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[Kai]

A change in polarisation voltage of 10% will have an effect of 1 dB on the output of the capsule and the signal to noise ratio...

To be exact, a variation of +/- 10% of the polarization voltage will change the sensitivity of a condenser mic by nominal 1.75 dB.
Practically the value can be a bit higher, as a higher polarization voltage attracts the diaphragma to the backplate and increases the level a bit further.

At the same time it stiffens the diaphragma's tension and thus reduces the amount of bass.
I have to admit, that with center tabbed diaphragms this effect is not very strong.

[klaus]

It is assumed that if you can afford a $15k+ mic, you have ears. Later this weekend I will adjust B+ and measure @100VDC and @107VDC (the current DC voltage relative to 120VAC from the wall, as the mic was designed for the then standard 110VAC household voltage in the 1950s)

Also: Bad idea to use a Zener as voltage limiter in NGs, unless it's positioned at the very beginning of the filter circuit. if not, it creates distortion on peaks. Found that out the hard way in the 1990s when customers started to complain.

The elegant way to custom-adjust to 107VDC output if an NG power supply is putting out too much DC: add a 10W buffer resistor inline between rectifier and first filter cap/dropping resistor. Usually the value will be around 200-500Ω to get to 107.

[RuudNL]

Interesting. What would be the reason for this distortion?
My idea was to add a high power zener, with a zener voltage that would never be reached as long as a microphone is connected, but still low enough to protect the microphone against overvoltage if some fool would connect the microphone while the power supply is 'on'.
Say: a zener voltage of 130 V (or so), or another 'safe' voltage that would not cause the zener to conduct under normal operating conditions.
(There is still a 1 mfd capacitor inside the microphone that does a certain amount of power line filtering.)

[afterlifestudios]

Thanks all. 

One of the reasons for the questions was to determine if I should adjust dropping resistors in the PSU.

My mains voltage here is typically 113-115VAC but on some days it's 118-120VAC (with no predictable pattern to the variance).

On "normal" days (113-115VAC) my 47's are currently under supplied at about 101.5VDC, while on 120VAC mains days it's close to the nominal 105VDC.

But since there seems to be no harm in oversupplying within a 10% range, perhaps I should adjust the dropping resistors for 105VDC at 113-115VAC, which is most common here.

(I think I will still do the variac test (which indeed will involve my ears, Klaus!).  Other than reduced bass, will an increase in polarization voltage also impact less easily measured things like transient response etc?  It's tricky to come up with truly consistent ways of repeating the source sounds for recording/testing at the different voltages, so confirmation bias is likely to invade the experiment.  But I will do my best.)

Thanks again for any thoughts...

[klaus]

Here are the parameters that are affected when you vary polarization voltage: noise floor, output level and distortion.
The first two are directly proportional to the voltage applied to the backplate: the more you prime the capsule, the hotter its output, the better the signal-to-noise ratio.

The amount of mechanical capsule distortion, and therefore distortion of audio output is directly proportional to the amount of electrostatic attraction exerted by the pol. voltage applied to the backplate: the higher the voltage pulling the diaphragm towards the backplate, the more capsule distortion.

To understand the interrelation between pol. voltage and mechanical capsule distortion, think of this with an example:

Let's say, the mic's capsule picks up a perfectly undistorted sine wave of 1kHz. generated by a loudspeaker.
Ideally that sine wave swings an equal amount above and below the x-axis as represented on a graph. Put another way, the sine wave will produce an equal amount of compression and expansion of air molecules, relative to no signal, or pressure equilibrium.

But when there is polarization voltage pulling on only ONE side of an otherwise equal amount of diaphragm movement back and forth from the zero-movement midpoint, the diaphragm will stay just a fraction of a second longer in the vicinity of the backplate than at the other extreme of the swinging process.  Another word for that non-linear swing of the diaphragm is distortion.

Neumann and most other manufacturers of large diaphragm mics have settled on 60-63VDC polarization voltage for capsules with the typical 6µ Mylar/PET diaphragms. That voltage has been determined to be an acceptable compromise between capsule output, noise floor and distortion. You go much higher, distortion becomes audible, much lower, the noise floor suffers.

Note that Microtech Gefell used 75VDC on its PVC M7 capsules. Why so much? The 10µ PVC diaphragm can handle that level of pol. voltage without major distortion.

[RadarDoug2]

Klaus, strictly speaking, the noise floor does not change with an increase in polarising voltage, but the output level changes, thus giving an increase in signal to noise.

[klaus]

I disagree, and ask readers' opinions on that: 
The processor's system noise, particularly that of the tube, does of course increase relative to a lowered capsule output. You can add to that the self noise fo having to increase preamp gain. These are all constant noise sources that do not change with changing levels of polarization voltage.
But the capsule's self-noise also increases with lower polarization voltage.

