Lowest noise amps

In article , Winfield Hill wrote: [....]

Yes, an amplifier where the feedback to the source is taken from much later in the circuit is a better way to go. You usually want to follow the cascoded pair of JFETs with a low noise PNP stage.

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kensmith@rahul.net   forging knowledge

Just that. Control the BJT cascode voltage from the JFET source.

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 Thanks,
    - Win

Errr... Methinks you are incorrect in saying that if the capacitor plate moves by amount X, that the capacitor varies as 1/X. That is not possible; moving the plate by 0.1 micrometer cannot give a capacitance change a thousand t1mes more than a movement of 0.1 millimeter. Your statement implies that a zero movement gives an infinite change of capacitance (this naturally is in the limit).

So, start with the formula for *total* capaciance, where everything is kept constant except the dielectric thickness X: C=K/X where C is capacitance in some reasonable units like picofarads, and K is a constant that includes fixed things like dielectric constant, plate area, etc. HERE AND ONLY HERE does the *total* plate seperation X change the

*total* capacitance by 1/X (aka "by definition". *BUT* The sound *changes* that seperation, and one wished to know how the capacitance changes. Hmmmm..... Sounds like high math...maybe, hummmm, differential calculus, maybe???? So, d(1/X) = -(1/X*X)dX . My, oh my! Sounds like it is LINEAR - that is to say, the capacitance changes the same way that X changes! What a massive surprise! . . . . . . . . . *NOT*

Dear Spehro,

Thank you for your guidance. The PDF file shows 2 nozzels at the top (A and B), however the the daikin model that I have only has one nozzel. Is that ok?

Just out of interest, quality wise, is "Sauer Bibus" the best available in the market?

Thank you,

Spehro Pefhany wrote:

Does anyone recognise these:

This is the inside of one:

Some of them have tuning-slugs. I picked them up in a box of assorted junk at a radio rally.

The markings are two letters (AA, PA, PM, FD, VR ... etc) followed by three digits always beginning with an 8 i.e. 8xx, sometimes with a siffix e.g.

803a. The date codes are 1979 or 1980.

I'm a little concerned that the white substrate material might be BeO - especially after prising one open (!)

On 20 Aug 2005 06:06:20 -0700, Winfield Hill wrote in Msg.

What does 'bootstrapping the cascode off the source' mean?

robert

In article , Robert Latest wrote: [...]

Vcc ! / \\ / \\ ! ! \\! ! !--+---/\\/\\/-- GND e/! ! ! ! ! --- ! --- ! ! !- ! ---->! ! !-+---- s !

Asssuming there is lots of feedback brought into the source of the JFET, all three terminals of the JFET have exactly the same AC signal on them so the input impedance is infinite.

The highest input impedance I've ever been able to do at a few KHz was

200G. To do this, absolutely everything near the first stage has to have the same signal on it.

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kensmith@rahul.net   forging knowledge

In article , Robert Baer wrote: [...]

I think perhaps I was unclear in the statement. The sound changes X from some initial value. At rest the two plates are at some distance.

This is how all of the high end microphones are in fact made.

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kensmith@rahul.net   forging knowledge

I suggest you go back to college.

Yes; what i have seen is a solid "front" with holes or slots, and a metallized mylar diaphram is stretched over the "back" of that piece. The plastic is the insulator, and a conductive ring makes the electrical contact to the metallized surface *and* stretches the mylar uniformly. It is rare that the metallized mylar is placed between two solid (but perforated) plates.

I read in sci.electronics.design that Robert Baer wrote (in ) about 'Lowest noise amps', on Mon, 22 Aug 2005:

I suggest you explain instead of just distributing your opinions.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Back in the 60's when I worked at the University of Rochester Accoustics Lab part of my job was replacing the aluminized Mylar diaphragms in a custom microphone designed for -3 dB @ 2 MHz (for shock wave measurements). This was the only Mylar diaphragm microphone in the lab, and it was a real pain because of the pathetic stability of Mylar; the diaphragms needed to be changed once a month or so to maintain reasonable performance. Real measurement microphones (Bruel & Kjaer, etc.) designed for reproducible response over a long term always use solid metal foil diaphragms. The slotted front cover is always removed for any precision measurements as it severely degrades the performance of the microphone; measuement microphone are always provided with response curves with and without the cover, the curve without being the good one, the curve with being for harsh field applications where the cover must be left on to protect the diaphragm.

Regardless of diaphragm material there is always an air gap between the diaphragm and the rear electrode which changes thickness with diaphragm movement, exactly as JW and KS have described. Perhaps some low end mics use the Mylar as the insulator between the capacitor plates (at the edge support), but I have not seen this design; usually the stationary electrode is insulated from the mic body and the aluninized Mylar or metal diaphragm (moving electrode) is connected to the mic body. The Mylar serves only to support the aluminum moving electrode. Parallel plate capacitor theory is a decent 1st approximation even though the diaphragm deflects into a shape closer to spherical (with a very large radius) than planar (the edges do not move).

I will attempt to post a description of condenser microphones with construction diagrams from Acoustical Engineering by Harry F. Olson (1957) to ABSE. Construction details have changed a bit since then but the principle of operation has not.

How about getting off your horse and joining the discussion?

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 Thanks,
    - Win

Not an opinion. No explaination needed for anyone that can read and knows a little math.

Methinks he is moaning about a missing minus. Perhaps picky in a discussion of amplitude, where use of absolute value could reasonably be presumed.

Did my PDF ever show up on ABSE?

My sentiments entirely !

Graham

I read in sci.electronics.design that Glen Walpert wrote (in ) about 'Lowest noise amps', on Tue, 23 Aug 2005:

Oh, my goodness. I shall have to leave s.e.d immediately, in disgrace! Oh, the SHAME. A missing MINUS!

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

It is a simple differential of the equation for capacitance as a finction of the plate seperation. The capacitance change has a simple linear relation to the change in seperation (leaving out constant multiplier). And someone is tying to imply that it is 1/(X*X)...

No need for such sarcasm, John. I'm sure we can all think of many much better reasons why you should leave SED in disgrace!