DIY RF Condenser Mics

[emrr]

Ok! Maybe I missed it but there was also a Shoeps RF mic series, CMT 20.

https://taperssection.com/index.php?topic=196830.0

 

[rogs]

Ok! Maybe I missed it but there was also a Shoeps RF mic series, CMT 20. Details here, don’t know if it’s viewable to non-members.

https://taperssection.com/index.php?topic=196830.0

Some fascinating details there from DSatz.... thanks for the link.
Never heard of a Schoeps RF mic before. Looks like it was a nightmare to assemble though!
(Still don't know if that thread is visible to non-members though? -- I've been a member there for several years! )

 

[emrr]

It's opening in some other browsers, so I guess for today, it's visible.

 

[abbey road d enfer]

It's visible for me, and I'm just a passer-by.

 

[Winston OBoogie]

Visible for me too and interesting to see, never knew there was once a Schoeps RF mic. Thanks Doug.

 

[abbey road d enfer]

never knew there was once a Schoeps RF mic.

Neither did I, although I'm not a historian. I wonder if KH has some knowledge ot it.
It seems Philips, Peter J. and Schoeps came to the same conclusion. Confronted with the stubbornly low-impedance of bipolar transistor circuits, they resorted to RF techniques.

 

[Tim Campbell]

This thread has sparked my interest and so I bought these to learn a bit more.

 

[rogs]

The early Sennheiser RF mics - like the MKH405 and 406 you've shown - appear to have used utilised a phase modulation system, a version of which was used by Uwe Beis' in his earlier DIY RF mic project here: RF-Condenser Microphone Circuit

According to Hibbing : https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf Sennheiser changed over to Amplitude Modulation with the 1980s re-design used for their later MKH 20/30/40 series.

Perhaps they had decided to follow Baxandall's ideas after all - some 20 years later! http://www.jp137.com/lts/Baxandall.RF.mic.pdf

Be interesting to see what you discover with your 405 and 406 mics.
I wonder if they use the same Sennheiser 'push pull' type capsule of the later MKH models?
That is one thing I have no access to -- a low tension 'push pull' capsule.
(As this has been very much a simple experimental 'hobby' project, I've restricted my budget to cheap Chinese capsules).

What I have discovered is the ones that collapse when a polarisation DC of more than 50V is applied, seem to perform best for this RF type project.
(I'm guessing they're lower tensioned capsules?) .....
That is one problem with using cheap capsules of course -- there's no way of knowing what a detail like membrane tension is going to be!

 

[emrr]

I wonder if they use the same Sennheiser 'push pull' type capsule of the later MKH models?

They do not.

What I have discovered is the ones that collapse when a polarisation DC of more than 50V is applied, seem to perform best for this RF type project.
(I'm guessing they're lower tensioned capsules?) .....

Ah, good thought, possibly a clue.

 

[emrr]

This thread has sparked my interest and so I bought these to learn a bit more.

I'd have thought you'd seen a fair number of these over the years. What is the view from your side of the pond? Not common?

 

[Tim Campbell]

I'd have thought you'd seen a fair number of these over the years. What is the view from your side of the pond? Not common?

They are fairly common but my work has been concentrated on traditionally polarized capsules and so never gave RF mics much thought except when I have used MKH40's I have always been impressed by their clarity. Symetrical capsules are also interesting. The MKH80 is an incredible mic.

 

[ricardo]

> rogs said:

I wonder if they use the same Sennheiser 'push pull' type capsule of the later MKH models?

They do not.

Are you sure about that?

Du..uh! I've just looked at MKH 104 105 404 405 805 circuits and they show 'single' sided capsules

Hibbing's AES paper describing their 'new' push pull capsule was Mar 85, Hamburg so you might be right.

Single Diaphragm, Push pull is the sensible way to design a capsule for an AM mike ala Baxandall. My single remaining brain cell can't quite figure out if this is equivalent to the 'new' Sennheiser circuit.

I tried Baxandall's circuit while doing the Mk4 Soundfield and was very impressed with the noise performance .. at least as quiet as the Sennheisers circa 1980. Alas 4 'similar' oscillators in close proximity was too difficult for me to sort out.

 

[rogs]

Single Diaphragm, Push pull is the sensible way to design a capsule for an AM mike ala Baxandall. My single remaining brain cell can't quite figure out if this is equivalent to the 'new' Sennheiser circuit.

I have constructed a 'figure of 8' version of the RF.AMX10 circuit by using a dual membrane capsule. I replaced C4 with the second side of the capsule. So the bridge is essentially 'balanced' in this configuration.
The self biased JFET infinite impedance detector sits 'naturally' around the cut off point, so works well even at low levels of AM modulation.

As far as I can see, this configuration should also work OK with a 'push pull' capsule?...
• A symmetrical 'push-pull' capsule is one membrane and two back plates.
• A 'standard' dual membrane LDC capsule is essentially the opposite.... one backplate and two membranes.

