DIY RF Condenser Mics

[Fastunov]

10 boards arrived

 

[Fastunov]

10 boards arrived

 

[rogs]

Couple of further 'tweaks' -- ( I've updated the project notes to version 4.2 )

• Now that R2 is no longer a link, C6 only decouples the oscillator supply. So I've now fitted a 100nF cap into the 'vacant' D1 location, to provide additional HF decoupling across C15. Probably overkill ? .... but it's only a few pence!

• I was wondering if C5 and C10 fitted as 220nF extended the LF response too far? Depends on what impedance you're working into of course, but with the BC557B emitter followers having a typical Hfe of around 280, I thought it might be better to try 100nF.

Not exactly straightforward to measure the frequency response of an RF mic circuit without the capsule connected, so I referred back to the wisdom of Peter Baxandall, and knocked up a little interface based on his ideas around fig.6 on page 593 (PDF sheet 6) of his 1963 paper. (Copy HERE ).

I've attached a schematic of my testing interface below.

Connecting to my Sound Devices USB 1.5 preamp, I find that with C5 and C10 fitted as 100nF the LF -3dB point is around 20Hz.

Initial test suggest a pretty linear frequency response (within 0.5dB) up to around 18KHz, with a -2dB drop in level at 20KHz.
The initial THD+n figure at 1KHz reads as 0.045% -- although as the source reads 0.0285%, it may be slightly better. So not too bad.

Experiments (as always!) continue ...

 

[abbey road d enfer]

• I was wondering if C5 and C10 fitted as 220nF extended the LF response too far? Depends on what impedance you're working into of course, but with the BC557B emitter followers having a typical Hfe of around 280, I thought it might be better to try 100nF.

I wouldn't worry so much about LF response. With 100n, the +3dB point is about 12Hz and response at 20Hz is -1.3dB.
However I would be much more concerned about the impedance unbalance at LF.
See attached table.
It is considered good practice to maintain impedance unbalance under 20 ohms, for adequate CMRR.
An unbalance of 20 ohms limits the max CMRR to 50dB with an otherwise perfect mic input.
See how 220n barely scratches it, when 100n fails.
That's why I chose 470n in the latest builds of my Micbooster.

 

[rogs]

Once again, I had followed Schoeps lead ... he has 100nF for both capacitors -- (although he does go to the trouble of recommending matching emitter follower transistors! )
I think I should probably return the value of C5 to 220nF, and leave C10 as 100nF.
As the PCB has had ongoing project mods, it is not easy to fit C10 with the larger size 220nF capacitor very neatly!

According to your table, C5 as 220nF and C10 as 100nF should still keep the Z value differences at less than 20....

 

[Khron]

To be fair, in balanced connections, impedance is the whole point; whether the signal happens to be symmetrical / present on the "cold" line as well is a nice-to-have, but not a necessity.

 

[abbey road d enfer]

The table in my last post makes no reference to the respective levels of the outputs, only to their impedances, and shows that, although the midrange impedances are quite well matched, at low frequencies (where it matters the most) the deviation can be a problem.

 

[shot]

Back to the capsule question!
Are you using the same capsule in every version and simply changing the parts on the pcb, or you always assemble new mic with new capsule? Have you ran into problems with different capsules?
I'm asking because I'm struggling with three mics that I'm attempting to build. They sound like sh*t and after going back and forth with components, even rebuilding one mic from the scratch back to original v1.0 circuit it's always the same - noise noise noise!

I've built my first RF back when this journey started with aliexpress 67-ish capsule (rogs recommended) and v1.0 circuit. That mic is stellar! It is loud and it has insanely low amount of noise. Basically preamp noise dominates and mic's noise feels as non-existent. And this mic's output it like 12-15db louder than any other RF mic I've built!

Almost year later I ordered two more capsules and I've put them in a drawer for some time. Few months later I ordered two more. Always the same aliexpress capsule. And I've built three mics with those newly bought capsules - they all have too much noise. At some point I finally got the tester tool that can measure small capacitance so I've tested three of those capsules and they have 33pf, 44pf and 48pf of capacitance. Really low! I didn't measure the first capsule I bought that is in the good sounding mic. But I'll do that when I have time.

I'm actually not sure if my noise problems are down to the capsule, but I'm out of ideas and I'm more and more towards blaming the low capacitance of the capsule for it. It's output level is probably too low so SN ratio gets reduced. Btw, no problems tuning T1/T2 on them.
I wouldn't be surprised if aliexpress ships whatever they have in stock so no consistency at all.
And I don't have at hand any other spare capsule right now to test



Luka

 

[abbey road d enfer]

so I've tested three of those capsules and they have 33pf, 44pf and 48pf of capacitance. Really low!

