DIY RF Condenser Mics

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.

EDIT: There is now a full DIY RF Condenser mic project here -- www.amx.jp137.com -- which is based around the 400+ posts now in this thread (as at Dec 2019)....


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?.....

There is at least the one early AKG RF mic, there’s a BBC paper that addresses it and the earliest MKH.

EmRR said:

There is at least the one early AKG RF mic, there’s a BBC paper that addresses it and the earliest MKH.

Click to expand...

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...

I've gone a little crazy buying MKH mics in the last couple years, it's worth pursuing. 

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  !

EDIT: (October 2019)  --- there are some notes here:  www.amx.jp137.com  which detail a completed project mic that came about as result of the 300+ posts this thread has  now grown into. ---
Those notes are an attempt to rationalise the twists and turns this thread has taken over the last 7 months.... my thanks to all those who have contributed.

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

Gerard said:

Did you get any further with your build?

Click to expand...

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!

EDIT: As result of help and ideas from the 300 + posts now in this thread,  the project has moved on to now becoming a  'repeatable' DIY building project. ..
See my notes here: www.amx.jp137.com

As a first try of an unproven design, I'm impressed!

Gerard said:

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.

Click to expand...

I tried out Great Guru Baxandall's circuit circa 1980 hoping to use it on the Soundfield Mk4.  It was VERY quiet.  At least as quiet as the Senheissers of the day.

Alas, it was impossible to prevent 4 oscillators in close proximity from beating.  AM introduces sidebands and in fact has very similar 'frequency shifts' to narrow band FM

rogs, I'm impressed by your success.  I took a long time to get good results.  I tried a 4046 solution too and got about 20dBA spl noise so gave up and went back to HiZ for the Soundfield Mk4

Hey Ricardo, Good to see your words again.

Am I being crazy in thinking I could take four (or eight) buffered feeds out of a single oscillator for a four (or eight) channel Ambisonic mic? Like a TetraMic or OctoMic, but diy and using an RF arrangement. I'm thinking that if ever rogs is confident enough to post the circuit diagram for his version, I would try to make a multi-channel Ambisonic version, with all the channels sharing a feed from a single oscillator (to save on component space), with each feed from the oscillator buffered separately. I'm probably crazy! Even if it is impractical, I would like to make a couple of RF mics, just to see how different they sound from say 'Alice' mics or 'Simple P48' mics.

I'm still using the Brahma mic you calibrated for me a few years ago as my main mic for recording the choral societies I sing in. My recordings have had lots of favourable comment; people are impressed with the sound. Thank you.

Gerard

Gerard said:

Am I being crazy in thinking I could take four (or eight) buffered feeds out of a single oscillator for a four (or eight) channel Ambisonic mic? Like a TetraMic or OctoMic, but diy and using an RF arrangement. I'm thinking that if ever rogs is confident enough to post the circuit diagram for his version, I would try to make a multi-channel Ambisonic version, with all the channels sharing a feed from a single oscillator (to save on component space), with each feed from the oscillator buffered separately.

Click to expand...

That's what I tried to do for the Mk4 Soundfield.  The problem is you have 4 capsules in close proximity, each radiating slightly different frequencies and beating.  Except when in a completely silent field, each capsule will radiate slightly different sidebands.  Even Great Guru Baxandall thought I was on a hiding to nothing.

If you do manage to shield them from each other, I will be insanely jealous and will grovel at your feet.

Even if it is impractical, I would like to make a couple of RF mics, just to see how different they sound from say 'Alice' mics or 'Simple P48' mics.

Click to expand...

That's certainly worth having a go.  I found his circuit at least as quiet as the Senheissers of the time.

But today, zillion G resistors & LN FETs are MUCH more easily available than circa 1980.  Then I had problems finding 1G resistors which actually obeyed Ohm's Law.  Most of them were more like crappy diodes.

Also Electret Capsules are far less susceptible to damp than externally polarized capsules.  If I was making mikes commercially today, I would get in bed with one of the better Far East makers like Transound or Primo to improve their capsules.  In 1980, the only 'capsule' I was jealous off was the Shure SM81 cos there was no way Calrec could afford the Electret technology then.

Ah well. One to try when I retire and have some time on my hands. I trust you will still be around to grovel in 5-10 years time? Or to laugh at my abject failure! 

I've been experimenting a little more, and have managed to improve the signal to noise ratio by around an extra 10dB from my first prototype..
The main improvement has been made by using a more suitable IF can - one where there is no internal capacitor of the wrong value already fitted!.....

A short 'raw' audio sample here:  www.jp137.com/las/RF.AMX2.sample.wav

I've also drawn out the circuit and made a few notes.  (details attached). 
I can add some construction details in due course if anyone is interested?...

EDIT: The attached circuit has now been modified to improve the performance.
You can find a schematic of the latest (slightly simpler!) version here:  www.jp137.com/lts/RF.AMX5.pdf

FURTHER EDIT:  There are some more recent project notes here:  www.amx.jp137.com :

Rogs, well done! The sample sounds good. I am definitely interested in trying to build this. I have a few BM800-type mics as donors, so any details of where you encountered any problems would help. I doubt I could build it into a pencil body; but I might try.

That is very interesting. Consider me interested!

rogs said:

I've been experimenting a little more, and have managed to improve the signal to noise ratio by around an extra 10dB from my first prototype..
The main improvement has been made by using a more suitable IF can - one where there is no internal capacitor with a value you don't really want!...

A short 'raw' audio sample here:  www.jp137.com/las/RF.AMX2.sample.wav

I've also drawn out the circuit and made a few notes.  (details attached). 
I can add some construction details in due course if anyone is interested?...

Click to expand...

I grovel at your feet o guru rogs 

yes please to constructional details 

Count me interested!
Kindest regards,
zephyrmic

Great stuff!
I can't wait to experiment with this circuit myself.
(Already ordered the inductors...)
Added is a first PCB layout I am going to use for testing.

in this project, as the inductors could pickup all sort of electromagnetic fields, would it be a good idea to shield  them (for example with mu metal screens), or is it not critical ?