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Project Specific Discussions => Microphones => Topic started by: Gerard on February 24, 2019, 12:37:47 PM

Title: DIY RF Condenser Mics
Post by: Gerard 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.
Title: Re: DIY RF Condenser Mics
Post by: rogs on February 25, 2019, 07:10:05 AM
EDIT: There is now a full DIY RF Condenser mic project here --  www.amx.jp137.com   --  which is based around the 300+ 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?.....
Title: Re: DIY RF Condenser Mics
Post by: EmRR 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.
Title: Re: DIY RF Condenser Mics
Post by: rogs 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...
Title: Re: DIY RF Condenser Mics
Post by: EmRR 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. 
Title: Re: DIY RF Condenser Mics
Post by: rogs 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  !

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.
Title: Re: DIY RF Condenser Mics
Post by: Gerard 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
Title: Re: DIY RF Condenser Mics
Post by: rogs 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!

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
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 18, 2019, 10:01:23 AM
As a first try of an unproven design, I'm impressed!
Title: Re: DIY RF Condenser Mics
Post by: ricardo on March 20, 2019, 04:13:01 AM
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.
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
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 20, 2019, 01:05:58 PM
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
Title: Re: DIY RF Condenser Mics
Post by: ricardo on March 20, 2019, 10:03:57 PM
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.
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. :o

Quote
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.
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.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 21, 2019, 08:51:41 PM
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!  ;)
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 24, 2019, 05:38:27 PM
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 :
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 24, 2019, 08:52:09 PM
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.
Title: Re: DIY RF Condenser Mics
Post by: cyrano on March 24, 2019, 11:25:14 PM
That is very interesting. Consider me interested!
Title: Re: DIY RF Condenser Mics
Post by: ricardo on March 25, 2019, 09:43:27 PM
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?...
I grovel at your feet o guru rogs  :o

yes please to constructional details  :)
Title: Re: DIY RF Condenser Mics
Post by: zephyrmic on March 25, 2019, 10:29:27 PM
Count me interested!
Kindest regards,
zephyrmic
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 26, 2019, 04:24:35 AM
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.
Title: Re: DIY RF Condenser Mics
Post by: granger.frederic on March 28, 2019, 11:46:37 AM
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 ?
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 28, 2019, 11:54:03 AM
The coils have their own screening can.
Title: Re: DIY RF Condenser Mics
Post by: Khron on March 28, 2019, 03:09:19 PM
There's that, plus they're already inside the metallic body of the mic, which at least in theory, should be grounded...
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 28, 2019, 04:56:36 PM
I've posted some notes on the project prototype details here:

http://www.jp137.com/lts/RF.AMX2.Documents.pdf

EDIT: The document linked to above has been updated, as improvements have been made to the prototypes.

You can find the latest schematic here: www.jp137.com/lts/RF.AMX5.pdf 

 
As I mention in those notes, the project would become a lot easier to construct using a PCB -- so I look forward to seeing how Ruud gets on with the design layout he's listed in post # 18....

... but it's been fun trying out the prototype to prove the concept.....
Title: Re: DIY RF Condenser Mics
Post by: kingkorg on March 28, 2019, 07:44:34 PM
A bit off topic, but what a lovely capsule!

I just wonder how they achieve different patterns.

https://cvp.com/pdf/sennheiser_mkh_series.pdf

EDIT:
And i'll answer my own question, they use two back to back capsules.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 29, 2019, 04:31:30 AM
As soon as I have results, I will let you know.
I tried to keep the HF part as far as possible away from the LF stage.
Also there is some grounded copper between the oscillator and the LF emitter followers.
Let's wait and see!
Are there other forum members who are going to experiment with this circuit?
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 29, 2019, 05:29:29 AM
As soon as I have results, I will let you know.
I tried to keep the HF part as far as possible away from the LF stage.
Also there is some grounded copper between the oscillator and the LF emitter followers.
Let's wait and see!
Are there other forum members who are going to experiment with this circuit?

I look forward to seeing how you get on. It will certainly be interesting to see how your layout affects the  noise levels.......I'm pretty sure they should be better than my stripboard layout!

I'm not sure the oscillator circuit is optimised for low noise either...... I shall be very interested to see what you discover.

If things go well, will you be making the Gerber files available for those of us who would also like to try a  PCB version ?
Title: Re: DIY RF Condenser Mics
Post by: Khron on March 29, 2019, 07:11:42 AM
I'd kinda lean towards doing a full groundplane-fill on the outside edges of the board (including the screw hole pads), instead of that thin (double) loop.

Maybe the 100nf in the middle could go parallel to the two 100K's slightly to the right? And the 10k near the bottom, 180deg rotated and parallel to the diode? :)

Swap positions between the two 47R and 47uH's, and the resistors can then go more towards the left, resulting in shorter total trace length?

I'm afraid my "PCB OCD" is showing, i'm sorry... ;D

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.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 29, 2019, 07:26:58 AM
It is the first try, there will be plenty of room for improvement!
First, let's see if and how it works.
I think a PCB has an advantage over stripboard because you have less parasitic capacity.
10 MHz isn't really high frequency. (I have built circuits that worked on hundreds of MHz.)
But...time will tell!
Title: Re: DIY RF Condenser Mics
Post by: Khron on March 29, 2019, 07:55:07 AM
I know, i know, it's kinda like mixing - sometimes you need to take some time off, and come back and see it with new eyes :)
That definitely often happens with my board designs ;D And/or a "second opinion" can have similar value.

Nothing wrong with getting closer to getting it right(?) the first time though.... Right? :P

But i'm also on the list of people looking forward to the result of these experiments ::)
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 29, 2019, 04:37:41 PM
It is the first try, there will be plenty of room for improvement!
First, let's see if and how it works.
I think a PCB has an advantage over stripboard because you have less parasitic capacity.
10 MHz isn't really high frequency. (I have built circuits that worked on hundreds of MHz.)
But...time will tell!

I'm thinking it might be an idea to try out a 12 MHz oscillator, rather than 10MHz?.....

That would permit the omission of  the 22pF cap C3 which - being in parallel with the bridge capacitors -  will tend to attenuate  any capacitive changes from the capsule ...

Operating at 12MHz would also mean changing C4 to 68pF and C8 to 33pF, but that should (hopefully) allow the overall Q to be largely maintained, as the inductors will still be tuned near the centre of their specified range.....

I don't have a 12MHz crystal to hand at the moment to check the idea out, but it may help to improve the sensitivity a little...so maybe worth a try?....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 29, 2019, 07:45:26 PM
Hi Rogs, Ruud, For a simpleton like me - it's been too many years since I last made up a mic circuit - are there any special considerations in selection of components? Any of the caps that should be a particular type (C0G/NP0 or polystyrene or whatever)? And for the crystals, any recommendations on series resistance or load capacitance or any other consideration? And any special requirements for the inductors L1/L2.

Also, would you, Ruud, be willing to make some PCBs for sale? I no longer have the facility to make them myself.
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 29, 2019, 09:56:52 PM
I built the prototype with 'standard' stock parts I already had in my  hobby electronics kit ...So..
• Vishay K series MLCC with class 1 C0G  diecletric for C1,2,3,4,and 8
•Multicomp polyester 63V  MCPBSFC  series for C5,6,9,10,11, and 12
• Crystal is an IQD LF A143K -- (HC49  -30pf - parallel resonant)
• Inductors are type 5u3H  (10mm coils) from Spectrum Communications here in the UK.  (I notice the Ebay stock level of these devices is now 0, so you may need to buy from Spectrum directly - see here:  http://www.spectrumcomms.co.uk/Components.htm
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 30, 2019, 04:38:19 AM
First I will do some experiments myself.
I have crystals for 8, 10 and 12 MHz available. (Sennheiser used an 8 MHz crystal in their MKHmicrophones.)
If the result is successful, I can publish the Gerber files. (At https://jlcpcb.com you can get 10 PCB's for $2.)

For those who can't wait: here are the Gerber files I used for the prototype. (Rename .JPG to .ZIP!)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 30, 2019, 07:14:34 AM
I have already ordered some coils. The eBay vendor says he has thousands of them! But for bulk orders to contact his website.

My question concerning any specific requirements for L1/L2 actually referred to the two inductors at the connection to J1, the XLR connector.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 30, 2019, 07:17:44 AM
Ruud, thanks for posting the Gerber files. I'll see if I can get some boards made here in Ireland.
Title: Re: DIY RF Condenser Mics
Post by: Khron on March 30, 2019, 07:18:12 AM
I'm quite sure those are only against (conducted) EMI / RFI, so they don't need to be anything special.


My question concerning any specific requirements for L1/L2 actually referred to the two inductors at the connection to J1, the XLR connector.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 30, 2019, 07:39:09 AM
Ruud, thanks for posting the Gerber files. I'll see if I can get some boards made here in Ireland.

The PCB fits this microphone body: https://www.wgtcenter.com/microphone-accessories/shell/diy-accessories-for-u87-microphone-condenser-classic-neumann-vintage-body.html

Type "B" (the smaller one.)
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 30, 2019, 08:06:45 AM
Sennheiser used an 8 MHz crystal in their MKHmicrophones....

That was my starting frequency too, but  - unlike Sennheiser - I don't have the expertise to design and build custom inductors!  :)

The Spectrum 5u3H has so far been the most useful 'off the shelf' IF can  I have found, so the oscillator frequency needed to be  adapted to match those, as far as possible.

I've had quite useful results at 10MHz, but - if my calculations are right - it should be possible to improve the sensitivity by  some 6dB  by using  12MHz, and the revised capacitor values?
I shall be interested to see what you discover....

Thanks for the preliminary Gerber files...
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 30, 2019, 08:42:12 AM
My question concerning any specific requirements for L1/L2 actually referred to the two inductors at the connection to J1, the XLR connector.
The 47uH inductors were fitted because that's what I had to hand.....The value is not critical -- anywhere from 47uH to 220uH should do. 
The potential dividers formed with C11 and C12 will attentuate any conducted stray RF pretty effectively...
The dominate source of any extra stray RF is likely to be ground borne - or possibly radiated -  and  hopefully not a significant amount.

As I stated in my notes, this is not a finished design, but rather a 'proof of concept' ,  to hopefully allow a practical design to be developed.  Ruud has already taken up the mantle!  :)

I think we've probably already advanced further in these few posts than the contributors did in the  70+ posts in the Yahoo thread you started a couple of years ago?  :)
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 30, 2019, 09:53:51 AM
The first thing I would like to have a closer look at is the biasing of the oscillator transistor.
Assuming the voltage over C6 is 12 Volts, the base of Q1 is at 10.9 V.
This would mean a DC voltage of ~10.3 V at the emitter of Q1.
This DC voltage theoretically causes a DC current of 10.3/470 = 21.9 mA.
But since the microphone is phantom powered, this current can't be delivered.
I would suggest a bias of Q1 in such a way that the voltage at the emitter is in the order of ~ 1/2 * U C6.
Maybe it would be a good thing to add a capacitor in series with R5, so there is only AC over T1.
(Any DC current through T1 is a waste and because T1 is tuned to resonance, the impedance as seen by the oscillator should be pretty high.)
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 30, 2019, 11:57:43 AM
As I mentioned in my notes earlier, I can quite understand that the oscillator may well need some  revision.
The Colpitts oscillator 'block' was taken  as is from an online source - not sure where now? -  so it may well need to be tweaked.

I did fit R4 to allow the oscillator to run without the inductor connected, and in that mode you can measure around 6vp-p  at the emitter.
For those without a 'scope,  I included  DMM  'dynamic DC'  readings at  Q1  when the oscillator is running without T1 fitted:

Q1 collector 33.2
Q1 base 28.8
Q1 emitter 29.3

So the current  is not too high in 'dynamic' mode.

I should perhaps try again to AC couple the input to T1 -- I didn't have much luck the first time I tried doing that...

I look forward to seeing your suggestions in due course.



Title: Re: DIY RF Condenser Mics
Post by: RuudNL on March 30, 2019, 12:08:50 PM
Maybe it is an idea to use an external oscillator, to see what the advantage could be of a higher oscillator frequency?
I will see what I can do to maybe optimize the circuit a bit.
(I just did a test with a 12 MHz oscillator. I got 9 V p-p at the output with a supply voltage of 12 V and a current of 2 mA.)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 30, 2019, 12:29:49 PM
Ruud: Thanks for the heads-up about the microphone body size required. I have a number of BM800 bodies, intened to be donors for 'Alice' or Homero Leal's 'Pimped Alice' circuits; but they are not long enough for this PCB. I'll get some of the bodies you suggest.

Rogs: Yes, this thread has sparked much more interested and positive response that my previous attempt. A big thanks to you and Ruud, and others who have responded, for rising to the bait with such enthusiasm. I'm looking forward to building a couple of these, even in their present state of refinement; maybe more, if the final design proves to be particularly good. Unlike you and Ruud, I have not done much circuit building in the past decade; I don't have a big supply of components, so I must temper my enthusiasm until the parts are delivered.  :)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on March 30, 2019, 12:35:53 PM
A bit off topic, but what a lovely capsule!

I just wonder how they achieve different patterns.

https://cvp.com/pdf/sennheiser_mkh_series.pdf

EDIT:
And i'll answer my own question, they use two back to back capsules.

KingKorg: Are you referring to the Sennheiser capsules or the one Rogs used? The Sennheiser ones are not two diaphragms with their backplates set back to back; rather they comprise a single diaphragm sandwiched between two symmetrical backplates, to provide a balanced output. As the diaphragm moves, the capacitance on one side rises proportionally as that on the other side falls.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 02, 2019, 07:13:06 AM
Maybe it is an idea to use an external oscillator, to see what the advantage could be of a higher oscillator frequency?
I will see what I can do to maybe optimize the circuit a bit.
(I just did a test with a 12 MHz oscillator. I got 9 V p-p at the output with a supply voltage of 12 V and a current of 2 mA.)

I'm still waiting for my 12MHz crystals!  (should be here today?)  but in the meantime I built an external version of the oscillator in my schematic, to check out my original component selections....
Using a 12V supply, and including both inductors and their associated 'load' capacitors - I measure 20V p-p across C8, (without D1 connected ).

It's a complex load.....the crystal looks like  a series inductor/resistor/ capacitor  - in parallel with another capacitor! - and the Q of that is 'tamed' (or more accurately 'damped') a bit by  by R3.   I can  measure c.19V p-p sine wave  between base and ground.


At 10MHz the impedance of T1 secondary (which we are using here as a non centre tapped primary) is around 700 Ohms -- which is understandably slightly  lower than the value of 792 Ohms  specified for 7MHz operation. 
R5 is fitted to both allow control of the current flowing in the emitter circuit, and to reduce the Q of the load on the emitter, to help improve the linearity of the sine wave output - and  to permit more controlled tuning ! 

With  the oscillator fed from a 12VDC supply , I can measure about 11V p-p at the emitter, and as I  mentioned earlier this translates into around 20V p-p across C8 off load.   This is more than adequate to drive the rectifier pretty effectively.  In fact, I has tried using a BAT85 diode at one stage, but I think that's probably overkill in this case.

From a 12V supply, the 'loaded' oscillator draws about 10mA, and produces a pretty linear sine wave output ( it's a bit difficult to measure actual harmonic distortion at 10MHz with my simple test  equipment here!)

Powering the oscillator from phantom power will inevitably require a  compromise, as the effective DC supply is 48 V fed through 3.4K . The values selected will cause the oscillator to draw about 8mA when powered from  phantom power - and this seems to be a decent balance between current drawn and oscillator voltage output.

I think the only change I might make with a 12MHz crystal is to possibly reduce C1 to 100p to improve linearity.
What does remain to be seen if is the loss of  system 'Q'  from using the higher frequency renders any increase in voltage swing across the bridge lost in the reduced output ?....

I did try again to AC couple the input of T1 to the oscillator, but had way too much loss with any cap I tried ?.....
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 02, 2019, 09:36:05 AM
I am still waiting for the inductors. As soon as I have them, the experiments will start here!

I did try again to AC couple the input of T1 to the oscillator, but had way too much loss with any cap I tried ?.....

Did you try to lower the value of R4 when you used AC coupling?
It should be possible to AC couple T1 without losing too much signal. Anyway, there is no need for DC current through T1.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 03, 2019, 09:15:47 AM
I am still waiting for the inductors. As soon as I have them, the experiments will start here!

Did you try to lower the value of R4 when you used AC coupling?
It should be possible to AC couple T1 without losing too much signal. Anyway, there is no need for DC current through T1.

R4 was originally included to allow the stripboard to be tested before the inductor sub assembly was added. I did try and reduce it  to increase the output volts ready for  my attempt at AC coupling to T1,  but of course that increases the current drawn by the oscillator. In addition, AC coupling this output to T1  produced a  much less linear sine wave, so I returned to direct coupling -  with  R5 in series to control the oscillator current. Much better sine wave!

Then of course you're back into the compromise required by the limitation of the current available  form the phantom power!

With the values I've currently selected for my prototype, you can measure a decent sine wave of c. 12v  p-p at each end of T1 secondary (with respect to ground).
The same sort of voltage appears across the secondary of T2 - but that will of course vary, depending on the values of the bridge imbalance.

The 5u3H inductors are not of course a perfect match for this task - just the closest I've found so far - but the 'step up' voltage gain available from both inductors use in the present configuration do seem to provide a useful amount of AM modulation to make the project worth experimenting with....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 03, 2019, 10:47:49 AM
• Inductors are type 5u3H  (10mm coils) from Spectrum Communications here in the UK.  (I notice the Ebay stock level of these devices is now 0, so you may need to buy from Spectrum directly - see here:  http://www.spectrumcomms.co.uk/Components.htm

In case people hadn't noticed, the eBay link given in rogs' write-up is to Spectrum Communications eBay outlet. I have written to the owner and he tells me he has lots ('thousands') of coils, not just the few listed on that eBay page. At the link rogs noted above, he also has many different coil specifications, for frequencies up to 50 MHz.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 03, 2019, 11:16:11 AM
Damn, still no inductors in the mail today...  :-\
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 03, 2019, 03:55:39 PM
...and still no 12MHz crystals for me today either!     :(   

I  note that Spectrum have now put some more stock onto their Ebay shop  although - as Gerard notes-  buying from Spectrum direct is another option with their higher stock levels - and slightly cheaper prices!  :)

There are useful details on both Spectrum's own site, and on their Ebay shop page with  information on the coil specs.

One thing I've found useful is the comment on the Ebay page about the range of tunable inductance for these 5u3H coils. -- 3.0uH to 7.5uH

And that got me trying out something new....... which seems to allow for another 6dB or so of improved sensitivity.

My prototype schematic has the capacitive loadings on both T1 and T2  set to around 47pF - which is the recommended load for 10MHz.

By  omitting C3 - changing C4 from 47pF to 68pF - and changing C8 from 47pF to 33pF - it is just possible to tune the inductors to around 7.5uH (the max limit).
As C3 was in effect across the 'bridge' , it acts  as a parallel  load to the bridge, and reduces sensitivity.
By removing it - and effectively changing the bridge  loading to around 33pF -  the sensitivity is improved.

Running at 12MHz should allow those coils to be retuned to around 5u3H , so I shall see if that improves the sensitivity any further. It could be that operating a 7MHz coil at 12MHz is a step too far .....we shall see.

So already there will be changes to the original schematic, in due course.

Once Ruud (finally!) gets his inductors,  there may be yet more improvements ...

All good experimental fun......  :)
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 04, 2019, 04:17:40 AM
Just an idea...
Why not add an extra detector (D1, R7,C9), but now with the diode reversed.
Then connect the output to the (now grounded) leg of C5.
Theoretically, this would increase the output level ~6 dB. (And maybe even improve the signal to noise ratio.)
Also the load on T2 would become more symmetrical.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 04, 2019, 06:17:43 AM
Just an idea...
Why not add an extra detector (D1, R7,C9), but now with the diode reversed.
Then connect the output to the (now grounded) leg of C5.
Theoretically, this would increase the output level ~6 dB. (And maybe even improve the signal to noise ratio.)
Also the load on T2 would become more symmetrical.

I did try grounding the centre tap of T2 secondary, and adding a second detector - connected to the presently unused Q2 base - so that the detector essentially became a full wave rectifier.
Unfortunately, the reduction of the 'free' step up voltage gain created by the higher Q in the resonance  of the non centre tapped secondary  T2/C8  dropped the sensitivity a lot.
I haven't tried introducing a second detector without the grounding centre tap........that might maintain the resonance of the secondary, so should be worth a try?.....

EDIT:  A preliminary test seems to indicate that adding a second detector diode and time constant network across T2 secondary - and removing the ground connection - still reduces the Q of the resonant  secondary network quite dramatically.
So I think any attempt to use a balanced detector may need to be a bit more sophisticated - which is a shame! :)

Certainly the most impressive improvements made so far involve maintaining the high Q tuning options of both inductors.

Removing C3 -  and changing the values of C4 and C8 - saw a dramatic improvement in sensitivity -  and thus signal to noise ratio.

Both my prototypes are now more sensitive than any of my dynamics - but not as good as most of my condensers..  (yet!  :) )


Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 04, 2019, 09:16:29 AM
T2 is (roughly) loaded witt 10K/1nF.
I can imagine that the Q will be lower with two 'detector' circuits in parallel.
But maybe it is possible to increase the value of R7 (10K) while keeping the R-C time constant about the same.
A higher value will however raise the noise contribution of the PNP output stage.
It feels very frustrating that I can't test anything here, since the inductors still haven't arrived...
(No idea why it takes so long to ship something from GB to the Netherlands!)
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 04, 2019, 10:35:33 AM
Looking at it slightly differently, T1 secondary is presently only loaded with 10k/1nF for 50% of the time  :)

I was thinking about  grounding the T2 secondary centre tap again, and looking at possibly fitting tuning caps to each half of the winding separately -- then re-introducing  a detector to each half again.....  That might help restore some of  the missing 'Q' ?

As I've mentioned before, these inductors are not perfect for this task - merely the closest I've found. 
The loading on them in this configuration is complex though - way above my level of maths! -  I just think that it's probably only the noise free gain available from the tuning that makes this project even vaguely useful.
Without the the 'Q' of these reactive components I suspect the noise levels would render the mic unusable.....

It's annoying you haven't received the inductors -- they were dispatched for next day delivery here...
Mind you, Spectrum is only about 30 miles down the road !  :)
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 04, 2019, 01:51:17 PM
Most RF microphones I know of, use an FM discriminator principle.
If this current approach doesn't lead to the success we want, it might be an idea to have a closer look at the FM discriminator principle. (By the way: most commercial microphones use some way of amplification in the circuit and since the current experimental design only uses emitter followers, it is IMHO no surprise that the output is only slightly higher than a dynamic microphone. As such, I think that is already a success!)
I have a RF microphone here, designed by the NRU (the technical department of the Dutch radio) in the 60's.
The design is very simple: only one transistor (BC109) en a ferroxcube core.
Oscillator is free running at ~8 MHz.
Unfortunately, no schematic or technical details are available.
I was told that the microphone capsules were made by Neumann, but the electronics were developed 'in house'.
The only thing I know, is that there is a trimmer to 'null' the DC output (before the coupling capacitor to the transformer) without any signal.
Output isn't very high, probably because they wanted not too much difference in output compared with dynamic microphones.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 04, 2019, 03:54:34 PM
I've now tried out a 12MHz oscillator......Not really much different from 10MHz -- possibly 1 dB less sensitive?

Both crystal frequencies fall within the inductor tuning range -- and the fact that the 12M is further away from the specified centre frequency (7MHz) probably accounts for a slight loss of Q  even though it is tuned  more towards it's centre  5u3H value.
Swings and roundabouts here I think....

I have been able to improve the sensitivity by another 3dB or so by tuning the inductors manually against an external tone, rather than with a 'scope.
Even using a 10M 15p  divide by 10 scope lead will load the test point and  cause errors in maximum resonance tuning.

I've got the noise floor down to below -60dB, which I think is quite good for such a simple design.

Here's another  'raw' short speech sample: www.jp137.com/las/RF.AMX312M.sample.wav

 -  (I think  the sibilance comes from the fact that the capsule is currently a cheap Chinese K67 clone with no EQ)

Next thing - to have another look at the balanced detector idea...

 

 

Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 04, 2019, 04:19:11 PM
Excellent result for such a simple circuit!
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 04, 2019, 06:21:23 PM
Since the value in question is more a relative one than an absolute one, might an e-field probe be an option, in order to avoid loading down the circuit?

Seems like even a cobbled-together one works well enough to be (somewhat) useful :)
https://www.youtube.com/watch?v=2xy3Hm1_ZqI
https://www.youtube.com/watch?v=nImoQcoqkuQ

I have been able to improve the sensitivity by another 3dB or so by tuning the inductors manually against an external tone, rather than with a 'scope.
Even using a 10M 15p  divide by 10 scope lead will load the test point and  cause errors in maximum resonance tuning.

Title: Re: DIY RF Condenser Mics
Post by: rogs on April 05, 2019, 06:02:05 AM
Since the value in question is more a relative one than an absolute one, might an e-field probe be an option, in order to avoid loading down the circuit?

Seems like even a cobbled-together one works well enough to be (somewhat) useful :)
https://www.youtube.com/watch?v=2xy3Hm1_ZqI
https://www.youtube.com/watch?v=nImoQcoqkuQ

Fascinating project! ...... in this case I'm not sure how useful it would be?
 I can see a problem trying to isolate radiation of the variable RF output (from the IF transformer can) from that of the fixed oscillator..

In actual fact, it's not difficult to calibrate, by placing the mic  right up against a headphone transducer running a tone, and looking at the output of the preamp meter....
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 05, 2019, 06:35:10 AM
T2 is (roughly) loaded witt 10K/1nF.
I can imagine that the Q will be lower with two 'detector' circuits in parallel.
But maybe it is possible to increase the value of R7 (10K) while keeping the R-C time constant about the same.
A higher value will however raise the noise contribution of the PNP output stage.

I've now tried a number of combinations - including grounding the T2  secondary centre tap, and creating two separate half wave rectifiers , each tuned to one half of the secondary.  Only half the voltage swing for each half of course, so no real advantage there.

Isolating T2 secondary from ground and adding a second reversed diode with it's own CR network works OK,  but the return path for each half cycle  is then via the second diode and network ... so  losses there.  Again, no real advantage in voltage gain

Increasing the value of the resistors does of course up the output voltage swing - but increases the noise - as you've already mentioned.

I do think the idea of balancing the rectifier network and using both of the Schoeps outputs is a more elegant solution, but tests so far have only increased the component count, with no real gain in voltage swing - a least, not without  creating extra noise as well....

The most important improvement so far has come from removing C3 and changing C8.  Both of which improved the Q of the system -- that's where it seems  the most 'noise free' gain can be found!
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 05, 2019, 07:45:46 AM
Finally! Now let the fun begin...
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 05, 2019, 07:53:37 AM
Isolating T2 secondary from ground and adding a second reversed diode with it's own CR network works OK,  but the return path for each half cycle  is then via the second diode and network ... so  losses there.  Again, no real advantage in voltage gain
Is there any advantage in looking again at the rectifier design used in the Baxandall article? I am not an electronics engineer, but I see he was using two transistors as rectifying switches, with the switching signal derived from the oscillator. But maybe I'm speaking from the wrong orifice.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 05, 2019, 08:53:06 AM
Finally! Now let the fun begin...
:)  :)

Is there any advantage in looking again at the rectifier design used in the Baxandall article? I am not an electronics engineer, but I see he was using two transistors as rectifying switches, with the switching signal derived from the oscillator. But maybe I'm speaking from the wrong orifice.

There may well be an improvement by adopting Baxandall's balanced bridge option ... although it's moving 'up a level' in design  expertise..
I did wonder about  using 74HC4052 analogue multiplexers to try out  Baxandall's idea, but whether  switch noise would be a problem? ... (you can't get at the 'gates' on 4000 series CMOS, to slow the switching transients down - although they're not always too much of  a problem). And then there's working out the extra inductors that may be required for oscillator coupling ----

A more complex project I would think ....might be worth looking into though ?....if you're feeling brave!   :)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 05, 2019, 08:59:24 AM
A more complex project I would think ....might be worth looking into though ?....if you're feeling brave!   :)

I'll let you know if I'm feeling brave when the components arrive (it's so long since I made anything I think I threw out or gave away most of my 'stock' of components) and when I create some free time.

Like I said, I'm no electronics engineer - I'm a steam systems designer, professionally - and my electronics knowledge is more than a little rusty.  ;)
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 05, 2019, 11:57:35 AM
This afternoon I constructed the first test version.
To my surprise everything worked the first time.
Output isn't bad, much higher than a dynamic microphone!
I found the best way to adjust the cores of the coils is with a test tone.
I held the microphone close to one half of a headphone with a 1 KHz tone on it, while measuring the output signal.
Noise isn't a problem. I think a lot of cheap condenser microphones are noisier than this design!
I used a K47 capsule for my tests.
The only problem I have with the design, is the current draw of the oscillator.
The voltage on the pins of the XLR connector is in the order of 22 V, so that would mean a current of ~8 mA.

I have changed some component values in my prototype.
Title: Re: DIY RF Condenser Mics
Post by: homero.leal on April 05, 2019, 12:16:04 PM
This afternoon I constructed the first test version.
...
The voltage on the pins of the XLR connector is in the order of 22 V, so that would mean a current of ~8 mA.

That's really great RuudNL!Congrats! This has been an elusive design for DIYers, but now it starts to be feasible task!  Big thanks to Rogs and Gerard for their work and sharing the info with the group!

Quick question... is it really a problem having 8mA of current draw, on a P48V mic?

Thank you and regards!

HL

Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 05, 2019, 01:00:46 PM
Quick question... is it really a problem having 8mA of current draw, on a P48V mic?

No, not really... It is just the idea that it is a much higher current than most condenser microphones.
Also the dissipation in the phantom power resistors is higher than with other microphones.
But the quality is surprisingly good and self noise is much lower than I had expected.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 05, 2019, 03:11:55 PM
This afternoon I constructed the first test version.
To my surprise everything worked the first time.
Output isn't bad, much higher than a dynamic microphone!
I found the best way to adjust the cores of the coils is with a test tone.
I held the microphone close to one half of a headphone with a 1 KHz tone on it, while measuring the output signal.
Noise isn't a problem. I think a lot of cheap condenser microphones are noisier than this design!
I used a K47 capsule for my tests.
The only problem I have with the design, is the current draw of the oscillator.
The voltage on the pins of the XLR connector is in the order of 22 V, so that would mean a current of ~8 mA.

I have changed some component values in my prototype.

Glad your first build has gone well Ruud..... Certainly good to see a DIY RF biased microphone idea  enter the hobby domain - at last!

I have been impressed with how well such a simple circuit performs  - I had a feeling that these coils could have a spec that was close enough to be useful for this task!

I think the problem with the fairly high oscillator current  is down to the fact that the  impedance of the secondary of the Spectrum coils (used as the primary for T1) is a bit too low to be ideal for the task required.
But I found that the addition of R5 (which I note you have changed to 560R) allows for a reasonable balance between voltage and current drawn. R5 also permits the sine wave available at the emitter to be much better quality than trying to drive the emitter with a purely inductive load. At least that's what I've found.

One thing that I've not mentioned in this thread is how this whole concept relies on an unbalanced bridge. If for example the value of C4  exactly matched the capsule value, there may not be enough of a bridge imbalance to allow  the rectifier diode to conduct effectively.
Highly unlikely of course - and I've now fitted a BAT85 in place of the 1N4148 to help minimise any such effect.

But it does perhaps point to trying out Baxandall's balanced version - which Gerard mentioned above - as a natural progression of the project.

As you have now confirmed, this circuit works well for a simple project - certainly better than I expected when I first started it!

I'm waiting for some of your PCBs to arrive from JLCPCB - hopefully on Monday next. 
I'm getting tired of making stripboard and hand built coil assemblies for my prototypes!  :)

Couple of new observations:
 
•The addition of the second rectifier to create a balanced output actually reduces the sensitivity slightly, so I shall stick with half wave for the time being. In addition, the extra AC voltage available in that mode might prove to be useful  if the bridge does end up closely balanced !
• I have decided to stick with 10MHz for the time being. Although using 12MHz does permit a higher Q tuning, the amplitude is reduced by being further away from the designed frequency of the coils (7MHz), and the sensitivity is slightly reduced as a result.

Maybe one day we shall discover a more suitable IF can?... Trouble is there doesn't seem to be many that have appropriate characteristics, and not have an internal capacitor fitted.
In theory, it's supposed to be possible to remove those caps, but I've never had any luck with that....
 



Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 07, 2019, 07:49:04 AM
Are there any 12MHz (or higher) coils available, even with internal capacitors? Maybe we will have to see if the capacitor can be removed. However, my preliminary searches have not shown much.

I wonder if any of the other coils available from Spectrum Communications might be suitable? If you see any possible candidates, I'm willing to try doing a capacitorectomy, to see if it is feasible.
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 07, 2019, 08:23:55 AM
Would anything like this be even remotely suitable?

https://www.ebay.co.uk/itm/SUMIDA-10-70-MHZ-IF-TRANSFORMER-COIL-PINK-WITH-MULTIPLE-WINDINGS-2-PIECE-OFFER/173863899325?epid=1930905707&hash=item287b1890bd:g:7qEAAOSw-jhUFyee

Since it's spec'd "for" 10.7MHz and the primary(?) inductance is in the order of 3-4uH...  ???
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 07, 2019, 08:48:24 AM
Rogs, Ruud, might the coil suggested by Khron work better at 12 MHz? It does have a capacitor, but I note that one end of the capacitor is not connected to any coil - see the description on the sellers web page (https://symkrilag.uk/shop/10x10mm-shielded/sumida-1070-mhz-if-transformercoil-pinkwith-multiple-windings-2-piece-offer/); it's also slightly cheaper direct from his website than from his eBay outlet.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 07, 2019, 12:22:35 PM
As far as I know, the core material of those coils has a pretty broad frequency range.
There are even capacitor values given for frequencies between 3.8 and 14 MHz.
Q factor will slightly change, because the L-C ratio changes.
If you are not faint-hearted, you could even open the enclosure of the coil and reduce the number of turns...
(In fact I have used only the coil formers in the past, and rewound the coils to suit my needs!)
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 07, 2019, 01:34:11 PM
There are 3 important parameters that the inductors need to  comply with for this design to work effectively:

• The secondary winding of T1 needs to be centre tapped ( lots of IF transformers  are tapped  near one end of the coil... These won't work very well - if at all! )

• The  capacitive loading of T1 secondary  will ideally be around 30 to 35pF (when used with a capsule with a capacitive value around 60/65pF) .
That secondary load is formed by the capsule and the series capacitor C4 ....For maximum sensitivity there does not want to be another capacitor in parallel with that winding.

• There needs to be a 'step up' turns ratio of around 1:4 between the (untapped) primary and the (centre-tapped) secondary windings.

All the other parameters are to be 'fitted' around those 3....
 
My first prototype used Toko  3894 cans , which were 2.6uH loaded with an 82pF cap. It worked - but not well.

The Spectrum 5.3uH coils will resonate at around 12MHz  with a 33pF  capacitive load. Their fundamental frequency is specified as 7MHz, so 12MHz is some way from that.

If you select a 2.6uH coils, the 33pF load  would resonate at around 17MHz   - a  bit too high, IMHO!
 
As Ruud points out, these coils have a fairly broad  frequency range, but the Q will drop a little as you move away  from the specified centre frequency. 
Overall though, I think 5.3uH coils are to be preferred for this task over 2.6uH ones.

Another thing to consider is the complex  loading  presented by the coil assembly as a whole.

 T1 secondary is also loaded by T2 and it's associated components.  T2 is in turn loaded by the rectifier - and 'looking back' into T1. ......It's a complex set up, the maths of which are way beyond me!

I have found the highest sensitivity - and hence the lowest noise - using a 10MHz  oscillator for my prototype.
Ruud used a different capsule, and preferred 12MHz.

There are many combinations that may well work OK... but you do need to  make sure that whatever you use for T1  has a centre tapped secondary winding, and will function - at a sensible frequency - with a 30pF to 35pF load, and without another capacitor in parallel.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 07, 2019, 04:16:13 PM
I did try out one more version of a balanced rectifier that came pretty close to the same sensitivity as the present 'half wave' version....
More elegant I think, but not quite so good, sensitivity wise -  so no cigar!  :)
Title: Re: DIY RF Condenser Mics
Post by: ricardo on April 07, 2019, 10:24:02 PM
One thing that I've not mentioned in this thread is how this whole concept relies on an unbalanced bridge. If for example the value of C4  exactly matched the capsule value, there may not be enough of a bridge imbalance to allow  the rectifier diode to conduct effectively.
Highly unlikely of course - and I've now fitted a BAT85 in place of the 1N4148 to help minimise any such effect.

But it does perhaps point to trying out Baxandall's balanced version - which Gerard mentioned above - as a natural progression of the project.
Ruud & rogs, I'm in awe of your progress in such a short time.

I think you guys are at about the stage of the 'first experiments' at the end of page 1 of Baxandall's article.  I'm guessing your noise level is about that of a 'good' Schoeps HiZ circuit.

This is definitely worthwhile as it comes with better tolerance of humidity

The next stage is to try GG Baxandall's circuit itself to get SOTA noise.  It's nearly 50 yrs since I played with it so my memories are quite hazy.  Then I could call on the Great Guru himself for help.

About the only useful thing I can dredge up is that his 1mH inductors in the output filters are non-critical and any convenient RF chokes will do.  Operating at 10MHz means you can use inductors 3x smaller.

Also your Schoeps double EF output stage makes this filter even less critical.

I await developments with jealous interest :o
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 08, 2019, 05:51:27 AM
I think you guys are at about the stage of the 'first experiments' at the end of page 1 of Baxandall's article.  I'm guessing your noise level is about that of a 'good' Schoeps HiZ circuit......

This is definitely worthwhile as it comes with better tolerance of humidity.....

The next stage is to try GG Baxandall's circuit itself to get SOTA noise.  It's nearly 50 yrs since I played with it so my memories are quite hazy.  Then I could call on the Great Guru himself for help......

I can now say that - noise wise - my own 'no.1 prototype' is now on a par with my  own Schoeps Hi-Z mics .... and not that much noisier than my Rode NT1 .
( I always keep  2  prototypes going when experimenting with RF circuits -- and never apply  mods  to both at the same time -- A golden rule I learned many years ago  :) )

Ricardo, I have to thank you for your comments in the original  Yahoo micbuilder's group thread a couple of years ago.
In among the ' flotsam' of that long thread (over 70 posts) there were 2 comments that proved inspirational to me...

 Post #4 from Umashankar:   " 'It would be fun to build the Baxandall version with the 25 mm capsules. One just needs coil details (I think they were not included in the WW article) best would be if one can use IF cans, ready made"

and - most importantly - your own comments in post #36, which included the following:

"Can I suggest some of you guys actually read Baxandall's paper(s)."   (absolutely essential reading IMHO!) .. and

"PLL allow accurate very small bandwidth receivers without yucky Inductors .. but you lose the Q multiplication of genuine Reactive bits on good stuff like S/N and power output" (The underlining was  in your original comment).

I had tried  experimenting with  4046 PLL and basically  got nowhere.  It was when the original poster of the Yahoo thread  -Gerard - started this thread that my interest was re-kindled.
I initially tried some Toko IF transformers, but they weren't really suitable, and it was only when I came across  these Spectrum coils that I though there might be a chance to get some useful results.   I was immediately reminded of your  comment about reactive Q .

After I published my initial  circuit here, Ruud 'took up the mantle' as well, and we have both now produced  useful prototypes .
We only seem to have a few differences in ideas on component values - only to be expected when using different capsules.
In addition, I know Ruud is keen to reduce the oscillator current.

Whether the project will now move on to the 'next stage' as it were, time will tell? 
I have already  bought a 5/65pF variable cap  to help create a balanced bridge...but in the short term, I have to try and catch up on my other chores....
My wife can't quite see why I spend so much time making microphones......'How many do you actually need?' - she keeps asking  :)

P.S. I am extremely jealous of your having had  contact with Baxandall himself   :)


Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 08, 2019, 07:55:31 AM
'How many do you actually need?'

That's I question I have been asking myself many times!
At the moment I have 80 of them...
Title: Re: DIY RF Condenser Mics
Post by: gyraf on April 09, 2019, 04:07:02 AM
 :o following from the sideline....
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 09, 2019, 06:16:51 AM
I have a feeling we should investigate the FM discriminator principle!
Title: Re: DIY RF Condenser Mics
Post by: Tubetec on April 09, 2019, 07:10:23 AM
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


Heres the link to RFmics chapter of the book ,

http://www.filedropper.com/rfmics



Title: Re: DIY RF Condenser Mics
Post by: rogs on April 09, 2019, 08:49:16 AM
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....
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 09, 2019, 08:53:54 AM
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....
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 09, 2019, 09:17:53 AM
What the..? Hey that's not fair! DHL shipping took about a week, and cost like 30$ :D 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....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 09, 2019, 09:37:08 AM
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.
Title: Re: DIY RF Condenser Mics
Post by: Tubetec on April 09, 2019, 10:15:39 AM
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



Title: Re: DIY RF Condenser Mics
Post by: rogs on April 09, 2019, 11:11:40 AM
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....

 
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 09, 2019, 12:26:49 PM
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.)
Title: Re: DIY RF Condenser Mics
Post by: ricardo on April 09, 2019, 01:56:37 PM
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"
Title: Re: DIY RF Condenser Mics
Post by: Tubetec on April 09, 2019, 02:55:29 PM
Hi Ricardo , link seems to work fine for me ,
If you still have no luck ,I'll find a different site to host it .

Title: Re: DIY RF Condenser Mics
Post by: rogs on April 09, 2019, 03:24:28 PM
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
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 09, 2019, 03:59:44 PM
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'.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 09, 2019, 05:52:56 PM
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.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 09, 2019, 05:54:52 PM
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
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 09, 2019, 06:06:05 PM
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.
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 09, 2019, 06:12:52 PM
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.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 09, 2019, 06:51:13 PM
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.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 09, 2019, 07:24:58 PM

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.
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 10, 2019, 02:02:46 AM
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.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 10, 2019, 06:25:02 AM
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.
Title: Re: DIY RF Condenser Mics
Post by: cyrano on April 10, 2019, 08:56:15 AM
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.
Title: Re: DIY RF Condenser Mics
Post by: lampas on April 10, 2019, 10:46:16 AM
I am sorry, but would someone would be so kind and with simple words explain me point of and advantages or disadvantages of the RF condenser mics ?
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 10, 2019, 10:54:56 AM
The most important advantage is that an RF microphone has no problems with high humidity. (Condensation)
That is because the capsule has a very low impedance at the (RF) working frequency.
Also the voltage on the capsule can be much lower than with a LF condenser microphone.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 10, 2019, 11:21:30 AM
An added advantage is that AC bias, being less than DC bias, does not exert as much pull on the diaphragm. It does give designers some freedom as to  increasing sensitivity (by reducing the gap) or increasing max SPL
Title: Re: DIY RF Condenser Mics
Post by: lampas on April 10, 2019, 11:38:53 AM
Thank you very much!
Title: Re: DIY RF Condenser Mics
Post by: EmRR on April 10, 2019, 11:48:08 AM


https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf
Title: Re: DIY RF Condenser Mics
Post by: Tubetec on April 10, 2019, 08:46:07 PM
Thanks for that link Doug ,
brings us right up to date with Sennheisers state of the art.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 16, 2019, 04:24:59 PM
There is quite a range of capacitive values for  different LDC capsules, and I've been experimenting with a couple of different types  - a C12  type and a K67 ...  The K67 measures as c.55pF, and the C12 as c.90pF.
Clearly these are different enough in value to look to see if certain component changes might improve performance.

In addition, I've been looking at the oscillator again, and have come to an experimental conclusion that I know Ruud won't like!  :)
The Spectrum 5.3uH coils are not matched exactly to what is really needed  - but they're close...  I decided to reduce the value of R5 from 470R to 47R, and to change R6 from 100R to 47R . In addition I linked out R2 .
This has the effect of increasing the current drawn by the oscillator slightly (to around 9mA  - or 4.5mA from each leg- so still within the 48V phantom power spec limits).

However the extra volts presented to the primary of T1 - in conjunction with a couple of 'capsule specific'  component changes - make for a quite dramatic further improvement in sensitivity, and with no measurable increase in the noise floor.

This improvement of the sensitivity is in the order of a further 10dB, taking the noise floor down to around -70dB ...quite respectable for such a simple design, and on a par with many quite high quality Schoeps type Hi-Z designs.

I've attached a  'capsule specific' schematic for the C12, detailing the changes. (The K67 schematic is attached to the next post. )

Edit:  The value of R6 as 47R may be too low for some phantom power supplies .
A value of R6 as 1K is probably a more useful value to start sensitivity experiments with.


 
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 16, 2019, 04:30:32 PM
And here's the schematic for the K67 type capsule

Edit:  The value of R6 as 47R may be too low for some phantom power supplies .
A value of R6 as 1K is probably a more useful value to start sensitivity experiments with.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 16, 2019, 07:09:10 PM
I am impressed with your success - and frustrated. My capsules have not arrived yet, nor the PCBs. I hope they get here before the Easter break; I am eager to try making these mics and trying them out.

Well done, Rogs. A very impressive performance from a basic, fairly unrefined (undeveloped?) design. You have put a lot of work into it and got the result.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 16, 2019, 07:14:16 PM
The Spectrum 5.3uH coils are not matched exactly to what is really needed 
What inductance values do you think would be better? Is it worth opening up and trying to rewind the coils for a better match?
Title: Re: DIY RF Condenser Mics
Post by: EmRR on April 16, 2019, 08:59:43 PM
I am glad to see this explored. 
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 17, 2019, 04:40:35 AM
What inductance values do you think would be better? Is it worth opening up and trying to rewind the coils for a better match?
I think the inductance value is pretty much spot on  for this task ... the tuning range of 3 to 7.5uH specified for these Spectrum coils allow for resonant capacitor values  to fall within our requirements here.

The actual loads  within the inductor configuration we are using here are complex - way beyond my maths -  but there is still useful experimentation  to be done here, I feel.

I'm looking now at the balance between the phantom power feed impedance, and the actual oscillator current requirements...
Perhaps increasing the value of  R6, while keeping R5 value low, for example? 
It looks as if it may be possible to reduce the oscillator current to around 4 or 5mA, without losing too much 'gain' ..

It would be useful to get the oscillator current down, if only because one of the main uses of an RF mic is outdoors -- where the phantom power is likely to be battery powered!

Experiments are continuing... :)

EDIT: A change in value for R6 from 47R to 4k7  seems to be a suitable compromise.  Each leg of the phantom power supply is then only required to supply around 3mA - so only a power dissipation of around 70mW  in each 6k8 resistor.

You could increase the value of R6  further, but the sensitivity seems to drop off more significantly if the DC supply to the oscillator falls below c. 3V

Changing R6 from 47R to 4k7  will drop the sensitivity by a little over 6dB  -  but it does reduce the current drain by around 40%....


Title: Re: DIY RF Condenser Mics
Post by: Khron on April 17, 2019, 06:39:20 AM
Not that i know anything about oscillators, but i was starting to wonder - would it help anything, switching (no pun intended) to a JFET or a MOSFET for the oscillator itself (Q1)?
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 17, 2019, 07:44:40 AM
This improvement of the sensitivity is in the order of a further 10dB, taking the noise floor down to around -70dB ...quite respectable for such a simple design, and on a par with many quite high quality Schoeps type Hi-Z designs.

That is a significant improvement!
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 17, 2019, 08:19:39 AM
Changing R6 from 47R to 4k7  will drop the sensitivity by around 6dB ... but it does reduce the current drain by around 40%....
What effect does this change have on the noise level?

I suspect anyone using this outdoors might prefer to choose the higher sensitivity and low noise over longer battery life. But maybe there's compromise in between that maintains very low noise with acceptable battery life.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 17, 2019, 08:54:38 AM
Not that i know anything about oscillators, but i was starting to wonder - would it help anything, switching (no pun intended) to a JFET or a MOSFET for the oscillator itself (Q1)?
I did try using a JFET, but  had more success with a bipolar.   The 'Q' components of the oscillator - the crystal and T1 -   are the ones which are the most important parameters, and I found it easier to  configure for a more linear sine wave using a BC549 than with an FET....
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 17, 2019, 08:59:28 AM
That is a significant improvement!

Yes -- there was a lot of  'wasted' voltage with R5 being 470R.  It does need to exist to help linearise the emitter load, but 47R is a lot less wasteful of 'precious' T1 voltage swing.
Downside is more oscillator current - I'm currently looking at the best compromise between sensitivity and oscillator current, as you can read in my post #111 above....
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 17, 2019, 09:16:18 AM
What effect does this change have on the noise level?

I suspect anyone using this outdoors might prefer to choose the higher sensitivity and low noise over longer battery life. But maybe there's compromise in between that maintains very low noise with acceptable battery life.

The noise level will be affected by however much is introduced by  turning up the preamp to compensate for the loss of sensitivity - so in theory  around 6dB.
But if that is from a very low preamp noise floor it may not be a problem..

The increase in oscillator volts which become available as R6 is reduced creates essentially 'noise free' gain, so if the extra current drain is acceptable, the  extra sensitivity will be welcome.

The simple mod of selecting different values of R6 to determine the oscillator amplitude should make it fairly simple to  choose the  best compromise for whatever use you have planned...

Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 17, 2019, 09:45:01 AM
The simple mod of selecting different values of R6 to determine the oscillator amplitude should make it fairly simple to  choose the  best compromise for whatever use you have planned...

That sounds wonderfully promising! Oh, where are those PCBs and capsules? China Post, please hurry!
Title: Re: DIY RF Condenser Mics
Post by: mhelin on April 18, 2019, 01:46:27 AM
I don't know anything about RF stuff, but regarding FM solution would it be too simple to use an oscillator like used on the common FM bugs (is it Hartley oscillator though in FM bugs only single coil is used) with the Foster-Seeley FM demodulator/discriminator so that the oscillator coil is actually the L1 in the demodulator:

http://aaronscher.com/Circuit_a_Day/week_by_week/August_2016_FM_Foster_Seeley_detector/FM_Foster_Seeley_Detector.html
There is also ltspice emulation asc file for the circuit:
http://aaronscher.com/Circuit_a_Day/week_by_week/August_2016_FM_Foster_Seeley_detector/documents/Foster-Seeley-Simulation.asc

L1 and L2 are "lightly coupled" which means the coils are in close proximity but are not really a single transformer (if I understood correctly).
The condenser capsule could replace C1, which is the parallel capacitor like in this FM bug:

http://www.next.gr/uploads/542-b12fce8580.gif

Also in some FM bugs the capacitor on base is replaced with a varicap diode which is then feed the audio signal to modulate the frequency, so I guess it's another spot for the capsule connection.

http://danyk.cz/stenice.gif
http://educypedia.karadimov.info/library/fm2w.gif

Wonder how Rode did it on NTG3...
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 18, 2019, 04:06:06 AM
I don't know anything about RF stuff, but regarding FM solution would it be too simple to use an oscillator like used on the common FM bugs (is it Hartley oscillator though in FM bugs only single coil is used) with the Foster-Seeley FM demodulator/discriminator so that the oscillator coil is actually the L1 in the demodulator:

http://aaronscher.com/Circuit_a_Day/week_by_week/August_2016_FM_Foster_Seeley_detector/FM_Foster_Seeley_Detector.html
There is also ltspice emulation asc file for the circuit:
http://aaronscher.com/Circuit_a_Day/week_by_week/August_2016_FM_Foster_Seeley_detector/documents/Foster-Seeley-Simulation.asc

L1 and L2 are "lightly coupled" which means the coils are in close proximity but are not really a single transformer (if I understood correctly).
The condenser capsule could replace C1, which is the parallel capacitor like in this FM bug:

http://www.next.gr/uploads/542-b12fce8580.gif

Also in some FM bugs the capacitor on base is replaced with a varicap diode which is then feed the audio signal to modulate the frequency, so I guess it's another spot for the capsule connection.

http://danyk.cz/stenice.gif
http://educypedia.karadimov.info/library/fm2w.gif

Wonder how Rode did it on NTG3...

It would be interesting to see how a practical build of something like his works out --To create a high quality FM mic, you need to use an  oscillator with really low phase noise....
There's a lot of expertise among the members here, so maybe someone might like to give it a try?....

I've never found schematics for any of the Rode RF mics online -- I've always assumed they've taken a similar approach to Sennheiser, but maybe not?.....
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 18, 2019, 06:35:12 AM
That sounds wonderfully promising! Oh, where are those PCBs and capsules? China Post, please hurry!

China post is usually pretty quick here in mainland UK  - apart from during Chinese New Year !  :)

A further thought on the value of R6.  There are likely to  be  some minor variations between the phantom power supplies provided by different pre-amps / recorders.
I've found using 1K or higher seems to keep the current drain below 8mA (4mA per leg)...which is OK with all the phantom powers I've tried
Going as low as 47R will  let you grab a couple of extra dB in sensitivity, but it might be pushing some phantom supplies to their limit?.......
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 18, 2019, 09:16:24 AM
I've never found schematics for any of the Rode RF mics online -- I've always assumed they've taken a similar approach to Sennheiser, but maybe not?.....
In my research about RF condenser microphones, before I started this topic here in GroupDIY, I found references to three or four different RF strategies for condenser microphones:
1. Use the mic capsule to modulate the RF oscillator directly. I think no-one uses this method.
2. Use the mic capsule to modulate an RF bridge, with and AM demodulator (Baxandall article in Wireless World 1963, and Rogs design here)
3. Use the mic capsule to modulate an FM demodulator. (Sennheiser use this method)
4. Use the mic capsule to modulate a phase discriminator. (Uwe Beis' design)

Three and four above may be the same; I don't have enough theory or electronics design knowledge to know if they are the same, or even variants of the same principle.

AKG also made an RF mic in the 1960s; I have found little about it other than mention of its existance. Ruud mentioned an "RF" mic designed by the Netherlands broadcaster (NOS?), also in the 1960s; apparently quite a lot of them were made
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 18, 2019, 09:27:44 AM
AKG also made an RF mic in the 1960s; I have found little about it other than mention of its existance........

That may have been the one that was mentioned in posts #2 and#3 of this thread......
this one maybe?...  https://ia800503.us.archive.org/3/items/bbc-rd-reports-1966-32/1966_32.pdf
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 18, 2019, 02:46:43 PM
The article I referred to, may have been published in "Omroep Technische Mededelingen nr. 1, 1961"
The Dutch radio and television relied on the NRU (Netherlands Radio Union) for the development and design of their technical installations.
As far as I know, some of the NRU microphones were designed by Professor Geluk, that is why they were often referred to as "Geluk microphones".  (Freely translated: "microphones that will bring you luck"...)
I have searched to find this article in "Omroep Technische Mededelingen", but thus far I haven't found it.
At the moment a friend of mine, with good connections in the broadcasting world, is trying to obtain a copy of the article.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 19, 2019, 12:00:22 PM

A further thought on the value of R6.  There are likely to  be  some minor variations between the phantom power supplies provided by different pre-amps / recorders.
I've found using 1K or higher seems to keep the current drain below 8mA (4mA per leg)...which is OK with all the phantom powers I've tried
Going as low as 47R will  let you grab a couple of extra dB in sensitivity, but it might be pushing some phantom supplies to their limit?.......

OK further to that - and as a result of further attempts at optimising oscillator current,  I've decided that the best compromise for the present circuitry  means several changes of component values. 
(Ruud's prototype PCBs are still valid - you just don't fit all the  part now!)

So --
• R1 is now 220k (was 100k)
• R2 is now a link (was 100R)
• R4 is not fitted (was 10k)
• R5 is now100R (was 470R)
• R6 is now 1K (was 100R)
• C1 now 220pF (was 100pF)

EDIT: I have removed the instruction to leave out R3 (1M) -- there can be some oscillator instability at certain settings if it's not fitte....

With those values, the phantom power drain is around 3.5mA per leg - so the 6K8 resistors now dissipate around 85mW each

The sensitivity now allows the noise floor to be >-65dB below 0dB FS, using a preamp gain of 20dB


Title: Re: DIY RF Condenser Mics
Post by: rogs on April 22, 2019, 04:21:34 PM
I've now built a number of prototypes - using Ruud's prototype PCB -  with different capsules and configurations.

What is becoming clear is that my attempts at reducing oscillator current as much as possible had side effects with oscillator stability and noise levels.

I've now built 3 prototypes of this latest version of the circuit: http://www.jp137.com/lts/RF.AMX5.pdf   which has proven to be the best so far.
The phantom power current drain is around 7mA - so 3.5mA per 'leg' - (only about 80mW dissipation  in the phantom power 6k8 resistors)

The noise floor is now approaching -70dB , which I don't think is too bad for such a simple circuit.

(Short audio speech sample here: http://www.jp137.com/las/RF.AMX5.wav )

My latest prototype is an RF end addressed cardioid mic...... a 34mm cardioid suitable for outdoor use maybe?....
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 22, 2019, 08:18:23 PM
I assume that noise floor was measured with a fixed capacitor replacing the capsule?
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 22, 2019, 09:17:01 PM
I assume that noise floor was measured with a fixed capacitor replacing the capsule?

No  - test was made  using the parameters I mentioned in post# 125  --

The actual test was made by playing a 1KHz tone directly into the mic capsule - via a Sennheiser headphone transducer - with the level set to record around -0.5dB below full scale into my Tascam DR100 Mk3, with the mic pramp gain set at 20dB.
 
The test tone was stopped and transducer was then removed , with the recorder continuing to record the ambient room noise (late at night).

The resultant level was around -70dB below that full scale tone recording. No other levels were changed.

There is a copy of the recording here:  http://www.jp137.com/las/FS.1k.wav
(I have removed the noises made by the removal of the transducer)

If you open the recording in your DAW and increase the gain of the 'silence'  by 40 or 50dB, you can hear the clock ticking in my room  (about 3 metres away) -- and my breathing!

I thought the noise floor was quite good for such a simple circuit?....



Title: Re: DIY RF Condenser Mics
Post by: Khron on April 23, 2019, 06:05:21 AM
The reason i asked was because i remembered Henry Spragens having compared the electrical noise floor of some Schoeps-ish mic circuits with and without the capsule attached, and the noise of "just" the air hitting the diaphragm swamped the circuit noise by quite a few orders of magnitude.

Just trying to say the SNR of this might potentially be even higher / better than what you've measured :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 23, 2019, 07:16:19 AM
The reason i asked was because i remembered Henry Spragens having compared the electrical noise floor of some Schoeps-ish mic circuits with and without the capsule attached, and the noise of "just" the air hitting the diaphragm swamped the circuit noise by quite a few orders of magnitude.

I do remember reading Henry's  comments on this....... Certainly the ambient noise 'swamps' the actual noise floor, and if I hadn't made the recording without an 80Hz  HPF the 'room rumble' would have made the result quite a lot worse!

I think there is an element of 'Schoeps noise' as well - although it's not too significant. 

As with the best of the DIY Schoeps circuits, I think we're entering the realms of the law of diminishing returns here. .....

Yes, the circuit is noisier than my reference mic (a new 'black' type Rode NT1)  but not by a lot -- and it is already ambient noise that dominates the floor....

As a comparison, the noise floor is about 20dB better than an unmodified BM800,  measured with the same setup ......

And as I mentioned earlier, I don't think it's too bad for such a simple circuit? ....
Title: Re: DIY RF Condenser Mics
Post by: mhelin on April 23, 2019, 08:51:45 AM
How do you compare the sound in comparison with a regularly DC biased one? At least the electrostatic force caused by DC bias is avoided so the output should be a lot less non-linear. Some (https://repository.tudelft.nl/islandora/object/uuid:6e23606d-c5a6-4635-8f8d-ea3b2b630763/datastream/OBJ/download) claim this is the main reason for the nonlinearities - at least in context of MEMS capsules but they are not so much different.
Title: Re: DIY RF Condenser Mics
Post by: Khron on April 23, 2019, 09:07:12 AM
At least in theory, i would imagine the resonance of the diaphragm is shifted down in frequency (as opposed to when it's constantly pulled by the bias voltage). Whether that's audible (and/or to what extent) or not, is another story...
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 23, 2019, 09:11:30 AM
I don't know anything about RF stuff, but regarding FM solution would it be too simple to use an oscillator like used on the common FM bugs Simulation.asc
This solution is discussed in the Sennheiser document, and the limiting factor here is phase noise of a free-wheeling oscillator. Getting very stable amplitude is much easier than stable phase (or frequency). That's why they decided to switch to variable demod of a fixed-amplitude signal.
Title: Re: DIY RF Condenser Mics
Post by: homero.leal on April 25, 2019, 07:08:39 PM
Just trying to say the SNR of this might potentially be even higher / better than what you've measured :)

Yes... I agree with Khron! It will be interesting to do a quick test with a small C0G ceramic cap replacing the capsule, just to check for noise floor sans ambience noise.

And also, what do you think about THD? Sergei Steshenko noted  on a personal message, that a possible advantage of RF mics could be that they doesn't need a JFET as impedance converter. If JFET gate to drain capacitance causes THD to increase and this circuit doesn't need a JFET, shouldn't THD be better? Just wondering.

Regards!

HL

Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 25, 2019, 07:37:32 PM
PCBs have arrived, but not the RK67 capsules! However, some C12 capsules intended for another project also have arrived; they might get sacrificed for the RF mic project.

Now to find some time when I'm not at work and not singing. Hopefully at the weekend after rehearsals (three long rehearsal workshops this w'end, for three different groups!).
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 25, 2019, 07:52:16 PM
At least in theory, i would imagine the resonance of the diaphragm is shifted down in frequency (as opposed to when it's constantly pulled by the bias voltage). Whether that's audible (and/or to what extent) or not, is another story...
Remember that, although different (AC vs. DC), AC bias does not eliminate diaphragm pull. It is generally lower because AC bias does not need to be of the same amplitude as DC bias, but it's still producing electrostatic pull.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on April 26, 2019, 07:10:21 PM
I'd love to have another go at Baxandall's RF mike but as a beach bum, I don't really have the facilities or inclination.

But a couple of points ...

Quote
1. Use the mic capsule to modulate the RF oscillator directly. I think no-one uses this method.
2. Use the mic capsule to modulate an RF bridge, with and AM demodulator (Baxandall article in Wireless World 1963, and Rogs design here)
3. Use the mic capsule to modulate an FM demodulator. (Sennheiser use this method)
4. Use the mic capsule to modulate a phase discriminator. (Uwe Beis' design)
Beis is a nearly exact copy of one of the Sennheiser versions.

Quote
As far as I know, some of the NRU microphones were designed by Professor Geluk, that is why they were often referred to as "Geluk microphones".  (Freely translated: "microphones that will bring you luck"...)
Baxandall references several Dutch papers which are almost certainly related to this.

Quote
With those values, the phantom power drain is around 3.5mA per leg - so the 6K8 resistors now dissipate around 85mW each
The maximum power you can draw from P48V is when you take a total of 7mA.  The voltage would have dropped to half so this is about right.

Quote
i remembered Henry Spragens having compared the electrical noise floor of some Schoeps-ish mic circuits with and without the capsule attached, and the noise of "just" the air hitting the diaphragm swamped the circuit noise by quite a few orders of magnitude.
In https://groups.yahoo.com/neo/groups/micbuilders/files/Mic%20Measurements/ (https://groups.yahoo.com/neo/groups/micbuilders/files/Mic%20Measurements/) Zephyr.pdf I show detailed noise measurements of several HiZ mikes on pages 10 & 11.  You have to join.  These are 'constant relative bandwidth' measurements eg like a 1/3 8ve spectrum analyser

You see the red  '1G noise' dropping at 3dB/8ve and the white 'resistor noise' rising at 3dB/8ve.  The 'acoustic resistance' noise is white like resistor noise.

I can't remember doing a spectral plot of Baxandall's circuit circa 1980 but it is likely that these RF circuits don't have '1G' noise.  But '1G' noise isn't all bad.  It sounds very much like ambient noise so is less objectionable than white 'resistor / acoustic noise'.

My 1980's Calrec 2050 designs have quite high '1G' noise cos they have effectively 500M or less (Good 1G resistors were rare in dem days) but you only notice this when you compare them with something better ... like the Sennheisers or Baxandall.

Quote
If JFET gate to drain capacitance causes THD to increase and this circuit doesn't need a JFET, shouldn't THD be better?
There's a Sennheiser paper that goes into detail on THD sources but the main reason for their better THD at high spl is their 'symmetrical' push pull capsule.  Dem capacitive pads used by all & sundry aren't really that audible.

Quote
Remember that, although different (AC vs. DC), AC bias does not eliminate diaphragm pull. It is generally lower because AC bias does not need to be of the same amplitude as DC bias, but it's still producing electrostatic pull.
In most (all?) RF mikes, the AC bias is push-pull so has 'no nett effect' on diaphragm tension.

