Sennheiser MKH800 / MKH80 capsule (ks80)

[jarvis]

The new MKH8000 series has a couple of disc shaped PCBs, that are sandwiched together with the capsule in front of them. You can roughly make them out in the drawing that Sennheiser publishes on their service page.

Best
Jannis

 

[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?

Spectrum coils received today, as well as the coils from Aliexpress.

I opened one of the Spectrum ones, and it seems it would be pretty easy to replicate them using the Ali stuff. Not sure about core material of the Chinese coils, but we'll see. The Spectrum ones use 105um (awg38?) wire.

What about the ratio of the coils, they are 4:1. How does that affect stuff?

The way they are wired in your circuit secondaries are in the 5uH range. Is it just secondaries that count? What about primaries? I'm thinking if we manipulate the ratio we could squeeze some more juice?

It is also that lower ratio primaries lay on top of just half of the secondary, the other half is on a spool below. Wonder what would happen if both were wound under the primary.

Edit:
I replaced the core in Spectrum one for Aliexpress core and got just slightly (.8uH) maximum value. So I guess the cores from Ali are appropriate. I wound one of the Ali inductors with thinner awg42 wire i have the same way Spectrums are wound and got similar results. Slightly higher DC resistance. No idea how this could impact the results.

 

[rogs]

My Aliexpress cores have arrived, but I'm still waiting on the enamelled copper wire. I think I shall try and just copy a Spectrum coil initially (center tapped secondary wired on two adjacent spools -- primary on top of top spool ) and see how close I am.

When it comes to the best ways of creating alternative values, I'm thinking it may well be partly a 'suck it and see' approach....
The Spectrum coil lists near the top of THIS PAGE may give some idea of the number of turns needed for alternative values?

What is more difficult is to know exactly in which direction the turns ratios need to be varied, for optimising results.
The fact that T2 primary is fitted across the bridge makes calculating the total load on T1 rather complex, and the tuning of T2 core is dependent to some extent on the tuning of T1 -- and vice versa.
I would think it maybe possible to derive better options, to suit different capsule values? .....

As I have said on several occasions, my theoretical knowledge is simply not up being able to offer any 'best approach' ideas.
The Spectum coils worked out far better than I ever thought they would -- especially for capsules above 70pF in value --- but I am the first to confess my good results were pure luck, when it came to inductor design details.....

My understanding is that there are a number of variables that make calculations around the whole inductor assembly difficult to simulate .... so it looks as if good old fashioned experiments with 'actual parts' may be necessary, to try and understand better exactly what is the best way to optimise things, for any given capsule.....

 

[kingkorg]

My Aliexpress cores have arrived, but I'm still waiting on the enamelled copper wire. I think I shall try and just copy a Spectrum coil initially (center tapped secondary wired on two adjacent spools -- primary on top of top spool ) and see how close I am.

When it comes to the best ways of creating alternative values, I'm thinking it may well be partly a 'suck it and see' approach....
The Spectrum coil lists near the top of THIS PAGE may give some idea of the number of turns needed for alternative values?

What is more difficult is to know exactly in which direction the turns ratios need to be varied, for optimising results.
The fact that T2 primary is fitted across the bridge makes calculating the total load on T1 rather complex, and the tuning of T2 core is dependent to some extent on the tuning of T1 -- and vice versa.
I would think it maybe possible to derive better options, to suit different capsule values? .....

As I have said on several occasions, my theoretical knowledge is simply not up being able to offer any 'best approach' ideas.
The Spectum coils worked out far better than I ever thought they would -- especially for capsules above 70pF in value --- but I am the first to confess my good results were pure luck, when it came to inductor design details.....

My understanding is that there are a number of variables that make calculations around the whole inductor assembly difficult to simulate .... so it looks as if good old fashioned experiments with 'actual parts' may be necessary, to try and understand better exactly what is the best way to optimise things, for any given capsule.....

As always thanks for invaluable information. Awesome part is that there aren't so many turns involved, and these Ali cans seem to be a great platform. I'm sure we'll get there. I can't let this capsule just collect dust.

 

[kingkorg]

@rogs @Tim Campbell

I made a wrong assumption about the connections of the capsule thinking simply one is diaphragm and the other backplate. However I was confused there was also separate connection for the ground.

