Sennheiser MKH800 / MKH80 capsule (ks80)

[kingkorg]

Snatched this one off eBay for cheap. My hopes were i would be able to use it with DIY RF Condenser Microphone RF microphone designed by @rogs .

Sadly the capsule measures just 20pf (50pf, i measured at wrong terminals, diaphragm is grounded) which makes it unsuitable for that circuit.

Just to test it, and see if there's any life in it I put it in my lowest pol. voltage mic with just 15v applied to the capsule. Lo and behold it worked! Next thing i put it on my testing mic body made out of CM60 with 60v pol. voltage, and the capsule doesn't complain at all! The fet might though as the capsule puts out insane amount of output. Which was expected.

As the Sennheiser document on MKH series design explains, the capsule is underdamped, and has a bell shaped frequency response which gets corrected electronically, with inverse eq curve.

Since it has so high output the noise performance with 60v is remarkable. But as it pushes the fet really hard THD isn't.

Rear rejection in the midrange is insane!

Here's the uncompensated frequency response of the capsule.
Red is 0°, green is 180°, blue is just for reference - 0° of Neumann's k105 in the same body so you can see how much more output Sennheiser has. Naturally when inverse curve EQ is applied it affects both 0° and 180° response.


I'll have to think about how I'm going to use it. Probably dual out mic, with variable polarization voltage to control the sensitivity.

I don't think i will be taking this apart, since I doubt I would be able to put it back together. Once compensated for FR it sounds killer! Here are some pics.

 

[rogs]

What a shame the capacitance value is so low!....... Not really surprising perhaps, given the size of the capsule.
The Spectrum inductors specified for the AMX10 project do rather limit the capacitive range of suitable capsules.
(Still looking for an alternative range of suitable IF cans -- no luck so far)

It might be worth trying the capsule in parallel with a 47pF cap, with 68pF fitted for C4.
OK, it would seriously drop the sensitivity, but as the capsule seems to have a really low tension membrane, it might still produce a useful output?

Or --- maybe try a figure of eight configuration, with the two backplates connected across T1 secondary and the single centre membrane terminal connected to the non grounded end of T2 primary....
If you then fitted C3 on the PCB (not normally fitted) as a 47pF, and omitted C4, you might still get a usable output?

From what you've discovered, it would seem that Sennheiser probably don't need to worry about applying that much RF to the capsule to still get a suitable modulation range.... I'm beginning to see how they can get such remarkably low noise figures!

 

[kingkorg]

What a shame the capacitance value is so low!....... Not really surprising perhaps, given the size of the capsule.
The Spectrum inductors specified for the AMX10 project do rather limit the capacitive range of suitable capsules.
(Still looking for an alternative range of suitable IF cans -- no luck so far)

It might be worth trying the capsule in parallel with a 47pF cap, with 68pF fitted for C4.
OK, it would seriously drop the sensitivity, but as the capsule seems to have a really low tension membrane, it might still produce a useful output?

Or --- maybe try a figure of eight configuration, with the two backplates connected across T1 secondary and the single centre membrane terminal connected to the non grounded end of T2 primary....
If you then fitted C3 on the PCB (not normally fitted) as a 47pF, and omitted C4, you might still get a usable output?

From what you've discovered, it would seem that Sennheiser probably don't need to worry about applying that much RF to the capsule to still get a suitable modulation range.... I'm beginning to see how they can get such remarkably low noise figures!

I am also wondering why this loosely tuned diaphragm doesn't colapse at 60v even with heavy plosives. Although it could be that the diaphragm isn't tuned loose at all, it could be the absence of damping is enough to provide high output even at low frequencies.

I also think i remember reading in this Sennheiser document that voltage under test procedure applied to some MKH capsules is 8-50v. Have you seen this document?

 

[kingkorg]

MKH30 schematic which seems like a half of this capsule shows 24V HF pol. voltage at capsule. Not sure if this is directly translatable to 24VDC.

 

[rogs]

I hadn't seen that particular Sennheiser document, so thanks for that......

I don't think the earlier MKH mics, like the 415 - or even the 416 - used 'push-pull' capsules, so they may well be quite a lot different?

The MKH30 - and associated models from that range - certainly do use push pull...
As you say, the schematic shows '24VHF' on both backplates, and I think we can assume that means 24V of RF, rather than any DC component. (Any DC would be essentially grounded by the associated inductor windings). The central membrane terminal is shown connected directly to ground.

24V is a lot more RF than is present on the AMX10 project - that only uses about 3 or 4V of RF - but then I'm guessing there needs to be a higher level of RF to accommodate the bias required by the demodulation diodes?....

I don't think those push pull capsules are used on anything other than Sennhesier RF mics, so I'm thinking you may be into 'uncharted territory' when applying any DC bias to a push pull capsule?....

