Gerard

DIY RF Condenser Mics
« on: February 24, 2019, 12:37:47 PM »
Has anyone come up with a practical, buildable DIY RF condenser mic, like a Sennheiser MKH-series mic? I am aware of Uwe Beis' rf mic page at https://www.beis.de/Elektronik/HF-Mic/HF-Mic.html. But, as Uwe said when I contacted him, the coils make building it a non-trivial task; also, he said he no longer has any record of how he wound the coils.

I would like to find a buildable diy rf mic circuit, to try building a condenser mic that doesn't rely on extreme high impedance to keep noise managable, and to improve humidity tolerance. I've got in mind to try doing an Ambisonic RF condenser mic, possibly with the 4 (first-order) or 8 (higher-order, like a Core Sound OctoMic) channels sharing a single oscillator to reduce component count and the condenser capsules acting in the demodulator circuits for the individual channels.

A further idea would be to go digital in the demodulating stage, perhaps in s SBC like a Raspberry Pi, with output directly in 24 bit digital format. However, that woul need a separate power supply as it would probably exceed the capacity of phantom powering

However, as I am more a mechanical engineer with limited electronics design experience, I may be dreaming up something that cannot be realised in a diy scenario.


rogs

Re: DIY RF Condenser Mics
« Reply #1 on: February 25, 2019, 07:10:05 AM »
I'm afraid this one comes up at regular intervals on different mic forums.....
The most detail I have ever read regarding the coils in the Beis circuit was a response to someone on the Yahoo micbuilders group who emailed Uwe Beis  about the inductor details a couple of yeas ago. The reply read:

"I cannot encourage to build this circuit. It becomes too complex, it is not just winding the inductors and reproducing the circuit. And it's not a final design, it is just the intermediate result of experiments. Anyway, I want to answer your questions as far as I can. It is a very long time ago that I made it
:
Can you provide winding details for L1 and L2?...
I'm afraid that I didn't write it down. L1 primary must fit to the oscillator frequency, secondary are very few windings (1, 1½, 2 or so as far as I remember). L2 is similar. The core material is crucial: I used Siferrit (later  Epcos) K1, which has been since a long time no longer produced but possibly still available somewhere. Also important is the wire: I got best results with wire wrap wire: Rather thin with a thick PTFE insulation.

Are there any special requirements for L3 and L4?...
Not at all.

What values did you use for C3 and C4?...
I guess C3 << C2 and C4 >> C3, C1 possibly not used. It all depends on experiments!

Have you made any significant changes to the circuit since you built it?...
Not at all.

And finally, though I think it’s not critical, what microphone capsule did you use, or at least, what was its nominal capacitance? ...
As far as I remember 15 to 100 pF or so, but I tried two completely different capsules, one small diaphragm from a Sennheiser RF microphone and the home made large diaphragm one on the photo.

Could one build a multi-capsule microphone using a single, common oscillator?..
I can't see anything that argues against this idea. But an advantage might be that interferences (beatings) between different oscillator frequencies would be avoided by design".

So as you see, no precise construction details - sadly!

I did try some experiments using 74HC4046 PLL, but it's not much good for this task ---the oscillator was far too noisy...
I'm guessing things like a Rasperry pi might have similar limitations?

This is not an easy DIY project, as far as I have been able to establish .....

The Sennheiser schematics probably offer the best ideas on the concept, but again sadly  - but understandably - there are no inductor details available...


EDIT: Looking again at my notes, I see the replies above I copied from the  'micbuilders' forum were addressed to someone also called 'Gerard' --- probably you, I'm guessing?.......If so, sorry to repeat stuff you already knew! ...

 Still, posting it in a new location might  attract some new interest?.....
« Last Edit: March 04, 2019, 03:58:09 AM by rogs »

EmRR

Re: DIY RF Condenser Mics
« Reply #2 on: February 25, 2019, 05:51:51 PM »
There is at least the one early AKG RF mic, there’s a BBC paper that addresses it and the earliest MKH.
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

rogs

Re: DIY RF Condenser Mics
« Reply #3 on: February 25, 2019, 07:26:59 PM »
There is at least the one early AKG RF mic, there’s a BBC paper that addresses it and the earliest MKH.