I'd really like to encourage more work on Baxandall's circuit.  I can't seem to find the  transformer core data I collected when I was in touch with Beiss.  The cores that Beiss used and those I used circa 1980 are all Unobtainium for decades.

Today my starting point would be to use the
- smallest RM cores of the correct frequency,
- 1 or 2 turns for the smallest windings (the base windings for the oscillator & the switching transistors)
- 'Bifiliar' secondary winding to give about the right frequency
- emitter winding to give 25V rms on the secondary

I'm nervous about this as this is going back 30+ yrs and I don't like posting stuff I haven't actually tried.  :o

I think its also possible to do a Baxandall on rogs' circuit at 10MHz with the Spectrum Comms cores but my doodlings are all too complicated at present .. with at least 3 transformers.

I like rogs' Schoeps type output.

BTW, if you use BC560 or other 'high' voltage PNPs (BC560 at Vceo 45V is marginal but I would risk it), D2 & 3 don't have to be Zeners.  1n4148 is fine but DON'T LEAVE THEM OUT.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 26, 2019, 07:31:22 PM
In most (all?) RF mikes, the AC bias is push-pull so has 'no nett effect' on diaphragm tension.
Agreed. However, the original comment was made in view of the actual experiments, that use conventional single backplate capsules.
A dual backplate capsule could certainly be made to operate with opposite DC biases, resulting in zero diaphragm pull (at rest).
Title: Re: DIY RF Condenser Mics
Post by: ricardo on April 26, 2019, 07:43:46 PM
Quote from: abbey
Remember that, although different (AC vs. DC), AC bias does not eliminate diaphragm pull. It is generally lower because AC bias does not need to be of the same amplitude as DC bias, but it's still producing electrostatic pull.
In most (all?) RF mikes, the AC bias is push-pull so has 'no nett effect' on diaphragm tension.
Duu..uh!  Abbey is right.

The diaphragm 'pull' on an RF mike is a 'rectified' version of the RF voltage.  So a 20V rms RF bias (28Vp) would have a nett 'pull' equivalent to about 14V DC.  But the 'bias effect' on a Baxandall or rogs mike, is likely about 56V ... making many dodgy assumptions about circuits  8)
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 26, 2019, 08:32:30 PM
Yes... I agree with Khron! It will be interesting to do a quick test with a small C0G ceramic cap replacing the capsule, just to check for noise floor sans ambience noise.

A quick test with the C12 capsule replaced with a  100pF mlcc C0G capacitor shows that  the noise floor is around  -84dB using the same gain set up  as previously.
This recording of the noise floors ( amplified by +75dB in my DAW) in this file :  www.jp137.com/las/RF.AM.noise.floors.75db.wav 
is in 3 parts...all made using the external mic input of my Tascam DR100Mk3, with the gain set to minimum (+20dB) and no pad. 
• The first 8 seconds is of the un-terminated mic input  (c. -87dB )
• The next 10 seconds is with  the mic circuit  'output' section active only  (D1 cathode grounded) ..(c. -89dB)
• The last 10 seconds is with the ground removed from D1 cathode.. (c. -84dB)

So yes, the noise floor is improved with the capsule removed  .... The PCB assembly was not fitted within the grounded mic body for this test - and you can hear a small amount of hum in the final section.
So it may actually be slightly better than the recording suggests, when fitted within the mic casing itself..... 
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 26, 2019, 08:40:17 PM
However, some C12 capsules intended for another project also have arrived; they might get sacrificed for the RF mic project.
You may need to make C4 a little larger when used with a C12 type capsule. 
My C12 capsule measured around 90pF (as opposed to around 65pF for a K67, and around 56pF for a K47) so I fitted C4 as 2x47pF in parallel.
(A single 100pF would also probably work OK) .

This should give a value for the 2 series bridge capacitors of around 47pF,  which is ideal for a Spectrum 5u3uH inductor, when used with a 10MHz oscillator.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on April 26, 2019, 08:59:28 PM
Those are spectacular noise figures. As good as good studio mics and approaching ultra low noise instrumentation mics (B&K say 4 dB above reference 0 dB for their very lowest noise mics).

Thanks for the comments on the value of C4 with a CK12 capsule. I had assumed as much. I haven't measured my CK12s from Wgtcenter yet, but the datasheet says 70 pF +/- 4 pF.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 27, 2019, 04:21:53 PM
And also, what do you think about THD? ........
....If JFET gate to drain capacitance causes THD to increase and this circuit doesn't need a JFET, shouldn't THD be better? Just wondering.

Like many folk, I simply don't have a transducer that  has a low enough THD in its own right to allow for accurate THD figures to be measured.
What I can do is to take the best one I have  - a Sony 7506 headphone earpiece transducer - and do some comparison tests.

It's simple enough to create a low distortion source tone --I used a Neutrik MR1 minirator, in conjunction with  Hypex UCD power amp. The output of that is fed to the line inputs of my Sound Devices USBPre,  and the output of that is opened up in a software spectrum analyser. That gives a THD reading of c. 0.002% for the source.

Connecting the output of the power amp to my Sony phones instead - and connecting the several  test mics in turn to the Sound Devices mic preamp - will show that introducing the transducer - and a test mic - will derate the THD distortion by a factor of around 10.
How much of that is down to the mic and how much to the Sony transducer, I have no way of measuring?....

Nevertheless, taking comparison THD  readings  with several mics show that the new experimental RF mic actually seems to perform pretty well....

Using a 1KHz sine source of around -12dB, I placed each mic in turn close to the headphone transducer, and set the SD preamp in each case to read -12dB on the meter.
Using that set up, I obtained the following THD readings...

• Shure SM58 - 0.028%
• AKG D202-E1 - 0.025%
• Rode NT1 (new 2014 model) - 0.015%
• RF.AMX5 prototype mic fitted with C12 type capsule - 0.012%
• RF.AMX5 prototype mic fitted with K67 type capsule - 0.016%
• C12 capsule with 'standard ' Schoeps type  JFET impedance converter - 0.019%
• K67 type capsule with 'standard' Schoeps type JFET impedance converter - 0.018%

So there's not a lot in it - at least with this set up. 
Neither AKG or Shure publish THD figures in their specs, and Rode only quote the THD (around 1%) for max output, so the manufacturers data doesn't help much here..

I do think the experimental RF mic comes out pretty well for THD, on a par with the other mics....



Title: Re: DIY RF Condenser Mics
Post by: RuudNL on April 28, 2019, 04:17:29 AM
Wouldn't it be an idea to use one or more varicaps to simulate the capsule to do distortion measurements?
I suppose you would need a varicap with a high C/U ratio, to operate the varicap in the most linear region.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 28, 2019, 05:51:14 AM
I remember that Uwe Beis used a varicap construction to simulate the capsule in his design (see the bottom of this page: https://www.beis.de/Elektronik/HF-Mic/HF-Mic.html )  but discovered that it wasn't particularly linear -- which doesn't really help with THD measurements !  :)

I think that the fact that the prototype holds up quite well against other mics - including commercial ones -  in the comparison tests gives some confidence that THD is not likely to be a problem.

The only possibility I can think of that might create linearity  differences between samples is if the 2 bridge capacitors (the capsule and C4 ) end up being identical in value.  That might mean that there could be insufficient 'standing' RF voltage from bridge imbalance to allow the rectifier to always conduct in a linear way.   
Extremely unlikely in my opinion, and any effect related to that is further minimised by using a BAT85 or other Schottky diode as the rectifier....
Title: Re: DIY RF Condenser Mics
Post by: homero.leal on April 28, 2019, 10:26:28 AM

I do think the experimental RF mic comes out pretty well for THD, on a par with the other mics....

Thank you very much Rogs for your work, tests and info that you have shared with the group. Great work, greatly appreciated!

Will try it very son...

Kind regards!

Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on April 28, 2019, 12:27:07 PM
The only possibility I can think of that might create linearity  differences between samples is if the 2 bridge capacitors (the capsule and C4 ) end up being identical in value.  That might mean that there could be insufficient 'standing' RF voltage from bridge imbalance to allow the rectifier to always conduct in a linear way.   
This comment prompted me looking again at your schemo. I think that, with two equal caps, the resulting RF voltage would be zero; as a result, the audio output would be a full-wave rectified version of the acoustical signal. I believe you actually operate your circuit with the detector de-tuned enough to stay on one side of the V vs. F response.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 28, 2019, 01:27:48 PM
This comment prompted me looking again at your schemo. I think that, with two equal caps, the resulting RF voltage would be zero; as a result, the audio output would be a full-wave rectified version of the acoustical signal. I believe you actually operate your circuit with the detector de-tuned enough to stay on one side of the V vs. F response.

Yes, the bridge always needs to be slightly out of balance  in this configuration..... Baxandall himslef makes that very point on the first page of his paper:  http://www.jp137.com/lts/Baxandall.RF.mic.pdf

Using a BAT85 diode means that there only needs to be a minimum amplitude difference of around 300mV or so  to allow the rectifier to conduct effectively, and the amplitude variations created by the modulation are only in the order of a few millivolts on top of that, so it doesn't need to be much of an imbalance in the bridge. 
Ideally the capsule capacitance needs to be slightly larger than the series capacitor for maximum sensitivity, but in all the 4 prototypes I've built so far there has never any issue with the bridge being  too close to being balanced to cause any problems.

As Ricardo has mentioned earlier, the next step would logically be to move on to a balanced bridge design - as Baxandall did himself - but in the meantime, this simple version has so far worked  better than I ever thought it would !
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 28, 2019, 01:34:47 PM
Thank you very much Rogs for your work, tests and info that you have shared with the group. Great work, greatly appreciated!
Will try it very soon...
It's been fun trying it all out..... Baxandall's original concept is very simple,  and I thought it might be worth trying it out as a simple unbalanced bridge first....

Using these inductors, I managed to get a much higher 'Q' from the system than I first imagined, and it has turned out to give much better results than I  originally expected. .... Definitely worth the effort, IMHO!  :)

.....And a big thanks to Ruud for his test PCB design (Gerber files attached to post #32) ...Using those PCBs for protoypes is a lot simpler than my original stripboard version
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 28, 2019, 02:26:28 PM
I've made a few notes on the inductors, and the 'tuning' options I adopted for them in this project:

www.jp137.com/lts/RF.AMX5.inductor.assembly.pdf
Title: Re: DIY RF Condenser Mics
Post by: ricardo on April 28, 2019, 06:09:13 PM
rogs, do you by any chance have measured voltages, DC and hopefully RF, at various points of one of your circuits?

I'm still doodling to see if your circuit can be Baxandallised simply using the Spectrum Comms inductors.
Title: Re: DIY RF Condenser Mics
Post by: rogs on April 28, 2019, 07:59:14 PM
rogs, do you by any chance have measured voltages, DC and hopefully RF, at various points of one of your circuits?

I'm still doodling to see if your circuit can be Baxandallised simply using the Spectrum Comms inductors.

Using the component values shown in the RF.AMX5 schematic ( copy here: http://www.jp137.com/lts/RF.AMX5.pdf ) --- fitted with a K67 type capsule -  I measure the following voltages at different points:

DC volts (with respect to system gnd) ...
• 16.4V at the junction of R6 and C7. 
•    9.3V at the junction of R6 and C6.....So the oscillator and associated circuitry draws around 7mA.

AC volts at 10MHz  (with respect to system gnd)

• 13V p-p at the emitter of Q1
• 12V p-p at the T1 primary end of R5
• c. 15V p-p at each end of T1 secondary ( slightly different values at each end)
• 1.5V p-p at T2 primary  (junction of C4 and the capsule)
• 2.5Vp-p at D1 anode.

These voltages are approximate ... I'm using a x10 10MOhm/15pF oscilloscope lead,  which  loads the measuring points by a small amount at 10MHz.
(The actual calibration of the inductors is carried out audibly - by tuning the cores for maximum audio level - with a tone applied to a  test transducer next to the capsule )

The measured voltages for my C12 type prototype -  which presents a bigger capacitive load to the bridge - were somewhat lower, even though that version is  actually slightly more sensitive....
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 01, 2019, 05:09:55 PM
A couple of points that have been mentioned throughout the thread -- the high oscillator current and the lack of a balanced audio output...

It suddenly occurred to me that there may be a single change that could help solve both those problems ....

That is to fit a JFET 'infinite impedance' detector in place of the diode rectifier.....

Advantages include:
• Much higher Q from T2   - load is now only a JFET gate
• Balanced output by using the JFET as an audio phase splitter as well.

Fitting that idea to the existing circuit , and the voltage swing now available at T2 secondary when the system is tuned can be way too high for the FET (15 -20V p-p) .

One obvious solution- increase R6 to reduce the oscillator DC supply.

Result  is  the oscillator current is now cut by around 50% -- and the sensitivity is increased by about another 6dB

Provisional schematic  is attached... (It may be that some of the new values still need to be 'tweaked'...)

The only disadvantage so far is that it's now a bit more tricky to get it all to fit onto Ruud's original prototype PCB .... :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 01, 2019, 06:18:31 PM
Well, i was sorta-kinda close... ;D

Not that i know anything about oscillators, but i was starting to wonder - would it help anything, switching (no pun intended) to a JFET or a MOSFET for the oscillator itself (Q1)?

Jokes aside though, great progress / evolution!  :D
Title: Re: DIY RF Condenser Mics
Post by: homero.leal on May 01, 2019, 10:17:21 PM

The only disadvantage so far is that it's now a bit more tricky to get it all to fit onto Ruud's original prototype PCB .... :)


But now, as there is a JFET in the signal path, THD may increase specially if jFET has a high gate-to-drain  capacitance. :-\

Also, no biasing requred for JFET Gate?

Regards!

HL


Title: Re: DIY RF Condenser Mics
Post by: rogs on May 02, 2019, 04:44:49 AM
But now, as there is a JFET in the signal path, THD may increase specially if jFET has a high gate-to-drain  capacitance. :-\

Also, no biasing requred for JFET Gate?

Regards!

HL

I haven't measured any changes in THD yet -- although as I mentioned earlier, I'm not sure how much of the distortion in my set up is from the audio transducer anyway?

The FET gate is effectively grounded, and I can measure c. 5V across both the drain and source resistors - so there's around 1mA of current  flowing in  the FET. 

 With the reduced oscillator current, the lower modulated 10MHZ carrier presented to the gate from T2 secondary should allow the FET to operate within it's linear region......

Another advantage is that there are now fewer volts across the capsule itself.....

If this does turn out to be a backward step - distortion or noise wise --we can always go back to the diode! ....That's the beauty of an experimental project !    :)

Title: Re: DIY RF Condenser Mics
Post by: rogs on May 02, 2019, 05:53:56 AM
Well, i was sorta-kinda close... ;D
:) :)

Jokes aside though, great progress / evolution!  :D
Thanks -- yes, it's turning out to be an interesting project ...it certainly now performs way better than I first thought it would..

I'm looking forward to seeing how the results from others  experimenting with the same idea turn out.....
 
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 02, 2019, 09:53:34 AM
The 'FET detector' may be a big step in the right direction!
Have to try this as well! (And maybe design a new PCB for it...)
I think you have almost reached the maximum performance from such a simple circuit.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 02, 2019, 11:45:28 AM
But now, as there is a JFET in the signal path, THD may increase specially if jFET has a high gate-to-drain  capacitance. :-\
Cgd is a source of THD in Hi-Z (DC bias) condenser mics, not with this implementation of RF bias. You can't generalize.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 02, 2019, 03:40:38 PM
The 'FET detector' may be a big step in the right direction!
Have to try this as well! (And maybe design a new PCB for it...)
I think you have almost reached the maximum performance from such a simple circuit.

Well the initial test results are encouraging! .......

At the moment, I'm using BM800 bodies for my prototypes, so there's quite a lot of  PCB 'trimming' required each time....
I've been testing out a configuration with a capsule mount that allows me to build a 34mm  capsule into an end address cardioid mic ... and the body and headbasket of the BM800 are ideal for that...

So, Ruud if you do consider a new PCB, a rectangular one of 65mm x 35mm  which will fit straight into 'standard' BM700/800  body would be a real help.
I realise it would probably mean some upright components, and wouldn't be as elegant as your first  design....... but it would save a lot of 'hacking'....
....Just a thought !  :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 02, 2019, 04:22:55 PM
"Only" using 7.5mm pitch at least for diodes would free up some space. I use that pitch for 1/4W resistors too, on my boards - a smidge "tight" for some tastes, but saves some board space too :)


I realise it would probably mean some upright components, and wouldn't be as elegant as your first  design....... but it would save a lot of 'hacking'....
....Just a thought !  :)

PS: If someone could confirm the outside dimensions and pin grid / pitch of those IF transformers, i'd be ready to whip up a quick board design ;D And i have my older boards to consult for holes to fit precisely BM800 bodies  ::)



[A couple hours later]

Found some apparently-similar-footprint inductors in one of the stock Eagle libraries, so... ;D

I went with a ground plane on both sides, as well as some via "shielding" of the signal going to the JFET, to hopefully minimize any pickup from the oscillator section. That may very well introduce some extra capacitances here and there, so there's a good chance some cap values in the front-end might end up needing tweaking.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 02, 2019, 09:11:19 PM
Looking good !  :D

Couple of  points .....
• I think you have the values of the 100n and the 1n caps at the top of the board reversed ?-- No big deal (the're the same size!)

• The Spectrum IF cans are supposed to be identical to the Toko 10K series  - so on a 7mm matrix
(see here: https://www.bec.co.uk/downloads/10K%20Series.pdf )...

 The 1.5mm pin size shown for the locating lugs is a bit tight... those holes really need to be c.1.7 or even 1.8mm

• The 47uH inductors are more like 1/2W resistors in size. I think they would need to be fitted on the back of the PCB in the present configuration.

• Same with the 47uF 50V electrolytic... ideally it would be nice to have enough room to have the option of being able to use the 8x8mm  Multicomp MCMHR series ( http://www.farnell.com/datasheets/1520837.pdf )  if possible ?

Am I right in thinking that because it's an Eagle design, you have to buy the PCB from them... They don't give you the option to generate standard Gerber files?......Or have I got that mixed up with someone else?

But as I say - looking good..... nice job!  :)

Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 04:51:38 AM
- That's just where the designation is on the silkscreen, just outside the caps' outline; i recreated the latest schematic.
I just figured i'd go with "functional" instead of "pretty", at least the first time around ;D

- I'll have to edit that package a bit then, no biggie. [Edit - turns out it's quite a bit bigger; the initial one was around 7x7mm-ish)
- Fair enough, i'll spread things out a bit in the output section.

You must be confusing it with some other package. Unless they went that route with later versions - i'm still on 6.3.0 :P It's got a Gerber export script, which i've used before to get boards done in China.

How's this, then?
https://www.dropbox.com/s/c3hx3a0w3odip2w/Screenshot%202019-05-03%2014.04.06.png?dl=0
[I tried attaching an updated version here, but it "failed security checks" - go figure  :o )
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 07:29:05 AM
Looking even better!  :D

Couple of further points...

• The capacitor across the top 4k7 needs to be 1nF, and the one connected to the top BC560 base needs to be 100nF....  just an ident change, so no biggie...

• Because of the height and position of the coil cans, the PCB will need to be mounted below the frame.... I think that the 10nF caps  in their current location may hit the corner frame  mountings?... and maybe the coils at the other end?...

(There's a picture of my latest modified PCB from Ruud's prototype design which shows the problem here: www.jp137.com/lis/BM800modRUUDpcb.jpg )

Like you I can't seem to attach anything to posts at the moment because of 'security issues' ...???

• It would be nice if you could add an extra capacitor (2.5mm spacing)  across T1 secondary?
There's  a schematic showing the extra cap ( C3 - marked 'SOT') - here:  www.jp137.com/lis/RF.AMX8.jpg

Good news about the Gerber files.....  my mistake!
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 07:54:11 AM
There's no ident change necessary, trust me :D

I shuffled the output components around a bit more, and the "top" edges of the cans (ie. towards the headbasket) are 3.8mm from the edge of the board.  Would they need to be even further away from the edge?

Is the server storage for attachments full, or something? :o

[Later edit] Reshuffled the oscillator section a bit too...
https://www.dropbox.com/s/l5neky8tw94i0rf/Screenshot%202019-05-03%2016.32.27.png?dl=0
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 09:57:58 AM
There seems to be number of slightly different BM800 frames around.....

I've taken a photo of the worst one I can find (corner 'fill in' wise ) and there is a jpg of the internal dimensions of it here:

www.jp137.com/lis/BM800.frame.dimensions.jpg




Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 10:02:21 AM
Hmmm... With those two caps between the cans,  ~25mm's about as close as i can squeeze them together.

But how "tall" (left-to-right, in that frame photo) are those corner-fill-ins, on the top and bottom corners?

https://www.dropbox.com/s/k1ylsyqh6vhxn4c/BM800frame.png?dl=0


EDIT: Actually, nevermind - i scooted the two mounting holes a couple mm to the left (towards the cans), and shortened the other end of the board by 1-2mm.

So now, the centers of the mounting holes are a smidge under 25mm from the headbasket-side of the cans, so about 1mm to spare before those corner-pieces :) The other end might still be under question, though...
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 10:50:44 AM
The dimensions you illustrated are c.5.5mm....

The one thing that I'm still not quite clear about.... The IF cans footprint is on a pretty 'standard' 7mm spacing...

What I'm not sure about is how big the holes are.

The  two mounting 'tags' on the cans  need around 1.7mm - or even 1.8mm if those will fit!
The pin hole sizes need to be 1mm ideally...
You might get away with 0.9mm for guaranteed new cans, but if you ever need to re-use a can, the holes need to be 1mm...

They look quite small on your schematic??
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 10:59:01 AM
https://www.dropbox.com/s/1isek1kegec5x4k/Screenshot%202019-05-03%2017.55.40.png?dl=0

For the cans, i went with the dimensions in the Toko datasheet you linked on this page, but i can edit the footprint to make the holes bigger, sure :)

But now i gotta get my rear end to rehearsals, we've got a gig next friday to prepare for ;D
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 11:17:17 AM
Looking good now! ....
Yes I think the TOKO hole dimensions are a bit too tight for the Spectrum Coils.....I had to drill out the tag mounting holes on Ruud's prototype PCB....
That's not too bad  with a single sided PCB -- not such a good idea for plated though holes on double sided board!   :)

And 1mm holes (with appropriate sized pads) for the pins will give bit of leeway for those coils that are 'less than perfect'....
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 01:25:36 PM
Can pins 1.8mm drill, winding pins 1mm drill.

https://www.dropbox.com/s/4ruzx87w8799aly/Screenshot%202019-05-03%2020.22.57.png?dl=0

PS: The board outline is 63 x 37mm, and the screw holes are 28.5mm away from the left-side edge (can edge).

Also, i "fattened up" the footprint for the output series inductors (10mm pitch, plenty of space on the sides).
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 04:08:51 PM
Looking better and better!  :)

I think the only query I have now  is the overlapping of the two negative terminal holes for the different sizes of electrolytic  -
It looks like the nearer hole would take out about 50% of the further hole pad!... maybe an optical illusion?
The 3.88mm (larger) spacing is the preferred option if one  of the holes does have to go....

I think it's maybe time to try it out ....  :)
   
Are you going to publish the Gerber files? .. .. I'd love to order a few from China ASAP..
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 04:24:27 PM
Running Itead's DRC does indeed give an error on that overlap, but it's a "drill distance" error, and it looks like the two holes would overlap the tiniest bit. Keep in mind the holes are 0.8mm diameter.

https://www.dropbox.com/s/6ztknaex5971beb/Screenshot%202019-05-03%2023.22.47.png?dl=0

I wonder, does your PCB fab of choice have some sort of checking-service before placing the actual order to make sure it's feasible?
Or some way to mention that this one detail is "ignorable"?

And sure, exporting the Gerbers is only a few clicks away. Should the silkscreen only contain the component outlines and values, or..?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 04:54:27 PM
I was going to order some prototypes from JLCPCB ...they do check the Gerber files, but I'm not sure what happens if they encounter what they see as 'errors'..

Re: the ident ... the ideal option is to include  the outlines and component references as shown on the schematic ...'R2' -- 'C4' -- 'T2'.
That way it's simple to change the  values on the schematic -, if necessary - without making the PCB then show the wrong value.....

But if that involves a lot of work, I think probably the outlines only, with a component reference listing on a separate document..

I can add the schematic references onto the latest  layout png if that helps at all?...
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 05:07:06 PM
When i drew the schematic (you have to do that first, in Eagle), i used the same designators as in the latest schematic, so that should all be fine :) I'll just have to "drag out" the designators so they don't overlap the outlines or pads.

https://www.dropbox.com/s/q7dp73lcvvpbbq4/Screenshot%202019-05-04%2000.04.40.png?dl=0

How's this, for a silkscreen then? (Ignore the little "+" things, those are just the "origins" of the designator items, they don't show up in the Gerbers)

https://www.dropbox.com/s/po13lzckqoqyvpq/Screenshot%202019-05-04%2000.13.51.png?dl=0
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 05:45:53 PM
Superb !  Exactly what I had in mind ( see here:  www.jp137.com/lis/RF.AMX8%20ident.jpg     :)  )
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 06:05:35 PM
Great minds think alike, or... something-something ;D

So let's see then - top silkscreen, top soldermask, top layer, bottom layer, bottom soldermask, and drills.  That should suffice, since there's no bottom silkscreen, right?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 07:38:48 PM
Sounds good to me...
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 07:51:54 PM
Looks good on JLC's Gerber viewer :)

https://www.dropbox.com/s/sc1y3m0l4uvwxs8/RFmic_gerber.zip?dl=0

I also added some "soldermask stop" areas on the top of the board, to help ensure as good a ground-contact with the chassis as possible. Not sure if or how much that does, but it definitely hasn't hurt my Schoeps + DC-DC BM800 builds.

Granted, if one was to "cheat", there's a script somewhere for panelizing (ie. copying multiples of one board within a footprint, and correcting the designators - in this case, one could get double the boards for the same price, since they fit within the 10x10cm "cheap limit" :D  )
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2019, 08:11:28 PM
Great stuff - and thanks for the Gerber zip files ..... I shall order 5 from JLC ( I won't try and 'cheat' -- yet!   :) )

Thank you so much for your efforts ... If this board works as well as it should, it'll be a huge step forward in making  this simple (but hopefully not bad!  :) ) 'hobby' RF biased mic into a cost effective buildable  reality....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 03, 2019, 08:13:53 PM
Can I suggest you add Rogs' circuit designation (RF.AMX8) to the silkscreen layer, to we can distinguish between different versions of the PCB.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2019, 08:18:39 PM
As i mentioned, with the added groundplane on the top layer, i fully expect some capacitances to possibly need tweaking, but i guess time will tell :)

Compared to the tacked-on mod-board, i'd expect a bit less of the HF / RF to end up in the JFET (or the output PNPs), so at least in theory, the output should(?) end up a bit "cleaner". Whether that's audible or not, is a totally different discussion ;D

Can I suggest you add Rogs' circuit designation (RF.AMX8) to the silkscreen layer, to we can distinguish between different versions of the PCB.

That's actually a good point :) Zip file updated. I went "fancy" and added that on the soldermask layer ;D
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 04, 2019, 04:24:12 AM
I was awake last night and thought about the 'new' circuit. (Sometimes that happens...)
The part from the FET to the output looks very much like the classical Schoeps circuit.
Now, in most Schoeps circuits, there is a trimmer potentiometer to set the FET bias.
In the 'new' circuit the bias of the FET is the result of the FET parameters and the current through R4.
Wouldn't it be better to add a trimpot to set the working point of the FET, so that it can work in the most linear region?
Just a thought...
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 04, 2019, 08:17:47 AM
I agree that the new infinite impedance detector  FET - especially as it's  being used as a phase splitter as well - looks very much like  the Schoeps configuration..
However in this case, there is no DC bias of the FET gate.
The gate is grounded - from a DC point of view - by the secondary winding of T2. 

The 10MHz modulated carrier signal created by the bridge imbalance is amplified by the turns ratio (and tuning ) of T2.
This signal is presented to the gate and will swing + and - to ground.

It's possible to measure what appear to be a static DC bias across the 4k7 drain and source resistors, which suggests that a 'DC' current of around 1mA is flowing in the FET.
In reality this is actually using a DMM to look at the 'rectified' modulated carrier, which is effectively turned on and off  every half cycle.

So in effect, the optimum setting is determined by tuning for the maximum amplitude of the waveform presented at the gate...

The actual THD appears at present to be actually slightly better than a conventional Schoeps equivalent  circuit (using the same type of  capsule, FET and transistors) - even when that circuit has been optimised with a gate bias adjustment resistor.

The detailed maths of the infinite impedance detector are way above my 'pay grade'   :) ....but there are a number of articles online which describe the concept. I first got the idea from reading Rod Elliott's audio website (see here:  http://sound.whsites.net/articles/am-radio.htm   - about half way down the page ) which is quite readable!
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 04, 2019, 10:46:17 AM
I would like to try this. (Decouple the gate from ground for DC)
And see if there could be any improvement. Or not...
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 04, 2019, 11:31:27 AM
Hmmm..I'm not sure that would do very much?.......

Biasing the FET accurately in the 'standard' Schoeps mode will allow the (relatively) tiny voltages created by the capsule capacitive changes to be applied accurately to the most  linear part of the FET curve.   
I've found a 1MOhm multi turn pot across the source resistor can be adjusted to  help reduce second harmonic distortion.....

In this AM modulation case we have severall volts of AC being applied to the gate.... The challenge  seems to be to pitch the peaks of that voltage into the 'right' part of the FET  curve......  That 'sweet spot' is simple enough to find using your 'tune by tone' technique....

EDIT -  Not quite true ...... if the 'sweet spot' was not at  maximum amplitude (and therefore maximum Q), there would be 2 'sweet spots' - one each side of the max amplitude setting - and there isn't! 
It is easier to see that T2 is actually tuned  for maximum 'Q' - and therefore maximum  amplitude  -  by applying the 'scope to a lower impedance measuring point .. namely the bridge junction. 
You can then see there that the maximum voltage across T2 primary does coincide with maximum sensitivity.   So the voltage at the FET gate is quite a lot higher - and is essentially a linear sine.
The  drain and source resistors  provide negative feedback to help keep things under control.

The point is, there is no actual DC gate bias to adjust  in this configuration - as far as I can see...  :)

THD is measured at around 0.02%  at a nominal  -10dB output from my SD preamp. How much of that is distortion from my Sennheiser headphone transducer, and how much from the mic I can't measure....
 
 
 
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 05, 2019, 05:17:33 PM
...  fit a JFET 'infinite impedance' detector in place of the diode rectifier.....

Advantages include:
• Much higher Q from T2   - load is now only a JFET gate
• Balanced output by using the JFET as an audio phase splitter as well.

Fitting that idea to the existing circuit , and the voltage swing now available at T2 secondary when the system is tuned can be way too high for the FET (15 -20V p-p) .

One obvious solution- increase R6 to reduce the oscillator DC supply.

Result  is  the oscillator current is now cut by around 50% -- and the sensitivity is increased by about another 6dB

Provisional schematic  is attached... (It may be that some of the new values still need to be 'tweaked'...)

The only disadvantage so far is that it's now a bit more tricky to get it all to fit onto Ruud's original prototype PCB .... :)
rogs, this is indeed a major development.  But I can't help pointing out that you could replace all the RF stuff with a single 1G resistor  8)

Jokes aside, you need to ask what you get for this complexity.
You should certainly get the first but you must be careful not to lose the second.

Making the Schoeps output 'balanced' introduces R10 4k7 and its noise. (The other Schoeps noise sources are much less)

Whether this is significant depends on the sensitivity of the source and its inherent noise output.  A louder source makes this noise less important as  its noise output drowns the extra resistor noise.  But if the source is very low noise .. like a good LDC .. the resistor noise can become dominant.

Zephyr.pdf in https://groups.yahoo.com/neo/groups/micbuilders/files/Mic%20Measurements/ (https://groups.yahoo.com/neo/groups/micbuilders/files/Mic%20Measurements/)
has measurements of a very low noise variable pattern CK12 using Zapnspark's SP2PCM circuit.  You have to join.

You clearly see the effect of the extra resistor as patterns are switched because the LDC has low inherent noise.  The omni has the most noise cos the 2k2 noise is not diluted.  Cardioid doesn't have this resistor at all and Fig8 has it diluted.

A single noise measurement doesn't show this cos this is a HiZ mike and the '1G' noise dominates A-weighted measurements.  But the hissy resistor noise may be the most irritating noise cos the '1G' is very similar to ambient noise so less noticeable and this is in the critical 4kHz area where our hearing is most acute.

