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Author Topic: U47: Does Supply Voltage Have An Effect On Sound?  (Read 2336 times)

afterlifestudios

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U47: Does Supply Voltage Have An Effect On Sound?
« on: September 07, 2020, 11:55:02 am »

Is there anything wrong with connecting my NG PSU to a Variac and adjusting the voltage to experience the differences in tone?  Then adjusting dropping resistors to hit the measured target? 

My B+ on an average day here (mains voltage 115VAC) is about 100VDC.  I thought this might be an easy way of seeing if it’s worth changing the dropping resistors to get it up to 105VDC?  Or just do it?
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uwe ret

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Re: U47 B+ voltage effect on sound
« Reply #1 on: September 07, 2020, 04:33:25 pm »

There will be no harm as long as you will remain within around 10% of the nominal 110V AC. However, a 5 V difference in the B+ will have negligible, if any, audible effect on the acoustic properties of the microphone. I suspect the rectifier in your NG power supply may aged and developed higher than nominal internal resistance. Be careful when attempting to replace it with a modern silicon diode bridge, its much lower resistance may jeopardize the mains transformer by excessive in-rush current upon power-up. A suitable resistor, or better yet, a NTC thermistor in series should be used.
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RuudNL

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Re: U47 B+ voltage effect on sound
« Reply #2 on: September 08, 2020, 09:24:54 am »

The topic starter mentioned that his mains voltage is 100 Volts, instead of 115 Volts AC.
That is 87% of the voltage the power supply was made for.
I think it is obvious that the DC output voltage of the power supply will now be lower too.
If DC output was 105 Volts at 115 Volts AC input, one could expect something in the region of 91 Volts DC now.
IMHO the exact voltage is not very critical.
As uwe ret mentioned already, a voltage of + of - 10% is accepatble in most cases.
I have an original Neumann U47 power supply here, that delivers 110 Volts DC. (Under load.)
Since the filament of the tube is underheated, I do not worry about this.
The reduction of the polarisation voltage and plate voltage on the tube in your case can be neglected in my opinion.
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uwe ret

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Re: U47 B+ voltage effect on sound
« Reply #3 on: September 08, 2020, 11:24:04 am »

The OP stated that his PS output is 100VDC at a mains of 115VAC. That makes for a rather inconsequential 5% difference to the nominal 105VDC.
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RuudNL

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Re: U47 B+ voltage effect on sound
« Reply #4 on: September 08, 2020, 12:03:11 pm »

My misinterpretation. But anyway: no problem!
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afterlifestudios

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Re: U47 B+ voltage effect on sound
« Reply #5 on: September 09, 2020, 12:28:02 am »

Thank you, gentlemen.  Yes, that correct.  100VDC B+ on my NG (loaded) with mains at 115 VAC. 
I’m more curious about the sonic effect of the resulting change in capsule polarization voltage, than the VF14.
I’ll give it a try, keeping the Variac so that the measured B+ is within 10% of 105VDC.
Thanks again for your thoughts.
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RuudNL

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Re: U47 B+ voltage effect on sound
« Reply #6 on: September 12, 2020, 04:00:33 pm »

I am interested in your findings!
I don't suppose + or - 10% will make an audible difference, but I never tried...

By the way: I measured the unloaded voltage of the original Neumann U47 power supply.
The unloaded DC voltage of my power supply is 378 Volts!
So it is very important to connect the cable and microphone first before switching on the power supply!
The capacitors in the power supply will hold the voltage for a long time.
And I am sure the VF14 wouldn't like a burst of 378 Volts! (Which means theoretically a filament voltage of ~126 Volts...)
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afterlifestudios

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Re: U47 B+ voltage effect on sound
« Reply #7 on: September 12, 2020, 09:38:01 pm »

Yes indeed.  No “hot swapping” here.  If one my NG PSU’s gets turned on unloaded, I drain the caps with a big resistor and measure for any stored voltage before connecting the mic. (Either that or wait 20 mins, as those caps can hold a charge for quite a while.)
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Jim Williams

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Re: U47 B+ voltage effect on sound
« Reply #8 on: September 13, 2020, 12:20:38 pm »

Place a 1 meg 1/2 watt resistor on the output of the psu caps to ground to drain them when shut off. It also provides a load if the mic is not connected.
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RuudNL

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Re: U47 B+ voltage effect on sound
« Reply #9 on: September 13, 2020, 02:42:39 pm »

I am going to install a 10 Watt/120 Volts zener across the output.
This will reduce the output voltage to a safe value, even with no load.
When loaded with the microphone, the voltage will be lower than 120 Volts anyway.
(So the zener doesn't 'exist' with the microphone connected.)
And even when the microphone gets connected with capacitors charged to 120 Volts, this won't hurt the microphone.
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Jim Williams

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Re: U47 B+ voltage effect on sound
« Reply #10 on: September 14, 2020, 12:15:51 pm »

Zener diodes create a lot of rf hash and noise. Place a fast .1 uf film cap across it to filter it.
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RuudNL

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Re: U47 B+ voltage effect on sound
« Reply #11 on: September 14, 2020, 12:31:28 pm »

True, when they are 'zenering'...
But under the zener voltage a zener diode is virtually non existent.
(A .1 uF capacitor won't help very much to reduce zener noise in the audio frequency band.)
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Jim Williams

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Re: U47 B+ voltage effect on sound
« Reply #12 on: September 15, 2020, 11:21:29 am »

Most diode noise is ultrasonic.
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klaus

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Re: U47 B+ voltage effect on sound
« Reply #13 on: December 04, 2020, 11:54:58 pm »

Just reading this now for the first time (where have I been?)

