Interesting audio circuit...

Phil Allison · 2011-05-15T03:30:16+00:00

** Hi to all,please see link to schem and pic of a Sennheiser "RF" condenser mic design from the 1960s.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 actionNo such evil issues exist with RF types so it is a surprise that so few exist - only Sennheisers AFAIK..... Phil

May 15, 2011 29 Replies

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John KD5YI 15 years ago

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

John

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Robert Baer 15 years ago

See nothing about FM; the closest maybe is phase modulation.

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Phil Allison 15 years ago

** See the words " FM modulator " ??

You must be totally blind.

..... Phil

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Phil Allison 15 years ago

** That is EXCELLENT !!

Thanks a lot.

..... Phil

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Phil Allison 15 years ago

"Jan Panteltje"

** 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

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Phil Allison 15 years ago

"Phil Allison"

** Some facts are now clear:

formatting link

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

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

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

** That's how a ratio detector works.

** 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

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Jan Panteltje 15 years ago

On a sunny day (Mon, 16 May 2011 11:21:00 +1000) it happened "Phil Allison" wrote in :

You really need help with your head.

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Jan Panteltje 15 years ago

On a sunny day (Mon, 16 May 2011 13:20:53 +1000) it happened "Phil Allison" wrote in :

I am glad you got it now, sorry if I was rude.

Yea, well, I was wondering if anybody could get away with patenting negative feedback.. Looks like they can.

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Phil Allison 15 years ago

"Jan Panteltje"

** I have already taken back the above remark.

Get over it.

.... Phil

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Reinhard Zwirner 15 years ago

Phil Allison schrieb:

[...]

That's right - until now:

Bye

Reinhard

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