Even if the good capsule measured ar 60-80pF, the one with 33pF would sound only about 6-8dB below, and have no more noise.
You should probably look elsewhere. Can You check the RF level at T1's primary, against teh one that workd good?

 

[Vac11]

To Shot: Same experience here. I built two of this RF mics and both were/are noisy. I used k47 capsule from Getwin.

 

[rogs]

I have built around 10 of these mics, and all have had acceptable noise floors. The lowest noise and best sensitivity came from a version using a 3 micron edge terminated capsule, which measured at around 90pF.
That same capsule would also collapse if you applied more than around 50V DC from a 'conventional' circuit. So I have concluded it probably has a lower tension membrane - which is ideal for an RF mic!
It also allows the system to be run at 8MHz, which is closer to the 7MHz specified on the inductor spec sheet.... So the Q is probably a bit higher.

I have tried several 'K.67' type Aliexpress capsules and have found them to be quite sensitive. Those have all had a capacitive value of around 65pF.
I have also tried a couple of Chinese'K.47' style mics - again with values around 60/65pF - but these were a lot less sensitive (by around 10dB).

If you use a capsule with a significantly lower capacitive value, I've found the bridge imbalance is often too extreme to allow a 'useful' calibration point. With the other bridge capacitor being significantly larger than the capsule, the resonant inductor tuning point generates a higher level of RF carrier - and with a smaller proportion of capsule capacitive value 'variation'.
This results in a lower modulation amplitude, with a higher RF carrier level, and this will tend to generate more noise.
I've also found that a significantly unbalanced bridge seems to allow several 'peaks and troughs' in the inductor tuning range - with no obvious dominant peak setting.
With a higher value capsule - and a more balanced bridge set up - there tends to be very much a dominant 'peak' setting -- which is often quite dramatically obvious. A much improved signal to noise ratio as a result of course...

Ideally, using a capsule with a capacitive value of - say - 40pF you would probably get better results from an inductor value of 10 or 12 uH. Unfortunately I've not been able to locate any such device off the shelf -- although I imagine some experienced hobbyists might consider 'winding their own'?....
Using a 5.3uH inductor with such a low capacitive value will require a lower value bridge 'balancing' capacitor. That will require a much higher oscillator frequency -- which will probably result in a much reduced system 'Q' -- and higher noise!

The more recent project revisions have concentrated on reducing the system current. One major advantage of RF condenser mics is that the low impedance allows for outdoor use, and low current drain is a useful feature for battery powered recorders.
The lower oscillator voltage does reduce the system signal to noise ratio a little -- but not dramatically.

The project notes still include a link to the original circuit set up -- see HERE .

As I say, I've had my best results from a low tension 3 micron 90pF capsule. And the bridge being almost balanced seems to give the best results. (The self biased infinite impedance detector sits automatically at 'Vp' so it doesn't need much to turn it on and off!)

 

[shot]

Even if the good capsule measured ar 60-80pF, the one with 33pF would sound only about 6-8dB below, and have no more noise.
You should probably look elsewhere. Can You check the RF level at T1's primary, against teh one that workd good?

Yeah, I'll do that!
I just need to remember to bring back the good one from the studio back on my home bench

 

[shot]

I have built around 10 of these mics, and all have had acceptable noise floors. The lowest noise and best sensitivity came from a version using a 3 micron edge terminated capsule, which measured at around 90pF.
That same capsule would also collapse if you applied more than around 50V DC from a 'conventional' circuit. So I have concluded it probably has a lower tension membrane - which is ideal for an RF mic!
It also allows the system to be run at 8MHz, which is closer to the 7MHz specified on the inductor spec sheet.... So the Q is probably a bit higher.

I have tried several 'K.67' type Aliexpress capsules and have found them to be quite sensitive. Those have all had a capacitive value of around 65pF.
I have also tried a couple of Chinese'K.47' style mics - again with values around 60/65pF - but these were a lot less sensitive (by around 10dB).

