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Author Topic: Dull sounding vintage U87  (Read 13857 times)

Timsplace

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Dull sounding vintage U87
« on: August 30, 2012, 01:33:40 pm »

I have a vintage U87 (not AI, but one with battery compartment).  It has seen no use in the last 3-5 years (I have a pair of newer AI's that are used regularly).  Upon pulling it out recently I found it to be excessively DULL (and noticeably lower output than the AIs) .  The capsule doesn't look particularly dirty.  I'm considering selling this one, so I'm looking for the most cost effective way to get it up to spec.

Thanks

Tim Reisig
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klaus

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Re: Dull sounding vintage U87
« Reply #1 on: August 30, 2012, 03:43:18 pm »

Two very different issues with very different solutions may me at play here:

1. The mic is defective. Compare its sound to U87 of similar age as yours. Do not compare it to a U87Ai (see below). If, in comparison, it still sounds "duller than another, similar U87, it should be evaluated by Neumann or a seasoned independent professional.

2. You have expectations (because of comparing apples to oranges) that are unrealistic. Do not compare the sound to a U87 to that of a current-issue U87Ai. They are hardly related in output, noise floor, headroom and timbre. The U87Ai's output is 10dB to 24dB higher (depending on the older U87's output strapping and pad settings).

The U87Ai's overall performance parameters give it a more aggressive, "modern" sound, that, depending on the listener, is either interpreted as positive of negative.

If you end up having a well-working U87 whose relative mellow high end you just don't appreciate, sell it to any of the many aficionados of this model.
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Klaus Heyne
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Timsplace

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Re: Dull sounding vintage U87
« Reply #2 on: August 30, 2012, 04:56:46 pm »

Thanks for the valuable input.  Makes perfect sense.  I'll try and dig up a similar vintage (serial 40000ish) mic to compare.  Now that I'm thinking about it, if the mic IS functioning correctly and just sounds DIFFERENT, maybe it makes more sense to sell one of my 2 AI's (that sound close, but not really close enough for true stereo micing).

Hmm.

Thanks again.  REALLY appreciate the expert advice.

Tim
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usattler

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Re: Dull sounding vintage U87
« Reply #3 on: August 31, 2012, 02:56:27 pm »

Unless the U87 in question is defective, once you equalize the inherent difference in sensitivity by raising the gain (+11 dB), the microphones should sound resonably similar. After all, they share the same acoustic and mechanical capsule construction.

For a detailed analysis of the differences between the U87 and U87A please contact me via email at usattler@neumannusa.com. Unfortunately the formats allowed for attachments are rather limited and exclude .pdf or .doc  :(
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Uwe Sattler
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klaus

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Re: Dull sounding vintage U87
« Reply #4 on: August 31, 2012, 03:11:17 pm »

Hello Uwe,
Excellent details. Thank you! (Oops- you just removed the data. But I could possibly attend to the unintentional strike-throughs in your original information you copied.)

We have only one disagreement about (dis)similarities of the sound between the two U87 models: the capsules.

They HAVE changed in a couple of significant ways over the last two decades, and I have posted about this issue previously: A full low end in current production K870 is increasingly hard to get.

While I disagree with the late Steven Paul that the first major change in K87/870 construction caused audible changes (replacing the 40µ spacer rings between diaphragms and backplates with lapping the gap into the backplates), I have experienced over and over again the second, truly noticeable, capsule manufacturing change which happened in the early 2000's and which caused a loss of low end:

Either the backplate lapping height has changed (reduced low end, due to slightly reduced capacitance) or, more likely, diaphragm tension was raised (raising resonance frequency, cutting low end).

As to the "why" I can only speculate: tightening of tensioning tolerances to reduce rejects/outliers, which saves Sennheiser money.
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Klaus Heyne
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radiovinhet

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Re: Dull sounding vintage U87
« Reply #5 on: September 03, 2012, 02:52:34 pm »

Comparing 4 u87's that i have... 2 U87i and 2U87Ai (90's), adjusting gain to sound equal, i've found that u87Ai have a little little more air.
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klaus

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Re: Dull sounding vintage U87
« Reply #6 on: September 03, 2012, 08:48:16 pm »

Now, compare your 2 U87 to current-edition U87A. And report back!
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Klaus Heyne
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usattler

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Re: Dull sounding vintage U87
« Reply #7 on: September 04, 2012, 09:08:57 am »

Another attempt at posting the analysis of the differenced between U87 and U87A. Inexplicably, I can not get rid of the strike-through (which incidently does not show in the preview) ...

The Differences Between Neumann U87 and U87A:

Frequently revived discussions and speculations about the subject of Neumann U87 VS U87A caused me to do some reverse engineering and research. As a result, I beliece to be in a better position to provide the answers to the often misunderstood differences in the specifications for the original U87 and the current U87A microphones.