 

[Tim Campbell]

This is brilliant.

 

[kingkorg]

Just a couple of closeup pictures.

 

[rogs]

I take it those two photos are both of the same side..... What does the other side look like?

 

[kingkorg]

I take it those two photos are both of the same side..... What does the other side look like?

They are identical. The fat plate in between is i guess delay network as described in the documents.

 

[Eric B]

I too bought a Sennheiser capsule through eBay from a gent in Romania. I was going to apply it to Rog's circuit but haven't had a chance to do so due to a family emergency that is consuming all of my time this month and more. I'd like to discuss with you off group if you will.

 

[kingkorg]

I too bought a Sennheiser capsule through eBay from a gent in Romania. I was going to apply it to Rog's circuit but haven't had a chance to do so due to a family emergency that is consuming all of my time this month and more. I'd like to discuss with you off group if you will.

Sure, drop me a dm. However i don't know much about RF stuff.

 

[kingkorg]

@rogs What spec of the inductors would be appropriate for your circuit to work with a 20pF capsule?

 

[rogs]

@rogs What spec of the inductors would be appropriate for your circuit to work with a 20pF capsule?

Sadly, I'm no academic when it comes to designing inductors...... quite a lot of variables I'm not sure how to resolve effeciently!

In simplistic terms - when used with a 10MHz oscillator - I would guess you need to use a transformer with an inductance of around 12.5uH .....
I don't think Spectrum has anything suitable in their published range, and - outside the UK - their shipping costs have now made their inductors pretty costly anyway.

I've sent away for a pack of these: https://www.aliexpress.com/item/1005003604363316.html ( not yet arrived)
It may be possible to experiment using those formers, and perhaps the internal structure of the Spectrum coils as a template for some alternative - and cheaper! - options ?
The first step would probably be to just try and copy a Spectrum coil as a starting point?

 

[kingkorg]

Sadly, I'm no academic when it comes to designing inductors...... quite a lot of variables I'm not sure how to resolve effeciently!

In simplistic terms - when used with a 10MHz oscillator - I would guess you need to use a transformer with an inductance of around 12.5uH .....
I don't think Spectrum has anything suitable in their published range, and - outside the UK - their shipping costs have now made their inductors pretty costly anyway.

I've sent away for a pack of these: https://www.aliexpress.com/item/1005003604363316.html ( not yet arrived)
It may be possible to experiment using those formers, and perhaps the internal structure of the Spectrum coils as a template for some alternative - and cheaper! - options ?
The first step would probably be to just try and copy a Spectrum coil as a starting point?

Awesome, thanks!

 

[kingkorg]

Sadly, I'm no academic when it comes to designing inductors...... quite a lot of variables I'm not sure how to resolve effeciently!

In simplistic terms - when used with a 10MHz oscillator - I would guess you need to use a transformer with an inductance of around 12.5uH .....
I don't think Spectrum has anything suitable in their published range, and - outside the UK - their shipping costs have now made their inductors pretty costly anyway.

I've sent away for a pack of these: https://www.aliexpress.com/item/1005003604363316.html ( not yet arrived)
It may be possible to experiment using those formers, and perhaps the internal structure of the Spectrum coils as a template for some alternative - and cheaper! - options ?
The first step would probably be to just try and copy a Spectrum coil as a starting point?

Ordered some PCB for your project, coils and those from Ali. It will be really exciting to see where i get with these Sennheiser capsules. One thing that keeps bugging me is if this whole issue with coils can be avoided by using some OpAmps. I have very limited knowledge about RF, but it would be worth learning more about it and explore the idea.

 

[rogs]

One thing that keeps bugging me is if this whole issue with coils can be avoided by using some OpAmps........

This is true .. much easier to just use opamps!

The main advantage to be gained from using RF circuitry is the huge drop in impedance of the circuitry surrounding the condenser capsule.
Essential when designers started using bipolar transistors to replace valves (tubes).... Once FETs started to appear, everyone apart from Sennheiser resorted back to conventional Hi-Z circuitry again. As Sennheiser had no history of making Hi-Z condenser mics beforehand, they alone stuck with the RF concept.

RF condensers are the answer to problems with Hi- Z circuitry and outdoor use - especially in high humidity situations. Only Rode have bothered to join Sennheiser in that market. And I think that all current RF commercial mics use SDC capsules?

I just fancied the idea of trying the RF concept out with LDCs, when Gerard asked if anyone had built a 'hobby RF' mic... No real reason, other than curiosity!

It was your Sennheiser capsule acquisition that prompted me to try my latest version..... I have fitted one of Ari's 'Flat K47' capsules in a figure of 8 configuration....That capsule has 2 properties that made me think it was worth a try...
• Both sides are matched within 1dB
• Each side has a capacitive value of around 68pF
It actually works quite well. Unlike other variations, there is no problem with the bridge actually being 'balanced'..... Because as the capacitance of one side gets larger the other get smaller - so the bridge can never reverse polarity..... It's a bit like Sennheiser's 'push pull' concept (well, sort of!)