If you use a capsule with a significantly lower capacitive value, I've found the bridge imbalance is often too extreme to allow a 'useful' calibration point. With the other bridge capacitor being significantly larger than the capsule, the resonant inductor tuning point generates a higher level of RF carrier - and with a smaller proportion of capsule capacitive value 'variation'.
This results in a lower modulation amplitude, with a higher RF carrier level, and this will tend to generate more noise.
I've also found that a significantly unbalanced bridge seems to allow several 'peaks and troughs' in the inductor tuning range - with no obvious dominant peak setting.
With a higher value capsule - and a more balanced bridge set up - there tends to be very much a dominant 'peak' setting -- which is often quite dramatically obvious. A much improved signal to noise ratio as a result of course...

Ideally, using a capsule with a capacitive value of - say - 40pF you would probably get better results from an inductor value of 10 or 12 uH. Unfortunately I've not been able to locate any such device off the shelf -- although I imagine some experienced hobbyists might consider 'winding their own'?....
Using a 5.3uH inductor with such a low capacitive value will require a lower value bridge 'balancing' capacitor. That will require a much higher oscillator frequency -- which will probably result in a much reduced system 'Q' -- and higher noise!

The more recent project revisions have concentrated on reducing the system current. One major advantage of RF condenser mics is that the low impedance allows for outdoor use, and low current drain is a useful feature for battery powered recorders.
The lower oscillator voltage does reduce the system signal to noise ratio a little -- but not dramatically.

The project notes still include a link to the original circuit set up -- see HERE .

As I say, I've had my best results from a low tension 3 micron 90pF capsule. And the bridge being almost balanced seems to give the best results. (The self biased infinite impedance detector sits automatically at 'Vp' so it doesn't need much to turn it on and off!)

I was always able to find a tuning point. Never had a situation that any of the capsules won't even tune. But noise is mostly constant, in some positions it falls off. But at that position sine wave response also falls of significantly, so in practice SNR at that point is even worse.

I've tried with different values for C4 and C8 and they just change the tuning point. And they change the timbre of the capsule. At some values mic gets darker (usually too dark) and at some values it starts to loose low end. It can actually benefit to pick an exact value based on a particular capsule to get a tone you like! And after most of capacitor swaps I was still able to retune the inductors. But that damn noise was in most cases the same. Maybe better or worse by few db's but still very noticeable.

I need to bring back the first mic I've built back to the bench and do some measuring and compare it to those newly built ones.

 

[rogs]

I was always able to find a tuning point........

I can confirm that I too have always been able to find a tuning point.....sometimes several, with a particularly 'unbalanced' bridge.
It's whether it is the primary tuning point - and with a high enough 'Q' - that seems to make the difference!

The 5.3uH Spectrum coils have a specified inductance range of 3 to 7.5uH. How far the 'Q' drops towards the edge of those limits is not specified?

The selection of the appropriate tuning capacitor for T2 has always seemed fairly straightforward..... 68pF for 8 MHz and 47pF for 10MHz should allow the inductor to be tuned at pretty much the middle of its range. (5.8uH for 8MHz and 5.4uH for 10MHz ).
The JFET doesn't really present any additional loading (unlike to the earlier diode rectifier did!)

The maths surrounding the tuning of T1 are way out of my league . Abbey did attempt a simulation of the inductor assembly earlier in this thread, but - as he said - there are so many unknown variables it's only possible to make an approximation.

The simple capacitive load across T1 secondary is C4 and the capsule in series. So, with both the capsule and C4 as 68pF, we would get a load of 34pF which is just within the specified range of the inductor at 7.45uH.
In fact T2 primary is fitted across the bridge -- and that makes things a lot more complex. Certainly, with a 65pF capsule and C4 as 68pF I have always found it possible to find the optimum tuning point of T1 - with the inductor core somewhere near the centre of its range.

I have only briefly tried out capsules with smaller values, and found the tuning much less precise (although there were some 'peaks' ). The system 'Q' certainly seemed a lot lower.
I decided there would probably be a need for alternative inductors to get useful improvements but - as I mentioned earlier - I haven't been able to locate any as yet.

I suspect there maybe be some major component changes required to get good results from capsules with lower capacitive values. Either higher oscillator frequencies or higher inductor values -- or maybe both?

There has been a lot of expert input on this thread, and it maybe that someone has already tried alternatives, and not yet posted their results?
I think that's why a forum like this is so useful for the experimental nature of this kind of project.....

 

[mykiss]

I can confirm that I too have always been able to find a tuning point.....sometimes several, with a particularly 'unbalanced' bridge.
It's whether it is the primary tuning point - and with a high enough 'Q' - that seems to make the difference!

The 5.3uH Spectrum coils have a specified inductance range of 3 to 7.5uH. How far the 'Q' drops towards the edge of those limits is not specified?