My guess is that RF mikes don't have '1G' noise .. ie quieter .. but the remaining noise is white so may be more noticeable.

rogs, is AMX7 louder than a HiZ Schoeps using the same capsule?

Is anyone in Oz going to make this with a CK12 capsule?  I'm happy to do detailed measurements of response & noise if I can borrow the mike for some weeks.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 05, 2019, 06:14:16 PM
rogs, is AMX7 louder than a HiZ Schoeps using the same capsule?
Yes - by around 6dB.

The lowest noise commercial mic I currently have is a Rode NT1 (latest 2014 incarnation) and that  has a published Equivalent  Noise Level (A weighted) of 4dB ...The prototype RF.AMX7 has a noise floor  - measured, sadly, without the benefit of an anechoic chamber - as approximately  6dB worse than that.

But.... the RF.AMX7 is around 15dB more sensitive than the Rode - so it already comes over as a pretty quiet mic!

The most impressive improvement I've noticed from trying out the infinite impedance detector comes from  effectively releasing the full 'Q' potential of the tuned  T2 secondary, by removing the diode detector load.

The next 'tweaks' to be tried are  to find the optimum balance between the oscillator output level - the FET load resistors - and the available phantom power.

The inductors are specified as having a Q of 85 at 7MHz . We are using them here at 10MHz, so it will probably be a bit less.

We're also using them 'backwards' --( i.e. it's the secondary we need centre- tapped, not the primary.) 

T2 primary has a Z of around 750R, and with the impedance of the 2 bridge caps  at around 250R each, T2 doesn't sem to load the bridge too much.
The turns ratio of these transformers - in the mode we are using them here - is around 1:5, so there's quite lot of 'noise free' gain available .....certainly way more than I managed to get from my first prototype, using TOKO cans...

It seems to be turning out to be quite a useful little mic, for such a simple concept . (Thanks, Mr. Baxandall!  :) )


Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 05, 2019, 10:27:19 PM
I was awake last night and thought about the 'new' circuit. (Sometimes that happens...)
The part from the FET to the output looks very much like the classical Schoeps circuit.
Now, in most Schoeps circuits, there is a trimmer potentiometer to set the FET bias.
In the 'new' circuit the bias of the FET is the result of the FET parameters and the current through R4.
Wouldn't it be better to add a trimpot to set the working point of the FET, so that it can work in the most linear region?
Just a thought...
In the Schoeps circuit, bias has to be trimmed for putting the FET in its most linear region.
In the RF circuit, demodulation is a consequence of the non-linear characteristic of the FET; an extreme case would be a perfectly linear FET, where no demod would result.
I think this has to be investigated; I guess the signal at the source of the FET is not the same as that at the drain (source output is under heavy NFB when signal at the drain follows the x power 3/2 equation).
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 06, 2019, 04:25:47 AM
I think this has to be investigated; I guess the signal at the source of the FET is not the same as that at the drain (source output is under heavy NFB when signal at the drain follows the x power 3/2 equation).
The drain and source measure the same ...... using a DMM  one can measure around 4.5V 'DC' across both source and drain resistors...I appreciate it's not actually 'DC'  but the 'half' wave' rectified carrier which appears across both 1nF capacitors.
The audio appears to be the same  amplitude at each output  (inverted with respect to each other of course)

The drain and source of the J113 FET are interchangeable - according to the spec -I must try one with the drain and source connected the other way round!

I'm no theoretical engineer -- I leave the maths to the clever guys !  :) --   but combining this infinite impedance JFET detector with a phase splitting role appears to generate a symmetrical output from both drain and source?

It seems to me as if this is more like a switching 'sample and hold' technique being employed here... but I may well be wrong of course!  :)

Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 06, 2019, 06:48:56 AM
The audio appears to be the same  amplitude at each output  (inverted with respect to each other of course)
Indeed; that's the way it must be, since there is no current in the gate. I got somewhat carried away. :-[


Quote
It seems to me as if this is more like a switching 'sample and hold' technique being employed here... but I may well be wrong of course!  :)
I would really think it's the non-linearity of the transfer function that demodulates. Investigating that takes thorough experimentation that is not easy to carry out in DIY conditions.
A first approach would be to evaluate the audio output voltage as a function of RF level at the gate (for a constant modulation index, of course).
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 06, 2019, 10:39:14 PM
Quote
is AMX7 louder than a HiZ Schoeps using the same capsule?
Yes - by around 6dB.

The lowest noise commercial mic I currently have is a Rode NT1 (latest 2014 incarnation) and that  has a published Equivalent  Noise Level (A weighted) of 4dB ...The prototype RF.AMX7 has a noise floor  - measured, sadly, without the benefit of an anechoic chamber - as approximately  6dB worse than that.

But.... the RF.AMX7 is around 15dB more sensitive than the Rode - so it already comes over as a pretty quiet mic!
We can argue about measurements & noise weightings but this make your mike

4dBA + 6 - 15 =sorta  -5dBA spl

assuming 2014 NT1 has a 'similar' (made in Sydney) capsule  to your CK12 clone.  So yours is one of a literal handful of mikes with sub 0dBA spl noise  :o

Quote
The most impressive improvement I've noticed from trying out the infinite impedance detector comes from  effectively releasing the full 'Q' potential of the tuned  T2 secondary, by removing the diode detector load.

The next 'tweaks' to be tried are  to find the optimum balance between the oscillator output level - the FET load resistors - and the available phantom power.
7mA is the 'optimum' power to take from P48V ...  with 5mA probably a safe maximum to cater for old N*** sh*t  8)

Your noise results show more S/N improvement won't be needed.

However, "15dB louder than NT1" means a 10-20dB pad would certainly be useful at times to prevent the mike overloading.  Maybe just a resistor across T2 secondary.

You should also check that the increased sensitivity doesn't kill your HF response too much from the high Q

Quote
My guess is that RF mikes don't have '1G' noise .. ie quieter .. but the remaining noise is white so may be more noticeable.
How does the noise of the NT1 sound compared to AMX7?  1G noise is 'red' ... so sounds like ambient noise in a quiet place but 'muffled'.

Can you post 15s of each as a WAV file with the same preamp gains?

For noise measurements, I wrap the mike in towels, put them in a stout wooden box and bury the box under a pile of blankets on a bed with a heavy weight on top.  I record 15s late at night and listen to each recording to make sure there are no insects & distant cars.  Fridge, fans & clocks disabled.

At Calrec, I did the same except the box sat on top of our anechoic wedges.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 07, 2019, 07:55:16 AM
• It would be nice if you could add an extra capacitor (2.5mm spacing)  across T1 secondary?
There's  a schematic showing the extra cap ( C3 - marked 'SOT') - here:  www.jp137.com/lis/RF.AMX8.jpg
Forgive a simplistic question from a mere 'mechanical', but what is this C3 ('SOT') for, and what value had you in mind for it?

Also, following Ricardo's suggestion earlier today, that there could be an option to add a resistor across T2 secondary to form a pad, what values would be appropriate for a 10 dB pad and for a 20 dB pad?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 07, 2019, 09:03:46 AM
Forgive a simplistic question from a mere 'mechanical', but what is this C3 ('SOT') for, and what value had you in mind for it?

Also, following Ricardo's suggestion earlier today, that there could be an option to add a resistor across T2 secondary to form a pad, what values would be appropriate for a 10 dB pad and for a 20 dB pad?
C3 is included to allow for the  tuning of T1 to be maintained within the recommended inductor parameters. It is only likely to be required where the capacitive value of the capsule to be used is less than c.60pF.
Alternatively, one could try  using a higher frequncy for the oscillator.  That might reduce the 'Q' overall though ... so this is still subject to being tested.   (One of the reasons I included the word  'experiment' in the original project title!  :) )

There are a few notes on this in the pdf I linked to in post #150    -- but this is now quite a long thread, so they were easily missed! 
 ...(see here: http://www.jp137.com/lts/RF.AMX5.inductor.assembly.pdf )

Re: pads .....  I think I would be inclined to reduce the sensitivity  - if necessary - by increasing the value of R2  - or possibly R3  -(http://www.jp137.com/lis/RF.AMX8.jpg (http://www.jp137.com/lis/RF.AMX8.jpg)) , rather than by de-tuning T2.?.. but I'm sure someone will correct me if that's not a good idea!

I don't currently have any values for the figures you've suggested ... again, something to be tested out as the experiments progress..  :)

EDIT: On reflection, I think Ricardo's idea to load T2 is the right one....
 
As a rough guide (within 1 dB or so)  fitting a 3k3 resistor across T2 secondary will attenuate the signal by c. -10dB.
Fitting a 1k resistor will  attenuate by c. -20dB.

Reducing  the sensitivity by modifying the oscillator amplitude is not as straightforward as it first seemed, with a relatively high impedance power supply (i.e. phantom power) ... too many variables.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 07, 2019, 04:54:45 PM
Plenty of room to hang a resistor from the JFET gate to ground (and the associated pin-header to connect a switch, if needed / desired) :)

EDIT: On reflection, I think Ricardo's idea to load T2 is the right one....
 
As a rough guide (within 1 dB or so)  fitting a 3k3 resistor across T2 secondary will attenuate the signal by c. -10dB.
Fitting a 1k resistor will  attenuate by c. -20dB.

Reducing  the sensitivity by modifying the oscillator amplitude is not as straightforward as it first seemed, with a relatively high impedance power supply (i.e. phantom power) ... too many variables.

Title: Re: DIY RF Condenser Mics
Post by: rogs on May 07, 2019, 07:25:59 PM
We can argue about measurements & noise weightings but this make your mike
4dBA + 6 - 15 =sorta  -5dBA spl
.....assuming 2014 NT1 has a 'similar' (made in Sydney) capsule  to your CK12 clone.  So yours is one of a literal handful of mikes with sub 0dBA spl noise  :o


.....Can you post 15s of each as a WAV file with the same preamp gains?

........For noise measurements, I wrap the mike in towels, put them in a stout wooden box and bury the box under a pile of blankets on a bed with a heavy weight on top.  I record 15s late at night and listen to each recording to make sure there are no insects & distant cars.  Fridge, fans & clocks disabled.

I think I may have been a fraction too optimistic with my noise figures  -- at least at 1KHz!.....  I had recorded the 2 mics by equalising the gain in the preamp before recording..... not really a very good idea of course....

I have re-done the recordings - taking on board your advice on how to create a home made 'anechoic chamber'!!   :)

The Rode NT1 is an example of the new 2014 'black' version, and is fitted with Rode's HF6 edge terminated capsule.

The RF.AMX8 prototype uses a Chinese 'C12' type edge terminated capsule.
(No where near as flat as the Rode - but not as bad as most cheap Chinese K.67 types)

The sound source is a single Sennheiser headphone earpiece, mounted within 30mm of both capsules (as close as possible to being equidistant).

The recording was made on my Tascam DR100Mk3 recorder, with an 80Hz HPF applied, to keep out the worst of the 'room rumble'.....


I have attached 4 'raw' 15 second waveforms - exactly as recorded - no adjustments made at all.

www.jp137.com/las/NT1.left.RF.AMX8.right.1KHz.wav
www.jp137.com/las/NT1.left.RF.AMX8.right.5KHz.wav
www.jp137.com/las/NT1.left.RF.AMX8.right.10KHz.wav
www.jp137.com/las/NT1.left.RF.AMX8.right.pink.noise.wav

As the file titles suggest, the Rode NT1 is recorded onto the left channel, the RF.AMX8 prototype onto the right.

As you can see, the 1KHz file shows roughly equal noise floor on both channels, once the signal gain has been increased on the Rode left channel recording to match the signal level on the right.

On the 5KHz, 10KHz  and pink noise recordings the RF.AMX8 noise floor comes out slightly lower than the Rode 

I think the reason for that is probably the non-linear frequency response of the 'Chinese' C12 type capsule....

Nevertheless, I think the RF prototype come out pretty well  for such a simple circuit.....

Title: Re: DIY RF Condenser Mics
Post by: rogs on May 07, 2019, 08:03:06 PM
Plenty of room to hang a resistor from the JFET gate to ground (and the associated pin-header to connect a switch, if needed / desired) :)

I think an  SPDT 'On - Off - On' with the common connected to ground  should do the trick......
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 07, 2019, 08:06:57 PM
Is that for a two-step pad (-10dB and -20dB)?

In the mean time, i added a single 2k2 (split the difference) to the board.

I think an  SPDT 'On - Off - On' with the common connected to ground  should do the trick......
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 07, 2019, 09:11:57 PM
Is that for a two-step pad (-10dB and -20dB)?

......In the mean time, i added a single 2k2 (split the difference) to the board.

Most of my devices have just a single pad... Usually -10dB - (although my Sound Devices unit has a -15dB pad which is quite useful)... I must check, but I'm guessing your 2k2 resistor will probably be about -15dB?..

I'm wondering if it would be a good idea to change the board ident to 'RF.AMX9'  as well ...... otherwise we shall have 2 versions of RF.AMX8 --- and that's probably not a good idea?....  :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 07, 2019, 09:25:15 PM
Duly noted re: the revision renaming.

The three-position on-off-on switch idea threw me off.
Surely, if the attenuation can be achieved by only connecting a resistor from the T2 secondary (ie. the JFET gate) to ground (ie. the other end of the T2 secondary), an SPST on-off switch should suffice, right? :)

On-off-on = middle does nothing, clicked in one direction does nothing, clicked in the other direction activates the pad. Doesn't make that much sense, does it? :P

And re: resistor value, like i said, i figured "split the difference", if 1k = -20dB and 4k7 = -10dB... 2k2 should then be somewhere in between :D

Most of my devices have just a single pad... Usually -10dB - (although my Sound Devices unit has a -15dB pad which is quite useful)... I must check, but I'm guessing your 2k2 resistor will probably be about -15dB?..

I'm wondering if it would be a good idea to change the board ident to 'RF.AMX9'  as well ...... otherwise we shall have 2 versions of RF.AMX8 --- and that's probably not a good idea?....  :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 05:46:15 AM
The three-position on-off-on switch idea threw me off.
Surely, if the attenuation can be achieved by only connecting a resistor from the T2 secondary (ie. the JFET gate) to ground (ie. the other end of the T2 secondary), an SPST on-off switch should suffice, right? :)

On-off-on = middle does nothing, clicked in one direction does nothing, clicked in the other direction activates the pad. Doesn't make that much sense, does it? :P


I was thinking of a switch like model 1MS3 on page 2 of this data sheet......  http://www.farnell.com/datasheets/1935700.pdf
So - pin 2 (common) is connected  to ground, and pins 1 and 3 are connected to the free end of each resistor (other end of both  resistors are connected to the gate).
Then the centre position connects neither resistor to ground - and both 'On' positions each connect one of the two resistors to ground....
Those switches are quite cheap too. ... (Strangely enough the double pole versions are even cheaper than the single pole??)
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 08, 2019, 06:17:18 AM
The three-position on-off-on switch idea threw me off.
Surely, if the attenuation can be achieved by only connecting a resistor from the T2 secondary (ie. the JFET gate) to ground (ie. the other end of the T2 secondary), an SPST on-off switch should suffice, right? :)

On-off-on = middle does nothing, clicked in one direction does nothing, clicked in the other direction activates the pad. Doesn't make that much sense, does it? :P


An on/off/on switch would allow two different attenuations like 10 and 20dB.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 08, 2019, 06:55:10 AM
I had asked for confirmation about that hunch, but i got none :P


An on/off/on switch would allow two different attenuations like 10 and 20dB.

Is that for a two-step pad (-10dB and -20dB)?


Most of my devices have just a single pad... Usually -10dB - (although my Sound Devices unit has a -15dB pad which is quite useful)... I must check, but I'm guessing your 2k2 resistor will probably be about -15dB?..
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 07:13:10 AM
Ahhh .. my fault .. sorry!  :)

Ricardo had mentioned attenuating by both -10 dB and -20 dB. That would require 2 resistors of course -- and an 'on -off - on'  switch similar to the  type I linked to.

When you added a single resistor to the PCB layout, I then mentioned - in passing - that most of my devices  actually had only one pad --
Many others have 2 of course. I think that is where I confused the issue.

Two is more comprehensive of course -- and it's only one more resistor, and not much (if any) extra cost for the switch.
 If it's easy to do -   two resistors and a 3 way connection header sounds like a good idea...

Apologies again for the confusion..
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 08, 2019, 07:41:24 AM
How's this, then?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 07:55:06 AM
Looks good! ----

 
I've just had confirmation from DHL that my RF.AMX8   PCBs from JLC should be here tomorrow.....

I'll be trying one out as soon as they get here  :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 08, 2019, 08:23:47 AM
Next step, figuring out how to do multipattern? ;D
Title: Re: DIY RF Condenser Mics
Post by: cyrano on May 08, 2019, 08:32:26 AM
Next step, figuring out how to do multipattern? ;D

Yes, yes, yes...

Please, please, please  ;D
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 09:06:24 AM
Next step, figuring out how to do multipattern? ;D

What happens if you were to fit a double sided capsule across T1 secondary, with  the centre terminal connected to  T2 primary, and perhaps with a small capacitor across one side of the capsule to unbalance the bridge a smidgeon?...

I've no idea what would happen -- just saying?   :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 08, 2019, 09:56:57 AM
I would imagine that should be easy enough to test out with the soon-to-arrive boards? ::)

What happens if you were to fit a double sided capsule across T1 secondary, with  the centre terminal connected to  T2 primary, and perhaps with a small capacitor across one side of the capsule to unbalance the bridge a smidgeon?...

I've no idea what would happen -- just saying?   :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 11:56:13 AM
I would imagine that should be easy enough to test out with the soon-to-arrive boards? ::)

Would be -- if I had any double sided capsules! ..   :)
(I have a couple on the way from wgtcenter)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 08, 2019, 12:37:39 PM
How will you combine the inputs from the two capsules? I imagine anything other than a straightforward placing of one capsule on each half of the centretapped coil, with or without one side inverted, will affect the Q significantly.

Just a thought to get you all thinking before Rog's capsules arrive... or before I order some of the appropriate PCBs from JLCpcb
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 08, 2019, 03:51:58 PM
What happens if you were to fit a double sided capsule across T1 secondary, with  the centre terminal connected to  T2 primary, and perhaps with a small capacitor across one side of the capsule to unbalance the bridge a smidgeon?...

I've no idea what would happen -- just saying?   :)
I think it's time to do some math instead of shooting in the dark.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 04:23:10 PM
I think it's time to do some math instead of shooting in the dark.

I've been wondering if someone is looking to take up that 'mantle'! .... :)

I understand there's lots of expertise on this board - Maybe one or two of the more expert  members might care to join in with some serious engineering advice on how  best to move the project forward to the next stage?...

There's only so far 'techs' like me can go, with just instinct and 'hands on' experimentation....

But in my own defence, it was only ever intended to be an experimental 'hobby' project - as I made clear in my early notes...
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 08, 2019, 04:31:22 PM
I understand there's lots of expertise on this board - Maybe one or two of the more expert  members might care to join in with some serious engineering advice on how  best to move the project forward to the next stage?...
At the moment I'm on a trip but soon I'll be home and have some time to do simulations.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 08, 2019, 07:27:53 PM
Rogs, we are in awe of how much you have done and achieved with this project. Without your enthusiasm and skill it would still be at the same stage as the thread on the Mic Builders forum, going nowhere. It has been successful beyond my wildest imagining, even at this early stage. Thank you. I hope you keep developing this and that others can contribute the heavy lifting of mathematical theory and simulation.

And that's not to ignore the PCB designs from Ruud and Khron and the experienced suggestions from Ricardo and others. Thanks, guys. Keep it up. This project already knocks the Alice design into a cocked hat; let's make it into a really great DIY microphone project.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 08, 2019, 08:16:45 PM
Oscillators and RF are way out of my league, but i do like to think i can whip up a half-decent PCB layout every now and then ;D

PS: To be fair, i believe the Alice merely was a lightly modded Schoeps circuit, so... ::)

That being said, i'm also looking forward to hearing from the more educated and/or experienced among us, on this  :)

Rogs, we are in awe of how much you have done and achieved with this project. Without your enthusiasm and skill it would still be at the same stage as the thread on the Mic Builders forum, going nowhere. It has been successful beyond my wildest imagining, even at this early stage. Thank you. I hope you keep developing this and that others can contribute the heavy lifting of mathematical theory and simulation.

And that's not to ignore the PCB designs from Ruud and Khron and the experienced suggestions from Ricardo and others. Thanks, guys. Keep it up. This project already knocks the Alice design into a cocked hat; let's make it into a really great DIY microphone project.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 08, 2019, 08:44:33 PM
And that's not to ignore the PCB designs from Ruud and Khron and the experienced suggestions from Ricardo and others. ....

...Many, many  thanks to Ruud and Khron... an absolutely vital part of the project..... and to Ricardo with his expert suggestions.

And to you of course ... for having another try at getting this project going, after the  not very positive response you got from the  earlier Micbuilders thread!

I'm hoping to receive some of Khron's version 8 PCBs tomorrow, and I'm intending to build a couple up straight away...
Hopefully, we shall be one step nearer to an actual repeatable, cost effective project build!

I do look forward to seeing what the experts come up with, maths and 'simulation' wise....
I would think that simulation software has improved since I last tried using it some 10 years ago.
I had a go at some audio circuits  with something called 'LT Spice'  -  which was hopeless!...
It produced 'designs' that may well have been correct mathematically, but were often hopeless from a practical build point of view.....too many things like layout, grounding and sensible component values it couldn't cope with well at all.....
I gave up on it at that time, and went back to experimenting with actual circuitry !

I'm sure things will have improved by now... it'll be interesting to see what improvements the simulations  suggest...



Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 09, 2019, 04:24:50 AM
This is my analysis of the files that rogs posted in #196.

They were done by loading the file(s) into Audacity and splitting the stereo file into Left (Rode) and Right (AMX8) files.

You select the part of the file in Audacity and then Analyse 'Plot Spectrum' with 'Hanning window', 16384 & 'Log frequency'.  Then Export saves the 8192 point spectrum as a text file.

I read this and plot it out with various bells & whistles .. in this case as though it is a constant bandwidth analyser eg 1/3 8ve though this is more like 1/20 8ve or finer.

With this, pink noise appears flat, white noise rises at 3dB/8ve with frequency .. and '1G' noise which is 'reddish' falls at 3dB/8ve

Alas, my programs are DOS which will soon be extinct but this beach bum is too lazy to move into the 21st century and learn a modern programming language.

In each case Blu is the Rode NT1 and Red is AMX8.

The 2 top curves are the Pink noise part of the files in an attempt to see whatis  the difference in response as a guide to matching sensitivities.

You can see the danger of trying to match sensitivity with a single frequency.  I prefer to match sensitivities using music and voice and then noting what my figures give when measured.  In this case, AMX8 is about 15 or 16dB louder than NT1.  It has more bass below 100Hz and less between 400 & 1k5 Hz.  The plots use 16dB matching.

I don't believe the response above 3k5Hz at all as this depends on mounting, room bla bla.  I have my own tools for 'anechoic' response above 200Hz but alas, they are also DOS programmes.  :(

You can clearly see the effect of rogs 100Hz filter on both response & noise.

rogs, how are the mikes mounted for the Pink Noise measurements?  Is this your 30cm measurement?

The interesting bit is the lower curves where I've increased the gain by 40dB.  Remember, this is with the 16dB matching so represents the noise when the 2 mikes are adjusted for the same sensitivity.  I've overlaid 3 noise curves out of the 4 files to check we are not looking at (very quiet) ambient noise.  The 10kHz files had more LF noise than the other 3 so I've discarded it as probably ambient.

NT1 has classic '1G' noise merging into white 'resistive' noise at about 2kHz.

I'll need to do more work (eg listen to the noise) to see if my guess that RF mikes 'don't have 1G noise' is valid ... but ... AMX8 has between 3 & 4dB less noise than NT1 below 1kHz.

Their noise spectrums crossover at 3k1Hz and above that AMX8 is noisier.  The noise is 'whiter than white' so not something I can identify immediate.  It might be the result of some response peaking due to the RF gubbins which we haven't soused out. 

This is sorta borne out by AMX8's greater HF response compared to NT1 but I'd like to see a proper 'anechoic' response before pontificating further.  There's a great temptation to EQ the extra HF which would then make AMX8 quieter all round  8)

So I think AMX8 is at least as quiet as NT1 though not as much as originally thought.  Still a great achievement rogs  :o

Gotta go but I'll write more later

AMX8 has less noise below 3k1 Hz.  As it also has more bass below 100Hz, it is clearly quieter than NT1 at mid & low frequencies.



Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 05:49:45 AM
Unless i'm horribly mistaken, i believe the test-capsule used is a single-sided K67 clone, which may well explain some(?) of the things on the higher end of the spectrum... Possibly.

[Edit]

Well, then again, if need be, one could consider Henry Spragens' method (http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html) of toning down the top end in a Schoeps circuit (with a high-shelf attenuation) :) Not quite sure yet where i could shoehorn that in (3 resistors and a cap), on the existing board size, though ;D

Actually, maaaaaaybe just. But that'll involve four vertical resistors, though :P
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 07:48:47 AM
....rogs, how are the mikes mounted for the Pink Noise measurements?  Is this your 30cm measurement?

30mm  measurement... The mics were packed into a box stuffed with towels,  with the Sennheiser transducer placed within 30mm of each capsule..
The box was covered in blankets , and stuffed under a duvet in a quiet bedroom.
I do believe it may have started raining outside slightly during the latter (10KHz and pink noise) part of the experiment ...That may account for the  increase in noise level in the 10KH file.

It really was just intended  as a  simple comparative noise floor test ....The AMX8 capsule was a cheap Chinese edge terminated C12 type .. so not very linear.
I'm not sure how linear the Sennheiser transducer is either?.... It was from an old pair of HD450s

And of course packing everything into a tiny space  doesn't allow for variations  that may be caused by the different cardioid status formed by the different mounting arrangements (the C12 in the BM800 body is constructed as an end address unit)

Many thanks for taking the trouble to analyse the results... it certainly gives an independent confirmation that  my own results were  not total nonsense!

I'm not sure about adding EQ to the mic itself?.....  I've never been convinced that a simple passive HF roll off can hope to compensate accurately for the complex frequency response of these various cheap capsules...
I've always preferred to either add EQ in post for recorded files -- or add active EQ into the mic channel chain - at line level - for 'live' use.
I've also found that adding a passive low pass filter before the output emitter followers tends to add noise!..

As usual, lots more to investigate and experiment with..... thanks again for your advice and comments.....

Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 08:05:23 AM
Unless i'm horribly mistaken, i believe the test-capsule used is a single-sided K67 clone, which may well explain some(?) of the things on the higher end of the spectrum... Possibly.

[Edit]

Well, then again, if need be, one could consider Henry Spragens' method (http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html) of toning down the top end in a Schoeps circuit (with a high-shelf attenuation) :) Not quite sure yet where i could shoehorn that in (3 resistors and a cap), on the existing board size, though ;D

Actually, maaaaaaybe just. But that'll involve four vertical resistors, though :P

No the capsule was a Chinese edge terminated C12 style 'clone' .....not quite as peaky as a K67, but still pretty 'HF heavy'...

I've not had much luck with Henry's passive EQ in the mic itself. .... The addition of a simple passive low pass will certainly 'tame' the HF -- but not necessarily correct the complex frequency response of the capsule very accurately ?
I've found it also adds noise - especially if the series resistors  are more than about 1k.

I prefer to add EQ in post for recordings, or add active EQ -  at line level - into the mic channel for live situations (as I mentioned in my previous post to Ricardo)

On a different note ......the PCBs are here! ... and looking good.  ;D

I may have to file out the mounting holes a bit ... although I may get away with it if I select a suitable BM800 frame (the mounting hole spacings do vary slightly)
I  dug up a few of the original BM800 PCBs (no idea why I kept them ---just a bit of a hoarder I'm afraid   :)  )  and they seem to solve the different spacing problem by placing the mounting holes  on 30mm centres, and making the hole diameter 3mm (it may even be 1/4" - I'm not sure?) ...
Anyway, it's only a minor issue...

I shall put one together as soon as I've finished my other chores (once I've started, I shall probably stay in my study until bedtime!!)

As I say, looking good so far..... again, many thanks for your efforts....
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 08:15:02 AM
I'm not sure how linear the Sennheiser transducer is either?.... It was from an old pair of HD450s

Well, since both mics were tested using the same signal source, the analysis is relative (as opposed to absolute), so the linearity, or lack thereof, of the signal source is (or should be) pretty much irrelevant :)

I've also found that adding a passive low pass filter before the output emitter followers tends to add noise!..

For what it's worth... http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/10_EQ_Pt.4_-_Noise.html

On a different note ......the PCBs are here! ... and looking good.  ;D

I may have to file out the mounting holes a bit ... although I may get away with it if I select a suitable BM800 frame (the mounting hole spacings do vary slightly)
I  dug up a few of the original BM800 PCBs (no idea why I kept them ---just a bit of a hoarder I'm afraid   :)  )  and they seem to solve the different spacing problem by placing the mounting holes  on 30mm centres, and making the hole diameter 3mm (it may even be 1/4" - I'm not sure?) ...
Anyway, it's only a minor issue...

Duly noted, i've now spread out the mounting holes to 29.85mm (to stick to some sort of grid), and made them 3mm.

I also took the liberty to make some room in the middle, in preparation for the high-shelf. At the end of the day, if one doesn't wish to employ it, one can just solder in wire jumpers where the base resistors would be :)

PS: 1/4" is 6.35mm; i'm guessing that was supposed to rather be 1/8" (3.175mm) ;D


PS(2): Re: Chinese CK12's - i don't know about "peaky"; i ran some sweeps on mine, and if anything, the response is more like "tapered", with a relatively "straight" (or constant / consistent) rise towards the top end.
https://www.dropbox.com/s/blkhgb8eq0z07y9/Screenshot%202019-01-11%2020.06.30.png?dl=0
(Three separate mics, capsules from the same batch, all relative to my calibrated measurement mic.)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 09, 2019, 09:36:46 AM
Wow! What a lot of new messages since Ricardo's posting this morning. Some of them may supersede my thoughts below.

This is my analysis of the files that rogs posted in #196.
What a fascinating preliminary analysis. And it opens the way for a discussion about how we want to trim the frequency response of Rogs' circuit design. I, for one, wait with bated breath Ricardo's fuller analysis. I also note that this preliminary analysis is the result of measurements on a single example with a C12 capsule. Those of us who are making early examples of the mic should measure ours to an agreed common method and upload the results for Ricardo (or someone with equal knowledge and capability) to analyse, to come up with a typical set of measurements.

Here are a few random thoughts:
Maybe others have thoughts to add too?
 ;D
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 09, 2019, 09:45:39 AM
Well, then again, if need be, one could consider Henry Spragens' method (http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html) of toning down the top end in a Schoeps circuit (with a high-shelf attenuation) :) Not quite sure yet where i could shoehorn that in (3 resistors and a cap), on the existing board size, though ;D
That is what I was thinking - both Henry's method and the space limitation for fitting in more circuitry. If necessary, we could consider a longer donor body, with room for a longer PCB; either the longer version of the one of the type Rogs mentioned for his first (stripboard) build, or a BM900 (looks longer; I haven't seen one in the metal).
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 09, 2019, 11:10:12 AM
I've not had much luck with Henry's passive EQ in the mic itself. .... The addition of a simple passive low pass will certainly 'tame' the HF -- but not necessarily correct the complex frequency response of the capsule very accurately ?
I've found it also adds noise - especially if the series resistors  are more than about 1k.

I can confirm this. Years ago I tested this and the addition of 10 K resistors between the FET and the PNP transistors gave an audible increase in self noise. On his website Henry states that the increase in noise level is so low that the noise caused by the air molecules on the capsule is higher, but that is not my experience.
The good thing is that with 820 pF it approaches the needed frequency compensation for a K67 capsule.
(An American company is still selling this mod as an "update" for certain Chinese microphones.)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 11:22:02 AM
Well, as i mentioned, i can still squeeze in the two series resistors with relative ease, but arguably for the sake of flexibility, the trimmer / resistor, and/or the capacitor may be easier to attach on the bottom side of the board.

That is what I was thinking - both Henry's method and the space limitation for fitting in more circuitry. If necessary, we could consider a longer donor body, with room for a longer PCB; either the longer version of the one of the type Rogs mentioned for his first (stripboard) build, or a BM900 (looks longer; I haven't seen one in the metal).


