R/E/P > Klaus Heyne's Mic Lab

U47 schematic, component-by-component

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Oliver Archut wrote on Wed, 26 January 2011 02:44
Most of the statement are based on my own measurements (...)
The membrane material is the 2nd problem, most capsules are not 100% tensioned with reference to dead center of the capsule. There are only a few capsule out of any given series that collapse uniformly when the polarization voltage is pushed up (...) the less uniform the capsule collapses the more the distortion.

Interesting. Could you elaborate on your measurement setup and procedure? I figure that it takes more than just measuring the change of capsule capacitance to evaluate partial collapse. An how did you check for the distortion that you relate the "collapsing properties" to?

Additional the material itself is one factor itself for distortion, as an example a multiple layer membrane will produce less distortion (...) AKG rolled the mylar with a special pattern aside several other production advantages it did reduced the capsule distortion.

Interesting detail (BTW could it have been AKG's Karl Peschel who gave you the info?).

A source of distortion that hasn't been mentioned is the nonlinear property of the air volume inside the capsule as it is compressed by the excursion of the diaphragm. There's even research by people like Hibbing (of Sennheiser, e.g. an 1981 AES paper) and Pastill

Klaus, would you think that it makes sense to relocate the last portion of posts into a thread of it's own? Capsule distortion is obviously a rarely discussed yet important subject, which mabe shouldn't be sitting hidden in this thread...  

Oliver Archut:
Hello Volker,

the measuring test were build by myself, for collapsing test I modified a circuit that was suggested by Mr. Sattler/Sennheiser USA for measuring capacitance with active polarization voltage. The set up has a resolution of 0.1 pico farad. With a fine stepped power supply the nonlinearity of polarization vs. capacitance can be viewed while slowly working to the point of collapse.
A 2nd test setup to collapse capsules is a 1 to 80 step up x-former, that injects a square wave into the capsule and a finely stepped power supply, any capsule can be slowly collapsed.

As I said, the capsule that collapse unevenly had the higher distortion.

Measuring distortion, the capsule is hooked up to a Reaktanz circuit, either a tube or a transistor is operated as an electronic blind-resistor, this circuit will amplify the output of the capsule without being active in the circuit. It is technical speaking a parallel amplifier that follows the movement/capacity changes of the capsule.
Technical an amplified copy of the original signal without effecting the capsule signal. THD of this circuit itself is not measurable.

The signal source were electronic stimulated tuning forks.

It is still not the best set up in my view to quantify distortion.

Got a bunch of good info over the years from Karl, but the capsule material data I got out of old Telefunken data that was thrown out when the audio archive was closed.

The distortion papers you mention, they read well from a technical point, but in my view they try to describe chemical composition of wine (I slightly modified Klaus comparison here)
with finer and finer measurments of what is in it, without knowing what needs to be measured to quantify why the wine taste the way it does.

Capsule distortion one thread? More for a technical minded forum in my view.

Best regards,

Geoff Emerick de Fake:
The way distortion is spec'd by manufacturers has almost no informative value.
It's like mentioning distortion of a loudspeaker at 1kHz, when most of the time the actual problem is distortion at low and low-mid frequencies.
Everybody knows a mic diaphragm's excursion increases with the inverse of frequency, so the accepted 3% limit at 1kHz may become 30% at 100Hz.
In fact the construction of the capsule has an enormous influence on distortion; a center-terminated capsule certainly has a different behaviour than a side-terminated, if only because they don't rely on the same acoustic damping.


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