The selection of the appropriate tuning capacitor for T2 has always seemed fairly straightforward..... 68pF for 8 MHz and 47pF for 10MHz should allow the inductor to be tuned at pretty much the middle of its range. (5.8uH for 8MHz and 5.4uH for 10MHz ).
The JFET doesn't really present any additional loading (unlike to the earlier diode rectifier did!)

The maths surrounding the tuning of T1 are way out of my league . Abbey did attempt a simulation of the inductor assembly earlier in this thread, but - as he said - there are so many unknown variables it's only possible to make an approximation.

The simple capacitive load across T1 secondary is C4 and the capsule in series. So, with both the capsule and C4 as 68pF, we would get a load of 34pF which is just within the specified range of the inductor at 7.45uH.
In fact T2 primary is fitted across the bridge -- and that makes things a lot more complex. Certainly, with a 65pF capsule and C4 as 68pF I have always found it possible to find the optimum tuning point of T1 - with the inductor core somewhere near the centre of its range.

I have only briefly tried out capsules with smaller values, and found the tuning much less precise (although there were some 'peaks' ). The system 'Q' certainly seemed a lot lower.
I decided there would probably be a need for alternative inductors to get useful improvements but - as I mentioned earlier - I haven't been able to locate any as yet.

I suspect there maybe be some major component changes required to get good results from capsules with lower capacitive values. Either higher oscillator frequencies or higher inductor values -- or maybe both?

There has been a lot of expert input on this thread, and it maybe that someone has already tried alternatives, and not yet posted their results?
I think that's why a forum like this is so useful for the experimental nature of this kind of project.....

Is there a T1 T2 disassembly drawing? I want to copy it

 

[rogs]

Is there a T1 T2 disassembly drawing? I want to copy it

If you mean a mechanical drawing of the can and coil former assembly details? ... then no, I don't know of one -- sorry.

If you mean the details of the number of turns on each winding, Q value and impedances, etc, then scroll down THIS PAGE - to about 1/3 of the way down - and you can find details of the 5u3H coil listed there.

 

[rogs]

I had the chance to try out a capsule which measured 47pF. Even using a 12MHz oscillator - in an attempt to adjust for the smaller capacitive values involved - the system 'Q' is just too low, that far away from the specified inductor frequency of 7MHz.
Add to that a less sensitive capsule, and the noise figure was simply too high to make a usable microphone.

It really does look like the only way of getting reasonably good results, using capsules with smaller capacitive values than the 65pF to 90pF range suggested in the existing project schematic, is going to be with larger inductors.

To keep the oscillator frequency within the 8 to 10MHz range, it looks like it might be necessary to try and find an IF can with an inductance of around 10 to 15uH , rather than the 5.3uH value of the present cans.

'Traditional' IF cans are (understandably) much more difficult to find these days ...
It is possible to still find some of the old TOKO 10k range, but those with suitable windings are in the 2.6uH range --- and we are looking here for larger values ,not smaller!

It sort of brings things right back to the beginning of this thread....
It would seem that one of the main reasons there have been relatively few RF bias mic hobby projects - so far - has been due to a lack of appropriate inductors being available !

 

[rogs]

......... I've built my first RF back when this journey started with aliexpress 67-ish capsule (rogs recommended) and v1.0 circuit. That mic is stellar! It is loud and it has insanely low amount of noise. Basically preamp noise dominates and mic's noise feels as non-existent. And this mic's output it like 12-15db louder than any other RF mic I've built! ...........


Luka

I re-read this post from Luka, and it reminded me that I too had seen quite a variation in noise levels, with the several mics I've built....

As we have discussed above, you really need use capsules with a capacitive value in the range between 60 and100pF.
These need to be used in conjunction with RF oscillator frequencies between 7 and 10MHz to get the best from these particular inductors.
Outside this range, and the inductor assembly system 'Q' can drop to unacceptable levels, creating an excessively noisy microphone.

The noise level of the circuit is largely dependent on the amplitude of the RF signal present on the gate of the JFET. This level is not only a function of the oscillator output level, but also of the amount of 'bridge imbalance'.
If one attempts to balance the bridge as accurately as possible, then the smaller the amplitude of the RF signal on the JFET gate - and the lower the noise....
Unlike a standard diode demodulator you don't need much amplitude to energise a self biased JFET - it sits 'naturally' at Vp.