I removed the front plate from one of the sides just to realize it is there just for protection and holding the white fabric which is probably for plosives. Identical frontplate is under it.

There are 4 backplates (2 in front of the diaphragms, two behind) in total. Fifth is the thick one probably for pattern control.

Both diaphragms are connected together, and to the ground, and thick fifth chamber backplate.

The two connections in front of each side are for the "frontplate" and "backplate" for each diaphragm. So they are push-pull and the diaphragms are grounded just like in available MKH series service manuals. Only dual in this case.

So I took measurements again, and capacitance is about 50pF for each backplate to diaphragm. They are slightly unbalanced diaphragm to front, diaphragm to back backplate(6pf), which I wonder about if it's on purpose to create slight unbalance for the bridge.

What I did before was to measure the capacitance between the frontplate and backplate for each, and somehow it was so low. I also wonder how in the world the thing even worked since I basically connected just the plates, not the diaphragm.


Here's a rough drawing. I have no idea if what I just wrote makes any sense.

 

[rogs]

I've found it a bit tricky trying to do exact comparisons between my circuit ideas and the published Sennehiser schematics from the MKH series.
My schematic was derived originally from Baxandall's 'unbalanced bridge' concept.
The MKH 'push pull' concept is slightly different. in that it seems to rely on a balanced 'off' state.

Pages 5 and 6 of Hibbing's paper describes the 'new' push pull realisation from the 1980s :
https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf
There is a description of the Sennheiser schematics (and some capacitive values) on pages 5 and 6 (again!) of this document:
http://www.sdiy.org/oid/mics/Senheiser-MKHs-SM.pdf
Unfortunately, that series of service manuals only seems to include the use of capsules KS20 thru KS70 -- but not KS80

You might get some useful snippets from John Willett's paper here:
https://sound-link.co.uk/docs/Technology of RF Condenser Mics - LineUp 2003.pdf
This is not an easy subject. The RF concepts seem simple enough..... It's turning them into high quality microphones that's the tricky part!

One variation I have been experimenting with is using Baxandall's concept - but balancing the bridge.
In a conventional sense that simply doesn't work..... If you make the capsule and the balance capacitor exactly the same size, the modulation will simply continually try and reverse the polarity of the amplitude modulated output signal (the capsule value 'moves' about both sides of the balanced centre point )

However, there is one configuration where that doesn't happen. As I mentioned earlier, I have tried one of Ari's flat K.47 in a 'figure of 8' RF mic configuration.
The outputs from both sides of her capsules are balanced to within 1 dB, so the second side of the capsule becomes the bridge 'balancing capacitor'.

As the modulation increases the capsule capacitance of one side, that same modulation casued the second side capacitance to reduce.
So the centre point of the bridge moves about - like a 'capacitive potentiometer' wiper - but never reverses the polarity.
A 'push pull' equivalent (sort of! )

Works quite well. Ari's flat K.47 is not designed as an RF capsule, so the tension is a bit high to allow for a really low noise output, but it's not bad!

However, with my circuitry this only works for figure of 8 mics.

Looking at the Sennehiser MKH service schematics, I'm having trouble trying to read how they derive different polar patterns.
The circuitry looks very similar for all versions?.......

 

[kingkorg]

Looking at the Sennehiser MKH service schematics, I'm having trouble trying to read how they derive different polar patterns.
The circuitry looks very similar for all versions?.......

I believe the difference is in the capsule construction.

 

[kingkorg]

You might get some useful snippets from John Willett's paper here:

Quote from this document:
For example, the figure-of-eight MKH 30 microphone is virtually perfect (see figure 7);

I have already ordered a couple of our typical Ali edge terminated capsules, and at least in my head came up with an idea on how to modify them so that the diaphragm will be in the sandwich between the two backplates just like Sennheiser. The mod should be relatively easy, and would provide us with first large symmetrical, undamped, single diaphragm f8 capsule. I plan on making a video.

 

[Tim Campbell]

I did that with this capsule. The plate in front of the membrane is +, the membrane is ground and the backplate is - times 2.

 

[Tim Campbell]

How did the capsule sound and measure when wired properly?

 

[kingkorg]

How did the capsule sound and measure when wired properly?

