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Author Topic: Original AKG CK12 Capsules: Sound, Lifespan and Repair  (Read 17306 times)

Offline usattler

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #30 on: March 14, 2013, 09:11:52 am »
Quote
(Quote inserted by K.H., assuming that this was the subject of Uwe's response)

that suggests a reduced polarization voltage. The P-48 model has no oscillator, it's like an original U-87 taking the phantom voltage to the capsule, at a reduced level. Usually it's around 42 volts or so after all the series resistors and such. That is why the P-48 414 is about 5 db less output than an older 414 EB. Those use the internal oscillator to raise the polarization voltage to 60~62 volts.

All other things being equal, the difference in sensitivity resulting from the bias voltage change for a condenser capsule from 42 to 62 V would be 20 log (62/42), or barely 3.4 dB!
« Last Edit: March 14, 2013, 01:50:26 pm by klaus »
Uwe Sattler
Technical Director - Neumann|USA (retired)

Offline klaus

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #31 on: March 14, 2013, 03:13:40 pm »
Uwe, and others:
There are of course tonal differences, due to the different electrostatic attraction forces, between "properly" polarized LD polyester capsules (around 60VDC) and those that are under-polarized.

But, what about headroom considerations when under-polarizing? How is headroom affected? Seems to me, on one end, under polarizing puts less stress on the FET's limited input capacity (gate). But I often find that robustly polarized capsules perform better, with more robust dynamics.
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline mike zietsman

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #32 on: March 15, 2013, 04:35:04 am »
Do you get the same voltage (as a minus voltage) for the rear side (Figure eight)?

I vaguely remember the voltage being the same - the mic was set in cardoid though - would that affect anything?. I will double check asap and report back...

Offline Jim Williams

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #33 on: March 15, 2013, 12:20:03 pm »
All other things being equal, the difference in sensitivity resulting from the bias voltage change for a condenser capsule from 42 to 62 V would be 20 log (62/42), or barely 3.4 dB!

"Being Equal" is the key. This assumes capsule sensitivity is linear or pure log with voltage applied. I've never done this but I suspect the capsule output does not follow a linear voltage vs sensitivity curve. One could test this by measuring capsule sensitivity at various polarization voltage levels. The chart plotted from this would be interesting. I wouldn't be suprized if some bumps occured in the plot.

Offline usattler

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #34 on: March 18, 2013, 02:31:42 pm »
Within the operational range of a capacitive transducer its sensitivity is directly proportional to the polarization voltage:

         e(t) = Eo*c(t)/Co,     where

e(t) is the output voltage,
Eo is the DC-bias voltage,
c(t) is the change in capacity due to diaphragm movement,
Co is the transducer capacitance at rest without sound input, and
t is time

The operational range is limited at both ends by the distance between the fixed electrode (back plate) and diaphragm, and to a lesser degree by the diaphragm tension. Too much DC-bias and/or too little distance may lead to arc-over or 'fatal' attraction, too little of either will reduce sensitivity and signal-to-noise ratio.
 
I can neither see any reasonable explanation or physical cause for non-linearity in sensitivity or any frequency dependency due to changes in the polarization voltage, nor have I ever experienced, heard or measured such behavior. This independence is actually used in many condensator studio microphones to control the sensitivity through adjusting the bias voltage of the transducer, and with dual diaphragm capsules enables the creation of multi pattern microphones.
Uwe Sattler
Technical Director - Neumann|USA (retired)

Offline Tim Campbell

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #35 on: March 19, 2013, 09:00:41 am »
Uwe, like you, I haven't noticed this non-linearity. During construction of capsules I use variable polarization voltage to measure some parameters. I will say that different types of capsules react uniquely to changes in dampning and polarization. Because these two forces aren't interdependent I could imagine a situation where dampning may be able to overcome additional attraction when the membrane is closest to the backplate but not when it is farthest away or the opposite and this might possibly translate into slight non-linearity. Also in a capsule type where you have one membrane sandwiched between two backplates of equal attraction you may find a limit to output gain with greater polarization.

