OTish Distortion in Conderser microphones

Found this, maybe of interest to audio dudes here

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martin

Reply to
martin griffith
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I imagine that the effect is real, but there is way of using negative feedback to get around it.

You've got to be able to drive the microphone sensing membrane as an electrostatic speaker, and then you can use an AC-excited capacitance bridge to measure the capacitance of the microphone, and compare the instantaneous capacitance with the long term average.

Any deviation in measured capacitance then generates an electrostatic feedback signal to drive the capacitance back to its nominal value.

J.J.Opstelten and N. Warmholtz describe a pressure-seinsor based on this pronciple in App. Sci. Res Hague, volume B4 from page 329 in 1955, and again with J.J. Zaalberg van Zelst in volume B6 from page 129 in

1956.

I'm quoting the references from my Ph.D, thesis - which dates back some

25 years before we moved to the Netherlands.

The problem with using this technique in a pressure gauge is that the electrostatic field required to counteract more than about 0,2% of atmospheric pressure is enough to induce conduction across the gap. This shouldn't be problem with a microphone.

Obviously, the electrostatic restoring force/voltage is a linear function of the presure being counteracted. You've got to use a bridge excitation frequency a good bit higher than the higest frequency you'd want to follow - well over 100kHz, ata guess - and you'd want to design the feedback loop on the basis that you were sampling the capacitance of the microphone twice per bridge-excitation cycle rather than continuously.

As a project it would offer a microphone that would be expensive enough to fascinate the audiophool exploitation industry - pity I can't patent it.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

The author should have read up on the subject before publishing his simplistic and unrealistic (10% capacitance variation!) analysis of

1/x error in condenser microphones for two load cases (not two distortion mechanisms as he claims). A much better analysis can be found in Acoustical Engineering by Harry F. Olson, copyright 1957, for example. Other distortion mechanisms related to diaphragm and air gap dynamics limit diaphragm displacement with low distortion to the extent that 1/x error is very small. Never nonexistent, however, and I think his analysis of the effect of load capacitance on 1/x error is correct even if it is so simplistic as to be virtually useless. Other than that, I liked it :-).
Reply to
Glen Walpert

Problems:

(1) assuming 100M of input resistance of the amplifier is unreasonable. A good design will be at least 1G. With bootstrapping, it can be many G.

(2) 30Hz is not below the audio range and 60Hz certainly isn't. The distortion will largist at the 2nd harmonic.

(3) That is a huge signal you are assuming. What are you trying to record?

(4) 1pF is not an extreme case. A fairly simple input stage using 2SK170s will get under 1pF of effective capacitance, if the drop on the source resistor can be high enough. Values down to 0.25pF can be done with real components.

(5) You need to review the math by which you come up with the distortion. The capacitance varies as (1/X) but the voltage goes as (1/C) so the voltage varies as X for the unloaded case.

(6) You left out all the interesting effects.

(7) Your yellow background is annoying.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

"Glen Walpert" "martin griffith"

Found this, maybe of interest to audio dudes here

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** Given that a condenser mic capsule is typically polarised to circa 60 volts DC, a +/- 5% variation in capsule C produces +/- 3 volts peak variation. This equates to a just over 2 volts rms of signal available at the gate of the FET used to buffer the capsule to the outside world.

Also, typical studio condenser mics have a rated output voltage of circa 10 mV at 94db SPL, = 1 volt at 134 dB SPL = 2 volts at 140dB SPL. Specified ( pre-amp) overload levels of 150 dB SPL or more are commonplace.

The pre-amp circuitry in the body of such mics is of unity or more often

*less* than unity voltage gain - but huge power gain of course. So, a +/- 5% variation in capsule C is a very reasonable estimate of what happens in practice with high SPL sound sources.

However, the polarising voltage is fed to the capsule via a resistor of 1 Gohm (sometimes more ) - placing the 3dB down point at about 3 Hz for a

50 pF capsule. Hence, the second harmonic generation alluded to in the URL can only occur at the very lowest audio frequencies and become at all significant at extreme SPLs that are most unlikely to exist at those frequencies from a musical source.

Poking the mic down the port tubes in a 15inch sub woofer going full tilt not withstanding !

....... Phil

Reply to
Phil Allison

One could feed the capacitive mike into a short circuit (summing junction of an opamp)...

Reply to
Robert Baer

The linearity changes with capacitance change. The capacitance does not change when the voltage does not change. So to prevent voltage changes!