[afterlifestudios]

Note that Microtech Gefell used 75VDC on its PVC M7 capsules. Why so much? The 10µ PVC diaphragm can handle that level of pol. voltage without major distortion.

Do people with M7 U47 systems run them hotter for the noisefloor and output level benefits?  Or are there other concerns that make that unwise?   (Assuming one would just adjust resistor values (R7/R8 presumably) to raise polarization voltage and leave the NG alone.   

*Measuring the polarization voltage is beyond my equipment's limitations, so I guess I'm just asking out of curiosity...

Also, anyone have thoughts about my NG DC output levels in regards to my AC mains variations? (as per above.)

[klaus]

It would be unwise to increase polarization voltage on M7-PVC equipped mics. For one, aside of Thiersch's M7 capsules, PVC versions don't even exist anymore, All aftermarket M7-types use Mylar skins.

Another reason to leave the stock-voltage divider as is: any time you install a K47 in a U47 you would not be happy with a polarization voltage in excess of 60-63VDC, for the reasons mentioned above.

You can measure polarization voltage fairly accurately by using as test point the voltage divider, before the 100MegΩ high-impedance resistor.

[Kai]

Also: Bad idea to use a Zener as voltage limiter in NGs, unless it's positioned at the very beginning of the filter circuit.

Putting a Zener at the beggining of the circuit isn't the best idea too because:

• Without any dropping resistor in front of the Zener the only current limiting in case it gets active are the internal resistances of the transformer and rectifier.
The high current could break those, or the Zener, which usually changes into a short, then breaks transformer or rectifier - or fuse at best.

• Without the dropping resistors in front of the Zener the voltage limiting in case no mic is connected doesn't take place, you gain nothing.

...  if not, it creates distortion on peaks. Found that out the hard way in the 1990s when customers started to complain.

I have no logical explanation where this distortion should come from.
A e.g. 130 V Zener does nothing when connected to 107 V, it behaves as if it was inexistent.
Even if it were active it would just "stabilize" the B+ voltage.
Every voltage inside the U47 is decoupled by resistors and partly caps, so no back-effect on the audio could be explained.


The best position for the Zener, if someone thinks it's necessary, is in parallel to the last step filter cap in the NG PSU.
As there's a high value cap in parallel, any noise generated by the Zener is filtered out.

[klaus]

I concur with your positioning of a Zener. Certainly in theory. Mine were positioned at the output of the supply. Peak distortion on very loud sound sources was only eliminated when I removed them, after complaints. There went my theory.

I should have just responded: Who needs a Zener in an NG power supply? Why has that never come up as an improvement in the 70-plus years of its existence?
Never once.

[Kai]

I concur with your positioning of a Zener. Certainly in theory. Mine were positioned at the output of the supply. Peak distortion on very loud sound sources was only eliminated when I removed them, after complaints. There went my theory.

I should have just responded: Who needs a Zener in an NG power supply? Why has that never come up as an improvement in the 70-plus years of its existence?
Never once.

I wouldn't advocate "improving" a valuable vintage device anyway.

The U47 won't break, even if hot-plugged accidentially, so why care.
My reply mainly heads to NOT place a Zener immedeately after the rectifier.


To give peace of mind, a dummy load in form of a 47 kΩ / 5 W resistor could be solders into a Tuchel plug.
Used to discharge the - switched off -   PSU avoids waiting 20+ minutes 


The distortions caused by the Zener diode is still interesting to me.
Do you remember the value of the Zener you used, and what position exactly it was placed?

[klaus]

It was either a 110 or 120 V. Zener.
And I need to clarify my original intention: to more or less limit p.s. output to the prescribed B+ of the mic, rather than as an escape hatch for a run-away supply.

[Kai]

It was either a 110 or 120 V. Zener.
And I need to clarify my original intention: to more or less limit p.s. output to the prescribed B+ of the mic, rather than as an escape hatch for a run-away supply.

This might explain the otherwise hard to explain effect of creating distortion.

The U47 is built to work with unregulated power, this makes the PSU a part of the active amplification circuit.

If now some of the filter caps are old and do not fully filter (high ESR) any more, there might be significant amount of audio signal at B+.
An oscilloscope connected to B+ could show how much audio is present, when the U47 is driven hard + maybe feeding a low impedance mic preamp's input.

If there 's any audio, a conductive Zener will certainly cause distortions.

Interesting link discussing the irrelevance of possible overvoltages of U47 / NG:
http://www.moxtone.com/mU47_U47_part2.html

[klaus]

THAT part of the article you linked is acceptable.
The rest is filled with speculation and misinformation, probably because the author either has never had access to genuine U47 for testing (see his photos of aftermarket components supposedly the core of every U47, but in this case showing nothing but copy products unrelated to Neumann's originals) or because he has too little experience with U47/NG in the real word (for example: transformers in NG don't shorten with time and age, and output voltages creep up with time for entirely different reasons).

So read at your own  risk.