As a starting point, here is an extract from the relevant published data:

Parameter                              U87                           U87A                         relative difference U87/U87A

Sensitivity (cardioid) (S)          -42 dBv                      -32 dBv                      10.9 dB
                                            8 mV/Pa                    28 mV/Pa
equivalent self noise [N]           18 dBA                       12 dBA                       -6 dB
(weighted per CCIR 468-2)        25 dB                        23 dB                         -2 dB
S/N (CCIR)                             69 dB                        71 dB                          2 dB
max. SPL                               122 dB                       117 dB                       -5 dB
(with 10 dB pad)                     132 dB                       127 dB                       -5 dB
dyn. range of amplifier              104 dB                       105 dB                        1 dB
max. output (THD <0.5%)         200 mV                      390 mV
                                           -14 dBv                      -8.2 dBv                     5.8 dB
max. input (test port)              320 mV                       390 mV                      1.7 dB
amplifier gain (1 kHz) [Av]         0.625                        1 (unity)                     1.6
                                            -4 dB                        0 dB                           4 dB
Transducer capsule                  K87                           K870 (K67/87A)
Spare part number                   053224                      053225
Number of contacts                 4                              3
Nom. capacitance [Cnom]         50 pF                        50 pF
Capsule bias [UB]                    47 V                          60 V                           2.1 dB
feedback capacitor                  10 pF                        none
10 dB padding copacitor [Cpad] 560 pF                       270 pF                        -290 pF

Application od some basic algebra (sorry), physical fundamentals and schematic interpretation lets us investigate and explain these differences more clearly:

There is a simple well established formula for the sensitivity of a capacitive transducer:

        eo = UB*deltaC/CT

where eo is the transducer output in V, UB is the bias voltage in V and deltaC/CT is the relative change in capacitance (with deltaC the absolute variance in capacitance and CT the total effective transducer capacitance at rest, including all parasitic capacitances).

This formula can be expanded into the expression for condenser microphone sensitivity S:

        S=Av*UB*deltaC/(Cnom+CP)

wher Av is the amplifier gain (as a linear multiplier) and CP is the sum of all parasitic capacitances acting in parallel to the transducer's nominal capacitance Cnom.

From the known data for S and Cnom we can calculate the unknown values for deltaC and the total effective capacitance at rest CT, which does include the nominal transducer capacitance Cnom and parasitic capacitances CP of the circuit and construction. From the schematic diagrams we also know how much additional capacitance is added to cause a 10 dB reduction in sensitivity. With this we can cleverly rearrange the formula for sensitivity to deduct the value for parasitic capacitance CP in such manner as to eleiminate the unknown deltaC. We have established that the 10 dB lower sensitivity is due to the additional padding capacitor Cpad, it follows that:

        S*(Cnom+CP) = (S-10 dB)*(Cnom+CP+Cpad)

With Cpad = 560 pF for the 10 dB pad in the U87, we can calculate the value for the effective total capacitance for the transducer at rest to be 205 pF, which does include the 50 pF of the rear transducer:

        8*(50+CP) = 2.5*(50+CP+560)
        400+8*CP = 1525+2.5*CP
        5.5*CP = 1125
        CP = 205 pF

For the U87A capsule and its Cpad of 270 pF the corresponding value for the total effective capacitance at rest is calculated to be 78 pF:

        28*(50+CP) = 9*(50+CP+270)
        1400+28*CP = 2880+9*CP
        19*CP = 1480
        CP = 78 pF

From the schematic diagrams we learn that the rear transducer is NOT burdening the front of the capsule in the U87A! It is this difference in parasitic capacitance which accounts for most of the difference in sensitivity, Now we have a 2.1 dB boost as a result of the bias increase from 47 V to 60 V and 8.4 dB from the reduction in static capacitances:

        20log(CT of U87/CT of U87A) = 20log(205/78) = 8.4 dB

Adding it up nicely confirms mathematically the nearly 11 dB higher sensitivity of the U87A when compared to the U87.

Furthermore, 2 dB of the almost 6 dB increase in maximum output are explaied by the eleimination of the 10 pF feedback capacitor, with another 3.8 dB gained from the reduction in THD by the reduced capacitive load on the FET gate.

Here we now have a verification and valid explanation of the salient differences between the U87 and U87A. The analysis provides proof that the improvements justify the redesign. The higher sensitivity and lower self noise are an easy tradeoff against the slightly lower tolerance for maximum SPL.

Caution! Eliminating the 10 pF feedback capacitor in the U87 will NOT result in any benefit! The inherently higher parasitic capacitance from the parallel back half of the capsule is also responsible for a higher total harmonic distortion, which needs to be (partially) compensated for by this feedback mechanism.