I've certainly had my best (lowest noise) results where I've managed to match the load presented by the capsule to the value of C4 as accurately as possible. I've found that can result in only c.200mV p-p of RF on the gate.
I'm still investigating what the minimum RF carrier amplitude needs to be - I think a fully balanced bridge might create some interesting results, if the JFET is not turned on into the saturation region - and then off again - on each cycle?
I've not been able to actually balance the bridge that closely - so far!

It's not that easy to select the best value for C4. I replaced C4 with a variable capacitor, and then connected a scope lead to the JFET gate.
It's then possible to adjust the capacitor for minimum RF gate voltage, by observing the scope trace.
This is not entirely accurate of course because - even though the JFET gate is low-Z in this application - the scope lead input capacitance will load T2 secondary slightly.
But it is pretty close (my x 10 scope lead has an impedance of around 10M and an input capacitance of c.15pF).

EDIT: A better way - to avoid the scope lead 'loading' the inductor assembly - might be to measure the voltage developed across either the drain or source load resistors instead? Keeping that below 2V should allow for a reasonably low system noise figure.

I've managed to improve the noise floor by as much as 8dB on one mic, where the bridge imbalance was particularly extreme (but still fully functional!).

The concept of this mic is simple..... It looks like the 'tweaks' needed to guarantee a repeatably decent performance are a bit more complex....

One of my first prototypes had performed much better than my other early efforts..... I now know a bit more as to why!

 

[IvanP]

Hello all!

So, I've been down in this rabbit hole with ya for a couple of months. That is, I got into DIY microphones a few months ago and have been learning a lot. I'm no electronics designer, even though I can follow a schematic well, so please do forgive my ignorance at times.

With that out of the way, I took the liberty to re-read this whole forum and write down some questions and/or ideas or pieces of knowledge I've stumbled up, which I hope could aid in the development.


Firstly, I would like to ask @Tubetec if you could ever so kindly send me a copy of the Microphone engineering handbook by M. Gayford? It's truly an elusive book and I haven't been able to find it in digital anywhere, except your chapter 8. I've been enjoying microphone literature recently and it has often been very enlightening.


@shot, if you haven't sorted out your noise-inconsistency problem still.. If it's not caused by circuitry, maybe it's a problem with the quality of components? Have you personally verified all parts work? I've made a habit to do check every component, since I built my first DJJules pimped alice with a counterfeit PF5102 from ebay that arrived dead-on-arrival, and lost two days troubleshooting (and learning JFET theory. And problems with counterfeit transistors).
Also, I had the dumbest thing happen to my whilst recording, just the worst case of the spontaneous hums/noises - turns out it was a single loose strand of copper wire from the cable's shielding touching pin 2 or 3 inside the connector jack - a trim and a quick solder later all is good, but maybe it's something random like that then? Please do let us know if you have already found out the cause.


@rogs, I have to commend you for everything you have done on this project, it has been a delight to follow your progress, but also your website, which is extremely well done. I have plans to take inspiration from your website when I will be making my own, hopefully soon enough.

You certainly know more about what is going on in the electronics then I do, so I wanted to ask you if you could shed some light on.


- Coils/crystal. I ordered the Spectrum 5u3HH coils and some cheapo K87 china capsules, and all the capsules tested in the 42-52pF range, which got me concerned at first, but the microphone worked honestly extremely well with the capsule, within 2pF for C4, and 10MHz crystal.
I did try out to swap out the crystals, I do believe there were mentions of Ruud using even a 12MHz with success. Placing the mic between my headphones playing a 1kHz tone, 52pF LDC capsule + 51pf C4, Behrry UMC404HD input gain set the same, both T1 and T2 tuned after each swap to max, I got:
10,00MHz, -42dB
10,70MHz, -36dB
11,06MHz, -28dB
So, obviously there is an increase in sensitivity, but I still haven't gotten into the realm of microphone testing.
There was a mention of a C3 'SOT' (also what does that stand for?) compensating for capsules with less capacitance, but it adds noise then, right? I also think someone mentioned that even the lower capacitance capsules will works, just will be somewhat less sensitive with no more extra noise? Speaking of noise...


- Distortion. What software did you use to determine your distortion (THD%) for RF.AMX10 and competitors? The other day I was trying to learn what capacitors are for, following this neat article: Practical Test & Measurement - Stop Worrying About Coupling Capacitors!
I made the tester and my goal was to measure THD in different types of capacitors, with the intention to prove or disprove the polyester vs polypropylene cap noise superiority. However, I myself own a UMC404HD which is my first audio interface and one I'm extremely happy (especially for the price), but I couldn't get THD under 0,2%, no matter what I tried with the components, tester or inputs, but then realized it's definitely the UMC404's preamp (apparently a Midas preamp ain't a Midas PRO preamp).

