Capsule bias voltage vs. sensitivity

It would appear my google-fu skills might be lacking, or i just haven't used (all) the right search terms.

My main question is this - is there any simple formula for the relationship between bias voltage and sensitivity?

I'm thinkin something along the lines of, assuming the nominal sensitivity is @ 60v, how much lower would it need to be, to drop it by 10 or 20dB?

The reason for my curiosty is due to the implementation of the pad, in the Studio Projects C1 mic (which uses a Schoeps-style circuit).

There is a very good PDF at Neumann's site.
Look for "microphones"

Do you mean the 70-some-odd-page booklet?

The closest answer to my question that i've found would seem to be on page 36:

"The output voltage e(t) of a condenser microphone using dc polarization is
proportional to the applied dc voltage E0 and – for small diaphragm amplitudes
– the relative change in capacity"

So then, halving the bias voltage (say, from 60v to 30v) would yield a 6dB drop in sensitivity?

True.
-10 dB = 1/3 of the polarisation voltage etc.
Some people are worried if the polarisation voltage is 59 V instead of 60 V... The truth is that there is almost no difference!
(54 V means -1 dB re. 60 V)

Okaaaaay, that makes a whole bunch of sense

I was wondering how a pad (which does NOT involve paralleling a cap with the capsule) could be most easily implemented with a Schoeps-style circuit.

I'm thinking that varying the series input resistor to the DC-DC converter might yield the smoothest (pop-free perhaps, even?) transitions. The SP C1 pad switching seems to literally be a resistive divider on the output of the DC-DC stage, which feels a bit "crude" to me

PS: In situations where a pad would be needed, the reduction of the sound-to-noise ratio is less of a concern, right? Just double-checking my logic...

In the case of a pad the amplifier noise stays the same, but the input signal is lowered.
So the signal to noise ratio in fact gets worse! (For low level signals)
But...you are using the pad because otherwise the amplifier circuit would be overloaded.
At these very high levels amplifier noise is no issue anymore.
The moral of this story: never use a pad unless it is absolutely necessary!

That's pretty much what i figured, i just wanted a bit of confirmation that my thinking / logic was correct, so thanks

Hello,
i take the opportunity of this post to go a bit deeper into this argument.
would a 5k trimmer do the job in place of the 1,5k resistor (R10) in this schoeps like schematic (attached)?
One can adjust the pol. voltage also by varying the D3 zener diode value. What are the differences (advantages/disadvantages) of using this 2 methods apart that the trimmer can realize a countinuous variation instead of a fixed value of the diode method?
Is delta-C the capacitance change due to the diaphragm displacement following the incoming sound wave (the result of shorter distance from the diaphragm to the backplate)?
An incoming sound wave push the diaphragm closer to the backplate and, according to capacitance formula (C=eA/d), this will result in increasing the capacitance value. Is there a way to measure this change?
i'd appreciate any comments or answers from experts

What you could do  (and/or what i plan to do) is to maybe reduce that 1.5k resistor to, say, 1k or 470ohm, and insert a trimmer (or, as i plan to, a switch) AFTER that zener (ie. between it and the collector of the oscillator transistor). That should make it simple enough to "strangle" the voltage, in order to reduce the output bias voltage to the capsule.

Yes, deltaC is indeed the (temporary) change in capacitance of the capsule. The thing is, that capacitance will indeed increase on the positive half of the sound-wave (compression), and decrease on the negative half (rarefaction). I don't claim to be an expert, but from my understanding, the output of the impedance converter (be it a JFET or a tube / valve) is directly(?) proportional to the change in capacitance of the capsule.

That capacitance change is in direct relation to the displacement of the diaphragm, so it's not a constant / fixed value

Khron said:

What you could do  (and/or what i plan to do) is to maybe reduce that 1.5k resistor to, say, 1k or 470ohm, and insert a trimmer (or, as i plan to, a switch) AFTER that zener (ie. between it and the collector of the oscillator transistor). That should make it simple enough to "strangle" the voltage, in order to reduce the output bias voltage to the capsule.

Click to expand...

IMO, that isn't good idea because the relationship between the output voltage from the oscillator and his supply voltage isn't so simple and also power supply for the oscillator will not be stable anymore.  Why not use a voltage divider after R12?

Mmm... I guess you've got a point there. Although in my idea, worst case, the output voltage would end up being equal to the input voltage, as a minimum (if the oscillator doesn't start).

Either way, i have several mods planned for a bunch of my condensers, some of which include adding a DC-DC converter (in my sE 2200A's, for example), so some testing / breadboarding sessions are planned for after the holidays

Khron said:

What you could do  (and/or what i plan to do) is to maybe reduce that 1.5k resistor to, say, 1k or 470ohm, and insert a trimmer (or, as i plan to, a switch) AFTER that zener (ie. between it and the collector of the oscillator transistor). That should make it simple enough to "strangle" the voltage, in order to reduce the output bias voltage to the capsule.

Click to expand...

I think that this solution is on Microphone-Parts MP-V57 circuit. I'll try and post results, thanks. Any thoughts about how the overall circuit is affected differently by the 2 approaches (change resistor value or diode value)?

Khron said:

That capacitance change is in direct relation to the displacement of the diaphragm, so it's not a constant / fixed value

Click to expand...

so if it's not a fixed value how one can calculate the sensitivity of a mic? what's the right method to measure the sensitivity of a condenser microphone?

moamps said:

IMO, that isn't good idea because the relationship between the output voltage from the oscillator and his supply voltage isn't so simple and also power supply for the oscillator will not be stable anymore.  Why not use a voltage divider after R12?

Click to expand...

can you get a bit deep in the way the oscillator goes unstable? For what i know the output voltage of the oscillator depends just on supply voltage (Zener voltage) and the ratio between the 2 inductors.
Thanks

ghiatorino said:

moamps said:

IMO, that isn't good idea because the relationship between the output voltage from the oscillator and his supply voltage isn't so simple and also power supply for the oscillator will not be stable anymore.  Why not use a voltage divider after R12?

Click to expand...

can you get a bit deep in the way the oscillator goes unstable? For what i know the output voltage of the oscillator depends just on supply voltage (Zener voltage) and the ratio between the 2 inductors.
Thanks

Click to expand...

The supply voltage for the oscillator isn't Zener voltage anymore if there is a series resistor.  Usually some parts (bias resistors, etc) are determined in relation with stable and fixed power supply so if the power supply is varied, some static parameters may be altered too much.  So, IMO, the better way to change the polarization voltage is to use high ohmic voltage divider at the output of the existing oscillator-HV generator.

ghiatorino said:

Khron said:

That capacitance change is in direct relation to the displacement of the diaphragm, so it's not a constant / fixed value

Click to expand...

so if it's not a fixed value how one can calculate the sensitivity of a mic? what's the right method to measure the sensitivity of a condenser microphone?

Click to expand...

It is not a fixed value, but you could determine an RMS value for delta C at a certain SPL. It is far easier just to measure the output voltage of the mic in a test chamber with a known SPL at the mic's position. Or as good as you can get close to that, perhaps by comparing against a mic of known sensitivity, or against a measurement mic.

I think we are not on the right way...
my thoughts are that the DeltaC we are talking about is not the change in capacitance due to the incoming sound wave but the effect of the dc polarization voltage that creates an attracting force, pushing the diaphragm closer to the backplate, thus changing the overall capacitance...
any expert can clarify this?

source:
http://www.bksv.com/doc/be1447.pdf

Good source, moamps.