Would going with lower values for the series resistors (say, 1k or so) have any detrimental effects? I guess the cap value might need to go up in that case, but that's not the end of the world anyway, is it? :)

I can confirm this. Years ago I tested this and the addition of 10 K resistors between the FET and the PNP transistors gave an audioble increase in self noise. On his website Henry states that the increase in noise level is so low that the noise caused by the air molecules on the capsule is higher, but that is not my experience.
The good thing is that with 820 pF it approaches the needed frequency compensation for a K67 capsule.
(An American company is still selling this mod as an "update" for certain Chinese microphones.)
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 09, 2019, 12:00:27 PM
Besides the increase in self noise, my main objection is that low-pass filtering adds frequency dependent phase shift.
Also it influences the transient response and adds distortion. (I did measurements.)
It would be possible to reduce the value of the resistors in series with the coupling capacitors (and increase the value of the filtering capacitor), but that would mean a heavier load on the FET outputs.  Because of the higher impedance on the drain, this would also unbalance the AC levels on the output.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 09, 2019, 12:04:18 PM
I can confirm this. Years ago I tested this and the addition of 10 K resistors between the FET and the PNP transistors gave an audible increase in self noise.
This is absolutely predictible. Increasing the impedance seen by the PNP's by a factor of 5+ makes their noise current perceptible. In addition, it increases the circuit's output impedance, which is detrimental to the CMRR.
There are better options that have been discussed here and on the micbuilders list, particularly adding caps across the drain and/or source resistors or across the FET.

Quote
On his website Henry states that the increase in noise level is so low that the noise caused by the air molecules on the capsule is higher, but that is not my experience.
The white nature of the noise current makes it perceptible against the pink-ish molecular agitation (brownian) noise. Of course it also depends very much on the capsule's size and tuning.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 09, 2019, 12:15:40 PM
Besides the increase in self noise, my main objection is that low-pass filtering adds frequency dependent phase shift.
Any kind of analog EQ results in phase-shift. It is often desirable. Because the problem EQ endeavours solving has already created its own phase-shift. As long as the mic is a minimum-phase transducer (it is for the most part of its response), its phase response is a conjugate of its frequency response. When properly EQ'd, the resulting phase response is linear.

Quote
Also it influences the transient response
Same comment.

Quote
and adds distortion. (I did measurements.)
It shouldn't; if it does, it's because it's been poorly implemented. It is true that the little space available in mics often does not leave enough room for well-designed EQ.

Quote
It would be possible to reduce the value of the resistors in series with the coupling capacitors (and increase the value of the filtering capacitor), but that would mean a heavier load on the FET outputs.
Loading the drain has no negative consequences, since its a current source.

Quote
Because of the higher impedance on the drain, this would also unbalance the AC levels on the output.
Which is a non-issue. It has consequences on the CMRR, but that would be in a favorable way.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 12:17:37 PM
At the end of the day, the whole thing about balanced connections is noise rejection, isn't it? So the output impedance is arguably (or should be?) more important than the absolute symmetry between signals.

Otherwise, mics like the C414 TL's or Oktava MK012's, or other asymmetrical-output- wouldn't work (as well as they do) :P
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 03:28:34 PM
My first sample of Khron's original PCB (pre 'pad' version) is now assembled, using the component values for RF.AMX8 ... see here: http://www.jp137.com/lis/RF.AMX8.jpg

Works fine - exactly as expected!  ;D ... Everything fits... although  - as I mentioned earlier - you may need to open up the mounting holes a tiny bit with some BM800 frames. (Now corrected on latest issue)

I decide to fit C4 and the capsule connections on the track side of the PCB for convenience.

As Khron has already mentioned, the 7.5mm spacing for the 1/4w resistors is a bit tight ... but they do fit (see attached photo).
You could use MF12 1/8w resistors -- the highest power consumption is for  R2 at around 85mW, so no real problems with power ratings.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 03:38:52 PM
Not that i'm patting myself on the back, but that's "pwetty"! ;D

And since it works, i'm quite glad to hear the "tweaked" layout and extra ground plane on the top, didn't mess things up too-too much :P

It might be interesting to look at the output signal with an "RF" spectrum analyzer, i kinda-sorta wonder if there's any difference between the two iterations (ie. if any RF from the oscillator section "bleeds" or doesn't, into the output).
But that's just a "random" curiosity on my part :D

Would it help if i moved the C4 footprint to be parallel with C3 ("SOT"), between the cans?


PS: If one (really) wanted to, even on this pre-pad version, one could tack on an extra resistor off the JFET's gate, maybe "inside" a dab of RTV / silicone, and run the other end to a pad switch ::)


My first sample of Khron's original PCB (pre 'pad' version) is now assembled, using the component values for RF.AMX8 ... see here: http://www.jp137.com/lis/RF.AMX8.jpg

Works fine - exactly as expected!  ;D ... Everything fits... although  - as I mentioned earlier - you may need to open up the mounting holes a tiny bit with some BM800 frames. (Now corrected on latest issue)

I decide to fit C4 and the capsule connections on the track side of the PCB for convenience.

As Khron has already mentioned, the 7.5mm spacing for the 1/4w resistors is a bit tight ... but they do fit (see attached photo).
You could use MF12 1/8w resistors -- the highest power consumption is for  R2 at around 85mW, so no real problems with power ratings.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 04:15:01 PM
Not that i'm patting myself on the back, but that's "pwetty"! ;D

And since it works, i'm quite glad to hear the "tweaked" layout and extra ground plane on the top, didn't mess things up too-too much :P

It might be interesting to look at the output signal with an "RF" spectrum analyzer, i kinda-sorta wonder if there's any difference between the two iterations (ie. if any RF from the oscillator section "bleeds" or doesn't, into the output).
But that's just a "random" curiosity on my part :D

Would it help if i moved the C4 footprint to be parallel with C3 ("SOT"), between the cans?


PS: If one (really) wanted to, even on this pre-pad version, one could tack on an extra resistor off the JFET's gate, maybe "inside" a dab of RTV / silicone, and run the other end to a pad switch ::)

You should pat yourself on the back -- it's  a pretty elegant PCB!  :)  ... especially as it's ' issue 1' , as it were.

I don't 'feel'  there are any significant RF issues... it tunes well, and there is very little noise.

One little quirk  I have discovered ... the K67 capsule in my 'end address' version  need to be connected one way round.   
The connection to the bridge balance point - the low voltage 'end' -  needs to go to the edge termination, with the higher voltage to the centre terminal...It's function of the proximity of the edge connection to the head basket, which can cause some handling noise.
It's still a bit strange getting used to being able to operate the mic with the head basket removed - if necessary-  without incurring horrendous hum!.....still coming to terms with a low impedance LDC condenser capsule of course.

(You also no longer need a double mesh on the head basket either)...

I think C4 is OK where it is... it's easy to fit, and to change it if necessary ...I think in between the cans might be less accessible....

Do you have the Gerber files for your 'pads' version  (which I think will be  RF.AMX9 ?)...
I think the layout looks really useful, and it should be possible to add either pad by just fitting a jumper link to the pad header....
In the case of a BM800 it's no big deal to simply unscrew the case to gain access to the link......a lot easier than fitting a switch somewhere on that tiny casing!

I know you're continuing to develop ideas with the EQ, but  I would like a copy of the non EQ  version - but with pads - if it's possible? ...Only if it's easy to do though!  :)

Thanks again for your efforts.
I think we now may have a version that  might just be ready for others to try and build?....   :)
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 04:37:11 PM
Aw, shucks! ::) You'd better quit with all the flattery, before it goes to my head ;D

No, i hadn't yet exported the Gerbers, but as before, that's just a few clicks away :) Looks good on JLC's viewer.

https://www.dropbox.com/s/sc1y3m0l4uvwxs8/RFmic_gerber.zip?dl=0

Re: pads - that sounds like a great application for those computer-mainboard-like shorting-jumpers. If no attenuation's needed, just place it on the middle pin, at 90deg from either pad "option" (ie. "in line" with R11).
https://sc02.alicdn.com/kf/HTB1ia26KVXXXXXwXFXXq6xXFXXXq/227803161/HTB1ia26KVXXXXXwXFXXq6xXFXXXq.jpg



You should pat yourself on the back -- it's  a pretty elegant PCB!  :)  ... especially as it's ' issue 1' , as it were.

I don't 'feel'  there are any significant RF issues... it tunes well, and there is very little noise.

One little quirk  I have discovered ... the K67 capsule in my 'end address' version  need to be connected one way round.   
The connection to the bridge balance point - the low voltage 'end' -  needs to go to the edge termination, with the higher voltage to the centre terminal...It's function of the proximity of the edge connection to the head basket, which can cause some handling noise.
It's still a bit strange getting used to being able to operate the mic with the head basket removed - if necessary-  without incurring horrendous hum!.....still coming to terms with a low impedance LDC condenser capsule of course.

(You also no longer need a double mesh on the head basket either)...

I think C4 is OK where it is... it's easy to fit, and to change it if necessary ...I think in between the cans might be less accessible....

Do you have the Gerber files for your 'pads' version  (which I think will be  RF.AMX9 ?)...
I think the layout looks really useful, and it should be possible to add either pad by just fitting link to the pad header....
In the case of a BM800 it's no big deal to simply unscrew the case to gain access to the link......a lot easier than fitting a switch somewhere on that tiny casing!

I know you're continuing to develop ideas with the EQ, but  I would like a copy of the non EQ  version - but with pads - if it's possible? ...Only if it's easy to do though!  :)

Thanks again for your efforts.
I think we now may have a version that  might just be ready for others to try and build?....   :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 05:35:41 PM
Aw, shucks! ::) You'd better quit with all the flattery, before it goes to my head ;D

No, i hadn't yet exported the Gerbers, but as before, that's just a few clicks away :) Looks good on JLC's viewer.

https://www.dropbox.com/s/sc1y3m0l4uvwxs8/RFmic_gerber.zip?dl=0

Re: pads - that sounds like a great application for those computer-mainboard-like shorting-jumpers. If no attenuation's needed, just place it on the middle pin, at 90deg from either pad "option" (ie. "in line" with R11).
https://sc02.alicdn.com/kf/HTB1ia26KVXXXXXwXFXXq6xXFXXXq/227803161/HTB1ia26KVXXXXXwXFXXq6xXFXXXq.jpg

Many thanks for the Gerber files .. much appreciated..

It wasn't flattery --I genuinely think that's  a very elegant piece of work!  :)......   
A double sided through plated PCB  - with a ground plane - and prototypes available for less than £10?.....amazing !
It's all come along way from hand drawing and Letraset tape masters!!  :)

I think I shall order a few MF12 resistors,  and try them out on one of my other prototype PCBs

And I think I need to try and get my head round quite where this new Infinite Impedance version fits with regard to the bridge balancing....I don't think the bridge does necessarily need to be unbalanced any more? ...
 
The grounded gate and the 4k7 drain and source resistors will allow the FET to self bias........and are  ideally chosen to suit the IDSS of the FET and establish a reasonable 'Q' point.
The 1nF across these 4k7 resistors determines the HF cut off frequency (Fc - currently around 30KHz) . But  the actual 'measured DC' across the resistors will vary considerably, depending on the level of 10MHz 'unbalanced' voltage. ..
Now how this 'rectified ' DC - added to the original bias voltage - affects the FET biasing, I'm not sure? 
The actual demodulated audio signal is only of the change of amplitude across the R/C time constant, and is not dependent on a minimum DC level, as the original diode  rectifier was..

On top of that, the FET is also acting as phase splitter to provide a balanced audio output... It's busy  little device in this circuit!   :)


Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 05:41:21 PM
The cap across the drain resistor does indeed "perform" a high-cut, but (and i could be horribly wrong on this) the one across the source resistor will actually cause a high-frequency boost.

Could that potentially be the (or a) source (no pun intended) of the rising HF noise in Ricardo's analysis?  ??? Just speculating / spitballing here...

[Edit] Well, kinda-sorta - see attachment (plot is of the drain voltage).
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 05:49:20 PM
Adding a drain capacitor does indeed end up in a "net" high-cut, albeit very slight. Disconnecting the source cap accentuates the high-cut (note carefully, the scales on the left edge of the graphs).

PS: "Even" the drain-cap-only version is "only" 1.4dB down @ 20kHz (relative to the flat region, at -0.5dB or so), and 0.4dB down @ 10kHz.

PS(2): Admittedly, i have no clue how the input RF and the demodulation plays into that, but... :D
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 09, 2019, 05:52:14 PM
This is absolutely predictible. Increasing the impedance seen by the PNP's by a factor of 5+ makes their noise current perceptible. In addition, it increases the circuit's output impedance, which is detrimental to the CMRR.
There are better options that have been discussed here and on the micbuilders list, particularly adding caps across the drain and/or source resistors or across the FET.
 The white nature of the noise current makes it perceptible against the pink-ish molecular agitation (brownian) noise. Of course it also depends very much on the capsule's size and tuning.
In https://groups.yahoo.com/neo/groups/micbuilders/files/Ricardo/ChinaMod%2BU87hybrid/ (https://groups.yahoo.com/neo/groups/micbuilders/files/Ricardo/ChinaMod%2BU87hybrid/) I analyse adding bits across the drain resistor in detail.  You have to join.

Since then Abbey has provided me with an LTspice model of U87 which is more accurate than what I used with the linear circuit analysis DOS programme I wrote circa late 80's.  This beach bum needs to get off his arse and update ChinaMod+U87hybrid.doc but the findings there are all still valid.  In fact my mod is ideally suited to EQ for AMX8 with the capsule rogs used, where a HF shelf is appropriate.  Only a single cap is required.

I second everything Abbey & Khron says about Henry's EVIL resistors including stuff about Minimum Phase which was of great interest to me in 1980.  I would add that my EQ mod REDUCES NOISE while his increases noise.  If you look carefully at Henry's measurements, you'll see that he actually does have increased noise.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 05:56:39 PM
I have no idea whether much (if any) work on this topic had been done beforehand, but had i been the (only?) one who prompted that research / analysis, at the time? :)

I do recall having read about the overly-bright K67/K87 capsules being "tamed down" in the electronics. I figured if Neumann themselves "can" do that in their own circuit, and thus it's not "heresy" to alter the response of a given capsule, couldn't a similar effect be achieved with the nearly-ubiquitous Schoeps circuit? :D

PS: To achieve a high-shelf response, wouldn't one need a resistor in series with the capacitor, across the drain resistor? The cap alone would cause a (monotonous?) high-cut. But i stand to be corrected, of course :P

In https://groups.yahoo.com/neo/groups/micbuilders/files/Ricardo/ChinaMod%2BU87hybrid/ (https://groups.yahoo.com/neo/groups/micbuilders/files/Ricardo/ChinaMod%2BU87hybrid/) I analyse adding bits across the drain resistor in detail.  You have to join.

Since then Abbey has provided me with an LTspice model of U87 which is more accurate than what I used with the linear circuit analysis DOS programme I wrote circa late 80's.  This beach bum needs to get off his arse and update ChinaMod+U87hybrid.doc but the findings there are all still valid.  In fact my mod is ideally suited to EQ for AMX8 with the capsule rogs used, where a HF shelf is appropriate.  Only a single cap is required.

I second everything Abbey & Khron says about Henry's EVIL resistors including stuff about Minimum Phase which was of great interest to me in 1980.  I would add that my EQ mod REDUCES NOISE while his increases noise.  If you look carefully at Henry's measurements, you'll see that he actually does have increased noise.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 09, 2019, 06:27:13 PM
I do recall having read about the overly-bright K67/K87 capsules being "tamed down" in the electronics. I figured if Neumann themselves "can" do that in their own circuit, and thus it's not "heresy" to alter the response of a given capsule, couldn't a similar effect be achieved with the nearly-ubiquitous Schoeps circuit? :D
ChinaMod+U87hybrid.doc in MicBuilders does exactly this and includes the research that went into the choice of EQ.

Quote
Adding a drain capacitor does indeed end up in a "net" high-cut, albeit very slight. Disconnecting the source cap accentuates the high-cut
AMX8 already has this capacitor.  It is C13 1n which matches C9 in the source.  For the HF shelf to make AMX8 match NT1, just increase C13 to 5n6 or even larger ... assuming the pink noise responses are representative bla bla ... I would want to do 'proper anechoic measurements' bla bla ...

I'm moving towardsview that the HF lift in AMX8 being due to RF stuff.  This is cos the noise becomes 'whiter than white'.  I'm too lazy to investigate cos it make my one brain cell hurt.  :-[

In the meantime, I'm listening to the quiet parts of rogs files raised by 40dB.  I hear a couple of quiet 'clonks' which aren't due to the mike so I'll edit them out and redo the noise measurements to see if this makes a difference.

In the meantime, AMX8 does sound slightly 'hissier' than NT1 but at such a low level that it is almo...ost drowned by the '1G' noise.  No idea how  a RF mike has '1G' noise  :o
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 09, 2019, 06:34:37 PM
I hadn't read that document in sooooooome time, but according to the first paragraph, seems like it was indeed me that "prodded" (to some extent, anyway) that research ;D What have i done?.....

Jokes aside though - is the C9 essential to the functioning of the circuit, actually? I mean, apart from the (admittedly outside of audible range) HF boost it does...
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 09, 2019, 08:49:24 PM
- is the C9 essential to the functioning of the circuit, actually? I mean, apart from the (admittedly outside of audible range) HF boost it does...
C9 and C13 are part of the time constants formed together with the drain and source resistors R10 and R4.
C9 and C13 are charged every half cycle of the 10MHz oscillator, the voltage being determined by the amplitude of the changing bridge imbalance.
When the FET is turned off during every other half cycle, C9 and C13 start to discharge through R10 and R4.
These time constants determine how fast the circuit responds, and with the values set  one can expect an HF  (-3dB) cutoff frequency of around 30KHz.

In this configuration the FET  - together with R10/C13 and R4/C9 -  is performing 3 tasks simultaneously ---
• Acting as a self biased N channel JFET
•  2 x half wave rectifiers with opposite polarities (an infinite impedance detector)
• An audio frequency phase splitter
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 10, 2019, 01:33:07 AM
I hadn't read that document in sooooooome time, but according to the first paragraph, seems like it was indeed me that "prodded" (to some extent, anyway) that research ;D
I'd forgotten that.

Quote
In the meantime, I'm listening to the quiet parts of rogs files raised by 40dB.  I hear a couple of quiet 'clonks' which aren't due to the mike so I'll edit them out and redo the noise measurements to see if this makes a difference.
Done  that.  On the quietest of the quiet sections (the Pink Noise sample), something starts up just before the 'clonks' too.  rogs, have you got Central Heating or a Fridge which starts up 'silently'?

Editing out all this makes minimal difference to the noise.

Hence, NT1 and AMX8 with this capsule are about equivalent noise in practice.  NT1 has a bit more 'whoosh' and AMX8 a bit more hiss but these contributions seem about equal.

If C13 is increased to 5n6 or more, I'd expect response to closely match NT1 and noise 3 or 4 dB better all over.  That's assuming the pink noise measurements are 'representative' of the response of both mikes.  This should be easy enough to check.

What I'd really like to know is

We can test the first by putting that same capsule in a Schoeps HiZ circuit.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 10, 2019, 07:15:43 AM
Adding a drain capacitor does indeed end up in a "net" high-cut, albeit very slight.
It's a matter of value. Since the signal is low-passed only on one half, the ultimate attenuation is 6dB, so it's actually a shelf, rather than a low-pass. I would think this is perhaps better suited to fixinf the sibilance of some capsules than a true low-pass filter.


Quote
PS: "Even" the drain-cap-only version is "only" 1.4dB down @ 20kHz (relative to the flat region, at -0.5dB or so), and 0.4dB down @ 10kHz.
4.7k and 1nF results in a low-pass filter tuned at about 35kHz; no wonder it does not produce significant attenuation at 20kHz.

Quote
PS(2): Admittedly, i have no clue how the input RF and the demodulation plays into that, but... :D
Someone, I believe Rogs, mentioned the values chosen in the detector resulted in a 9kHz turnover frequency.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 10, 2019, 08:16:41 AM
4.7k and 1nF results in a low-pass filter tuned at about 35kHz; no wonder it does not produce significant attenuation at 20kHz.
 Someone, I believe Rogs, mentioned the values chosen in the detector resulted in a 9kHz turnover frequency.

The original diode based detector had a time constant of 10uS (10k and 1nF) resulting in a turnover at around 16KHz.

This preliminary  version using an FET as an infinite impedance detector used 4k7 resistors to better suit the 'Q' position of the J113 FET.
With the values of C9 and C13 unchanged, that turnover is now c.34KHz.
I'm sure experimenters will be able to change those values to suit their own requirements....

I am having trouble understanding how changing just C13 to 5n6 in this 'balanced' configuration would work?...
Surely that would simply make the value  of the R10 / C13 time constant around 26uS - and change the Fc on the Q2 side of the output to around 6KHz - while leaving the  R4/C9 (Q3) output side at 34KHz?....
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 10, 2019, 10:43:49 AM
The original diode based detector had a time constant of 10uS (10k and 1nF) resulting in a turnover at around 16KHz.

This preliminary  version using an FET as an infinite impedance detector used 4k7 resistors to better suit the 'Q' position of the J113 FET.
OK, my memory was vague and I didn't want to parse through the whole thread to be more correct.

Quote
With the values of C9 and C13 unchanged, that turnover is now c.34KHz.
Ok, these are the values used by Khron in his sim.
So that is essentially flat up to 17kHz.

Quote
I am having trouble understanding how changing just C13 to 5n6 in this 'balanced' configuration would work?...
  In that case, the drain output decreases by about 8dB at 13kHz, which is a factor 0.4, so the total differential output at this frequency is 1.4 Vin, which is 3dB below 2. The LF signal is 2Vin. So the response is a shelf that goes from 2 to 1 (0 to -6dB) with a -3dB transition at 13kHz.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 10, 2019, 05:34:57 PM
  In that case, the drain output decreases by about 8dB at 13kHz, which is a factor 0.4, so the total differential output at this frequency is 1.4 Vin, which is 3dB below 2. The LF signal is 2Vin. So the response is a shelf that goes from 2 to 1 (0 to -6dB) with a -3dB transition at 13kHz.

Thanks for explaining that...
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 10, 2019, 07:38:19 PM
If C13 is increased to 5n6 or more, I'd expect response to closely match NT1 and noise 3 or 4 dB better all over.  That's assuming the pink noise measurements are 'representative' of the response of both mikes.  This should be easy enough to check.
Finally resurrected my attempts to become a pseudo LTspice guru.  8)

C13 needs to be 10n for this capsule in AMX8 to match NT1 response.  Then it would have better noise all round.  Of course this holds only for this capsule and assumes the pink noise response  measurements are 'representative' .. bla bla

The shelf introduced is 4.6dB so this will still sound slightly brighter than NT1.  You could tweak this further with R10 but that would increase output level even further when it is already much higher than usual.

At these levels, some attention should be paid to the Schoeps output stage.  Overload is governed by 2 things
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 11, 2019, 04:06:25 AM
Many thanks for the new suggestions ...I shall  measure them in 'real life' as soon as I get a chance...
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 11, 2019, 06:42:01 AM
Many thanks for the new suggestions ...I shall  measure them in 'real life' as soon as I get a chance...
I've started the simulation but there are too many unknown ATM.
What DC voltage are you measuring at the PNP's collectors and at the BC549 ' collector?
What current through the FET?
What RF voltage at T1 and T2's pri and sec? (I want to determine the turn ratio).
Actually, the tuning of the RF xfmrs seems difficult to figure out since it must be off for proper linear demod but not too much as to offer good sensitivity.
I get to see how demod happens (via the FET's 3/2 characteristic) but my results are probably off by a whole order of magnitude.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 11, 2019, 08:40:17 AM
I've started the simulation but there are too many unknown ATM.
What DC voltage are you measuring at the PNP's collectors and at the BC549 ' collector?
What current through the FET?
What RF voltage at T1 and T2's pri and sec? (I want to determine the turn ratio).
Actually, the tuning of the RF xfmrs seems difficult to figure out since it must be off for proper linear demod but not too much as to offer good sensitivity.
I get to see how demod happens (via the FET's 3/2 characteristic) but my results are probably off by a whole order of magnitude.

• D.C. voltage at Q2 and Q3  (PNP) collectors: 25.3V

• D.C. voltage at Q1 (BC549) collectors: 4.8V

• Using a 10Mohm DMM, the measured 'DC' voltage across R10 and R4 (4k7) resistors (includes RF content of course) depends on the bridge imbalance:

-- With a C12 style capsule (measured as c.90pF), and a 2x47pF in parallel value for C4 , the voltage across R10 and R4 is 4V.

-- With a K.67 style capsule (measured as c.65pF) , and C4 fitted as a 68pF, the voltage across R10 and R4 is 2.5V. 

• I don't have any way of accurately measuring the RF voltages on the  transformers - my best scope lead ( x10  10Mohm 15pF) still loads the transformers significantly.
The turns ratio of the Spectrum 5u3H IF transformers used are included in the highlighted specs attached to this post.

• I'm sure you're right about the tuning settings. The most effective way of tuning both coils I've found so far is the one suggested by Ruud early on the thread.... Tune for maximum  sensitivity, using a tone  applied via a transducer placed against the mic.  Not necessarily a very 'scientific' method I'm afraid, but the only one I've found so far!.....
I did wonder earlier if the actual tuning was 'skewed' from maximum amplitude (post#186)  but decided there would be two optimum settings - one each side of the peak - if that was the case. ...However, so far I've only been able to discover one optimum setting for the tuning in each set up...
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 11, 2019, 09:44:05 AM
Next step, figuring out how to do multipattern? ;D

Maybe something along the lines of the attached .....(not tried at all yet!   :) )
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 11, 2019, 05:25:51 PM
• D.C. voltage at Q2 and Q3  (PNP) collectors: 25.3V

• D.C. voltage at Q1 (BC549) collectors: 4.8V

• Using a 10Mohm DMM, the measured 'DC' voltage across R10 and R4 (4k7) resistors (includes RF content of course) depends on the bridge imbalance:

-- With a C12 style capsule (measured as c.90pF), and a 2x47pF in parallel value for C4 , the voltage across R10 and R4 is 4V.

-- With a K.67 style capsule (measured as c.65pF) , and C4 fitted as a 68pF, the voltage across R10 and R4 is 2.5V. 

• I don't have any way of accurately measuring the RF voltages on the  transformers - my best scope lead ( x10  10Mohm 15pF) still loads the transformers significantly.
The turns ratio of the Spectrum 5u3H IF transformers used are included in the highlighted specs attached to this post.

• I'm sure you're right about the tuning settings. The most effective way of tuning both coils I've found so far is the one suggested by Ruud early on the thread.... Tune for maximum  sensitivity, using a tone  applied via a transducer placed against the mic.  Not necessarily a very 'scientific' method I'm afraid, but the only one I've found so far!.....
I did wonder earlier if the actual tuning was 'skewed' from maximum amplitude (post#186)  but decided there would be two optimum settings - one each side of the peak - if that was the case. ...However, so far I've only been able to discover one optimum setting for the tuning in each set up...
Thanks for these info. I'll try to make the most of it in my simulation.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 12, 2019, 05:09:46 PM
I'm sure you're right about the tuning settings. The most effective way of tuning both coils I've found so far is the one suggested by Ruud early on the thread.... Tune for maximum  sensitivity, using a tone  applied via a transducer placed against the mic.  Not necessarily a very 'scientific' method I'm afraid, but the only one I've found so far!.....
I did wonder earlier if the actual tuning was 'skewed' from maximum amplitude (post#186)  but decided there would be two optimum settings - one each side of the peak - if that was the case. ...However, so far I've only been able to discover one optimum setting for the tuning in each set up...
I suspect the capsule and C4 are just acting as a potential divider for the RF signal.  That would explain the single twiddle peak.  ie AM modulation is NOT due to the LC frequency changing.

If so, T2 is unnecessary.  It ups the voltage but this is already too high for comfort.  It also raises the impedance which raises the 'noise resistance'.  Removing T2 would also remove the noise contribution of the transformer and its associated bits.

We would need to do another noise & sensitivity test like #196 & #219 to confirm if the 'equivalent noise spl' is better.  It may be that T2 is what's introducing the HF lift on signal & noise but that might be wishful thinking  ;)

T1's centre tap is not used and the bottom of its secondary is earthed.  That means the capsule can be earthed too which is often convenient.  If C4 is variable it can be set for max sens.  dC in the capsule is most effective when C4 & the capsule are the same.

So alignment is to twiddle C4 for max sens, then T1 for the same thing.

Du.uh!  This is exactly GG Baxandall's procedure & his understanding of the situation :o

If T1 secondary CT isn't used, that opens up the possibility of using his phase sensitive detector.  Using 2 FETs instead of his 2 BJTs would simplify driving it.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 12, 2019, 05:52:40 PM
I suspect the capsule and C4 are just acting as a potential divider for the RF signal. 
They do act as a potential divider between an in-phase and an out-of-phase signal, so the basic unbalance determines the RF base level. perfect balance would mean full-wave rectification of the audio signal.

Quote
That would explain the single twiddle peak.  ie AM modulation is NOT due to the LC frequency changing.
My simulation (with all its imperfections, approximations and unknowns) shows that both the tuning AND the balance are changed. See Attachment, which shows voltage across C3 (47pF, C8 on the original schemo). Max gain here is about 20dB, which would be about 30vrms (estimating the output of the oscillator at 3Vrms (collector voltage 4.8V). But it looks like the whole circuit shows a maximum significantly lower than 10MHz; actually, at 10MHz, the RF signal is attenuated.

Quote
  dC in the capsule is most effective when C4 & the capsule are the same.
It may be but it's not linear operation when it comes to demod.

Quote
This is exactly GG Baxandall's procedure & his understanding of the situation
This seems to contradict posts #147 & 148.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 12, 2019, 07:01:14 PM
They do act as a potential divider between an in-phase and an out-of-phase signal, so the basic unbalance determines the RF base level. perfect balance would mean full-wave rectification of the audio signal.
Yes.  At perfect balance, it is a DSBSC signal which needs Baxandall's Phase Sensitive Detector to demod w/o distortion

But we don't twiddle C4 to balance the bridge in AMX8.  I think we have a much simpler situation where 'RF leakage from the wonky bridge' is amenable to simple 'diode' rectification.  It's not the LC that needs to be 'detuned' but the bridge.

My proposal maximises this and uses the full RF voltage, 'halved' by C4 & the capsule acting as a potential divider.  As we are not using a bridge & DSBSC, we can twiddle C4 for max audio output.  Gotta be careful as FET bias changes too.

There are problems.  The 'bias' for the FET is the rectified RF voltage.  If RF is 10Vrms, this is 7V across both  R4 & R10

In AMX8, we could twiddle C4 to get a desired bias across R4 & R10 from 'rectified RF leakage' from the slightly wonky bridge
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 12, 2019, 09:42:29 PM
In AMX8, we could twiddle C4 to get a desired bias across R4 & R10 from 'rectified RF leakage' from the slightly wonky bridge
Not sure I quite understand what you are saying; I'm just a dumb 'mechanical' engineer. What is the "desired bias" value. Could you please elaborate a bit.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 13, 2019, 12:17:34 AM
But we don't twiddle C4 to balance the bridge in AMX8.  I think we have a much simpler situation where 'RF leakage from the wonky bridge' is amenable to simple 'diode' rectification.  It's not the LC that needs to be 'detuned' but the bridge.
I agree totally. It calls into question the need for a bridge, since it needs to be very out of balance for proper demod to happen.

Quote
My proposal maximises this and uses the full RF voltage, 'halved' by C4 & the capsule acting as a potential divider.  As we are not using a bridge & DSBSC, we can twiddle C4 for max audio output.
I agree it makes more sense and makes the design simpler to build and to tune. ATM the interaction between the two tanks make the analysis quite difficult.

Quote
The 'bias' for the FET is the rectified RF voltage.  If RF is 10Vrms, this is 7V across both  R4 & R10
Check attachment. That's actually the source voltage vs. capsule capacitance. I have chosen a significant unbalance (fixed cap 68pF, capsule 78 to 86p); as a result, the actual RF voltage at the FET's gate is from 2Vpp to 4Vpp. The resulting source DC voltage varies from 1.8 to 2.7V. That's 0.9V, which would correspond to about 300mV rms or 600mVrms for the full diff output. That's not too different an order of magnitude than the same capacitance variation applied to a 1G resistor with 70V bias, 560mVpp IINM.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 13, 2019, 12:19:19 AM
Attached graph of RF signal at FET's gate vs capsule's variations
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 13, 2019, 12:28:13 AM
Attached graph showing FET current vs. RF gate voltage
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 13, 2019, 04:22:36 AM
Not sure I quite understand what you are saying;  What is the "desired bias" value. Could you please elaborate a bit.
I discuss FET bias in my MicBuilders FETbias.doc

Abbey has in fact answered your question but in a rather roundabout way.