It's not a particularly sensitive capsule, so the mic doesn't have an ultra low noise floor - although it's comparable with my OPIC opamp mics.

Sounds pretty good (the 'flat K.47' a really nice capsule - as others have already mentioned !)

I think those blank Aliexpress coil formers might offer some interesting options for future versions?... I've never found an alternative to the Spectrum coils that would work well 'off the shelf', but the option of trying out different winding arrangements - using blank formers -might make for some interesting new experiments?
Now - whether any of this is actually worth the effort is for people to decide for themselves?
I just like the idea of trying something a bit different, hobby wise.

Further RF experiments I have in mind are looking at creating a very low noise osciallator, to maybe drop the noise floor even further, and possibly even looking again at FM or phase modulation instead of AM....... just for the fun of it!
(OK, I realise that sounds really 'geeky' ! )

 

[kingkorg]

This is true .. much easier to just use opamps!

The main advantage to be gained from using RF circuitry is the huge drop in impedance of the circuitry surrounding the condenser capsule.
Essential when designers started using bipolar transistors to replace valves (tubes).... Once FETs started to appear, everyone apart from Sennheiser resorted back to conventional Hi-Z circuitry again. As Sennheiser had no history of making Hi-Z condenser mics beforehand, they alone stuck with the RF concept.

RF condensers are the answer to problems with Hi- Z circuitry and outdoor use - especially in high humidity situations. Only Rode have bothered to join Sennheiser in that market. And I think that all current RF commercial mics use SDC capsules?

I just fancied the idea of trying the RF concept out with LDCs, when Gerard asked if anyone had built a 'hobby RF' mic... No real reason, other than curiosity!

It was your Sennheiser capsule acquisition that prompted me to try my latest version..... I have fitted one of Ari's 'Flat K47' capsules in a figure of 8 configuration....That capsule has 2 properties that made me think it was worth a try...
• Both sides are matched within 1dB
• Each side has a capacitive value of around 68pF
It actually works quite well. Unlike other variations, there is no problem with the bridge actually being 'balanced'..... Because as the capacitance of one side gets larger the other get smaller - so the bridge can never reverse polarity..... It's a bit like Sennheiser's 'push pull' concept (well, sort of!)

It's not a particularly sensitive capsule, so the mic doesn't have an ultra low noise floor - although it's comparable with my OPIC opamp mics.

Sounds pretty good (the 'flat K.47' a really nice capsule - as others have already mentioned !)

I think those blank Aliexpress coil formers might offer some interesting options for future versions?... I've never found an alternative to the Spectrum coils that would work well 'off the shelf', but the option of trying out different winding arrangements - using blank formers -might make for some interesting new experiments?
Now - whether any of this is actually worth the effort is for people to decide for themselves?
I just like the idea of trying something a bit different, hobby wise.

Further RF experiments I have in mind are looking at creating a very low noise osciallator, to maybe drop the noise floor even further, and possibly even looking again at FM or phase modulation instead of AM....... just for the fun of it!
(OK, I realise that sounds really 'geeky' ! )

I think i wasn't clear. I was thinking about using opamps for the whole RF circuitry and get rid of the coils altogether.

 

[rogs]

I think i wasn't clear. I was thinking about using opamps for the whole RF circuitry and get rid of the coils altogether.

I found most 'conventional' opamps don't have the bandwidth for RF work -- although maybe there are specialised ones?
I did try looking into using RF phase lock loop ICs to avoid using inductors, but the noise figures were hopeless for this kind of project.

AFAIK, all current RF mics use inductors somewhere? --- the 'noise free gain' available from the 'Q' of inductive devices is difficult to resist!

 

[Tim Campbell]

Has anybody looked at the latest generation MKH mics to see what they use in such a small body? Also what about modern wireless systems?

 

[rogs]

Has anybody looked at the latest generation MKH mics to see what they use in such a small body? Also what about modern wireless systems?

I've only ever seen the MKH layouts from the service manauls of some 20 years ago.... I'm guessing it's all been updated with surface mount now- even if the circuits are the same?
I did do a screen capture of a couple of frames from the 'promo' video put out by Rode on their new NTG5 RF mic a couple of years ago.
Sadly, the internals were only on screen for a second or two -- and the frames are not high res.
As you can see from the attached 'montage' of the captures, we are looking at a single inductor housing, and surface mount devices throughout.
It does look like the semiconductors are individual transistors though.....

As for modern UHF wireless systems , I suspect much of their technology will come from the same 'camp' as modern phone designs.... So some pretty specialised surface mount packages there, I suspect?