Interesting audio circuit...

[Phil Allison]

** Hi to all,

please see link to schem and pic of a Sennheiser "RF" condenser mic design
from the 1960s.

http://www.waltzingbear.com/Schematics/Sennheiser/MKH-405.JPG

http://www.saturn-sound.com/images - microphones/sennheiser mkh405 - inside.jpg

The capsule is shown as "Mi" and has a capacitance of about 30pF.

The operation of the section involving D1 ad D2 is a tad mysterious - but
clearly is at the heart of the design.

There is an 8 MHz crystal oscillator driving ( pumping ?) the transformer
labelled L2 - so any idea that it is simply an FM modulator and detector is
hard to justify. The exact operation of the loop coupling *audio* from T3 to
the D1 & D2 network is interesting too.

Note how the 8MHz signal is coupled to L2 in two ways, via the 3 turn output
winding of L1 and a 2 turn winding on L2.

The design of L2 is a big part of the secret and my best guess is that is it
a phase modulator with D1 and D2 forming a phase detector.

BTW:

The design is of particular interest as so few ( true) condenser mics work
using RF, the vast majority apply a DC voltage to the capsule ( circa 50 to
90 volts) and operate by charge conservation - ie small changes in
capacitance causing corresponding voltage changes on the capsule. This
understandably means having extremely high impedances everywhere so the mic
works down to less than 20Hz - ie 2Gohm resistors for the DC bias and FET
preamps with 2Gohm input resistances. The mic capsule and surrounding
circuitry becomes highly vulnerable to the tiniest DC leakage current and
moisture ingress from human breath can put it completely out of action

No such evil issues exist with RF types so it is a surprise that so few
exist - only Sennheisers AFAIK.


..... Phil

 

[Phil Allison]

"Robert Baer"

I had thought of a variant: use the capacitance to vary the frequency of
an oscillator - thereby creating FM.

** Better re- read my post - Bob.



..... Phil

 

[Rich Grise]

I had thought of a variant: use the capacitance to vary the frequency
of an oscillator - thereby creating FM..

The first time I heard of that was about 50 years ago.

Anybody tried that?

Yes, there were even circuits available in books and magazines,
for hams. But doing it with a crystal makes it sound "tinny." ;-)

I've wondered about using an electret mic in the same function,
kinda like they sometimes use a varactor diode, AKA varicap.

Cheers!
Rich

 

[Phil Allison]

"Jan Panteltje"

Phil, that is a classic FM phase detector.
The F0 of the circuit changes if the membrame moves,

** Bull.

D1, D2 makes a phase detector, note L3, and the diode polarity, it is a
Foster-Seeley discriminator:

** That is an FREQUENCY modulation detetector.

http://en.wikipedia.org/wiki/Detector_(radio)

** Fucked link.



.... Phil

 

[Phil Allison]

"Jan Panteltje"

http://en.wikipedia.org/wiki/Detector_(radio)

** Yawnnnnn......................

Same fucked link.

I knew this was gonna happen.



..... Phil

 

[[email protected]]

And Jan's point was that the capacitor microphone was set up as an FM
oscillator, where changes in the microphone capacitance changed the
oscillation frequency.

The two diodes really looked like an FM detector and it would make
sense that microphone capacitance would control the oscillator (Fvco)
frequency.

However, I do not understand, why there is an 8 MHz crystal in the
circuit. A simple VCO+FM detector would make sense.

Is this kind of heterodyne system (Fvco-Fcrystal) to get a larger
relative frequency deviation from the FM detector. Not likely.

It does not look like a crystal discriminator either.

Apparently there is a fixed amplitude, fixed frequency (8 MHz) feed
into windings 7-8.

The microphone and winding 1-2 form an LC resonant circuit nominally
at 8 MHz.

The actual LC resonant frequency varies with air pressure. This
resonant circuit loading then determines, how much power will enter
the FM detector windings.

 

[Phil Allison]

"Bill Sloman"


** **** off - you retard wog pig or I will phone all your relatives here
in Sydney.

And that will not be funny for you.



.... Phil

 

[Phil Allison]

[email protected]

The two diodes really looked like an FM detector and it would make
sense that microphone capacitance would control the oscillator (Fvco)
frequency.

** That is what most folk think on first look.

circuit. A simple VCO+FM detector would make sense.

** The circuit has lotsa hidden subtlety.

Is this kind of heterodyne system (Fvco-Fcrystal) to get a larger
relative frequency deviation from the FM detector. Not likely.

** As the lovely Eliza Doolittle said.

It does not look like a crystal discriminator either.

** Huh??

Apparently there is a fixed amplitude, fixed frequency (8 MHz) feed
into windings 7-8.

** Do tell.

at 8 MHz.

** I think so that correct.

he details are what I am after.

Audio is so simple - ain't it ????




..... Phil

 

[axolotl]

he details are what I am after.