It's about setting good working conditions for the FET.  The voltage across R4 / 10 gives the current through the FET.  The voltage at Drain & Source gives the voltage across the FET.

In FETbias.doc, which describes simple FET circuits up to & including the Schoeps, this is set by the Source resistor & sometimes a potential divider for the Gate.

Here, the Gate voltage is set by the level of the RF presented to it.  Changing the level of the RF by tweaking the bridge will change the bias of the FET.

I don't know at present what the 'desired bias' is as the desired operating conditions are changing with each iteration.  But FETbias.doc tells you the sort of stuff you need to consider.

Which leads me into an important point which we've overlooked ...  In AMX8, R10, the Drain resistor is fed directly from the O/P BJTs.  But that means it is wide open to sh*t on the P48 supply.  The only 'smoothing' is C7 47u.

This may be OK with rogs Sound Devices but there are many prosumer recorders/soundcards (and some 'pro' ones) with terrible noise on their P48.  An important example is early TASCAM DR680 but I think the Mk2 has sorted this out.

Moving R10 & C13 to the other side of R2 and another 47u added across C6 which decouples the oscillator will help this.

Then you need to watch oscillator noise doesn't get into R10.  Also as the voltage available to the FET is now less, you need to be more careful with your bias.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 13, 2019, 07:15:28 AM
I discuss FET bias in my MicBuilders FETbias.doc
Thanks, Ricardo. I'll check it out and refresh my memory; I last read that doc several years ago.

Which leads me into an important point which we've overlooked ...  In AMX8, R10, the Drain resistor is fed directly from the O/P BJTs.  But that means it is wide open to sh*t on the P48 supply.  The only 'smoothing' is C7 47u.

This may be OK with rogs Sound Devices but there are many prosumer recorders/soundcards (and some 'pro' ones) with terrible noise on their P48.  An important example is early TASCAM DR680 but I think the Mk2 has sorted this out.
That is a very important point for me. I use an early model TASCAM DR-680 and, though I have added extra smoothing to the P48 supplies on channels 1-4, I have not been able to do so for channels 5 & 6. My Brahma and my Oktava MK-012 4D are both quite sensitive to noise on the P48 supplies, the Brahma especially so. I will probably be using the RF condenser mics on channels 5/6 with the sh*tty P48 supply smoothing, so they'll need to be resistant to noise on the phantom power supply.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 13, 2019, 10:53:11 AM
How about shoehorning in some capacitor-multipliers? In the simplest form, it's just an NPN, a resistor and a capacitor, and less than 1V dropped.

Sometimes, straight up adding capacitance just doesn't cut it.

Thanks, Ricardo. I'll check it out and refresh my memory; I last read that doc several years ago.
That is a very important point for me. I use an early model TASCAM DR-680 and, though I have added extra smoothing to the P48 supplies on channels 1-4, I have not been able to do so for channels 5 & 6. My Brahma and my Oktava MK-012 4D are both quite sensitive to noise on the P48 supplies, the Brahma especially so. I will probably be using the RF condenser mics on channels 5/6 with the sh*tty P48 supply smoothing, so they'll need to be resistant to noise on the phantom power supply.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 13, 2019, 11:24:39 AM
Do you mean in the mic or in the TASCAM? In the TASCAM DR-680 I have now got the P48 smooth enough on channels 1-4, where they are located on an auxiliary PCB that is relatively easily accessed; but I can't get adequate access to the main PCB to do the same for for channels 5-6 (or even to find the location of the P48 supplies for those channels.
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 13, 2019, 11:43:23 AM
In the Tascam. Is the layout really that convoluted, that the two pairs of 6.8k resistors can't be found (or accessed)? Cutting the trace and tacking on a cap multiplier should be relatively easy to achieve, i would imagine (or at least hope).

Do you mean in the mic or in the TASCAM? In the TASCAM DR-680 I have now got the P48 smooth enough on channels 1-4, where they are located on an auxiliary PCB that is relatively easily accessed; but I can't get adequate access to the main PCB to do the same for for channels 5-6 (or even to find the location of the P48 supplies for those channels.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 13, 2019, 12:46:42 PM
Unfortunately, yes it really is. Nearly a quarter of the TASCAM DR-680 is taken up by the battery tray (8 x AA batteries). It's small! Yet it packs six analogue inputs with phantom power, six analogue outputs, and a stereo S/PDIF in and out. It's possible, if not especially easy, to record all eight input channels if you put two inputs on the S/PDIF in. What with the physical size of the XLR/TRS sockets on channels 1-4, the TRS sockets for channels 5-6, six RCA connectors for the line outs and two more for the S/PDIF in and out, as well as a memory card slot, a 24 V power socket and that battery tray, a lot of space is used inside for connectors; it's very tightly packed inside.

The electronics is spread over a main logic board and three or four auxiliary boards, all tightly packed with SMD components. Apparently the switches on the top cover use pads on the main logic board as switching contacts; there doesn't appear to be room for conventional switches. To get at the main board requires removing some dozens of screws, cables, the auxiliary PCBs, external sockets and other overlaid parts.

I managed to get the main analogue input board out; it's the first one that comes out; that allowed me to add eight extra smoothing capacitors on the phantom power distribution to those sockets. But it has only the combined XLR/TRS connectors and P48 supplies for channels 1-4. The next two boards to come out were the DI/DO and AO boards; they did not have anything for analogue inputs 5/6. At that point I damaged the locking catch for a ribbon connector on one of the boards and am reluctant to dig further; but I did get far enough to see that the TRS connectors for channels 5/6 seemed to be connected to the main logic board, not to any more accessible auxiliary PCB.

But having got that far, it was apparent that removing the main board would likely be a significantly bigger task than all that had gone before as all the top panel switches would no longer be held in place, and possibly would no longer be held together. My courage failed at that point   :-\

Someone has reported that they managed to get TASCAM in Germany to accept to modify their DR-680, to make the phantom power quieter. But it seems to me, having read his posting about it, TASCAM only modified channels 1-4, i.e. added extra capacitors on that auxiliary board. I may be mistaken; I read it some years ago, when I had just got my DR-680 and discovered the problem. TASCAM would not even respond to my request for the same modification.

This is now the only reference I can find to the modification: https://www.gearslutz.com/board/showpost.php?p=9675848&postcount=29. Unfortunately the original was on Audiomasters Forum, which seems to have disappeared; I can't find that thread even on the WayBackMachine (I can see it in the index, but not bring it up).
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 13, 2019, 03:56:21 PM
I suspect the capsule and C4 are just acting as a potential divider for the RF signal.  That would explain the single twiddle peak.  ie AM modulation is NOT due to the LC frequency changing.

If so, T2 is unnecessary.  It ups the voltage but this is already too high for comfort.  It also raises the impedance which raises the 'noise resistance'.  Removing T2 would also remove the noise contribution of the transformer and its associated bits....

I tried out a sample with just T1 fitted, and with a nominal FET gate  resistor (c.100K?) to ground ...
Works OK, but it does reduce the sensitivity by around 24dB... In addition, there was a significant increase in the noise floor (not from the extra preamp gain required). I can only assume that - as the noise level varies with T1 tuning - it comes from the oscillator?.....
So I fitted T2 again!   :)
One other observation I've made...As discussed above, the bridge needs to be unbalanced slightly....
One would assume that dC would be a slightly larger percentage of the C4/ capsule series capacitors if C4 was slightly lower in value than the capsule. In my experiments, the reverse seems to be true.
With a C12 style capsule that measures around  100pF, making C4  115pF rather than 85pF actually increases the sensitivity by  around 6dB, which was a surprise.

One other thing I've noticed ... when tuning the coils for maximum  audio level it's interesting to note that when running the audio output through a spectrum analyser it's possible to 'tweak' the max setting  for minimum 2nd harmonic distortion, without affecting the out put level noticeably.....
Much along the same lines as 'tweaking' the bias on the 1G gate resistor with a multi-turn pot across the source of a Schoeps style impedance converter FET ....

I've been very  interested to follow the comments and observations you and Abbey have been making...
I look forward to see what results come from the mods and changes that are derived from your notes by the more experienced experimenters here on the board, as the project 'moves up a gear' as it were....
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 13, 2019, 05:11:40 PM
I tried out a sample with just T1 fitted, and with a nominal FET gate  resistor (c.100K?) to ground ...
I would think it has no effect on bias because the resistance of the xfmr's secondary is humpteen times smaller. However it may damp the tank a little and reduce sensitivity ...?

Quote
With a C12 style capsule that measures around  100pF, making C4  115pF rather than 85pF actually increases the sensitivity by  around 6dB, which was a surprise.
It seems normal to me, since the tank is then tuned with 115pf and 100pf in series, which is close enough to the nominal 47pF.
The main issue with this latest arrangement is that the tank is damped by the 100R from the oscillator so the response is quite broad and does not show the resonance that increases the RF level.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 13, 2019, 05:30:32 PM
I would think it has no effect on bias because the resistance of the xfmr's secondary is humpteen times smaller. However it may damp the tank a little and reduce sensitivity ...?

With T1 only fitted, the input to the gate is taken from the junction of C4 and the capsule -- which has no DC component to gnd.

Although I now know that some of the FET bias is a result of the rectified RF, I had assumed the FET gate would still need a nominal DC ground reference?...
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 13, 2019, 05:41:36 PM
With T1 only fitted, the input to the gate is taken from the junction of C4 and the capsule -- which has no DC component to gnd.
Aah...OK, then.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 13, 2019, 08:01:06 PM
I use an early model TASCAM DR-680 and, though I have added extra smoothing to the P48 supplies on channels 1-4, I have not been able to do so for channels 5 & 6. My Brahma and my Oktava MK-012 4D are both quite sensitive to noise on the P48 supplies, the Brahma especially so.
I thought Brahma did a mod which sorted this out and issued a recall for early mike users with the problem like yourself.

Did this not happen?
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 13, 2019, 09:00:58 PM
I thought Brahma did a mod which sorted this out and issued a recall for early mike users with the problem like yourself.

Did this not happen?
I never saw any mention of such a recall.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 14, 2019, 12:10:21 AM
I need to be very careful what I say.  I've a good grasp of noise in HiZ mikes but I'm a novice at RF stuff.  The Baxandall mike was my one serious RF project apart from a SMPS which worked well but was far too complicated.  40 yrs ago, I had the Great Guru to hold my hand too.

I never played with the 'simple rectifier' version .. going straight to his PSD.

There's a real danger I'm pontificating from the wrong orifice on RF stuff.  :o

I tried out a sample with just T1 fitted, and with a nominal FET gate  resistor (c.100K?) to ground ...

Works OK, but it does reduce the sensitivity by around 24dB... In addition, there was a significant increase in the noise floor (not from the extra preamp gain required).
I don't think the 100k is needed with a full wave rectifier or the PSD. Maybe with our half wave rectifier.  Half wave rectifier noise is complicated.  When conducting, the 'next' stage sees the tuned circuit which is low noise (?).  But when not conducting (most of the time), the O/P stage sees R7 4k7 which is noisy.

But IS the tuned circuit LoZ and hence low noise?  Or are we just benefiting from higher output?  How does your 100k affect this?

Perhaps its best to Series Tune the output of the bridge like Baxandall does, even with the simple 1/2 wave rectifier.

Quote
I can only assume that - as the noise level varies with T1 tuning - it comes from the oscillator?.....
Carefully re-reading Baxandall shows he thinks a Balanced Bridge & PSD will better deal with oscillator noise ... though I can't see how this has more than a limited effect.  And what is the noise of our Crystal controlled Colpitts compared to Baxandall's Hartley?

It may be having the bridge slightly wonky reduces oscillator noise cos less carrier wave.  In that case, my suggestion of using the 'full' RF would be noisier

Quote
... when tuning the coils for maximum  audio level it's interesting to note that when running the audio output through a spectrum analyser it's possible to 'tweak' the max setting  for minimum 2nd harmonic distortion, without affecting the out put level noticeably.....
Much along the same lines as 'tweaking' the bias on the 1G gate resistor with a multi-turn pot across the source of a Schoeps style impedance converter FET ....
The FET bias is almost wholly set by the rectified RF so this isn't surprising.  You ARE in fact tweaking the bias of a Schoeps mike.

Much of this is a beach bum talking to himself in an attempt to understand some of the black magic.  What we DO have is a benchmark which is AMX8 and we should not make any moves without looking back at this.

About all I'm sure of is that making C13 10n will reduce noise & flatten its response (with rogs capsule) compared to NT1  8)

But we should look at the effect of noisy P48 too as this & the very high output are IMHO, the 2 most important points to address.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on May 14, 2019, 12:40:14 AM
The FET bias is almost wholly set by the rectified RF so this isn't surprising.
Source voltage without RF carrier is 1.1V. Compare with 2.7V for extreme RF level. I would think the FET's threshold voltage to be a significant factor, as it is in a Schoeps-type circuit.

Quote
  You ARE in fact tweaking the bias of a Schoeps mike.
Regarding the role of impedance converter and phase-splitter, yes, but as a detector, I'm not so sure. Detection is a non-linear behaviour. I would think setting the op point is a balance between linearity as a basic audio amp and efficiency as a detector.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 16, 2019, 07:56:20 PM
....But we should look at the effect of noisy P48 too as this & the very high output are IMHO, the 2 most important points to address.

I have 6 mic input sources to hand  that include phantom power....  I had tried out the various versions of this project with 5 of them and had not experienced any problems, noise wise.
Having now tried the 6th  - a cheap Behringer mixer - and I can confirm that it does indeed introduce some hum  into the AMX8 prototype as currently presented.
I have no problem with my own Hi-Z Schoeps type circuit connected to the same input....so maybe time to copy the Hi-Z Schoeps voltage regulation? ....

Adding the components highlighted in blue on the attached schematic seems to solve the P48 noise problem.
In addition, it also makes the mic much more predictable  ---( I'd been having problems matching sensitivities between prototypes for example, with the constantly variable voltages playing havoc with calibration). 
Using a regulated supply (much like standard Schoeps of course) makes life a lot easier.
 
I've managed to add the necessary components to a small 'piggy back' strip board assembly added to Khron's AMX8 PCB - fortunately without any track cuts! 
No where near as elegant of course, but it does seem to prove that controlling the P48 variables - as suggested by Ricardo - does work well.

As to reducing sensitivity? --- Some of my early prototypes were indeed over sensitive - and not always under effective control! This new regulated version seems much more predictable.

There are a number of other component changes which  should help improve things a bit...
•C2 is now 47pF (improves 10MHz oscillator symmetry)
• R10 and R4 are now 2k2 which  - together with a new rail voltages of 9V -  sets the FET  bias such that there is now c. 3 volts across both R4 and R10, and 3V across Q4 D-S.  There should also be a little less noise by using a lower value resistor for drain and source resistors.

If it is still necessary to reduce the sensitivity further, then adding  pad resistors across T2 secondary  - suggested by Ricardo and implemented by Khron in his AMX9 layout - should do the job. 
Sadly, they will  not reduce the noise floor by the same degree of attenuation.

A way of reducing the noise floor - as well as the signal level - using internal attenuation, rather than an external pad remains one of the outstanding project objectives......
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 16, 2019, 08:41:25 PM
AMX9 layout updated w/ zener regulation :)

Not sure much else can be shoehorned into this BM800-sized board, though ;D
Maaaaaybe if i scoot the cans closer to the "top" edge of the board, and the output pads & filtering elements closer to the "bottom" edge...

PS: For cost-cutting reasons, the 12v zener could be made up of two 6.2v zeners, like the ones across the output PNP's.
One less BOM item to worry about, plus better chance of bulk discounts for multiples ;)
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 17, 2019, 04:36:43 AM
AMX9 layout updated w/ zener regulation :)

Not sure much else can be shoehorned into this BM800-sized board, though ;D
Maaaaaybe if i scoot the cans closer to the "top" edge of the board, and the output pads & filtering elements closer to the "bottom" edge...

PS: For cost-cutting reasons, the 12v zener could be made up of two 6.2v zeners, like the ones across the output PNP's.
One less BOM item to worry about, plus better chance of bulk discounts for multiples ;)

Wow! -- I really couldn't see how that was going to be achievable -- great job!  :)

Maybe the ident should now be RF.AMX10  ?..... and if you could post the Gerber files that would be great.....

Re. the zener....... I'm currently paying 0.04p (including VAT) for a 500mW 12V Zener -- so I thought - hang the extra expense!   :) 
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 17, 2019, 04:54:29 AM
Here ya go.

https://www.dropbox.com/s/sc1y3m0l4uvwxs8/RFmic_gerber.zip?dl=0

And the zener substitution was just a thought :P
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 17, 2019, 06:02:32 AM
Looking at the new Gerber files --- thanks again --I can't find C14? ...I'm not sure where it can go though ---I think it may need to be a 2.5mm MLCC  rather than a 5mm poly!

I think C7 and C15 will probably need to be mounted on the reverse side now ---- same as the electrolytics on some of the original BM800 circuit boards..
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 17, 2019, 06:14:45 AM
I sort of questioned the actual requirement / usefulness of C14, actually. The oscillator section has its own "bypass cap" as it were (C6),  and the JFET's not doing all that much in the HF range(?).

And since, with the added zener regulation, there's less need for "brute force" supply rail smoothing / filtering, won't 6.3mm diameter electrolytics suffice? Surely those could easily fit into those footprints :)

A 47u/50v on the "input", before R11 and the zener, and 100-220u/16v after that.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 17, 2019, 06:33:15 AM
I sort of questioned the actual requirement / usefulness of C14, actually. The oscillator section has its own "bypass cap" as it were (C6),  and the JFET's not doing all that much in the HF range(?).

And since, with the added zener regulation, there's less need for "brute force" supply rail smoothing / filtering, won't 6.3mm diameter electrolytics suffice? Surely those could easily fit into those footprints :)

A 47u/50v on the "input", before R11 and the zener, and 100-220u/16v after that.

C14 came from my earlier experiments with my Hi-Z Schoeps....... the low pass filter after the zener - R12 and C15 - gets rid of most of the zener noise, but electrolytics are not always that good at HF decoupling....... So I added C14 to get rid of any HF noise not 'captured' by C15.

As you say, probably overkill....

Yes, I'm sure 6.3mm electolytics would be fine..... I just tend to think in terms of what I already have in stock here!   :)
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 18, 2019, 12:34:16 AM
C14 came from my earlier experiments with my Hi-Z Schoeps....... the low pass filter after the zener - R12 and C15 - gets rid of most of the zener noise, but electrolytics are not always that good at HF decoupling....... So I added C14 to get rid of any HF noise not 'captured' by C15.

As you say, probably overkill...
You need C14 to stop stuff from the oscillator getting to R10 & the FET drain.

rogs, your blue extra bits in #276 & RF.AMX8Zen is the correct way to do things.  Many 'simpler' arrangements abound but (as you have found) lesser arrangements will fail at times.  It's not overkill

With the 'high' voltage BC560s, D2 & 3 don't have to be Zeners.  They could be 1n4148 or any number of SMD double diodes.  But you must have them or else a momentary cable short will zap the base emitter junctions.  The mike will still work but be forever noisy.

Don't forget to leave enough space for a bigger C13 to flatten response & reduce noise.  I think I said 10n with R4 & 10 4k7 so C13 should be 22n with2k2

Quote
If it is still necessary to reduce the sensitivity further, then adding  pad resistors across T2 secondary  - suggested by Ricardo and implemented by Khron in his AMX9 layout - should do the job.
Sadly, they will  not reduce the noise floor by the same degree of attenuation.
If my understanding of the noise sources is right, damping T2 secondary will reduce noise & signal equally for no difference in S/N

Some of you will know I have an obsession with simplification .. but never at the expense of performance or reliability.

The mods we are discussing here all improve reliable performance and we have good evidence they work and are worthwhile.  8)

My musings about feeding the rectifier directly from the bridge come under 'stuff that must be tested' but, IF they work they MAY improve & simplify stuff.  :o
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 18, 2019, 04:01:39 AM
Isn't the oscillator power already sort of "isolated" by R2 and C6? I suppose one could "guild the lily" and replace R2 with an inductor, for extra HF / RF filtering...

You need C14 to stop stuff from the oscillator getting to R10 & the FET drain.

Title: Re: DIY RF Condenser Mics
Post by: rogs on May 18, 2019, 08:06:31 AM
With the 'high' voltage BC560s, D2 & 3 don't have to be Zeners.  They could be 1n4148 or any number of SMD double diodes.  But you must have them or else a momentary cable short will zap the base emitter junctions.  The mike will still work but be forever noisy.
I had just copied the 6v2 protection diodes as fitted in the original Schoeps CMC5 schematic ... I see your point of  them simply acting as diodes, should either pins 2 or 3 be shorted directly to ground - but I had always assumed that using a zener was to protect against  excess voltage presented across the transistor, in the event of the 47uF cap being fully discharged.
If it was then presented with a 'live' phantom power 'plug in'  it might present excess voltage across the CE junctions before the capacitor was charged?......
A bit remote I would have thought? .... but  as Schoeps had fitted them - and at a cost of only at 3p per diode - I decided 'hang the expense'!  :)


If my understanding of the noise sources is right, damping T2 secondary will reduce noise & signal equally for no difference in S/N
Yes of course..... I had assumed that the dominant noise source might be the resistor noise  from the source and drain resistors, which is 'post' attenuator... but a quick practical test confirms your assumption that the noise floor is indeed reduced as well.
(I do need to confirm the actual values for the -10 and -20 dB pads ... the provisional values of 3k3 and 1k  on both the AMX9 and AMX10 schematics may not be correct...

My musings about feeding the rectifier directly from the bridge come under 'stuff that must be tested' but, IF they work they MAY improve & simplify stuff.  :o
I looked again at Baxandall's paper and found this comment on page 594:

"....Ordinary conversational speech at a foot or two corresponds to about 1 dyne/cm' alternating pressure, and this causes, with a typical modern electrostatic microphone element, a capacitance change in the region of 0.001 pF....."

I decided that's not a lot! -   and changes that small would need all the help they could get.......  :)
Hence my decision - at this stage - to stick with the 'noise free' gain  from the step up voltage - and 'Q' - provided by T2 ....

Isn't the oscillator power already sort of "isolated" by R2 and C6? I suppose one could "guild the lily" and replace R2 with an inductor, for extra HF / RF filtering...
...
C6 was fitted to decouple the collector of the oscillator transistor, which helps with the linearity of the oscillator waveform ...R2 was selected to allow the optimum oscillator level - at max 'Q' setting - to be determined.

The idea of C14 was to add extra decoupling  to keep any HF zener noise out, should the electrolytic not be good enough at high frequencies. 
In fact it would also act - with R2 - as a low pass filter to help keep any remaining oscillator noise away from the drain resistor as well.

In the past, I have had problems with HF noise and electrolytics, - but that was some years ago, and modern electrolytics are probably much better with HF..... old habits die hard!   :)
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 18, 2019, 11:07:51 PM
I had just copied the 6v2 protection diodes as fitted in the original Schoeps CMC5 schematic ... I see your point of  them simply acting as diodes, should either pins 2 or 3 be shorted directly to ground - but I had always assumed that using a zener was to protect against  excess voltage presented across the transistor, in the event of the 47uF cap being fully discharged.
If it was then presented with a 'live' phantom power 'plug in'  it might present excess voltage across the CE junctions before the capacitor was charged?......
A bit remote I would have thought? .... but  as Schoeps had fitted them - and at a cost of only at 3p per diode - I decided 'hang the expense'!  :)
The reason Dip. Ing. Wuttke used Zeners is cos he has quite low voltage PNPs as the outputs.  There's no problem with using Zeners though your 'high voltage' BC560s can do with plain diodes.

Quote
Yes of course..... I had assumed that the dominant noise source might be the resistor noise  from the source and drain resistors, which is 'post' attenuator... but a quick practical test confirms your assumption that the noise floor is indeed reduced as well.
This was what prompted my original reply.  But your extra 16dB sensitivity compared to Rode NT1 makes all that moot.

Quote
"....Ordinary conversational speech at a foot or two corresponds to about 1 dyne/cm' alternating pressure, and this causes, with a typical modern electrostatic microphone element, a capacitance change in the region of 0.001 pF....."

I decided that's not a lot! -   and changes that small would need all the help they could get.......  :)
Hence my decision - at this stage - to stick with the 'noise free' gain  from the step up voltage - and 'Q' - provided by T2 ....
Listening to the noise on your samples, I can barely hear those 'clonks' and something starting up with the signal magnified greatly in Audacity.

Extra noise free gain would only be an advantage to drown the noise of a less-than-stellar preamp.  Your Sound Devices is about as quiet as its theoretically possible.

Even the Rode's sensitivity makes it quiet with lesser preamps so yours is well above what is required for these.

I'm investigating Infinite Impedance Detectors.  Most seem to suggest larger Drain resistors eg 10k.

Was there some reason to go from 4k7 to your present 2k2?

I'm just trying to understand how all this works.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 19, 2019, 07:51:43 AM
The reason Dip. Ing. Wuttke used Zeners is cos he has quite low voltage PNPs as the outputs.
According to the data sheets, the CE breakdown voltage for Wuttke's BC416 and for the BC560s are the same .. at - 45V ....
 So  I'm probably just following Wuttke's lead and being ultra safe!   :)
 
Extra noise free gain would only be an advantage to drown the noise of a less-than-stellar preamp.  Your Sound Devices is about as quiet as its theoretically possible.
My Sound Devices unit is a (now rather old) USBPre 1.5 ... and yes, the preamps are quite quiet...
Not quite as quiet as my Tascam DR100Mk3 though...Even my cheap Behringer UMC404HD 'Midas' preamps give quite a good account  of themselves , noise wise.
So I'm a bit spoilt for choice with quiet preamps ...

I'm investigating Infinite Impedance Detectors.  Most seem to suggest larger Drain resistors eg 10k.
I came across the infinite impedance concept almost by accident, and thought it might be worth a try.  I was immediately impressed by the results over a simple diode rectifier...and that seems to be a common thought among those who have used infinite impedance detectors.
Usually for higher quality AM detection in radio, from what I've read..
I would guess that the drain resistor is selected to suit the  FET and the supply voltage?......

Was there some reason to go from 4k7 to your present 2k2?
I changed to 2k2 resistors after I had added the voltage regulation components, as they seemed to suit the FET requirements better, with a supply voltage now around 9V. 
That has certainly been the case with my Hi-Z Schoeps style circuit..... and the smaller value resistors seem to generate less noise - although I'm guessing that's marginal?

I'm just trying to understand how all this works.
Me too!  :)  ..... I think there is more to be done on calculating optimum FET biasing.
It looks like there is going to be a quite narrow range of ideal  FET gate voltage input from T2 ?...Too low and the noise level increases  as the FET doesn't seem to turn off completely in the +ve half cycles? ...Too high and the 'on' voltage swing seems to become non linear....
Problem is, it's the calibration of T1 - not T2 -  that  seems to determine the optimum setting for noise and distortion - which is slightly confusing to a simple hobbyist like me?...
 Investigations continue!
Title: Re: DIY RF Condenser Mics
Post by: Khron on May 19, 2019, 08:36:26 AM
I could be wrong, but calculating things will only get you so far with JFETs, what with them being not exactly tight-tolerance (or tightly-binned) devices. Hence the 1M trimmer in the Schoeps circuit, or the "hand-selected" source resistors in KM8x's and U87's.

But of course, having a starting point, or some values to aim for, would obviously help :D



Me too!  :)  ..... I think there is more to be done on calculating optimum FET biasing.
It looks like there is going to be a quite narrow range of ideal  FET gate voltage input from T2 ?...Too low and the noise level increases  as the FET doesn't seem to turn off completely in the +ve half cycles? ...Too high and the 'on' voltage swing seems to become non linear....
Problem is, it's the calibration of T1 - not T2 -  that  seems to determine the optimum setting for noise and distortion - which is slightly confusing to a simple hobbyist like me?...
 Investigations continue!
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 19, 2019, 09:48:49 AM
I could be wrong, but calculating things will only get you so far with JFETs, what with them being not exactly tight-tolerance (or tightly-binned) devices. Hence the 1M trimmer in the Schoeps circuit, or the "hand-selected" source resistors in KM8x's and U87's.

But of course, having a starting point, or some values to aim for, would obviously help :D

I'm sure you're right.... 'hands on' and you're  checking what actually works!...

The thing I'm currently looking at is how the adjustment of T1  is the one that appears to be performing almost identically to the adjustment of the 1M trimmer in the Schoeps circuit.... 
On a spectrum analyser, you can actually see the change in the  2nd harmonic distortion  - down to a very similar level to the Schoeps -  as you adjust the core of T1.
I would have expected it to be T2 (which is the one that changes the amplitude of the signal presented to the gate) that made the difference?...
EDIT: Bit of a red herring ... sorry!...... Yes, tuning T1 can appear to change the  level of distortion, but only because  changes the level of the oscillator output, and thus the amplitude of the T2 output as well.
So tuning either core can affect the sensitivity and the linearity as the FET bias is changed.   
There does appear to be only one 'sweet spot' -  although there are other (smaller) peaks as you tune for the optimum setting against a tone reference.

As I say, investigations continue...  :)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 23, 2019, 09:55:06 AM
My order of RF.AMX8 PCBs arrived yesterday, some time after Khron uploaded the RF.AMX10 PCB layout last week.  ::)  My previous PCB order, for the original diode detector layout, also arrived after Rogs had advanced the design to the 'Infinite Impedance Detector' circuit. It seems delivery of my PCBs are always a couple of generations behind the state of the design.  :(

All my present choral concerts will be finished early next week, so maybe I will have some evenings free and can get back to making some of the latest generation of this design.

Quick question: Which capsule seems to work best for choral music and which for solo voice? Has anyone tried them in real life? I notice Rogs started with the RK-67 type, but now seems to mention the C12. Is that becayse they are what is to hand, or is it as a result of trying them in a real recording environment and, if so, what were you recording?
Title: Re: DIY RF Condenser Mics
Post by: RuudNL on May 23, 2019, 01:27:40 PM
Off topic, but great to see how this project has grown since the start!
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 23, 2019, 02:28:32 PM
My order of RF.AMX8 PCBs arrived yesterday, some time after Khron uploaded the RF.AMX10 PCB layout last week.  ::)  My previous PCB order, for the original diode detector layout, also arrived after Rogs had advanced the design to the 'Infinite Impedance Detector' circuit. It seems delivery of my PCBs are always a couple of generations behind the state of the design.  :(

I know what you mean about PCB deliveries!.... I received my second batch of Ruud's original prototype PCB the same day as Khron's posted his first BM800  version...... :)

Only to be expected I suppose, when you have such useful contributions to a thread like this....... and I repeat, I don't think we should be anywhere near this far on without such great  input from others -- especially  Ruud and Khron !

I don't have any AMX10 PCBs yet, but I have managed to add the voltage regulator mods to the AMX8 PCB on a small stripboard - and without cutting any tracks on the PCB. (Photo attached).
It's only really important when used with poor phantom power supplies ... I can jot down a few notes if that would help?...


Quick question: Which capsule seems to work best for choral music and which for solo voice? Has anyone tried them in real life? I notice Rogs started with the RK-67 type, but now seems to mention the C12. Is that becayse they are what is to hand, or is it as a result of trying them in a real recording environment and, if so, what were you recording?

I've tried out K47, K67 and C12 style capsules, simply because that's what I have here.... They are only Chinese versions, sadly...

The C12 is the most sensitive and thus has the lowest noise floor. The K67 is almost as sensitive, but requires a different value for bridge balance capacitor... ( around 68pF for K67 and  68p in parallel with 47pF for the C12.

The K47 was lot less sensitive -- and surprisingly had the largest capacitive value, at around 100pF.

I've only done voices tests here so far, and I prefer the C12 personally... The K67 is too bright for my liking. 
I haven't done any  internal EQ testing -- I tend to prefer adding EQ externally -  so the AMX10 FET load values of 2k2 and 4n7 should give an HF -3dB cutoff frequency at around  16KHz.  Other values may suit different versions better?....
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 23, 2019, 03:18:15 PM
Off topic, but great to see how this project has grown since the start!
Not quite sure how far it would have got without the huge 'nudge' your prototype PCB gave it ! .... thanks again ... :)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 23, 2019, 05:28:35 PM
I have managed to add the voltage regulator mods to the AMX8 PCB on a small stripboard - and without cutting any tracks on the PCB. (Photo attached).
It's only really important when used with poor phantom power supplies ... I can jot down a few notes if that would help?...
Please; that would help me. My main recorder is a Mk. 1 TASCAM DR-680, which has noisy phantom power (I've added extra smoothing on the P48 supplies for inputs 1-4, but I can't do the same for inputs 5-6; I use inputs 1-4 for my Ambisonic mic).

