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[Hussyh]

Has anyone here ever attempted to mod a U87Ai back down to the lower capsule polarising voltage of the U87/U87i by disabling the DC to DC converter?

If so, is it a straightforward mod?

And...I know I know...why...right?

[klaus]

This basic idea is not as silly as one may think.
The price paid for the U87A's greater sensitivity and output is reduced headroom: a single FET mic can only handle so much input voltage (from the capsule's output) before it folds up. The additional capsule output voltage generated by the increased polarization voltage in the A model, in comparison to the old-style U87, translates to an even less forgiving behavior of the mic in the border region of SPLs than its already headroom-challenged predecessor: The U87A has more than 6dB lower headroom than the old U87!

So, in the end, it's a matter of trade-off: where, on the scale between self-noise, output level, and distortion, one would like to settle.

Can we turn a U87A into a U87? Yes, if you were to copy the power supply circuit of the old U87 model exactly. But you would likely be limited to cardioid-only operation, because the pattern switching signal routing in the head would require additional re-configuration- both, mechanical and electrical. And all of it, combined, would be cost-prohibitive. That's why you have the choice between two existing models.

[Hussyh]

Hi Klaus! And thanks for your comments on this.

You know, it's no so much the trade-off off noise/headroom that I am motivated by. It's more to do with what I feel is the subjective sonic feeling of this voltage issue. The lower voltage I believe allows the capsule to sound more generous and round in the low-midrange...less tight and over-damped if I can put it that way. It reminds me of power amps with lower-damping factors and the way that slight "release" of control over a woofer, considered by some to be technically a step in the wrong direction, actually sounds more musical to listen to. It might not be something that shows up clearly on a frequency response plot, but it does affect the way the mic responds dynamically to a singer...the interaction of the mic with the changing dynamic of a performance. Every single time I hear a comparison of an 87Ai to an 87, I hear the same thing. The Ai sounds somewhat clearer but in a non-musical way...too tightly controlled. Th lower voltage 87 breathes and swells in the meat of the vocal range and makes more of the "melody frequencies"...i.e. it carries the melody better I feel.

Cannot one simply swap out some resistor values post the DC to DC converter to experiment with various voltages? Perhaps not as elegant or minimalist as ripping out the DC to DC converter altogether and reconfiguring the circuit, but more straightforward, yeah?

[Kai]

...swap out some resistor values post the DC to DC converter to experiment with various voltages?

No, shunting to GND at the output of the DC-DC converter would cause excessive ripple.

I see 3 options to change the converter output voltage, that might need to be combined:

Version 1 (if it works the most elegant one):
- Remove D5.
- Change the value of D7 (62V Zener). You don't need to remove the original, piggy-back one (e.g. 48V)

Version 2:
- Remove D5.
- Increase the value of R20.

Version 3:
- Insert an (adjustable) series resistor into the path running across internal connector (2), the supply voltage of the DC-DC.
- Check both voltages +60V and -60V at the output of R10, R13 with a high impedance voltmeter (Ri > 10M, most DMM's can be used).
If they don't change symetrically, change the value of D7.


Regards
Kai

[usattler]

The difference in sensitivity between 60 V and 47 V capsule bias is only 20 log (60/45) = 2.12 dB. The more significant difference for achieving the higher sensitivity and lower self noise in the U 87A results from lowering the parasitic capacitance appearing shunting the transducer at the FET gate from 205 pF in the U 87 to 78 pF in the U 87A. A detailed description of the difference between U 87 and U87A can be found in the archives of the Neumann forum:
Differences between U87 and U87A:

Frequently renewed discussions and speculations about the subject of Neumann U87 VS U87A caused me to do some research. As a result, I believe 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 of the relevant published information:

Parameter                               U87                  U87A(i)              rel. difference U87A/U87
Sensitivity (cardioid)             -42 dBv          31.1 dBv          10.9 dB
Sensitivity (cardioid) S           8 mV/Pa         28 mV      
equivalent self noise N                                                           
weighted per CCIR 468-2)      18 dBA           12 dBA             -6 dB
unweighted                         25 dB             23 dB               -2 dB
S/N (CCIR)                          76 dB             82 dB                6 dB
max. SPL (0.5% THD)            122 dBspl       117 dBspl          -5 dB
(with 10 dB pad)                  132 dBspl        127 dBspl          -5 dB
dyn. range of amplifier           104 dB           105 dB               1 dB
max. output (<0.5% THD)      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 (1kHz) Av          0.625            1                  
                                        -4 dB             0                     4 dB
Transducer capsule               K87               K870 (K67.87A)     
Spare part number                053224          053225                  
# of capsule contacts           4                  3                                
Nom. capacitance Cn            2 x 50 pF       2 x 50 pF              
capsule bias UB                    47 V             60 V               2.1 dB
feedback capacitor               10 pF            none                        
10 dB padding cap. Cpad       560 pF          270 pF            290 pF (-6.4 dB)

Application of some basic algebra and physical fundamentals lets us investigate and explain these differences clearly:

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

   eo = UB * delta C/CT   

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

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

   S = AV * UB * delta C/(Cnom + CP)

Where:   AV is the amplifier gain (linear multiplier, not logarithmic relationship in dB), and CP is the parasitic capacitance
            (sum of all capacitances acting in parallel to the transducer's nominal capacitance Cnom).
   
We can calculate the the unknown values for effective capacitance at rest from the known data. This does include the nominal capacitance Cnom of the transducer and how much additional capacitance causes a 10 dB reduction in sensitivity. We shall  cleverly deduct the value of CP by rearranging the sensitivity formula in such a way as to eliminate the unknown delta C. We know the only change is in the additional 10 dB-padding capacitor Cpad, thus:

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

With Cpad =560 pF for the 10 dB pad in the U87 we can calculate the value of the all effective 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 + 560 + CP) = 400 + 8 * CP = 1525 + 2,5 * CP
   5.5 * CP = 1125       and finally CP = 205 pF

For the U87A and its Cpad of 270 pF this results in a value of effective total capacitance for the transducer at rest of 78 pF.

   28 * (50 + CP) = 9 * (50 + 270 + CP) = 1400 + 28 * CP = 1525 + 9 * CP
   19 * CP = 1125       and CP = 78 pF

From the circuit diagrams we learn that the rear transducer is NOT burdening the front of the capsule in the U87A! It is this difference in parasitic capacitances which accounts for most of the difference in sensitivity. Now we have the 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:
 
      20 log (CT of U87 CT of U87A) = 20 log (205/78) = 8.4 dB

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

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

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

A word of caution! Eliminating the 10 pF feedback capacitor in the U87 will NOT result in any benefit, since the inherently higher parasitic capacitance (mainly from the permanent parallel back element) is also responsible for significantly higher THD, which needs to be partially counteracted and compensated for by this feedback mechanism.

Some other interesting calculations from the known data allows deduction of the total capacitance change at the reference SPL (94 dB) and at max SPL, with and without the 10 dB pad.

For the U87 and 94 dBSPL the change in capacitance calculates to be 0.07 pF, 1.74 pF at 122 dBSPL and 5.5 pF at the maximum 132 dBSPL. Even though the capsules are acoustically identical, for the U87A these values are 0.06 pF at 94 dBSPL 0.08 pF at 117 dBSPL and 2.6 pF at the maximum 127 dBSPL.

[klaus]

... I am rather more interested in the subjective feeling I get from the microphone depending on what voltage the capsule is being hit with.

It is no "subjective feeling" you experience: any change in polarization voltage will result in an change in electrostatic attraction by the backplate. The correlation between polarization voltage and acoustic behavior of a mebrane has been downplayed by some in regards to its audible effect, but is an unquestionable mechanical fact.

As part of my modifications of U87A I vary the polarization voltage to help find the sweet spot for a given capsule's performance, considering that its manufacturing tolerance is significant, when expressed in frequency response variations alone (±2dB for the mic as a whole, with an overwhelming contribution to that 4dB swing by the hand-made capsule), in addition to other, more subtle effects.

