Interesting audio circuit...

** Hi to all,

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

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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

Reply to
Phil Allison
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I had thought of a variant: use the capacitance to vary the frequency of an oscillator - thereby creating FM.. Anybody tried that?

Reply to
Robert Baer

"Robert Baer"

** Better re- read my post - Bob.

.... Phil

Reply to
Phil Allison

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

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

Reply to
Rich Grise

On a sunny day (Sun, 15 May 2011 13:30:16 +1000) it happened "Phil Allison" wrote in :

Phil, that is a classic FM phase detector. The F0 of the circuit changes if the membrame moves, D1, D2 makes a phase detector, note L3, and the diode polarity, it is a Foster-Seeley discriminator:

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Reply to
Jan Panteltje

Will have to try and find AES Journal article that appears to be noted near top of the schematic...

13:1 and 14:1 Volume ??

boB

Reply to
boB

On a sunny day (Sun, 15 May 2011 13:30:16 +1000) it happened "Phil Allison" wrote in :

Phil, that is a classic FM phase detector. The F0 of the circuit changes if the membrame moves, D1, D2 makes a phase detector, note L3, and the diode polarity, it is a Foster-Seeley discriminator:

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Should be 'ratio detector' of course :-)

Reply to
Jan Panteltje

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. Try this link for the detector.

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though it doesn't give much detail. This link does

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but you've got to plow through a lot irrelevant but interesting stuff to get to the Foster-Seeley discriminator at Figure 9.22 on page 19.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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.

Reply to
upsidedown

"Bill Sloman"

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

And that will not be funny for you.

... Phil

Reply to
Phil Allison

snipped-for-privacy@downunder.com

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

** The circuit has lotsa hidden subtlety.
** As the lovely Eliza Doolittle said.
** Huh??

** Do tell.

** I think so that correct.

he details are what I am after.

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

.... Phil

Reply to
Phil Allison

The Sennheiser patent may provide some answers.

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Kevin Gallimore

Reply to
axolotl

s here

I wouldn't do that if I were you. Some of them do have influence, and if they complained to the police about your behaviour, the police might find it in thier hearts to do something to abate the nuisance. And the lawyer nephew seems to be good at lawyering.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

On a sunny day (Sun, 15 May 2011 04:28:24 -0700 (PDT)) it happened Bill Sloman wrote in :

NO NO, the OSCILLATOR is fixed, the discriminator fo changes. See my other reply to Phil.

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.

Reply to
Jan Panteltje

On a sunny day (Sun, 15 May 2011 14:57:52 +0300) it happened snipped-for-privacy@downunder.com wrote in :

See my reply to Phil.

Reply to
Jan Panteltje

a

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

Reply to
Fred Bloggs

a

I think this is more in line with what's going on there:

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Reply to
Fred Bloggs

is a

Yep, the diodes form not a Foster-Seeley detector but a double- balanced mixer. It looks like the microphone capsule acts as a 'drone' element, makes the L2 induction lead or lag the phase of the L1 inductor (which is the local oscillator). It's a pretty clever design.

Reply to
whit3rd

t is a

Nope- a single balanced mixer

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between the oscillator direct output and a Mic dependent phase shifted version of itself.

Reply to
Fred Bloggs

man

s a

Sorry about that. I wasn't paying enough attention to the actual circuit.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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