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Author Topic: U87ai capsule polarizing conversion  (Read 687 times)

Donn

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U87ai capsule polarizing conversion
« on: April 28, 2020, 04:21:01 pm »

I read an informative thread on the topic of the U87ai's capsule voltage from 60V to 45V found on this forum.

However, I was still left with a few questions:

First, (to Klaus): You have a way with words in describing sonic differences between mics or mic configurations. How would you describe the overall sonic difference between 60V and 45V capsule polarization? (I would also like to hear descriptions of the sonic differences from any other willing forum members who wish to add their opinions).

Second, what mechanical/acoustical effect of lowering the voltage contributes to this tonal difference?

Third, what can I expect in terms of increased headroom?

I'm aware there is a tradeoff in S/N ratio, but I'm hoping the difference in headroom and tonality may be well worth the tradeoff. 

Thanks for your replies,

(original thread can be found here): https://repforums.prosoundweb.com/index.php/topic,36194.msg531539.html#msg531539
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klaus

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Re: U87ai capsule polarizing conversion
« Reply #1 on: April 28, 2020, 04:29:46 pm »

Don,
In reverse order of your questions:

1. The U87Ai has a lower headroom of ca 6dB,  compared to the U87. With both topping out below 110dB, neither model is an example of a high headroom mic.
Add to that that when you get into the danger zone, a single-FET circuit lets you know you've reached the headroom limit rather suddenly and with ugly effects: harmonic distortion of a transistor is never pleasant

2. The higher the polarization voltage, the higher the output of the capsule, the stronger the electrostatic effect pulling the capsule towards the backplate. If you go much past the industry-established polarization voltage of 60VDC for 1" capsules with Mylar diaphragms, you will hear the crunch which that non-linearity produces.

What caused most manufacturers to settle on 60VDC? A polarized condenser diaphragm's excursion is never linear (it stays momentarily longer at one extreme of the excursion of moment (when closest to the charged backplate) than the other (farthest away from the backplate), because the electrostatic attraction of the backplate holds the diaphragm minutely longer there*.  So a good compromise between high output, low noise and acceptable distortion of the non-linear diaphragm excursion is around 60VDC.

The original U87 could not produce 60VDC polarization voltage, because only 48VDC was available from phantom powering, as no DC-to DC converter was used. So you end up with ca. 47 volt  at the capsule. Compared to 60VDC, 47VDC increases the headroom by ca. 6dB and lowers the capsule's output by ca 10 dB, which then raises the noise floor of the mic as a consequence as well.
Which brings me to the subjective sonic effect of the two levels of polarization voltage:

3. All things equal (which they never are, considering that a Neumann capsule has a ±2dB, i.e. 4db overall tolerance level), a 47VDC polarized capsule will sound softer in the mids, fuller in the bass, and overall less aggressive in dynamic response. Call it sweeter. The differences are very subtle and I would not make blanket quality statements that one type is preferable overall. Too many other factors of the two mics' differences would make such statement ludicrous.


* there are other factors that play a role in the overall response relative to a given level of polarization voltage that I do not understand fully: one could speculate that, in addition to lower linearity of the diaphragm's swinging process, the diaphragm's increased stiffness when higher voltage is applied is one cause of the hard sound that some report from mics with very high polarization voltage (I believe the U87Ai is among them tough the same 60V applied to say, a U67 does not have that effect), some MG models which apply 70+ volts to their mostly PVC-equipped capsules are among them.
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Klaus Heyne
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Donn

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Re: U87ai capsule polarizing conversion
« Reply #2 on: April 30, 2020, 02:43:06 am »

Thanks for the detailed and informative response, Klaus. I recently had the daughterboard of my U87ai removed in place of a single FET (although I would like a Fairchild 2N3819 installed soon). After I find a suitable 3819, I will also have the polarization voltage lowered. I'm looking forward to the increased headroom, and perhaps a subtle sonic benefit.

Donn
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uwe ret

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Re: U87ai capsule polarizing conversion
« Reply #3 on: April 30, 2020, 07:35:42 pm »

In my humble opinion headroom is not as relevant to signal quality as the device dynamic range and signal-to-noise ratio. I may have posted it in this forum before, but again I do offer a thorough analysis of the technical differences between the Neumann U 87 and U 87A models. Judge for yourselves ...
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klaus

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Re: U87ai capsule polarizing conversion
« Reply #4 on: April 30, 2020, 09:06:11 pm »

Uwe, it was gratifying to read your detailed description of the U87 and U87A specifications again (your link link at the bottom). One of the most valuable contributions top this forum, ever.

As strong believer in microphone circuits without negative feedback, I still never understood why Neumann omitted the 10pF feedback in the U87A, when that is probably the single most detrimental factor why that model's headroom went down by almost 6dB, compared to its predecessor.

That step back seemed uncharacteristic for a company with a strong emphasis on measurable specs, especially distortion specs?
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Klaus Heyne
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afterlifestudios

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Re: U87ai capsule polarizing conversion
« Reply #5 on: May 01, 2020, 04:13:08 pm »

Thank you Uwe.  What a great document.  I sent you a PM with a link to a pdf version where I took the liberty to reformat the layout and add a nice "table" for the specs etc to make it a little easier to read.  Let me know if that's ok to share, or if you'd rather just keep the "text doc" that you posted as is...

Either way, thanks again for the research and analysis.
JR
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gtoledo3

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Re: U87ai capsule polarizing conversion
« Reply #6 on: May 01, 2020, 06:51:19 pm »

I read Uwe’s analysis as stating that the reason for the feedback capacitor was to offset THD linked to the parasitic capacitance caused by the rear diaphragm, in the U87. Since the U87Ai does not have parasitic capacitance from the rear diaphragm, the context that led to it being there is longer applicable, hence its removal.

Thanks for that perspective, Uwe!
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klaus

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Re: U87ai capsule polarizing conversion
« Reply #7 on: May 01, 2020, 07:40:34 pm »

Parasitic capacitance does not address the larger issue of why many Neumann mics aside of the U87 (with the only split-backplate capsule of its kind) used negative feedback, and as early as the 1950s. Good examples: SM69 fet, all KM8x

My understanding is that the feedback was primarily installed to keep overall gain within a specified range, to comply with German Broadcast standards (see also M249, M254, starting in the mid-1950s.
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Klaus Heyne
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