As far as I can tell, none of the Sennheiser RF mics use an infinite impedance detector, but stick to conventional diode based demodulators.
Makes sense of course ... One of the main reasons for Sennheiser's early RF work was to find a way of creating Lo-Z condenser mic, so that they could use (bipolar) transistors instead of valves (tubes).
Once FET's came along, everyone else seemed to drop RF mic development. Only Sennheiser seem to have decided that the other RF advantages warranted carrying on commercially.
Having already obtained good results from their bipolar RF circuits, there was no need to 're-invent the wheel' as it were - so they saw no point in replacing demodulator diodes with infinite impedance JFET demodulators.
I have no idea whether the only other people who appear to have joined the RF mic world more recently - Rode - use infinite impedance demodulation?
(I've never been able to find any Rode RF schematics online -- either official or otherwise)..

I like to think that Baxandall might have considered an infinite impedance detector, as a simpler alternative to his bipolar switching technique?
And he does mention FETs in his 1963 Wireless World article -- but only acknowledging that 'in due course they may become available'.....

 

[Fastunov]

I was very interested in the development of Rogs, periodically came across it in the network, various forums and messages. Finally found the source and the site DIY RF Condenser Microphone.
The day before yesterday, I downloaded the zip folder of all the Gerber files from the site and made an order for 10 boards on jlcpcb.com and ordered two Bm-800s. I'm looking forward to the delivery.

I was interested in the project first of all by the possibility of placing a large membrane perpendicular to the axis of the housing, there is a great need for this combination.

However, I was faced with a lack of my knowledge, experience and skills ) And I will ask for your help. Since I am not strong in electronic components and their selection, the selection is even based on the list that is compiled in detail by Rogs on the site http://www.jp137.com/lts/AMX10.v.4.1.plus.PCB.Parts.List.pdf -help to make the assembly for the order on Mouser.com one list.I'm sure someone has already done this and please share )

 

[Fastunov]

Guys, please help me. I need the help of our community.
I was very interested in this project, and I wanted to implement it. Since I do not have the skills and experience, I will contact specialists for assembly, but I need to buy all the components and elements. I have already ordered and am waiting for delivery of 10 PCBs, on ebay I bought 10 T1 and T2 (Spectrum Communicaions 10mm 10K 7MHz RF/IF coils-Type 5u3H) and ordered the membranes.
It remains to order the entire element base, and then I was faced with the fact that I can not find everything on my own on Mouser.com or DigiKey.com .

Please help me build a BOM Mouser Link on the list of Rogs
http://www.jp137.com/lts/AMX10.v.4.1.plus.PCB.Parts.List.pdf

 

[rogs]

I have no idea whether all the exact components on my list are available from Mouse or Digi Key?.... Seems likely they will be though.

My list was prepared for the parts available from CPC here in the UK.
'CPC' is a Farnell company, and I think you should be able find all the same components on the Russian Farnell website?

My CPC (Farnell) list includes all the manufacturers series references.
If you have someone who is experienced enough to assemble an example of this circuit successfully, that person should be able to work from my parts list.

I wouldn't however necessarily recommend this project as a simple 'starter' project for a complete beginner with no knowledge of electronics.
It is not a commercial hobby 'kit', merely an experimental design that may benefit from further modifications.

The project began as one possible solution to the original thread query from Gerard, for a 'practical' RF bias mic. I have made it clear throughout that the circuit is experimental, and I have been grateful for the support and suggestions from the many contributors the the 400+ posts in this thread, who have helped with the development.

I have built several examples of the circuitry, all of which seem to work quite well.
The circuit seems to be quite easily 'repeatable' but - as I say - I'm not sure it can necessarily be recommended as a starter project for someone with no electronics experience at all?

 

[Fastunov]

Thank you so much for your response!

The company Farnell is in Russia and buying from them does not cause any difficulties. I would be happy if you would share the list of components you are talking about: -)
Yes, I have an experienced engineer who is able to perform the assembly, but I have to buy the components for him.

Your project interested me just because it is very flexible, the sound can be fine-tuned. Personally, I am not interested in banal commercial projects, I have a huge microphone fleet at work from Sennheiser (mkh series), AKG, Nuemann. And on the one hand, this is enough for me, but I want to expand the horizons of sound, I am looking for sound Alchemy. Do you understand me? Especially when applied to large membranes.

I plan to use your microphone in broadcasting sports events: football, handball, etc.

 

[rogs]

The company Farnell is in Russia and buying from them does not cause any difficulties. I would be happy if you would share the list of components you are talking about:

You have already seen - and posted a copy - of my list of CPC (Farnell) parts... All the manufacturers series types are included on that list.

You simply need to look at the catalogue and order the specific components you need from the component types on my list.
You will most likely need to buy the resistors in batches of 50, but most other components should be available individually.

 

[Fastunov]

You have already seen - and posted a copy - of my list of CPC (Farnell) parts... All the manufacturers series types are included on that list.

You simply need to look at the catalogue and order the specific components you need from the component types on my list.
You will most likely need to buy the resistors in batches of 50, but most other components should be available individually.

I understand the purchase scheme very well, it's not a problem. It's up to me to choose and not make a mistake, even knowing the markings.

I mean, can you do something like that?

 

[rogs]

You should be able to derive the exact parts you need from my list?
As I say, it does include both the exact part type and the manufacturer's series reference.

If you feel the need to prepare a full BOM yourself, then post a copy here before you order.
I'm sure one of us will be able to take a quick look, and compare it to the existing parts list....