To do this, the microphone must be short-circuited (can also be into a DC offset). The way to do this is feed the signal into the - input of an opamp that has capacitive feedback. This way this circuit does not function as voltage amplifier, but as CHARGE amplifier. The charge the sound delivers is amplified.

I designed an accelerationmeter based on a crystal device (capacitive) that had a very large frequency range this way.

Another advantage is that the signal is very low-impedant, this means low disturbances, low noise. And it is not sensitive to the length of the cable (the cable only sees a short-circuit), and it isn't sensitive to movement of the cable.

Feel free to ask any questions.

Pieter email: snipped-for-privacy@hoeben.com without the NOSPAM

Reply to
Pieter

Tanks for your eloquent expansion of my suggestion.

Reply to
Robert Baer

"Robert Baer"

** Yep - he made a real omelette out of one of your turds.

........ Phil

Reply to
Phil Allison

This statement is simply incorrect. It is also incorrect in a complex way too but we'll ignore this for now.

Actually, it is more likely to act as a slightly charge sensitive RF oscillator with a lot of noise.

No, it means very high noise. The input noise voltage of the op-amp divided by the impedance gives a noise current. Also the lower a resistor is, the more its noise current. When it comes to noise, you can't win, you can't break even and you can't get out of the game.

Try tapping on a coax with a little bias voltage on it connected to such a circuit. You will find that the circuit is sensitive to moving the cable. I've had to shock mount cables.

If a train is leaving the station at 11 miles per hour ..........

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Not when you do a proper design. The opamp is feedback capacitive, making it very stable. A small series resistor at the input may be wise.

Low sensitivity to outside disturbances.

There will always be noise.

The other circuits are sensitive. The one is decribe here isnt. When then is a short-circuit, there is no voltage. Where there is no voltage, that voltage can't change either. But it is best not to use phantom feeding, but a separate power supply cable. The phantom's voltage does interfere.

Two hours and 15 minutes.

Pieter

Reply to
Pieter

Putting a feedback capacitor on a high performance op-amp does not make it "very stable". You have to use a "unity gain stable" one in this sort of application. Without the small resistor you suggest, the op-amp is fairly likely to oscillate even if it is a "unity gain stable" one if the input cable is very long. The cable and capacitor look like a tuned circuit.

No, this is not the case. The circuit you suggest is this:

Noise ! ---Cnoise --- ! A ------------- Signal -[Zgen]---+-------! Z Amplifier !---- GND -------------

You can't improve the signal to noise at "A" by making Zamplifier low. How ever much you lower the amplifiers input impedance, you must also raise its voltage gain by too to maintain the same signal output.

Just for fun, lets assume that Zgen=Cnoise and the Signal(RMS) is equal to the noise(RMS).

High Z amplifier case: SNR = 1:1

Low Z amplifier case: SNR = 1:1

When there's no impedance, the current is infnite and thus the noise is infinite. It was exactly the sort of circuit you are suggesting that required the shock mounting of cables.

Try tapping of a length of COAX even with "no voltage" on it. You will still see spikes when you strike it. Just hook a BNC-BNC cable onto your scope's input and wack it against the work bench.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

that it does. unless its terminated.

I didnt do a very good job, but I tried to isolate both connector ends, and only whack the middle. simply waving an end about picked up a lot if noise. my crude attempt at isolation greatly reduce the resultant spike. its extremely sensitive at the connector.

I wonder if its because the open end is moving thru an otherwise stationary E field. so I wrap it in tin foil, and it makes no difference....

so whats going on here? is it simply VdC/dt, with dC/dt due to the pressure wave propagating thru the cable?

Cheers Terry

Reply to
Terry Given
[....]

If you hit it hard enough, you get triboelectric effects. This is the voltage generated when you mechanically break most materials. At lower hits, I believe, it is charges trapped in the plastic materials that is doing it. At the terminated end, the parts can move with respect to each other more easily.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

makes sense. same reason semiconductor physics tests all assume an un-stressed material.

Cheers Terry

Reply to
Terry Given

Not just triboelectric effects, piezoelectric effects as well. Picocoulomb/g accelerometer sensors require very special cables which require very special connectors in turn. I have had to use these and special microwave cables (vibration resistant) when testing waveguide relay switches under vibration. I think i still have a made-up cable somewhere. The tooling for terminating such an accelerometer cable is probably in excess of US$10,000 by now, 20 years ago the cable was US$20/ft.

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JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  
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Reply to
Joseph2k

as a general rule of thumb, everything is far more complex than at first suspected....

Thanks for some informative posts guys :)

Cheers Terry

Reply to
Terry Given

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