I look forward to seeing any final updates or comments. But mainly I look forward to trying out these mics and asking some professional recording friends for their comments. I'll post their comments and my own.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 24, 2019, 08:13:47 AM
Please; that would help me. My main recorder is a Mk. 1 TASCAM DR-680, which has noisy phantom power (I've added extra smoothing on the P48 supplies for inputs 1-4, but I can't do the same for inputs 5-6; I use inputs 1-4 for my Ambisonic mic).

Some notes on the actual experiment here:

www.jp137.com/lts/RF.AMX8voltreg.pdf

Certainly helps keep out the rubbish from the  noisy phantom power I have on a preamp here ( part of a cheap Behringer mixer)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 25, 2019, 08:03:10 PM
Thank you, Rogs. Much appreciated.

When things quieten down, I'll make up some of the RF.AMX8 boards with the add-on power supply stripboard and see how I get on. Gotta make up a power supply for a Sennheiser MKH 110 too - that's the one with a controlled frequency response down to 1 Hz (the MKH 110/1 allegedly went down to 0.1 Hz! That's more like a high-speed barometer). However, I'm taking part in two choral concerts tomorrow and I've got to finish and present the accounts and treasurer's report for one of my choral societies next week; so I probably won't get anything done electronics-wise before the bank holiday weekend.
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 25, 2019, 08:45:03 PM
Thank you, Rogs. Much appreciated.

When things quieten down, I'll make up some of the RF.AMX8 boards with the add-on power supply stripboard and see how I get on. Gotta make up a power supply for a Sennheiser MKH 110 too - that's the one with a controlled frequency response down to 1 Hz (the MKH 110/1 allegedly went down to 0.1 Hz! That's more like a high-speed barometer).

I look forward to seeing how you get on in due course .....

Re: the extreme Sennheiser LF response ...
You may have noted that Baxandall's paper  includes the observation (on page 594 in the section  'Linearity Measurements') that "this microphone system has a response extending down to zero frequency" ... So providing the coupling caps are large enough  I can't see why you couldn't get a response that extended down to 1Hz  from our mic?...
The precise  capacitor values would need to be based on the actual input impedance of the emitter followers - which depends in large part on the Hfe of the devices.. so some calcs to be done there...

And I'm not sure quite what you might use it for?...recording elephant infra-sound maybe?  :)
The lowest organ 32' C pedal has a fundamental of 16Hz , and that would push most  recorders to their limit....
But below that?... pretty specialised stuff I would think! 
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 25, 2019, 09:12:02 PM
Yes, it's an instrumentation mic and reputedly may have had a military application back in the '60s. A pair were sold recently on eBay by a recording hire company in Ireland (actually the later MKH 110/2, but otherwise I believe the same spec as my older MKH 110). Mine came from a theatre that was disposing of older equipment. I got it for recording big organs; though I agree it is rather overkill. It was not very expensive.

I want to get that "trouser-flapping" effect of the low pedal stops. If the mic works properly, next stop is to check the  recorder LF response. And then to hope the recordings are not overwhelmed by environmental infrasonic noise ::)

Yes, I am aware of Baxandall's comment re LF response; that was one of the reasons I started this project. It will be interesting to see how our mic compares with the MKH 110.  ;)
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 27, 2019, 01:11:30 PM
Yes, it's an instrumentation mic and reputedly may have had a military application back in the '60s.
Just checked. The Sennheiser Micro-Revue 70-71 (http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Sennheiser%20Micro-Revue%2070-71.pdf, page 35) refers to "military and research purposes". The frequency response curve looks suspiciously flat to 10 kHz, as if it comes out of the art department rather than the development or production department  :o
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 28, 2019, 07:58:48 AM
Rereading Baxandall's paper again last night, I thought it might be interesting to look at some of the references in his article. I found one that looks interesting in it's own right, though I haven't had time to read it in full. It is the paper by J. J. Zaalberg van Zelst: "Circuit for Condenser Microphones with Low Noise Level". It can be found at https://www.americanradiohistory.com/Archive-Philips-Technical-Review/40s/Philips-Technical-Review-1947.pdf, starting at page 357 (p.359 of the pdf file).

Another paper that is available, but only behind a paywall, is "Noise in Condenser Microphones" by Becking and Rademakers. It's at https://www.ingentaconnect.com/contentone/dav/aaua/1954/00000004/00000001/art00033;jsessionid=2pao1jlta9b09.x-ic-live-02

Has anyone found the article by Arends, either the original Dutch article or the German version (or an English translation)? This is reference 7 in the second part of Baxandall's article.
Title: Re: DIY RF Condenser Mics
Post by: cyrano on May 28, 2019, 10:29:00 AM
It's also available at ResearchGate:

https://www.researchgate.net/publication/262854727_Noise_in_Condenser_Microphones

IIRC it only requires a free subscription. I've found articles there before, but I don't know if this is true for all their papers. Anyhow, they're preferable over Ingenta in my mind. Ingenta is only in it for the money and the grab what they can get, just like Elsevier.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 28, 2019, 10:58:47 AM
Thanks. I'll try ResearchGate; though, as I am not an accredited researcher with a recognised institution, I have had very mixed success with them. I think I have only once or twice succeeded in getting a paper through ResearchGate.
Title: Re: DIY RF Condenser Mics
Post by: cyrano on May 28, 2019, 05:28:37 PM
I don't use it that often either. And I DON't remember the success rate, to be honest.
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 28, 2019, 05:37:07 PM
I want to get that "trouser-flapping" effect of the low pedal stops. If the mic works properly, next stop is to check the  recorder LF response.
For 'trouser-flapping', you need to record in STEREO with Fig-8s and have STEREO SUBS.

With omnis and mono, (most recording techniques with omnis are 'mono' below 50Hz) or single subs, you only get the unpleasant 'pressure-in-the head' feeling .. suitable for Arnold S. destroying the universe but not so nice for John Seb.

Alas, there are only a handful of mikes with significant Fig-8 response below 50Hz .. mostly my designs.

There's a secret code in TetraMic that unlocks its subsonic response if your recorder & other stuff is up to it.  But no one has complained about TetraMic's usual 25Hz limit.

I've designed speakers for cathedral organs so have some experience with 32' stops.  The biggest installation used to be the nave organ at Worcester which was only replaced a few years ago.  IIRC, it had 16 subs and 16 'organ' speakers.

It's claim to fame was when the organ grinder was dancing on the pedals at an early  recital, a 1 ton block of stone fell from the ceiling, fortunately in the aisle rather than on the audience.  :o
Title: Re: DIY RF Condenser Mics
Post by: Marik on May 29, 2019, 01:46:58 PM
Alas, there are only a handful of mikes with significant Fig-8 response below 50Hz .. mostly my designs.

Ricardo,

I assume we are talking condenser mics here, because ribbons do much better than that. Our MF65 has -0.2dB at 20Hz and newest AL95 with piston ribbon does even better...

Best, M
Title: Re: DIY RF Condenser Mics
Post by: ricardo on May 30, 2019, 03:08:12 AM
I assume we are talking condenser mics here, because ribbons do much better than that. Our MF65 has -0.2dB at 20Hz and newest AL95 with piston ribbon does even better...
:o

What distance are you measuring at?  Is the reference mike an omni?
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 30, 2019, 08:02:39 AM
Has anyone tried the TCS-1 capsules from JLI Electronics - for any microphone, not just for this RF design? (https://www.jlielectronics.com/transound/tsc-1). It appears to be significantly cheaper than the equivalent single-sided RK-67 from, say, WGTCentre; but if it's made by Transsound, it should be of decent and consistent quality.

Just a few points make me ask here before trying a couple of these capsules: JLI's shipping costs to Europe are very high (courier only); the specs on the JLI web page are very limited, but they give the frequency range as 100 Hz to 20 kHz; and they don't include a saddle, as WGTCentre does. That LF end of the range is rather high but they don't say how far below the nominal output it is at 100 Hz or how steeply it cuts off; maybe they use a thicker diaphragm.

Are they worthwhile trying?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 30, 2019, 08:48:45 AM
From the images, the Transsound capsules look pretty much like 'standard' Chinese K67 types.
The published specs of most of these capsules are just  made up by the marketing guys - not to be taken too seriously.
 
WGCenter are OK, but you have to hope they send the right parts (I have just received a double sided K47 when I ordered a C12 -- too much hassle to send it back though --I'll use it somewhere! :) )

I still think AliExpress are the cheapest for 34mm K67s..... usually with free shipping.

I've used these before:  https://tinyurl.com/y3oveux3

No saddles, but I find these  from Shapeways work well :    https://www.shapeways.com/product/AX4APE9SV/k87-saddle?optionId=65223254&li=marketplace

I've also had quite good results making my own from bits of 2mm and 4mm Neoprene rubber - and a dab of  super glue .... but only when the budget is really  tight!

EDIT: even cheaper one here:  https://tinyurl.com/y37tq3fm  - and apparently UK stock? ... 


Title: Re: DIY RF Condenser Mics
Post by: Marik on May 30, 2019, 10:33:29 AM
:o

What distance are you measuring at?  Is the reference mike an omni?

Cross-spectrum did it for us. It's been awhile, but from memory, measured at 80cm with proximity correction applied. Yes, the reference was an omni.

Best, M
Title: Re: DIY RF Condenser Mics
Post by: Gerard on May 30, 2019, 10:38:06 AM
Thanks, Rogs. I bought most of my 34 mm capsules from WGTCentre and, so far, they have sent me the right ones each time. Those AliExpress ones look like some I bought; mine came from Rayking (another company that also makes fairly decent electrets, though their 25 mm electret is reputedly not quite as good as the TSB-2555 from Transsound): https://rayking.en.alibaba.com/product/60573293486-803584280/34mm_capsule_microphone_Rayking_factory_large_diaphragm_condenser_microphone.html.

But I wonder how much these capsules vary from vendor to vendor; do they all come out of the same factory? Even if different Chinese vendors all make their own capsules themselves, is quality control better from some vendors? If so which? Transsound have a generally good reputation for electret capsules and claim ISO 9000 accreditation, but does that extend to the 34 mm capsule they offer? Perhaps someone out there knows or can answer from experience of having bought from different vendors and made a comparison.

I have sometimes wondered at the very much higher prices for capsules charged by, for example, Microphone Parts, Studio Products, ADK, Advanced Audio Microphones and others; most of them do not make their own capsules. Do they select capsules to meet a certain standard? or just charge what the market will bear?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 30, 2019, 01:57:48 PM
.......most of them do not make their own capsules. Do they select capsules to meet a certain standard? or just charge what the market will bear?

I think most of them will claim the former ... in reality, I suspect it's often the latter. 
Title: Re: DIY RF Condenser Mics
Post by: ricardo on June 06, 2019, 04:55:51 AM
I have sometimes wondered at the very much higher prices for capsules charged by, for example, Microphone Parts, Studio Products, ADK, Advanced Audio Microphones and others; most of them do not make their own capsules. Do they select capsules to meet a certain standard? or just charge what the market will bear?
Certainly Microphone Parts do a lot of selection for those they sell.

Has anyone in Oz made a variable pattern mike with a Microphone Parts RK12 and would be willing to lend it to me for a couple of weeks?  I can promise at least a detailed report.

 I made detailed measurements of Zephyr's CK12 mike from WGT IIRC. The results are in Yahoo MicBuilders Files.  You have to join.  It's not bad but there are a few things which could be better ..  notably the fig-8 response.

There's some evidence that the Microphone Parts RK12 won't have this fault but I need to get my paws on a complete mike to confirm this.
Title: Re: DIY RF Condenser Mics
Post by: zephyrmic on June 06, 2019, 07:32:53 AM
I certainly would be interested to find out what Microphone Parts C12 type capsule is like by comparison to the one you tested for me, which I very much appreciated. Would be good if a comparison can be  made. And it would add to the overall knowledge of this type of mic and give others a good basis as they consider what capsule to purchase in days to come.
Kindest regards,
zephyrmic
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 08, 2019, 05:59:39 AM
Just a quick note to say that I've tried out the figure of eight version of the idea I suggested in post#253.... (sketch attached) ..
Seems to work quite well.   Only tried out with a dual K.47 capsule so far...( I intend to try out a dual C12 as well)   
I did have to add a small capacitor (4.7pF) across one side of the capsule, to correct an imbalance between the output from each side.....Probably down to the fact that the centre tapped 5u3H winding is not bifilar wound?....

One step close to creating a 34mm condenser 'mid-side' mic suitable for outdoor use -- which I've always fancied trying.....
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on June 08, 2019, 06:56:08 AM
I did have to add a small capacitor (4.7pF) across one side of the capsule, to correct an imbalance between the output from each side.....Probably down to the fact that the centre tapped 5u3H winding is not bifilar wound?....
Are you sure about this? Do you not think it's due to capsule imbalance? Then the additional cap could be permanently affixed to the capsule and would not require an additional pole to the switch.
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 08, 2019, 08:43:08 AM
Are you sure about this? Do you not think it's due to capsule imbalance? Then the additional cap could be permanently affixed to the capsule and would not require an additional pole to the switch.

Good point -- I can reverse the capsule connections to check....
Title: Re: DIY RF Condenser Mics
Post by: Martin K on June 16, 2019, 03:54:41 AM
About the paper "Noise in Condenser Microphones" by Becking and Rademakers"; see this link
https://www.icacommission.org/Proceedings/ICA1953Delft/ICA01%20Proceedings%20Vol1%20OCROPT.pdf (https://www.icacommission.org/Proceedings/ICA1953Delft/ICA01%20Proceedings%20Vol1%20OCROPT.pdf).
Page 96.

Martin K
Title: Re: DIY RF Condenser Mics
Post by: Gerard on June 16, 2019, 08:19:47 AM
Thanks for that. I'll download it and read it later this afternoon.
Title: Re: DIY RF Condenser Mics
Post by: Gerard on June 16, 2019, 06:38:27 PM
Good point -- I can reverse the capsule connections to check....
Rogs, did you have time to try that yet? Any result or comment?
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 17, 2019, 03:48:30 AM
Rogs, did you have time to try that yet? Any result or comment?
No, I haven't taken things any  further at present.
I'm still waiting for a dual membrane C12 style capsule to try this out with.
WGTcenter have sent me a K47 type in error - twice! (I'm still trying to get refund on the second one..... I think I shall be avoiding WGTcenter in future!)

Hopefully, I shall be receiving a dual membrane C12 style from Rayking within the next day or two.....
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 17, 2019, 09:49:24 AM
Just a quick update .....
I have now tried reversing the connections on my dual K47 prototype, and can confirm that Abbey is right -- it is differences between the capsules...

I shall experiment further when my C12s arrive....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on June 17, 2019, 10:30:18 AM
WGTcenter have sent me a K47 type in error - twice!
I found reference to another supplier of capsules in another GroupDIY thread; a Guosheng, who makes microphones under the 3U Audio label. The thread suggests he makes his own capsules in both single-sided (RK-47 type) and double-sided (CK12 type) and also SDC capsules. The messages there suggest his capsules are good. They are certainly more expensive than, say WGTCentre. See https://groupdiy.com/index.php?topic=70549.msg905992#msg905992 and the following two messages, and https://www.ebay.co.uk/itm/112996573095. I have written to him to ask what he can supply (the thread I quoted is not particularly recent) and what prices.

That thread also mentions other capsules, though I have not investigated them yet.
Title: Re: DIY RF Condenser Mics
Post by: Martin K on June 22, 2019, 10:58:52 AM
Regarding the G. F. J. Arends paper in Omroeptechnische Mededelingen No.1 1961 as mentioned in Baxandall references;
A dutch radio historian kindly send me a copy.
I put it in Technical documents as "G. F. J. Arends paper, Omroeptechnische Mededelingen 1961".

Regards,
Martin K
Title: Re: DIY RF Condenser Mics
Post by: Gerard on June 22, 2019, 08:05:10 PM
Regarding the G. F. J. Arends paper in Omroeptechnische Mededelingen No.1 1961 as mentioned in Baxandall references;
A dutch radio historian kindly send me a copy.
I put it in Technical documents as "G. F. J. Arends paper, Omroeptechnische Mededelingen 1961".

Regards,
Martin K
Thanks for posting. But something seems to be wrong; I'm getting "404 - Attachment Not Found"
Title: Re: DIY RF Condenser Mics
Post by: Gerard on June 23, 2019, 08:04:58 PM
I see the re-upload. I have now been able to download it. Thanks.
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 29, 2019, 11:15:31 AM
I've now had a chance to try out Khron's latest RF.AMX10 pcb with different capsules.

The layout works fine - thanks again Khron!  :)

I did have some trouble with getting the noise floor as low as I had it with some earlier versions...but this turned out to be nothing to do with the PCB layout.

A bit more experimentation suggested that -  although any phase noise with the oscillator is not critical in this application - any amplitude modulated noise is. (Pretty obvious when you read Baxandall carefully of course! )

I've therefore made some component changes to try and improve the linearity of the oscillator, by increasing the voltage swing
 of the oscillator itself, and simultaneously reducing the reactive load  presented by T1 primary by increasing the value of the feed resistor.  Seems to provide better - and more consistent- results.

Details of the changed values (no track pattern changes required) here:  www.jp137.com/lts/AMX10-2txt.pdf

EDIT : 1.7.19: Couple of further value 'tweaks'.  Version is now :  www.jp137.com/lts/AMX10-3txt.pdf

I've also tried out a double sided C12 capsule to create a figure of 8 version.   Seems to work pretty well - although there can be a need to add a small capacitor across one side of the capsules to make both sides match (discussed previously).

What I think maybe more complex is making a switchable multi-pattern version.....
Changing modes would require changes in  'balancing' the capsule capacitive loads, and this in turn would probably require the inductors to be retuned to optimise the settings for each mode  ....  Not good!

For the time being, I'm intending to try out a 'Mid-Side' mic, using 2 x BM800 bodies:
One double sided C12,  mounted as a 'side address' for the 'Side' mic --  and one 'end address' single sided C12 for the 'Mid' mic .

A bit cumbersome maybe, but at least I shan't need to worry about any RF 'birdies'  - which could occur with all capsules mounted in a single enclosure.....
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on June 29, 2019, 12:22:11 PM
What I think maybe more complex is making a switchable multi-pattern version.....
Changing modes would require changes in  'balancing' the capsule capacitive loads, and this in turn would probably require the inductors to be retuned to optimise the settings for each mode  .... 
I suggest replacing the capsules that are not connected with fixed capacitors.
Title: Re: DIY RF Condenser Mics
Post by: rogs on June 29, 2019, 01:26:34 PM
I suggest replacing the capsules that are not connected with fixed capacitors.
I did include that idea on my initial sketches for the concept (copy attached)  but I can't see how to actually achieve it in practice? The high(ish) Q of the tuned circuitry is pretty selective --- you can only move the tuneable inductor core  a few degrees (say 30) for the output to fall dramatically - over 10dB.

In a 'figure of 8' mode, each capsule is connected across T1 secondary in series..... So T1 load will typically look like about 40pF. (Assuming each capsule is around 80pF - typical for a C12 type)

The same capsule in ommi mode will have both capsules effectively in parallel - so around 160 pF.  That will require a similar value cap to be added to (almost) 're-balance' the bridge. ...So the T1 secondary load would then be around 80pF...

With a 10MHz oscillator, the resonant value of inductance would then need to be around 3.16uH , as opposed to  around 6.3uH for a 40pF capacitor....

Both those values do fall within the specified limits ( well almost!), but would need a very different setting for the inductor core...

Unless I'm missing something here?.....
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 01, 2019, 04:49:34 PM
Couple of further 'tweaks' to the RF.AMX10-2 schematic in post #326 above.  Ref. is now RF.AMX10-3

Copy here: www.jp137.com/lts/AMX10-3txt.pdf
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 04, 2019, 04:59:50 PM
I've tried out several different capsules in various prototypes, and one thing that has become clear is just how different the sensitivity is between the different types.
K47 and CK12 style capsules seem to have a similar capacitive values (around 80- 90pF)  and similar sensitivity.
K67 style capsules tend to have a lower capacitive value - around 65pF -  and are about 10 to 12dB more sensitive.

Problem is - using this type of RF circuitry - the noise floor remains pretty much constant, regardless of the sensitivity of the capsule. 
So the S/N ratio using a K67 is around 10 to 12dB better than when using K47 or CK12 types.
Using a K67, the noise floor is around the same as with a 'standard' FET Schoeps type circuit...
Using a CK12 or K47 type, the S/N ratio is around 10dB less.
Which is a bit disappointing, if you need a really low noise mic  because - IMHO - the K47 and CK12 capsules tend to sound better than the K67 type.   
I know there are a number of things which affect the sensitivity of a capsule - the  thickness and tension of the membrane for example.
With the lower voltages employed with RF bias type mics, one could afford to use a lower tensioned membrane...which might be more sensitive?...... It's just finding out which types have that, as part of their construction....
 
EDIT:  I should add that  simply listing a 'type' of capsule is probably oversimplifying some of the above comments. 
I have tried out 2 different 'C12' style capsules (i.e edge terminated).  The Rayking type from AliBaba is listed as 6 microns, and has a sensitivity  similar to a K47.
A 3 micron 'C12' style from a UK supplier (no longer available) has a sensitivity more like a K67.... so I think there is probably quite a range of sensitivities -  even among similar 'types'....
Title: Re: DIY RF Condenser Mics
Post by: gyraf on July 15, 2019, 05:52:12 AM
..so is it the "dead capacitance" of the CK12-style design that swamps noise figure, or something else..?

Jakob E.
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 15, 2019, 05:16:41 PM
..so is it the "dead capacitance" of the CK12-style design that swamps noise figure, or something else..?
Jakob E.
I'm not sure I understand what you mean by 'dead capacitance'? ....
In the present configuration, the noise floor of the circuit varies considerably - depending on the inductor settings.

I tried removing the capsule and replacing it with a  similar value capacitor to the 'load' capacitor - in this case 68pf.

It was then possible to adjust both inductor slugs to find a very specific 'null' point where all the noise virtually disappears (It's then at least as good as my Schoeps circuit)
T1  and T2 both appear to be at their maximum 'tuned' settings at this null point.

Sadly, this is not the same null point that provides the maximum audio output with  a capsule fitted, with an audio source used for calibration.  There is an increase from the minimum noise floor by around 10dB or so at the maximum audio output setting.

The overall  signal/noise ratio is therefore determined by the sensitivity of the capsule employed.  --
My  3micron C12 and  K.67 samples are about 10dB more sensitive than the K47 - or indeed my 6 micron C12 from Alibaba.
And the signal/noise ratio of those latter two is obviously worse by the same factor.

I've not been able to discover so far what  causes this oscillator noise.  The oscillator output looks to be pretty sinusoidal - especially when measured at either end of the T1 secondary  winding.
The fact that it is possible to tune out most of the noise at the max 'tuned' settings does rather suggest that the signal at the gate of the infinite impedance FET is what is causing problems. 
Looking at the tiny 'residue' of the oscillator signal at both the drain and the source of the FET does show some non-linearity -   and the waveform at the drain is slightly different from the source.  At the minimum noise 'null' point these tiny (a couple of millivolts) residue signals appear to be at their maximum amplitude .

There may well be ways of further improving the application of the FET in this circuit -- it is acting as both rectifier and phase splitter - but I fear the technicalities of how best to progress  may be a bit above my 'pay grade', engineering wise.

As I have mentioned before, the circuit works way better than I had ever imagined when I first sketched it out - especially with high sensitivity capsules -  and I think we're now probably moving into the realms of the law of diminishing returns, trying to take such a simple concept very much further..

It would be nice to try and discover how the inductor tuning settings are  affecting the FET noise output though..... :)
Title: Re: DIY RF Condenser Mics
Post by: cyrano on July 15, 2019, 06:39:25 PM
A question that popped up while reading about your progress: "Why is it Sennheiser only uses RF for shotguns?"

I mean, could it be related to what you're finding in capsule sensitivity? Smaller diameter? Smaller capacitance?
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 15, 2019, 08:25:51 PM
A question that popped up while reading about your progress: "Why is it Sennheiser only uses RF for shotguns?"

I mean, could it be related to what you're finding in capsule sensitivity? Smaller diameter? Smaller capacitance?

The Sennheiser white paper on the MKH story : (https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf)  makes some interesting observations on some of these points.  Because the impedance of an RF capsule is low - and independent of audio frequency - it's easier to manipulate frequency response without adding any noise. So extending the bass response of small capsules is much simpler.
In addition, the directivity of smaller capsules  is better than larger ones, making them more suitable for shotgun use. 
Add to that the advantages of RF condensers for outdoor use, and I think Sennheiser have merely decided to concentrate on  developing smaller capsules  for their RF mics.

One further point I have read (although I haven't been able to quickly find the reference in that paper)  is that the much reduced voltage across the capsule in RF usage allows for less tension in the capsule membrane  - and thus higher sensitivity - in capsules used in this mode.
Certainly, my 3 micron C12 capsules seem to have a lower tension than other capsules I've tried.   They certainly don't seem to like anything much above 50V  polarisation across them in 'conventional' usage!

Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on July 16, 2019, 04:46:20 AM
It was then possible to adjust both inductor slugs to find a very specific 'null' point where all the noise virtually disappears (It's then at least as good as my Schoeps circuit)
T1  and T2 both appear to be at their maximum 'tuned' settings at this null point.
Since the detection sensitivity is based on the unbalance of the bridgeI would think it's normal to assume the circuit does not operate at it maximum sensitivity when noise is minimum. Knowledge of the various RF voltages is paramount in analysis. Unfortunately quite difficult since probing is invasive.

Quote
Sadly, this is not the same null point that provides the maximum audio output with  a capsule fitted, and an audio source used for calibration.  There is an increase from the minimum noise floor by around 10dB or so at the maximum audio output setting.
Exactly for the same reason. In addition, the capsule's  acoustic radiation noise comes into play.

Quote
I've not been able to discover so far what  causes this oscillator noise.  The oscillator output looks to be pretty sinusoidal - especially when measured at either end of the T1 secondary  winding.
Since it's amplitude demod, I would think distortion and phase noise to be largely eliminated. It may even work with a square wave... Frequency stability is necessary though, since it modulates the tuned bridge output.

Quote
The fact that it is possible to tune out most of the noise at the max 'tuned' settings does rather suggest that the signal at the gate of the infinite impedance FET is what is causing problems. 
Looking at the tiny 'residue' of the oscillator signal at both the drain and the source of the FET does show some non-linearity -   and the waveform at the drain is slightly different from the source.
Indeed non-linearity is necessary; if not there would be no detection. I don't think noise results from these non-linearities. Indeed the actual voltage amplitude is what governs the detection efficiency.

Quote
  At the minimum noise 'null' point these tiny (a couple of millivolts) residue signals appear to be at their maximum amplitude .
Seems to be consistent with "maximum tuned setting"

It would be nice to try and discover how the inductor tuning settings are  affecting the FET noise output though..... :)
[/quote] I believe you have everything to figure this out. Plot noise vs. RF level.
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 16, 2019, 05:26:21 PM
Many thanks for your comments and suggestions..... some more testing needed, I think!....

Title: Re: DIY RF Condenser Mics
Post by: cyrano on July 16, 2019, 06:21:14 PM
Thanks, rogs, for the link and the insight. Food for thought!
Title: Re: DIY RF Condenser Mics
Post by: gyraf on July 17, 2019, 06:56:56 AM
I'm not sure I understand what you mean by 'dead capacitance'? ....

It's that the "active", i.e. moving part of the diaphragm is a relatively smaller portion of the total capsule capacitance - mainly because of the membrane retaining ring being at front potential. Thought this might have upset the system.

But I see your explanation.

btw. I find this thread to be some of the most exiting things that has happened around here for a long time - following closely, but I have nothing to add at this time..

Jakob E.
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 20, 2019, 05:40:50 PM
...."It would be nice to try and discover how the inductor tuning settings are  affecting the FET noise output though".....
 I believe you have everything to figure this out. Plot noise vs. RF level.

You're right of course --  I should have given it a little more thought before I posted....

The 'no noise' null point  occurs simply because - at maximum tuned RF level - the FET is in saturation.
With no volts across the FET  the capacitors remain essentially fully charged during the whole cycle.
And that leaves only the noise from the 'Schoeps' output stage....

The oscillator output voltage needs to be as large as possible so that - in conjunction with T1 being tuned to maximum - the voltage across T1 secondary presents as large an AC voltage as possible across the bridge.
T2 is then tuned to allow the FET gate to be biased to the optimum setting.
 
This will not be at maximum of course - for the reason mentioned above - and will vary for each bridge and FET combination.
But there is an optimum setting which can be easily derived, by tuning for maximum audio from a source presented near to the capsule. (This approach was suggested by Ruud very early on in the project).

There is some noise introduced from the oscillator inductor assembly  - although I suspect it's constitution is complex, as suggested by Baxandall on page 595 of his paper.

What does make a huge difference to the overall performance is the sensitivity of the capsule. Using one of my cheap K67 Chinese capsules I can get a signal/ noise ratio similar to my Rode NT1, even though the RF circuitry  introduces more than 10dB of noise into the system.
My K47 - and some C12 capsules - appear to  have a much lower sensitivity, and thus a much reduced overall noise performance.

It was interesting to note that when testing an AMX10 PCB with two equal mlcc capacitors - and no capsule - for the bridge  components , the whole sub assembly is still microphonic.
Presumably something  to do with the IF cans although - as I mentioned above - I think the overall noise generating mechanisms may be complex to analyse.

In any event, using a high sensitivity capsule renders to noise floor low enough not to worry too much about!
Title: Re: DIY RF Condenser Mics
Post by: Khron on July 20, 2019, 05:47:22 PM
It was interesting to note that when testing an AMX10 PCB with two equal mlcc capacitors - and no capsule - for the bridge  components , the whole sub assembly is still microphonic.
Presumably something  to do with the IF cans although - as I mentioned above - I think the overall noise generating mechanisms may be complex to analyse.

The cans are a "maybe", but the MLCC's are a definite "yes" for being microphonic.
Title: Re: DIY RF Condenser Mics
Post by: rogs on July 20, 2019, 06:07:26 PM
The cans are a "maybe", but the MLCC's are a definite "yes" for being microphonic.

I'm pretty sure the 68pF caps I used for the test were NPO, and not X7R or Z5U .....

I  thought I had read that class 1 MLCC caps weren't very microphonic?... although I may have that wrong...

 
Title: Re: DIY RF Condenser Mics
Post by: Khron on July 20, 2019, 08:16:19 PM
Hmm... At least according to Wikipedia, class 1 ceramics (NP0 among them) should indeed not really be microphonic - i stand corrected.
Title: Re: DIY RF Condenser Mics
Post by: rogs on October 05, 2019, 09:02:23 PM
It's been a couple of months since there's been any more posts on this topic ....
I know that Gerard was hoping to complete a  version  using Khron's RF.AMX8 pcb layout -  as time permitted - and Ruud had already tried out an early version using his original prototype PCB ..
But whether anyone else has had a look at trying the idea out, I'm not sure?.

Meanwhile, I've rationalised  many of the ideas put forward by the various contributors to the topic, and have come up with a 'repeatable' version based on Khron's RF.AMX10 PCB layout.

I've made some notes on it here:

www.amx.jp137.com

Hopefully, they will inspire others to have a go ...I'd love to see what variations  other people might suggest.....
Title: Re: DIY RF Condenser Mics
Post by: Gerard on October 06, 2019, 03:12:11 PM
Hi Rogs,
Great set of pages on your website. And thank you again, and Kron and Ruud and everyone else who has contributed, for all the dedicated work. My own professional work as a mechanical engineer has taken me away from home to another country for some months before I could complete my mics; I was still waiting on some parts when the assignment came my way. I look forward to completing the mics when I am home from time to time.
Title: Re: DIY RF Condenser Mics
Post by: Khron on October 06, 2019, 03:37:38 PM
I'm just glad to have been able to put my (admittedly limited) skills to good use :) And i'm slightly sorry i can't really do a whole lot more in this direction, as it stands  :P
Title: Re: DIY RF Condenser Mics
Post by: shot on October 10, 2019, 05:02:50 PM
Fantastic job Rogs for putting up a webpage with all the info for this mic! This makes so much easier to digest all 18 pages of this journey.
I pulled a trigger and ordered pcbs and inductors. I thought to give it a try since I have BM800 body unused (two of them in fact!) and I'll experiment with some leftover capsules I think I have somewhere.
I don't need another mic but this is so tempting!