Rogs, have to tried to measure distortion with your UMC404HD? Their website doesn't specify THD levels (like Scarlett does) for the model, and I know Sound Devices are quality stuff. I don't know if the problem is in the tester I built or the limitations of a really-affordable sound card, because I would definitely like to give distortion testing a try.

I used REW to try and analyze cap distortion, but THD would always be way higher (THD 0,1-0,2% for caps assumed to be 0,01%), except for ceramics, which were about 0,2% - ceramics are definitely not recommended for audio signal paths.

Apparently, you could use REW to test microphone preamps (without the capsule) for distortion in this method. I'm not sure how, and haven't tried it, but maybe it could be useful in seeing where the problem lies, without the capsule.


- Padding. Is there a relationship, like equation, connecting the RP1 value to how much dB it will pad? I installed:
RP1 2,2k and got about 15db padding
RP2 1,0k and got about 20dB padding
but I didn't measure them too precisely, I will have to redo those tests.
I implemented Khron's idea to put the pins and the small jumper to act as a pad switch, but I put in a row of 4 pins instead, so that the jumper doesn't swivel when only on one pin (visible in the attached photo; padding is set off). Working great so far.
Speaking of which, @Khron, thank you for your PCB design contributions! I actually learned new things from your design, I'll send you a PM when I prepare questions and ideas about the PCB's design, and hopefully I can help contribute too.


- Multipattern. Rogs, you mentioned you don't think multipattern is feasible, but specifically in this, one oscillator design, right? Is there any point in perhaps making two oscillators run one dual-membrane capsule, connected to one JFET? I do believe there were mentions there might be interference from the other oscillator, but I'll be willing to test that out if you hook me up with a schematic. Alternatively, that is solved by using a housing bigger than a BM800 (i.e. I just got a fake U87 mic, and it definitely has enough real estate to ensure there is no interference.) I accidently ordered a three-wired capsule(front memb., back memb., with connected backplates) instead of a single membrane, so I'm invested now



I think this is more or less everything I had on mind. And please do let me know if I got something wrong.

P.S. I ordered the royal purple PCBs from JLCPCB. I adore how nice it looks and I've been enjoying how it sounds.

 

[shot]

@shot, if you haven't sorted out your noise-inconsistency problem still.. If it's not caused by circuitry, maybe it's a problem with the quality of components? Have you personally verified all parts work? I've made a habit to do check every component, since I built my first DJJules pimped alice with a counterfeit PF5102 from ebay that arrived dead-on-arrival, and lost two days troubleshooting (and learning JFET theory. And problems with counterfeit transistors).
Also, I had the dumbest thing happen to my whilst recording, just the worst case of the spontaneous hums/noises - turns out it was a single loose strand of copper wire from the cable's shielding touching pin 2 or 3 inside the connector jack - a trim and a quick solder later all is good, but maybe it's something random like that then? Please do let us know if you have already found out the cause.

Thanks for reviving this thread and your interest in this project!
I've pretty much gave up on it since I first made one mic and this one is stellar fantastic, and on the wings of the success I've started another four mics. Unfortunately all four of those don't sound remotely good as the first one. Actually they are all unusable noisy! I've chased all components and in the end figured that it's all about capsules. All capsules are bought from the same seller that Rogs recommended in this thread but first one was bought I think about year prior to other four capsules. Guess they changed the batch of capsules that they sell now.
I have put a "bad" capsule in the first mic (the "good one") and it was also noisy after calibration. Actually I could never get the resonant peak to get as high as I could with the first capsule. Measured the capacitance of those "bad" capsules and they turned out to be as low as thirti-ish picofarads.
I don't have money to try and test different sellers and different capsules in order to chase one that is compatibile with this mic. So I gave up. Actually I've started fooling around with the idea to try and ditch RF board from one and do a p2p version of moded Oktava MK-012. Just to try it out, maybe it'll work with those low capacitance capsules I have in four mics.

But just to be clear - this RF mic when done with proper capsule is fantastic. It is extremely low noise with high output gain. I did a location recording for a film few weeks ago and when this thing is connected into a low noise preamp you can record miraculously quiet things with it. No noise at all! Hopefully someone will find out one day what capsule to get in order to get it working. Or what to change to get it working with those ****** capsules they sell now...



Luka