Properly or previously wrongly wired it measured the same. Just a giant, smooth sad face curve. Just as described in the documents and in the first post of this thread. Compensated with eq for flat response i really liked it. Very good rear rejection.

When i wired it the wrong way, now we are talking about just half of the capsule in cardioid mode, frontplate was polarized at 15v (and later 60v) backplate 0V, and diaphragm was left dangling in the air. Somehow it still produced the same exact FR and even level. I guess the diaphragm found a way to find it's bias point out from the space...

Still waiting for all the parts for Rogs' circuit, cant wait to try it out in RF mode.

What is the capacitance of your capsule? I bet it is ideal for @rogs circuit.

 

[jarvis]

I did that with this capsule. The plate in front of the membrane is +, the membrane is ground and the backplate is - times 2.

Wow Tim,

that looks awesome! I've been really into the symmetrical capsule design lately and have been trying to make one myself but my progress is pretty slow, with lots of hiccups along the way. How thick are the backplates you made there, if you don't mind me asking?

Best
Jannis

 

[kingkorg]

Wow Tim,

that looks awesome! I've been really into the symmetrical capsule design lately and have been trying to make one myself but my progress is pretty slow, with lots of hiccups along the way. How thick are the backplates you made there, if you don't mind me asking?

Best
Jannis

I will try to explain my thought process behind modding standard china RK12 type.

Disassemble, drill through all the blind and through-holes, polish the backplates, then sandwich tensioned diaphragm between the halves using stock i presume 40uM spacers to separate the diaphragm from both backplates. Find a way to make connection to the diaphragm. You get + and - from backplates, and 0 from the diaphragm. And it will off course be F8 undamped capsule. Or even use a lathe to render backplates thinner.

I hope it gives you some ideas.

It could be possible to use additional (3rd) half of the stock backplate and put it behind one of the modified backplates to create cardioid. But i have no idea what kind of FR you would get. In that case it would be probably best to leave one of the modified halves - unmodified.

Or if you think the other way about it, use stock capsule, and then use third, drilled through the all holes backplate in front of it, which would hold the diaphragm in place instead of the tensioning ring. And be the "frontplate"

 

[Tubetec]

This discussion reminded me of the original Quad electrostatic speaker in many ways ,
moving membrane is essentially grounded , drive is applied differntially to the panels via a massive step up transformer .

The SSL2 pre has turned out best friends with my new 416 ,
silence is golden ,didnt you know

 

[Tubetec]

Interesting to note ,
In the P48 version 416 I have L1 is an SMD part , where in older versions its a traditional pot core type , L2 as we know is the multitapped one in the schematic .

It has several pairs of colourcoded wires exiting , I think its a four section winding ,
I wont be dissconecting the coils for measurement purposes , but I can give it a measure with the meter and LCR , if thats of any use , I can probably get a calipers onto the magnet wire if Im very careful, just from visual inpsection its looks like all the lead outs from the inductor are the same gauge wire , the Dc resistance measurements appear to be in the range of a few ohms ,

Doing a through hole version of this circuit within the confines of the MKH casework is a tough task ,
The SMD version on the other hand has tons of space , but would be impossible to do by hand due to the size of the components ,
I think we saw a picture of the Rode RF mic pcb somewhere back along , its remarkably similar to the Sennheiser down to the design of the main inductor core and handfull of supporting transistors ,

For test purposes maybe we can recreate one of the through hole designs , theres a certain era of MKH mic which uses still commonly available transistors ,
BC550 etc
Maybe I'll try Sennheiser for a service doc on the 416 , in any case I havnt identified if the second 416 is the T powered or Phantom version yet , Sennheiser should be able to confirm from the serial number exactly which kind it is ,

The throughhole version I have with the KS15-41 capsule incorporates three transistors and three inductors ,it doesnt have the crystal osccilator used in earlier models , in the later SMD version the third inductor has been replaced , a total of seven transistors are used in that circuit .