Offline usattler

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #36 on: March 19, 2013, 11:41:46 am »
Simply put: Why speculate what happens when pushing the past the limits of the intended operational range? For such applications a different transducer would have to be defined...
Uwe Sattler
Technical Director - Neumann|USA (retired)

Offline klaus

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #37 on: March 24, 2013, 04:13:14 am »
I can neither see any reasonable explanation or physical cause for non-linearity in sensitivity or any frequency dependency due to changes in the polarization voltage, nor have I ever experienced, heard or measured such behavior. This independence is actually used in many condensator studio microphones to control the sensitivity through adjusting the bias voltage of the transducer, and with dual diaphragm capsules enables the creation of multi pattern microphones.

Unlike you, Uwe, I experience frequently what happens when too much capsule bias voltage is applied:
There is a microphone modifier (who shall remain nameless), who specializes in "quieting down" tube mics by adding 20 volts of polarization voltage beyond the nominal amount traditionally applied to Mylar® LD capsules (around 60VDC).
The mics sound like hell, to put it politely; hard, honky and grainy. The sound immediately returns to its former sweetness once I reduce the voltage back to normal.

It's pretty obvious to me what goes on: too much electrostatic backplate attraction from the over-voltage freezes up the diaphragm movement.

So, in a perverse sense, what is gained in S/N and gain is lost in linear response, especially when complex musical sources are processed. I am sure you guys in Connecticut or maybe in Berlin have some fancy chamber and sensitive enough instruments, where it can be shown what happens to complex waveforms when a backplate is bombarded with too much voltage.

Quote
this independence (from a set capsule bias voltage) is actually used in many condensator studio microphones to control the sensitivity through adjusting the bias voltage of the transducer
But only within reason.  I know of no situation where more than ±60 volts are pulling on a diaphragm.

Quote
... and with dual diaphragm capsules (a variable bias voltage) enables the creation of multi pattern microphones.

Again, never are more than 60 volts electrostatic attraction present; even when the rear pattern control pumps 120VDC into the back diaphragm, the voltage differential between plates never exceeds 60VDC static pull.



« Last Edit: March 24, 2013, 04:23:27 am by klaus »
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline Tim Campbell

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #38 on: March 24, 2013, 07:59:41 am »
Just to add a little info, Brüel & Kjęr commonly uses 200v polarization voltage. Their capsules are omni with metal diaphrams and a large spacing between membrane and backplate.

Offline klaus

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #39 on: March 24, 2013, 02:35:25 pm »
That's why I specified LF mylar diaphragms which usually use ± 40µ spacing.  There's likely no sonic penalty to push as high as 200V if the diaphragm is small enough and its proportion of tension to backplate gap is properly chosen to withstand the effects of electrostatic suck.
« Last Edit: March 24, 2013, 02:37:51 pm by klaus »
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline usattler

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Re: Original AKG CK12 Capsules: Sound, Lifespan and Repair
« Reply #40 on: April 01, 2013, 08:34:13 am »
The operative words are: "WITHIN THE OPERATIONAL RANGE OF A CAPACITIVE TRANSDUCER" - that shouls exclude any abnormal biasing beyond this range, Some transducers are specifically designed for higher or lower polarization voltages than the 60 V common for large diaphragm Neumann capsules (and many clones). Due to their typically greater diaphragm tension pure pressure transducers (dedicated omni) often can tolerate higher bias than pressure gradient (bi-directional and cardioid) transducers with their low tensioned diaphragms. Diaphragm size and spacing to the stationary backplate are other determining factors for the proper bias. In any case, within the operational range the relationship between bias and sensitivity is strictly linear! Once any deviation from this physical law is encountered, the designed for operational range has been left behind, either by excess bias or (far less likely) excess SPL.
Uwe Sattler
Technical Director - Neumann|USA (retired)