There will be no harm as long as you will remain within around 10% of the nominal 110V AC. However, a 5 V difference in the B+ will have negligible, if any, audible effect on the acoustic properties of the microphone.

No harm? Agreed. The tube can handle almost twice the voltage it works under in this mic.
But effect on sound? Sorry. Every volt not getting to the capsule increases noise, lowers output and affects the dynamics of the mic.
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Klaus Heyne
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RuudNL

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #14 on: December 05, 2020, 08:20:54 am »

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...
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Kai

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #15 on: December 05, 2020, 10:50:48 am »

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.
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #16 on: December 05, 2020, 03:15:22 pm »

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.
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Klaus Heyne
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RuudNL

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #17 on: December 05, 2020, 03:58:14 pm »

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.)
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afterlifestudios

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #18 on: December 05, 2020, 04:04:55 pm »

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...

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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #19 on: December 05, 2020, 07:25:30 pm »

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.

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Klaus Heyne
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RadarDoug2

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #20 on: December 06, 2020, 01:37:48 pm »

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.
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #21 on: December 06, 2020, 01:59:57 pm »

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.
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Klaus Heyne
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afterlifestudios

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #22 on: December 06, 2020, 03:31:43 pm »


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.)
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #23 on: December 06, 2020, 07:25:19 pm »

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.
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Klaus Heyne
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Kai

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #24 on: December 06, 2020, 08:02:05 pm »

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.
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #25 on: December 06, 2020, 09:49:12 pm »

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.
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Klaus Heyne
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Kai

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #26 on: December 07, 2020, 01:50:18 am »

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?
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #27 on: December 07, 2020, 01:35:12 pm »

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.
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Klaus Heyne
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Kai

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #28 on: December 10, 2020, 01:03:40 pm »

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
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klaus

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #29 on: December 10, 2020, 02:06:16 pm »

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.
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Klaus Heyne
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Kai

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Re: U47: Does B+ voltage have an effect on sound?
« Reply #30 on: December 10, 2020, 05:39:24 pm »

... (for example: transformers in NG don't shorten with time and age, and output voltages creep up with time for entirely different reasons).
The author correctly mentiones that shorted windings in a transformer CANNOT work to boost output voltage, instead makes it unusable.
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klaus

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Re: U47: Does Supply Voltage Have An Effect On Sound?
« Reply #31 on: December 10, 2020, 08:49:06 pm »

Just to be clear: neither the author nor I LINKED the two. I should have put a semi-colon or period between the two statements. 
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Klaus Heyne
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Kai

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Re: U47: Does Supply Voltage Have An Effect On Sound?
« Reply #32 on: December 11, 2020, 12:30:03 pm »

A question that arises in this context:
Klaus, have you ever experienced a U47 capsule broken by overvoltage?

I guess 75 or 85 V (at worst) are too low for arcing and creating burned, conductive traces, but ...

But, what happens to the capsule when the tube filament breaks  - does that happen in a way that it becomes open circuit, practically?
You could see maybe 150 V bias without the tube load.

Ever faced something like that?
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klaus

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Re: U47: Does Supply Voltage Have An Effect On Sound?
« Reply #33 on: December 11, 2020, 01:48:35 pm »

I have not.
This brings up the old "let's fix a problem that should (but does not) exist" issue: to what extent should we design systems and their components for problems that in theory should exist but in practice never occur?

I am reminded of the many unnecessary replacements of filter capacitors in Neumann NG power supplies by "techs" not familiar with the model - the kind of tech who assumes electrolyte leaking out of the capacitors' vent holes in NG supplies indicates that they need to be replaced...

Or the "tech" who sees a crack in the plastic of the original blue rectifier of NU67 power supplies and assumes it's shot and must be replaced...

Or the "tech" who replaces all capacitors in mics, insisting a "recap" is in order...

In these instances and in many more, the level of the tech's experience, rather than broad theoretical ideas determine whether repairs are really needed. That's why the term "tech" sometimes warrants quotation marks.

One could even argue that applying a more rigorous scientific approach, rather than acting on "what we generally know to be true" would lead to the same conclusion (to act or not) as that of extensive experience alone*: "recapping" (i.e. replacing capacitors) of studio consoles makes sense. Components deteriorate from stress inflicted by heat cycling, current flowing through them, etc. It makes less or no sense to periodically replace components that are un-stressed and idling, with minimal current flowing through them (microphones).

Then there is the 800-pound gorilla in the room: a tech who sees a "problem" as justification to make money (subject for another thread).

*Hyundai records even the smallest technical or cosmetic flaw detected during final inspections off the assembly line: corrections are implemented immediately that benefit all future cars coming down the line. The practical augments, sometimes even drives, the theoretical.
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Re: U47: Does Supply Voltage Have An Effect On Sound?
« Reply #34 on: December 11, 2020, 05:57:57 pm »

Very few of my vintage mics have had a component replaced.
The longevity of those is astonishing.
That's true for a lot of stuff from that era.

Later on, about end of the 70s, components began to shrink in size.
Mainly electrolytic capacitors suffer from not enough knowledge in construction or even installation.
The killer are bent capacitor pins where they leave their case (e.g. to fit circuitboard mounting holes with wrong spacing).
This damages the seal and makes the cap leak for sure.

I guess these are the main reasons why it's necessary to recap lots of 1980s+ large scale consoles.
Only after the turn of the millennium things started to partly become better on the capacitor front.
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