I think this may be the paper you are referring to:  https://ia800503.us.archive.org/3/items/bbc-rd-reports-1966-32/1966_32.pdf

Sadly - and understandably - there are still no details on  the inductors. 
That seems to be the sticking point for those of us who are perhaps less technical than 'professional designers', but would nevertheless like to have a go at a practical DIY realisation of a low-Z RF modulated mic...

EmRR

Re: DIY RF Condenser Mics
« Reply #4 on: February 25, 2019, 08:58:35 PM »
I've gone a little crazy buying MKH mics in the last couple years, it's worth pursuing. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

rogs

Re: DIY RF Condenser Mics
« Reply #5 on: March 10, 2019, 12:49:11 PM »
I took another look at the  Yahoo 'micbuilders' group thread on this subject  (I believe the OP here (Gerard) started the thread there as well?..)

There were around 70 replies in that original thread and - in amongst the 'snipes and gripes' you always get when there are 'experts' involved  :)   -  there were a couple of posts that I thought might prove useful ?...

• Posts #4 & # 5 mentioned using  IF cans to solve the inductor questions.
• Post # 36 suggested further reading of  Peter Baxandall's 1963 paper on the subject..
 (there's a copy of that here :  http://www.jp137.com/lts/Baxandall.RF.mic.pdf )

So having read Baxandall's paper again - and thinking about using  IF cans as inductors - I've tried out a prototype device based on  the Baxandall AM concept  built into a BM800 body - using  Toko 10K 3894 IF cans to solve the inductor requirements.

Photo of the prototype internals attached -- and there's a 'raw' audio sample here :  http://www.jp137.com/las/RF.bias.prototype.mp3

Still quite a lot to finalise (prototyping 10MHz circuits onto stripboard is not an ideal approach :) ) but I think the concept could work quite well...
I can post a sketch of the circuit , but it doesn't have many values, and is still very much a work in progress at this stage?...

Still, maybe something different to try? --  rather than  trying to resolve the Beis circuit inductor values  !
« Last Edit: March 11, 2019, 11:23:15 AM by rogs »

Gerard

Re: DIY RF Condenser Mics
« Reply #6 on: March 17, 2019, 07:36:54 PM »
Hi Guys, Thanks for the comments, and apologies for not acknowledging sooner. I have been very busy with performances last month and the accounts arising from them since.  "Real" work is also busy!

Rogs, that was my email you quoted. But nevermind; it seems to have sparked your interest. And thanks for the link to the BBC report; I'll have a look at it tonight. Did you get any further with your build?

Gerard

rogs

Re: DIY RF Condenser Mics
« Reply #7 on: March 18, 2019, 07:10:16 AM »
Did you get any further with your build?

As I mentioned above -- still a 'work in progress' at this time......

Most of the comments regarding this concept as a DIY project seem to - understandably - revolve  around inductor details and values.

My intention at the moment is to try and utilise existing  commercial inductors in the form of IF transformers - and adapt the AM concept outlined in Peter Baxadall's paper.

Both Sennheiser and the Beis circuit seem to employ more complex inductors, which are used in conjunction with adapted ratio detectors  as part of an FM modulation technique....A bit above my  design 'level ' I'm afraid!

You can hear the limitations of my prototype in the audio clip I linked to above, but I'm currently trying out different oscillator techniques and  extra RF de-coupling to try and reduce the system noise and improve the modulation linearity...

With the apparent lack of any detailed models to work with, it's currently a bit of a  'suck it and see' approach at the moment.....

 I'll report back any improvements I manage to make  - if  any!

Gerard

Re: DIY RF Condenser Mics
« Reply #8 on: March 18, 2019, 10:01:23 AM »
As a first try of an unproven design, I'm impressed!

 

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