beyond my capabillity mathmatically  to  bother thinking about much .  I have a Behringer B2 body  I use for different test configuations , looks like your pcb might fit , although I might just allow you guys to 'cook in' the design a little more  first .

The author of the book makes reference to or hypothesises about 'the wireless tooth microphone ' in the book somehwere , he alludes to a mems or piezo based actuator/ pickup , basically a two way radio built into your tooth ,jaw bone conduction of both the persons voice for pick up  and also the abillity transmit  messages  directly into a persons head , you can well see why the military would be all over that sh!t.

Even the teeth have ears now ,and speakers too  , you just need to hold a transmitting  mobile phone beside your head every once in a while to charge up the battery  ;D 

http://www.filedropper.com/rfmics

 

[rogs]

That looks like a great job you did Rog , nice symmetrical layout , elegant simplicity .
What kinds of spl is it capable of handleing ?

I can take credit for the  circuit - but not the PCB! ...That was designed and laid out by RuudNL  ( see post #18 for the layout, and post#32 for the Gerber files)

The max SPL will be determined by the capsule I suspect -- I would think the AM modulation limits are likely to be way in excess of whatever max capacitance change the capsule can create  ....but none of that has been tested yet -  at this end....

 

 

[RuudNL]

Where to start with the inductors though? ...

Well, the core material of the 5.3 uH coils isn't the problem. (I think)
And at these frequencies you will only need a couple of turns.
So, you could strip he coil in such a way that only the coil former remains, and rewind it according to your own specifications.
I did this before and it can be done!
I am tempted to experiment with the Sennheiser MKH principle.
Maybe it is possible to get it working, maybe it will turn out to be a disaster...
I have looked at the schematic of the MKH 105 and I get the impression that the number of turns is written in the schematic?
Or am I mistaken?
(A small problem will be how to get 7 connections to a single coil... Sennheiser uses unshielded coils.)

 

[ricardo]

Heres the link to RFmics chapter of the book ,

http://www.filedropper.com/rfmics

Tubetec, this comes up with "can't find the server at www.filedropper.com"

 

[Tubetec]

Hi Ricardo , link seems to work fine for me ,
If you still have no luck ,I'll find a different site to host it .

 

[rogs]

I have looked at the schematic of the MKH 105 and I get the impression that the number of turns is written in the schematic?
Or am I mistaken?

I have to say that seemed likely when I first looked. Not quite sure how to read  the figures "20,5" shown against the (apparently) untapped winding marked  9and 10 though?....

The Sennheiser inductors looked a bit less complex  in the later schematics from around 2005  (copy of MKH 20 attached) -- but still pretty daunting without a reference starting point

 

[RuudNL]

The problem with the MKH20 is that it uses a very special type of microphone capsule.
The membrane moves between two perforated plates in such a way that while the capacity of the membrane to the front perforated plate decreases, the capacity of the membrane to the back perforated plate increases and vice versa.
It seems the perforated plates (you can't call them backplates anymore) are acoustically very 'transparent'.
Also interesting is that they don't use a crystal anymore in the later versions of the MKH microphones.
At the time a crystal was chosen to reduce the amount of phase noise, but it seems a free running oscillator is 'good enough'.

 

[Gerard]

The problem with the MKH20 is that it uses a very special type of microphone capsule.
The membrane moves between two perforated plates in such a way that while the capacity of the membrane to the front perforated plate decreases, the capacity of the membrane to the back perforated plate increases and vice versa.

I believe thats the case for all current capsules in MKH-series microphones. If that is the case, I'd love to see how they do pattern-shaping.

Reminds me I must look out and read some more of Manfred Hibbings' papers.

 

[Gerard]

I have to say that seemed likely when I first looked. Not quite sure how to read  the figures "20,5" shown against the (apparently) untapped winding marked  9and 10 though?....

I read that as simply the contental decimal format, i.e. 20.5 turns in UK/USA decimal format. Not 25 turns tapped at 20 turns. But I might be totally wrong  ;D

 

[Gerard]

http://www.filedropper.com/rfmics

Tubetec, I've downloaded this file three times, on different PCs and different networks. Each time, when I try to open it, I get a message that the archive is corrupt. Can you please re-upload it.

 

[Khron]

What did you try to open the archive with, then? Works perfectly fine with 7zip. Downloaded size is 20.3MB.

Either way, i put it up on my Dropbox, see if it does the same:
https://www.dropbox.com/s/3ffgcjq7dtxh1e1/RF%20mics.rar?dl=0

If it does, then the problem's on your side. Give 7zip a try, it opens just about anything and everything.

Tubetec, I've downloaded this file three times, on different PCs and different networks. Each time, when I try to open it, I get a message that the archive is corrupt. Can you please re-upload it.

 

[Gerard]

Thank you. The problem is a strange one. I can open individual pages in the rar archive from your Dropbox, so it seems Dropbox can open the archive, but if I try to download the whole archive, even from your Dropbox, , I get something that 7zip doesn't open.

However, as I say, I can download the individual pages, so Success!

What did you try to open the archive with, then? Works perfectly fine with 7zip. Downloaded size is 20.3MB.

Either way, i put it up on my Dropbox, see if it does the same:
https://www.dropbox.com/s/3ffgcjq7dtxh1e1/RF%20mics.rar?dl=0

If it does, then the problem's on your side. Give 7zip a try, it opens just about anything and everything.

 

[abbey road d enfer]

Also interesting is that they don't use a crystal anymore in the later versions of the MKH microphones.
At the time a crystal was chosen to reduce the amount of phase noise, but it seems a free running oscillator is 'good enough'.

It's pretty well explained in this very interesting paper. AM detection doesn't need a very stable frequency, but amplitude must be stable, which is easy to achieve. However it works well only with symmetrical capsules.

 

[RuudNL]

I believe thats the case for all current capsules in MKH-series microphones.

No, it is not. I know of the MKH20 and MHK40, that use this special capsule.
But MKH 105, 405, 415, 435, 416, 804 etc. use a 'normal' capsule.

 

[Gerard]

OK. Thanks for the clarification.

No, it is not. I know of the MKH20 and MHK40, that use this special capsule.
But MKH 105, 405, 415, 435, 416, 804 etc. use a 'normal' capsule.

 

[cyrano]

Tubetec, I've downloaded this file three times, on different PCs and different networks. Each time, when I try to open it, I get a message that the archive is corrupt. Can you please re-upload it.

Probably a recurring transmission error. Downloads/expands fine on my end.

Had the same problem a week ago, with a 25 GB archive. In the end, my provider admitted that their traffic shaping was interfering.

The download stopped with a "network error" every hour. When I tested over a VPS, it didn't. But the VPS was way too slow for such a big file, so I had to babysit the download, restarting every hour.