The Sennheiser patent may provide some answers.

http://www.google.com/patents/about?id=xRl0AAAAEBAJ&dq=3422225


Kevin Gallimore

 

[John KD5YI]

Diode polarities are not right for a frequency discriminator- so
either the shematic is in error or something else is going on...

No, Bill, it is a ratio detector as Jan pointed out. A *phase* detector.

John

 

[Phil Allison]

"Jan Panteltje"

It is very simple.
In a FM radio the transmitter frequency changes,
and you use a fixed tuned circuit in the ratio (FM) detector.
This is the other way around. The frequency is constant (xtal),
but the capacitance change caused by the moving membrane changes
the frequency of the ratio DETECTOR tuned circuit.

** Nonsense.

Wikipedia works here,.

** The link is to a dead page.


..... Phil

 

[John KD5YI]

"Jan Panteltje"


** Nonsense.



** The link is to a dead page.


.... Phil

Works fine here.

 

[Phil Allison]

"Robert Baer"

See nothing about FM;

** See the words " FM modulator " ??

You must be totally blind.


...... Phil

 

[Phil Allison]

"axolotl"

The Sennheiser patent may provide some answers.

http://www.google.com/patents/about?id=xRl0AAAAEBAJ&dq=3422225

** That is EXCELLENT !!

Thanks a lot.


...... Phil

 

[Phil Allison]

"Jan Panteltje"

NO NO, the OSCILLATOR is fixed, the discriminator fo changes.

If you ever adjusted an FM receiver, you can see how the output changes
if you adjust the discriminator (ratio detector).
It is fun, I designed soem nice ratio detector many years ago, fun stuff.

** Looks like that is just how it works.

Varying the ratio detector tuning with the mic capsule appears to give
better s/n ratio than any simple FM scheme - while the NFB loop makes the
whole thing far more linear at high SPLs.

Axolotyl posted a link to the patent which has lotsa detail.


.... Phil

 

[Phil Allison]

"Phil Allison"

** Some facts are now clear:

http://www.waltzingbear.com/Schematics/Sennheiser/MKH-405.JPG

http://www.saturn-sound.com/images - microphones/sennheiser mkh405 - inside.jpg

The capsule is shown as "Mi" and has a capacitance of about 30pF.

The operation of the section involving D1 ad D2 is a tad mysterious - but
clearly is at the heart of the design.

** Along with L1 and L2, they form a "ratio detector" or FM discriminator.

There is an 8 MHz crystal oscillator driving ( pumping ?) the transformer
labelled L2 - so any idea that it is simply an FM modulator and detector
is hard to justify.

** The tuning point of the ratio detector changes - not incoming the RF
frequency.

The exact operation of the loop coupling *audio* from T3 to the D1 & D2
network is interesting too.

** Simple audio frequency equalisation for natural roll offs at the ends of
the audio band inherent in the capsule design.

winding of L1 and a 2 turn winding on L2.

** That's how a ratio detector works.

BTW:

The design is of particular interest as so few ( true) condenser mics work
using RF, the vast majority apply a DC voltage to the capsule ( circa 50
to 90 volts) and operate by charge conservation - ie small changes in
capacitance causing corresponding voltage changes on the capsule. This
understandably means having extremely high impedances everywhere so the
mic works down to less than 20Hz - ie 2Gohm resistors for the DC bias
and FET preamps with 2Gohm input resistances. The mic capsule and
surrounding circuitry becomes highly vulnerable to the tiniest DC leakage
current and moisture ingress from human breath can put it completely out
of action

No such evil issues exist with RF types so it is a surprise that so few
exist - only Sennheisers AFAIK.

** Sennheiser * never * made conventional ( ie charged plate ) condenser
mics at all.

With the arrival of low noise Silicon transistors in the early 1960s, they
developed an RF circuit that enabled a condenser mic's capsule to operate
without the need for high voltages or vacuum tubes. Other makers simply
adapted their existing vacuum tube condenser mic designs to use low noise
FETs - when they arrived later in the 60s.

Sennheiser are still the only known maker of RF condenser mics - and
rightly claim them to be in a class of their own for their combination of
immunity from weather conditions, near perfect polar and frequency response
curves and very low self noise.



..... Phil

 

[Phil Allison]

"Jan Panteltje"

You really need help with your head.

** I have already taken back the above remark.

Get over it.


..... Phil

 

[Reinhard Zwirner]

Phil Allison schrieb:

[...]

** Sennheiser * never * made conventional ( ie charged plate ) condenser
mics at all. ...

That's right - until now:

<http://www.sennheiser.com/sennheise...ired-microphones_studio-recording-mics_504298>

Bye

Reinhard

 

[JW]

"Jan Panteltje"

** Yawnnnnn......................

Same fucked link.

I knew this was gonna happen.



.... Phil

The "(" breaks the link. This should work:

<http://en.wikipedia.org/wiki/Detector_(radio)>