 

[Tubetec]

After giving the internet a good scrape I found additional service
documents which I added in technical section ,
The one I cant seem to lay my hands on is the 416-P48U3 service doc ,

Ive been able to conclusively identify my second mic as a 416T , the one without the crystal ,
so just as well I didnt try plugging it into 48v without double checking ,
as you'll see from the schematics its BF240 and BC550/560 transistors are used ,there both still easily available types ,which is a plus for the diy constructor .
The quartz crystal version saves on the L3 inductance ,


Has anyone got an email address of Sennheiser service department handy ,
There seems to be a few spares available off the shelf , but no sign of new casework or internal windshield which is missing from both of my mics ,
I need a pair of top grills which they have for sale for around 30 euros ,
foam windsheild and box is readily available at a reasonable price .

The coils for the various mics were available as a spare part , but older models have long since seen service discontinued ,it might still be possible to find a coil an disect one ,

 

[jarvis]

I will try to explain my thought process behind modding standard china RK12 type.

Disassemble, drill through all the blind and through-holes, polish the backplates, then sandwich tensioned diaphragm between the halves using stock i presume 40uM spacers to separate the diaphragm from both backplates. Find a way to make connection to the diaphragm. You get + and - from backplates, and 0 from the diaphragm. And it will off course be F8 undamped capsule. Or even use a lathe to render backplates thinner.

I hope it gives you some ideas.

It could be possible to use additional (3rd) half of the stock backplate and put it behind one of the modified backplates to create cardioid. But i have no idea what kind of FR you would get. In that case it would be probably best to leave one of the modified halves - unmodified.

Or if you think the other way about it, use stock capsule, and then use third, drilled through the all holes backplate in front of it, which would hold the diaphragm in place instead of the tensioning ring. And be the "frontplate"

That's pretty much the idea I started out with but I got distracted pretty fast, since I wanted to learn everything about capsule making from the ground up.

So now I've got myself an old lathe and one of those cheap small CNC routers, that I want to use to drill the holes of the backplates with. Using the Sennheiser MKH capsules as a guideline, I modeled a large diaphragm capsule in CAD and now I'm fighting stability issues in the cheap router and some kinks in my old lathe. Since I don't have that much time on my hands at the moment, it's kind of a slow but really fun learning experience.

Best
Jannis

 

[kingkorg]

Here's initiatial report on this capsule in @rogs RF circuit.

Stock just capsule put in the circuit wont work - too low capacitance.

I added 47pF in paralel to both front plate and back plate. C4 is replaced with front plate (ca50pF) + 47pF. Backplate (ca50pf) + 47pf are connected at regular points where the capsule should be. I made sure there's some difference (under 10pf) between sides.

It works, but pretty high noise level. I guess useable for come applications. I will have to find a way to wind custom inductors.

RF polarized the capsule has more low end compared to the capsule polarized with DC. Makes sense, DC makes the diaphragm stiffer. There's some HF roll-off in the RF variant, which i believe has to do with the circuit itself. Other capsules i tested in this circuit behave the same.

Green - DC polarized capsule (No body/grille)
Purple - RF polarized capsule (In a u87 style body hence some ripples in FR)

 

[rogs]

............ There's some HF roll-off in the RF variant, which i believe has to do with the circuit itself........

The HF cut off frequency is determined by the low pass filters created by R4/C9 and R10/C13. With the values selected for V.5 the cutoff frquncy is 15.9KHz.

Yuo should be able to increase that by reducung the size of C9 and C13. You could of course change the values of R4 nd R10 as well/ instead of, but that will affect the cicuit DC current drain, and thus sytem DC levels. Probably easiset to start by reducing the vlaues of C9 and C13 .
In theoey, making then 470pF instead should increase the cut off to 33.8KHz .. so possibly a value like 680pF or 820pF might prove more suitable?......

 

[kingkorg]

The HF cut off frequency is determined by the low pass filters created by R4/C9 and R10/C13. With the values selected for V.5 the cutoff frquncy is 15.9KHz.

Yuo should be able to increase that by reducung the size of C9 and C13. You could of course change the values of R4 nd R10 as well/ instead of, but that will affect the cicuit DC current drain, and thus sytem DC levels. Probably easiset to start by reducing the vlaues of C9 and C13 .
In theoey, making then 470pF instead should increase the cut off to 33.8KHz .. so possibly a value like 680pF or 820pF might prove more suitable?......

Awesome. Will try.

I was also thinking about adding two 10k resistors in series after c5 and c10 bypassing them with cap and inductor to create "smiley face" curve to counter and flatten out the capsule response. But that would be the easy part, back to trying to figure out those inductors.