Keep up the good work!
I'll report how it sounds (or does it sound at all :) ) when pcbs arrive. It'll be a long wait since I went with low cost shipping option on jlcpcb, but I'm not in a rush.

:)

Luka
Title: Re: DIY RF Condenser Mics
Post by: rogs on October 10, 2019, 05:17:31 PM
Fantastic job Rogs for putting up a webpage with all the info for this mic! This makes so much easier to digest all 18 pages of this journey.
I pulled a trigger and ordered pcbs and inductors. I thought to give it a try since I have BM800 body unused (two of them in fact!) and I'll experiment with some leftover capsules I think I have somewhere.
I don't need another mic but this is so tempting!

Keep up the good work!
I'll report how it sounds (or does it sound at all :) ) when pcbs arrive. It'll be a long wait since I went with low cost shipping option on jlcpcb, but I'm not in a rush.

:)

Luka

Glad it helps! --- I have to admit even I would be hard pushed to recall all that's happened along the way .. Lots of changes as the experiments developed, and there was more input from different contributors.
It was time to rationalise the project.....

I've now built 3 exact copies of the  latest version, and they all perform virtually identically -- something which didn't always occur with the early prototypes!
There are slight differences between capsule sensitivities - maybe 1 or 2dB  - but that's about it!.

Look forward to seeing how you get on in due course...
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on October 15, 2019, 11:44:20 AM
I've made some notes on it here:

www.amx.jp137.com
The info available on the coils renewed my interest in simming the circuit. Finally got a meaningful sim.
There are two aspects of performance that need more investigation. One is the final tuning. It would be useful to know how you proceed and what the final values are for the inductors, once they are tuned. I know it's somewhat difficult, because measuring them implies disconnecting them, but that would be a significant marker.
The other element is the capacitor that "antagonizes" the capsule (C4 in the schemo). From the beginning I have been skeptic about its presence. I understand that it's a legacy of attempts to use a typical FM detector.
The reason I object to this topology is that, in an extreme case where the capacitor would be of the same exact value as the capsule, there would be zero output because the bridge would be perfectly balanced. Then, any variation of equal value but opposite sign in sound pressure would result in a rectified signal, thus very distorted. I understand capacitor C4 must be chosen different enough of the capsule's value for an acceptable performance, but how much different?
Instinctively, I would think that dispensing with C4 would give a better result in terms of distortion, so I simmed it, and so far, the conclusion is: it works, but it takes a significant re-alignment of the inductors. Counter-intuitively, omitting C4 shifts the alignment down. After re-alignment, it seems the sensitivity is not too different; again, that's according to sim.
I hope you, or someone else had the time and patience to do these experiments. I would have done it but I'm in the process of moving so most of my test equipment is not usable.
Title: Re: DIY RF Condenser Mics
Post by: rogs on October 15, 2019, 04:14:04 PM
The tuning procedure is quite straightforward, and I've found only needs to be repeated twice for best results.
Repeating further has so far proved unnecessary.

The microphone under test is arranged as per the top image on this page: http://www.amx.jp137.com/index-perform.html
The pre-amp in my set up is a Sound Devices USBPre, and the output from this is run through a software spectrum analyser, to allow a visual - as well as an audible - maximum audio setting to be observed.

With both cores set in roughly mid position, a 1KHz audio tone is applied to the transducer held right against the headbasket. 
It is set to simulate a typical speech level directly at the mic capsule.

T1 core is adjusted for maximum audio output. This can be quite low - depending on the uncalibrated T2 setting - but should be obvious.

T2 is then adjusted until a maximum audio output position is located. This point will be a dramatic 'peak'  output. There can be more than one peak discovered as the core is tuned, but there is always one dominant peak  which coincides with the minimum second harmonic distortion setting observed on the spectrum analyser.
It is not difficult to locate this dominant peak practically.

It is then desirable to check that the T1 setting is still at maximum, and then repeat the T2 calibration as well.

As you say, it is possible that the capsule and associated bridge capacitor could in theory be exactly the same value. In practice I have never encountered this. There appears to be a very wide range of 'bridge centre' voltages which  can be accommodated by adjusting T2 to optimise the FET bias setting.

The T1 setting is set for maximum oscillator volts into the inductor. The resonant setting  allows for the lowest oscillator current - at around 5mA.

The T2 setting seems to be more complex.  It is apparent that the optimal FET bias setting is not necessarily at maximum resonance of  T2.

On all prototypes so far (around 10) it has been possible to find the optimum output setting for a number of different capsule/ C4 values.  Even when the measured value of C4 has been very close to the capsule value, it has always proved easy to find the optimum setting for the associated FET.

The 5u3H inductors are specified as having a tunable range of 3 to 7.5 uH.
As long as the associated parallel capacitive loading falls with the range of 33pF to 82pF -- which covers most options in this case - then the inductors seem to perform as specified.

You describe a configuration that omits C4 --- are you thinking along the lines of the second sketch in the attached drawing - or have I misunderstood?



Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on October 15, 2019, 04:49:25 PM
As you say, it is possible that the capsule and associated bridge capacitor could in theory be exactly the same value. In practice I have never encountered this.
I think I haven't been clear enough. It is almost impossible to have a capacitor of the same exact value as the capsule, so tehre would always be an output, but this output would be quite distorted if the values were matched within about 1 pF.

Quote
You describe a configuration that omits C4 --- are you thinking along the lines of the second sketch in the attached drawing - or have I misunderstood?
That would be the one attached here.
I don't think the second arrangement would work reasonably; it would be a slope detector, which is known to be problematic in terms of linearity.
Title: Re: DIY RF Condenser Mics
Post by: rogs on October 15, 2019, 06:05:29 PM
I think I haven't been clear enough. It is almost impossible to have a capacitor of the same exact value as the capsule, so tehre would always be an output, but this output would be quite distorted if the values were matched within about 1 pF.
I clearly haven't experienced anything that close then!...
As I say, even with values apparently within a few pF of each other, I've always managed to find the 'dominant' tuning peaks I've come to expect, with respectable harmonic distortion figures  (less than 0.05%) 


That would be the one attached here.
I don't think the second arrangement would work reasonably; it would be a slope detector, which is known to be problematic in terms of linearity.
It was simple enough to try that quickly - simply removing C4.  Sadly it hasn't worked well. A serious drop in sensitivity (something like 30dB) and a very broad range of 'tuning' where there is no real change in amplitude, suggesting to me that we are no longer within the required resonant tuning range using these inductors.
There might be some improvement by connecting across the whole winding, rather than just  half? .... but in the present PCB layout the centre tap is grounded, so that will have to be tried with a modified PCB in due course.
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on October 15, 2019, 06:43:46 PM
I clearly haven't experienced anything that close then!...
As I say, even with values apparently within a few pF of each other, I've always managed to find the 'dominant' tuning peaks I've come to expect, with respectable harmonic distortion figures  (less than 0.05%) 

It was simple enough to try that quickly - simply removing C4.  Sadly it hasn't worked well. A serious drop in sensitivity (something like 30dB) and a very broad range of 'tuning' where there is no real change in amplitude, suggesting to me that we are no longer within the required resonant tuning range using these inductors.
I would think the main issue is that this connection changes so much the tuning it begs for changing the 47pF cap to a much lower value, probably about 10pF. That's because now, the dominant tuning is transferred to the 1st tank, that sees the capsule's capacitance instead of being virtually shorted.

Quote
There might be some improvement by connecting across the whole winding, rather than just  half? .... but in the present PCB layout the centre tap is grounded, so that will have to be tried with a modified PCB in due course.
I don't think so. Even with the half winding, there is largely enough gain, but the resonance is shifted so much lower it becomes impossible to tune the circuit using only the core adjustment.
There may be a problem with my proposal, though. Since the dominant alignment is controlled by the capsule's capacitance, proper alignment may require a change in inductance value or the use of a capacitor in parallels with the capsule (e.g. for SDC's), which would reduce sensitivity.
Title: Re: DIY RF Condenser Mics
Post by: rogs on October 16, 2019, 03:34:45 PM
As I've mentioned before here - and in my notes - the idea behind this project came from Peter Baxandall's 1963 paper
(see here: http://www.jp137.com/lts/Baxandall.RF.mic.pdf ).

In that paper, Baxandall's initial experiments were carried out using an unbalanced bridge, and a conventional diode detector. 
He then observed limitations with that approach, and moved onto using switched detectors and a balanced bridge. 
That, I suspect, would be the logical next step for this project to take..... Ricardo already made that observation in this thread some time ago.

What Baxandall didn't use in his unbalanced prototype was an FET infinite impedance detector, which I found to be  a significant improvement over  a standard diode detector - if only because it  essentially removed the loading from the tuned secondary of T2.
With the reality of unbalancing the bridge (if necessary ) simply requiring the addition of a single small extra capacitor, and the wide range of adjustment for optimum FET biasing from T2 tuning, I think  this simple design has worked out pretty well - certainly way better than I first imagined!

I do think that further  improvements are probably best attempted by moving on to Baxandall's balanced bridge concept.
Whether anyone feels like having a go? -- there's certainly the expertise  on this forum!

I think I'll probably give it a miss for the present ..... but I've said things like that before in haste !...  :)

 
Title: Re: DIY RF Condenser Mics
Post by: shot on November 07, 2019, 09:03:37 PM
Oh my! I made this mic!
It's not completely done but I managed to hear some sounds from it.

Unfortunately, I thought I had correct FET in my inventory but I was wrong. And since I didn't want to wait for days to get the correct FET delivered, I tested it with what I had.

I have put J111 and got a nice signal out of the mic, but at too low level. Tried to roughly calibrate T1 and T2 while speaking into the mic but still it was too quiet. Then I tried J201 and got a bit more level out of the mic (I had to recalibrate inductors) but this still seems too low level. I had to crank the preamp on my audio interface to the point it brought too much noise of its own. There's probably some noise from this FET, but I can't distinguish it from the preamp noise at this point. And it seems unstable since some noise an level fluctuations were present. But that's just me testing the mic with wrong FET so please disregard those observations. It's just me being impatient to hear what I've built. Guess I'll have to wait for J113 to be delivered. I cannot buy it locally so it'll be a week or so to wait for it.

I also had similar situation with Q1. I thought I had BC549, but it turned out that I was out of it so I put BC547B instead. I'll get the correct transistor tomorrow. Could it be that this one is actually the cause of low output level? Any other transistors I can try here?

In the BOM on amx.jp137 website it's not specified which variant of transistors should be installed (A, B,C...). For Q2 and Q3 I've installed BC557B with hfe matched to around 278. Should I use C variant that has higher hfe?

Even if it's not working as it should yet, I really like the sound of this mic! It has very well defined low end. And I must admit that I totally dig it's distortion in this uncalibrated stage of the build! Sounds very interesting. In a way it's different from other mics that I've built but still high end, even though it was very cheap to build it.

I'll swap the Q1 for the correct one and wait for the FET to arrive.
In the meantime, should I also put BC557C instead of BC557B?

:)

Luka
Title: Re: DIY RF Condenser Mics
Post by: Ironworks on November 08, 2019, 03:15:42 AM
It's been a couple of months since there's been any more posts on this topic ....
But whether anyone else has had a look at trying the idea out, I'm not sure?.
Hopefully, they will inspire others to have a go ...I'd love to see what variations  other people might suggest.....

I've been following your idea for a while, came across it while researching RF mics.
Excellent project!
I will be build a couple of these as soon as time permits.
BTW, on the matter of microphony  I would suggest this is the coils themself's, I have had this occur in some manufactures FM transmitters that used a similar style of coil in the oscillator. In our design we eliminated it by employing a custom sealed toroidial coil in a shielded box . This of course is not an option here!
Title: Re: DIY RF Condenser Mics
Post by: rogs on November 08, 2019, 05:58:46 AM
I have put J111 and got a nice signal out of the mic, but at too low level. Tried to roughly calibrate T1 and T2 while speaking into the mic but still it was too quiet. Then I tried J201 and got a bit more level out of the mic (I had to recalibrate inductors) but this still seems too low level. ...

The J111 is significantly different from the J113 in 2 important parameters - IDSS and Vgs, so I'm not surprised  that  it has not been possible to calibrate the FET to the optimum point.
The J201 should be a bit better - although it does have a lower IDSS  than the J113. It may be possible to improve the performance using that device by increasing the values of  R4 and R10  to - say - 10k to reduce the current in the FET. (You would also need to reduce the value of  C9 and C13  to 1nF to correct the HF cut of frequency)

I have tried a 2N5457 which seems to work OK . It's just that the J113 is easier to find, here in the UK.


I also had similar situation with Q1. I thought I had BC549, but it turned out that I was out of it so I put BC547B instead. I'll get the correct transistor tomorrow. Could it be that this one is actually the cause of low output level? Any other transistors I can try here?

The main difference  between the BC547 and the BC549 is the noise figure, which is probably not really a significant consideration in this instance. The difference is Hfe between the B and C variants may make some difference , although I have tried several  BC549C with Hfe differences of more that 50, and have not seen any change in oscillator amplitude measured at the emitter.
I would suggest that most NPN small signal BJTs will probably perform OK in this application...

In the BOM on amx.jp137 website it's not specified which variant of transistors should be installed (A, B,C...). For Q2 and Q3 I've installed BC557B with hfe matched to around 278. Should I use C variant that has higher hfe?

In this emitter follower configuration, matching the Hfe is probably more important that whether you are using B or C versions.

In my prototypes I am using the B version. The higher Hfe of the C version will effectively increase the input impedance to the devices, and thus - in conjunction with C5 and C10 - affect the extreme LF response of the mic  - marginally.
I would suggest that increasing the value of C5 and C10 to 220nF would have a much greater effect on the low  frequency response than using BC557C rather than a BC557B.
....In either case, you're probably only increasing extreme LF 'rumble' noise anyway!

The values shown on the website schematic represented a version that I found to give repeatable results , and could be built with components that were easily available from a single supplier here in the UK.

One further point.  I would strongly recommend applying a fixed tone at the microphone input to calibrate, rather than  trying to find the best settings from a speech input.

There should only be one 'sweet spot' for any given bridge/FET combination. However, there may well be other smaller 'peaks' discovered within the overall tuning range.
The optimum tuning point is to be found within a pretty small range of rotation (around 10 -15 degrees) . So the use of a fixed tone makes things much easier.
If you have the option of running the pre-amp output into a spectrum analyser, that makes thing even simpler!




Title: Re: DIY RF Condenser Mics
Post by: rogs on November 08, 2019, 06:07:20 AM
BTW, on the matter of microphony  I would suggest this is the coils themself's, I have had this occur in some manufactures FM transmitters that used a similar style of coil in the oscillator. In our design we eliminated it by employing a custom sealed toroidial coil in a shielded box . This of course is not an option here!
I had reached the same conclusion - that the microphony was from the coils themselves! 
It was only noticeable under very high gain settings during testing, and in reality has not proved to be a problem.
The most significant factor regarding noise from this  circuit is the sensitivity of the capsules themselves. 
Using a selection of cheap Chinese 34mm capsules - K67, K47 and C12 types - I have discovered that the K67 types tend to be much more sensitive than the others . By a factor of around 10dB.
As the noise level from the circuitry remains constant, the signal to  noise ratio of the K67 types is thus around 10dB better than the K47 of C12 types.. and on par with my Rode NT1   (self noise 4dBA ...  sensitivity -29dB)
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on November 08, 2019, 06:41:53 AM
In this emitter follower configuration matching the Hfe is probably more important that whether you are using B or C version.
Let's debunk a myth here.  :)
The two output transistors see different source impedances. The impedance from the drain of the FET is pretty much the drain resistor, but the impedance from the source of the FET is Re//Gm, which would typically result in about half.
With perfectly matched transistors with a hfe of 500, the output impedance at the emitters would be 9 and 7 ohms, with a hfe of 250, they would be 14 and 9.
You can see that a better CMRR can be achieved by having a transistor with hfe=500 in the drain path and one with hfe=250 in the source path.
Assuming the 47r resistors would attenuate the differences would be erroneous since CMRR depends on the actual absolute difference between branches.
To put things in perspective, Schoeps never worried about this, since HF CMRR is governed by the RF caps across the outputs, more than by the impedance of the active branches. LF CMRR is seldom an issue with mics.
Title: Re: DIY RF Condenser Mics
Post by: rogs on November 08, 2019, 04:01:34 PM
The two output transistors see different source impedances. The impedance from the drain of the FET is pretty much the drain resistor, but the impedance from the source of the FET is Re//Gm, which would typically result in about half.

I hadn't realised that there would be two different impedances at source and drain in this configuration...

In this  infinite impedance  detector mode, I had always rather thought of it as a 'switch'  - with the oscillator waveform switching between an 'off' state and then 'on' into the linear region of the FET every cycle.
This process charges the associated  drain and source capacitors to a level determined by the changing amplitude of the 'on'  signal , which are in turn discharged through the equal value source and drain resistors, during each half cycle 'off' period.
As the source and drain are interchangeable in this configuration, I had always imagined that the drain and source impedances were the same?
From what you write, I've clearly misunderstood the concept!
 
Title: Re: DIY RF Condenser Mics
Post by: shot on November 18, 2019, 09:04:14 PM
So I finished my mic!
In fact, I've made two mics.

I've tried 2N5457 before J113 got delivered and it sounds same as J113.
Since I've socketed all the transistors, I had a chance to try different transistors in it. For Q2 and Q3 it didn't matter what I've installed. At first I went with matched pair od BC557C, and after that I tried 557B, 560B, 560B. Matched and unmatched. It just didn't produce any different result. For Q1 I didn't have BC549 so instead I went with BC547C. I've tried 547B, 550B, 550C and they all did work but there was audible difference in low mid frequency range. It was subtle, but definitely noticeable. I settled with 547C that sounded the best to me. Totally subjective here.

I noticed that two capsules that I bought on aliexpress were very different from each other. Actually I would dare to say that first capsule is almost defective! I have installed it in the first mic and I was annoyed how it produced intermittent noise, sounding as if I was rubbing head grille of the mic. The level was low-ish and a bit dark. But when I closed the mic body noise dissapeared. That's odd since this mic doesn't have problems with shielding since it's not super high impedance. Anyway, I thought that this dark quiet signal is the sound of this mic. But after I populated second pcb and assembled the second mic, that one blew me away. First of all it was louder and brighter. It sounds very different from the sample Rogs posted on his performance page. This one is open without any kind of compression, contrary to Rogs example that sounds a bit like radio DJ's mic processing! And this mic measured 5.4db louder than the first one.
After that I populated new pcb and installed it in the first mic - since I convinced myself that I did something wrong on the first board. But to my surprise it sounds just the same. So it must be due to capsule.

So I guess I'll order more capsules and match two mics since they can easily be tuned to act as a matched pair. But considering how cheap parts and donor body is, I bet I'll even order another body so I can have third mic.
And I'm not saying that my first mic sounds bad - I can see it on some bass cab or some loud sources like amp or drums.

This mic is a real refreshment after spending big bucks building brand name mics and hunting rare or expensive parts for them.  Just few weeks ago I built FET847 with chinese capsule and this mic sounds almost as good. I'll have to do a serious test in my studio when I find some spare time to compare them since on my voice they sound very very close!

Big up to Rogs, Khron and all of you guys for this little victory! Humans win!!

:)

Luka
Title: Re: DIY RF Condenser Mics
Post by: rogs on November 19, 2019, 04:19:27 AM
Thanks for posting this report Luka.... It's good to read that you've had some successful results.. :)

I too have discovered that there can be quite a difference between capsules - even between those that are supposed to be the same type!

It's also useful to read that you have been able to try  out different semiconductors, and have discovered that - for the most part -  you are not restricted to very specific types.  The ones chosen for my schematic are all available from one UK  source - CPC -  which tend to have good prices and low delivery costs, hence my choice.

There are still some tweaks I'm trying out, to see if there are further mods to suggest, but it's good to know that the  present circuit has now been successfully completed by someone else!
Title: Re: DIY RF Condenser Mics
Post by: shot on November 19, 2019, 09:49:40 AM
I went ahead to test RF.AMX mic even more!
Came to my studio and I recorded myself talking. Then as a comparison I also did two more recordings into FET847 and U87 clone.
It's all in this link:
https://1drv.ms/u/s!AuvQ2JGkf8-IhtlfDhBG4ML_kFFyzQ
All files are 48kHz/24bit WAV packed in 70mb ZIP file
(I'm not native english speaker so excuise my accent)

Rogs, if you find it interesting you can host it on your page. I'll also try to have this file hosted on my OneDrive as long as I can.

Bottom line - after more testing I find RF.AMX seriously good!

:)

Luka
Title: Re: DIY RF Condenser Mics
Post by: rogs on November 19, 2019, 03:32:15 PM
Thanks for posting these samples Luka ...  :)
I've uploaded the zip file to my server, and added a link to it among the audio samples at the bottom of the audio performance web site page here:  http://www.amx.jp137.com/index-perform.html

Really interesting to hear how the mic sounds on speech with a different voice  from mine!   I think it stands up pretty well against the other mics you've recorded samples of as well....
The hi-mid is probably a bit too prominent of course ... but that's just a feature of the K67 Chinese capsules we're all used to by now!  :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on December 07, 2019, 05:38:32 PM
I've been looking at optimising some of the component values when using alternative capsules to the one presently specified for the project  - which is a 'standard'  Chinese K67 style capsule.

I have a couple of 3 micron  edge terminated 34mm capsules, which have a much larger  capacitive value than the 65pF of the K67.
At around 90pF,  this capsule really needs several component changes to work optimally with the existing PCB...

The use of an 8MHz oscilllator in place of the present 10MHz - for example - helps increase performance by improving the 'Q' of the inductors, as they are operated closer to their specified  frequency..

I have also taken the opportunity to experiment with reducing the current drain from the phantom power by around 40%.

The performance using these 90pF capsules has improved by quite  a measurable  amount..... a signal to noise ratio of over 80dB, and a sensitivity of around -12dB.... (probably now a little too 'hot' for some applications). 

But reducing the sensitivity can also reduce the noise floor even further of course ... which is useful!

I've made some extra notes - and a new schematic - a copy of which you can find here:

www.jp137.com/lts/RF.AMX10.new.notes.pdf


Title: Re: DIY RF Condenser Mics
Post by: rogs on December 09, 2019, 07:22:47 AM
Further to my post above - and to my notes - I have to confess that I cannot figure out the maths involved in working out the total loading presented to T1 primary by the inductor assembly as a whole ? -- ( As you can tell I'm no mathematician!)

I've attached a diagram of the relevant circuitry below..

It's easy enough to calculate the value of the series total of C4 and the capsule at around 47.4pF ..
But how does the introduction of T2  across the 'bridge' actually  affect the loading on T1?....
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on December 09, 2019, 09:02:05 AM
Further to my post above - and to my notes - I have to confess that I cannot figure out the maths involved in working out the total loading presented to T1 primary by the inductor assembly as a whole ? -- ( As you can tell I'm no mathematician!)

I've attached a diagram of the relevant circuitry below..

It's easy enough to calculate the value of the series total of C4 and the capsule at around 47.4pF ..
But how does the introduction of T2  across the 'bridge' actually  affect the loading on T1?....
It is very complex; interaction between tuned circuits is counter-intuitive and maths are tidy and often not very informative. Calc methods are better applied. That's what simulation does.
The most significant factor IMO is that combining these circuits yields in addition to a low-Q primary resonance a secondary resonance at about 8MHz that is very narrow so the slope of the voltage vs. frequency curve is very sharp, giving high sensitivity.
The lack of accurate parasitic data makes the results quite error prone.
See attachment; Red is tank input without T2, green same with T2 and purple is output at T2's secondary.
Resuts are probably +100/-50% from reality because I neglected parasitic capacitances, resistances and losses, also made an assumption about coupling coefficient.
Title: Re: DIY RF Condenser Mics
Post by: rogs on December 09, 2019, 09:58:56 AM
Thanks for that information Abbey .. and it seems to match up pretty well with a real world 'mock up' of the assembly I tried earlier, and helps makes sense of what I discovered.

It was using my original 10MHz design for the K67 capsule, so the 'Q' of the IF cans is likely to be slightly less than with an 8MHz  version (the coil data is specified for 7MHz operation).

I also need to make some allowance for some small scope lead loading error ( 10M and 15pF).

Even so, I can measure a gain of some 27dB between the input to T2 primary at the bridge junction, and the output of T2 secondary.

I think it's this kind of 'noise free' - 'Hi-Q' gain   that has made my results so much more impressive than I first expected from this simple circuit.

My thanks to the contributors on the original Yahoo Group Micbuilders thread who got me thinking about using IF cans and the benefits of  resonant 'Q' gain. (That would be Umashanker and Ricardo - thanks guys!)

Title: Re: DIY RF Condenser Mics
Post by: rogs on January 03, 2020, 11:28:51 AM
Interesting new 'RF bias' mic from Rode -- the NTG5:  https://www.rode.com/microphones/ntg5

Looks very nice -  and seems to get good reviews!  The price is good too -- especially when you consider what you get with the whole package

Impressive spec -- Thought I'd compare some of the parameters to the latest version of the RF.AMX10  - (the one with the 90pF 'C12' style capsule)..

The sensitivity of this latest version of the AMX10 is probably a bit too 'hot' at c. -11dB , so I added a pad to match it to the sensitivity of the NTG5  at  c. -23dB.
I added a single 47R resistor across pins 2 and 3 of the XLR to achieve this, rather than fit RP1 onto the PCB. That way you 'pad' the noise from the output stage as well....
That mod brings the equivalent noise figure of the mic down to 4dBA  ( this was confirmed by checking against my Rode NT1, which also has a noise figure of 4dBA).

So ...

The good:

• The AMX10 now  has a signal to noise ratio of 90dB - against the NTG5 figure of 84dB
• The AMX10 is  around one tenth of the cost of the NTG5

The bad:

• The NTG5 has a weight of 76g against the 280g of the AMX10
• The AMX10 draws 4mA of  current against the NTG5 2mA

The ugly:

• The NTG5 is a very elegant short shotgun mic, with a length of 203mm  and a diameter of only 19mm
• The AMX10 still looks like a BM800 !  :(


Still, at least the AMX10 signal to noise ratio is better!   :) ...



 
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on January 03, 2020, 12:25:14 PM
An SDC cannot be as quiet as an LDC. SDC seems to be a requisite for shotgun mics; I must admit a 17x1.5" mic would be embarassing...
Title: Re: DIY RF Condenser Mics
Post by: rogs on January 03, 2020, 05:36:22 PM
According to the Neumann notes here:  https://www.neumann.com/homestudio/en/difference-between-large-and-small-diaphragm-microphones  one of  the advantages of an SDC  is their excellent transient response and the 'reality' of the recorded audio. ...At the expense of slightly higher noise levels.

I think that most (all?) commercial RF bias mics are SDC shotguns ?...
The AMX10  is probably the world's first LDC  RF bias mic !

Not that there is any real demand for this type of mic  of course - but at least it is quiet!   :)   
Title: Re: DIY RF Condenser Mics
Post by: abbey road d enfer on January 03, 2020, 11:32:22 PM
According to the Neumann notes here:  https://www.neumann.com/homestudio/en/difference-between-large-and-small-diaphragm-microphones  one of  the advantages of an SDC  is their excellent transient response and the 'reality' of the recorded audio.
That is true. However, I see many people using LDC's for acoustic string instruments, where the difference is noticeable. I guess they love the presence peak in the upper midrange

Quote
I think that most (all?) commercial RF bias mics are SDC shotguns ?...
Since shotguns are often used in a difficult environment, RF bias makes sense.

Quote
The AMX10  is probably the world's first LDC  RF bias mic !
Indeed!

Quote
Not that there is any real demand for this type of mic  of course - but at least it is quiet!   :)
There could be a demand for a sub-zero noise shotgun...?
Title: Re: DIY RF Condenser Mics
Post by: rogs on January 04, 2020, 04:38:59 PM
There could be a demand for a sub-zero noise shotgun...?

Well the AMX10 has a noise floor 10dB lower than the much vaunted EM172 Primo electret capsule,  so it might find an application for use outdoors - in nature recording  for example?....

It's not a shotgun mic of course, but the cardioid pattern is pretty impressive in the end address configuration....
Title: Re: DIY RF Condenser Mics
Post by: gyraf on February 06, 2020, 05:28:26 AM
thanks
Title: Re: DIY RF Condenser Mics
Post by: rogs on March 30, 2020, 09:24:23 PM
The main project schematic and parts list website pages have both been updated to version 2 (v.2)
This includes the  current  reduction mods originally made to the 8MHz (90pF capsule) version, as described in the posts above.
Links to the previous schematic and parts list are included on the new pages for anyone already working with the original version.

The project website URL remains the same: www.amx.jp137.com
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2020, 04:15:09 PM
EDIT: Please see post #378 below -- the inclusion of  R13 47R as an attenuator between pins 2 and 3 of the XLR can cause distortion with some pre-amps, and has now been removed.
C5 and C10 are now 220nF.
Please see the revised schematic attached to post #379
The main project notes have been revised.


Title: Re: DIY RF Condenser Mics
Post by: Khron on May 03, 2020, 05:50:46 PM
Increasing R5/6 above 100k is not an option? Or what (detrimental) effects might that have?
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2020, 07:00:26 PM
I don't think  changing those for - say - 150k  resistors would make that much difference to the input impedance.
It's only going to be around 20k with a 100R  output load, using those transistors - so 100k resistors  are going to make that around 16.6k  150k resistors  would make it around  17.6k, so probably not really that  significant.

I do think I may need to investigate further whether it is a good idea to fit  R13, the 47R resistor across the output.
It works well enough as an attenuator, but I'm finding an increase in distortion figures at higher input levels using a separate test circuit
It looks as if  some pre-amps may not like having the differntial inputs - and the 2 x 6.8k phantom power resistors - effectively bridged with a 47R resistor...

It may be necessary to leave that out  and go back to adding the 'pad' resistors RP1 and RP2 in required.
Not quite such low noise floor of course..

It's not easy to test this whole circuit by simply substituting the capsule, as it is with 'standard' Schoeps type circuits -- I may need to try and knock up a test rig using a varicap diode.....?...

Still playing - and still finding out new things about this concept..  :)
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 03, 2020, 10:03:34 PM
Further to my 2 posts above,  I have now realised that the inclusion of R13 (47R) across pins 2 and 3 - whilst working fine as an attenuator - can cause distortion with some preamps. I have thus removed it from the schematic.
The sensitivity is returned to around  -12dB.
C5 and C10 are now  fitted as 220nF to lower the LF -3dB point to 20Hz with Q2 and Q3  BC557B transistors, working into a 300R load.
The main project schematic has been revised.
Sorry for any confusion... 
Title: Re: DIY RF Condenser Mics
Post by: rogs on May 10, 2020, 10:34:57 AM
I mentioned the new Rode NTG5 mic: https://ntg5.rode.com in my earlier post #368 above. 
Looks like a very nice mic - and the cheapest I've seen so far of the various commercial RF bias mics on the market.
(Not that many of course!)

I was impressed with their phantom power current drain figure of only 2mA, and wondered how well I could get the RF.AMX10 to perform only drawing that much current? -- The answer is, not too badly....

I had previously mentioned that the latest version of the AMX10 is a pretty 'hot' mic, with a sensitivity of  around -12dB. 
I wondered how the overall noise figure would be affected, if I reduced the sensitivity by around 10dB by reducing oscillator current.
A few quick experiments showed that the noise figure would only be degraded by around 5dB.

I've attached a schematic of a  version that  pretty much copies the NTG5, spec wise -- although the AMX10 does have an more extended LF response.
It's also - sadly - still much heavier and uglier.  :(
...But it is LOT cheaper  :)

By changing the values of  R3, R11 and R12 you can get to the required  spec.. a sensitivity of around -22dB, a total current drain of 2mA - and a S/N ratio of c.85dB.

You'll notice D1 has been re-introduced as a 36v zener. That's to protect Q4, which has a maximum Vgd of 35V. 
I found it necessary when using my dbx286s channel strip pre-amp, which has a phantom power voltage of  52V  (right on the upper limit of the phantom power spec)

I know we have discussed current drain at various times before in this project thread ... so I'm quite pleased to have been able to get it down to 2mA  :)

( Higher resolution PDF of the schematic here: www.jp137.com/lts/AMX10v.2.2mA.pdf  )