Some other interesting calculations with the date of the known technical specifications allow the deduction of the transducers' change in capacitance resulting from various levels of sound pressure level. For the U87 and 94 dBSPL the change in capacitance computes to 0.07 pF, 1.74 pF at 122 dBSPL and 5.52 pF at the maximum SPL of 132 dBSPL.

Even though the K87 and K870 capsules are acoustically identical, the corresponding values for the U87A are 0.06 pF at 94 dBSPL, 0.84 pF at 117 dBSPL and 2.67 pF at 127 dBSPL.

Amazing what can be extracted from the technical data!...
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Uwe Sattler
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Timsplace

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Re: Dull sounding vintage U87
« Reply #8 on: September 04, 2012, 11:30:25 am »

The difference between my U87 and U87Ai's is definitely more than "a little air".  Here's a couple shots of the capsule.  Kinda grimy.  Could that much dirt cause the dulling?  ALSO is there a way to confirm that this is the original capsule/diaphragm? 

Thanks
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klaus

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Re: Dull sounding vintage U87
« Reply #9 on: September 04, 2012, 01:16:07 pm »

The front side of this 1993 replacement capsule looks really dirty. If the backside sound is markedly better in the high end, then you have your answer, because the rear diaphragm is not quite as dirt-loaded.

Correction: both pictures show the same (front) side, as Uwe Sattler pointed out. Different light angles show a different level of what could be termed substantial contamination.
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Klaus Heyne
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Ernie Black

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Re: Dull sounding vintage U87
« Reply #10 on: September 04, 2012, 02:35:10 pm »

usattler, please note that using [ S ] (no spaces) renders the following text as strikethrough (this is BBCode for strikethrough). Hence, I have changed your post to (S). I know you intend to list Sensitivity as [ S ], but that will not work in the forum.
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Code Warrior of the Seven Galaxies

Jim Williams

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Re: Dull sounding vintage U87
« Reply #11 on: September 05, 2012, 11:54:04 am »

If you have suitable test gear like an Audio Precision, you can feed a signal through a 50 pf capacitor to the jfet and measure the frequency response of the mic circuit. That will tell you if there is an excessive roll-off in the mic electronics.

Also consider that the older A versions used a large value feedback capacitor that lowered the response to 16 k hz. The AI's re-tuned that to 20 k hz. The increased polarization voltage from 42 to 60 volts also increases the top end reach over distance as well as output levels.
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klaus

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Re: Dull sounding vintage U87
« Reply #12 on: September 05, 2012, 12:15:55 pm »

...Also consider that the older A versions used a large value feedback capacitor that lowered the response to 16 k hz.

I assume you meant "the older non-A versions"?
The high frequency eq of A versions has remained unchanged since the model introduction in 1986.
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Klaus Heyne
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radiovinhet

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Re: Dull sounding vintage U87
« Reply #13 on: September 05, 2012, 03:06:18 pm »

Jim, u87A don't exist.
We have u87 (tuchel), u87i (xlr) and the newer U87Ai (xlr)
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Timsplace

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Re: Dull sounding vintage U87
« Reply #14 on: September 05, 2012, 07:50:46 pm »

The front side of this 1993 replacement capsule looks really dirty.

"Replacement" meaning this is NOT the original capsule?  How do you know it's 1993?
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klaus

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Re: Dull sounding vintage U87
« Reply #15 on: September 05, 2012, 08:02:26 pm »

Marking on diaphragm ring: single digit which repeats every ten years (in this case "3"). 
Then add material and color features of hardware, to identify the decade, and you get the year of manufacture of capsule.
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Klaus Heyne
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usattler

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Re: Dull sounding vintage U87
« Reply #16 on: September 06, 2012, 08:24:51 am »

I am surprised nobody noticed that both pictures are of the same side of the capsule with only a slightly different angle of illumination! Even with decades of experience, from these photos I cannot conclude with any authority a possible acoustic difference between front and back...
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Uwe Sattler
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Timsplace

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Re: Dull sounding vintage U87
« Reply #17 on: September 06, 2012, 08:40:06 am »

Well,  I've done some MORE testing/listening, and with appropriate level matching, this mic is actually MUTCH more similar to my other 87s than I originally thought.  I'm not sure WHAT I was hearing the first time I brought it out.  UNLESS something changed from me disassembling/reassembling it.  Maybe the contacts on the capsule were dirty?  Thanks for all the input guys.

Tim
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klaus

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Re: Dull sounding vintage U87
« Reply #18 on: September 06, 2012, 12:28:00 pm »

I am surprised nobody noticed that both pictures are of the same side of the capsule with only a slightly different angle of illumination!...

Thanks, Uwe, I inserted your correct observation into my initial response.
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Klaus Heyne
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