But in the end, describing audible differences between old and new U87 models needs to include the capsule: You cannot pinpoint the source of the differences you hear to the amp alone, unless you eliminate the capsule as a variable.

That means, you MUST use the same capsule to confirm your findings for both amps! You can do that easily by connecting only the front system of the K87 capsule, bridging the two backplates, as it's done on the U87Ai. Then listen to the amp with the K870 hooked up the usual way. Or you modify the electric connections so that both capsules perform like they would in the original U87 environment (see the respective diagrams for both mics).

What you may find, especially when using K870 capsules made after 2005, is that the capsule itself may be a significant contributor to the changes you hear (see other threads and posts describing the performance changes of the K870 since the mid 2000s).

[Hussyh]

Hello Klaus, Kai, and Usattler...thanks to all of you so much for your comments on this.

Klaus has just messaged me saying that my last post, which was a reply to your posts, got accidentally deleted. I don't have a copy of it to re-post, but no matter.

Kai thanks a lot for the schematic and the 3 ideas on changing the voltage...It's a great start...

Usattler, yes I had seen your detailed analysis previously, but I am not technically able to infer much from it as I lack the necessary technical chops. If you could summarise what you are saying using slightly less maths I would be fascinated. I'm not very good or experienced with active electronic circuits...however I do have a good grasp of passive electronics due to my niche interest in passive filter/crossover design for loudspeakers.

[Hussyh]

Usattler one thing your post makes clear, if I'm understanding correctly, is that lowering the polarising voltage on the 87A will not result in the sensitivity/output penalty that one might think, as there is more going on than just the polarising voltage accounting for the difference in these parameters.

This sounds interesting to me, in that perhaps an 87A at lower voltage will have the best of both worlds...The tone of the 87...but with higher output.

[Klaus yes I know I know...the capsule the capsule...  ]

[klaus]

The difference in sensitivity between 60 V and 47 V capsule bias is only 20 log (60/45) = 2.12 dB. The more significant difference for achieving the higher sensitivity and lower self noise in the U 87A results from lowering the parasitic capacitance appearing shunting the transducer at the FET gate from 205 pF in the U 87 to 78 pF in the U 87A. A detailed description of the difference between U 87 and U87A can be found in the archives of the Neumann forum:
Differences between U87 and U87A:

Frequently renewed discussions and speculations about the subject of Neumann U87 VS U87A caused me to do some research. As a result, I believe 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 of the relevant published information:

Parameter                               U87                  U87A(i)              rel. difference U87A/U87
Sensitivity (cardioid)             -42 dBv          31.1 dBv          10.9 dB
Sensitivity (cardioid) S           8 mV/Pa         28 mV      
equivalent self noise N                                                           
weighted per CCIR 468-2)      18 dBA           12 dBA             -6 dB
unweighted                         25 dB             23 dB               -2 dB
S/N (CCIR)                          76 dB             82 dB                6 dB
max. SPL (0.5% THD)            122 dBspl       117 dBspl          -5 dB
(with 10 dB pad)                  132 dBspl        127 dBspl          -5 dB
dyn. range of amplifier           104 dB           105 dB               1 dB
max. output (<0.5% THD)      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 (1kHz) Av          0.625            1                  
                                        -4 dB             0                     4 dB
Transducer capsule               K87               K870 (K67.87A)     
Spare part number                053224          053225                  
# of capsule contacts           4                  3                                
Nom. capacitance Cn            2 x 50 pF       2 x 50 pF              
capsule bias UB                    47 V             60 V               2.1 dB
feedback capacitor               10 pF            none                        
10 dB padding cap. Cpad       560 pF          270 pF            290 pF (-6.4 dB)

Application of some basic algebra and physical fundamentals lets us investigate and explain these differences clearly:

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

   eo = UB * delta C/CT   

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

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

   S = AV * UB * delta C/(Cnom + CP)

Where:   AV is the amplifier gain (linear multiplier, not logarithmic relationship in dB), and CP is the parasitic capacitance
            (sum of all capacitances acting in parallel to the transducer's nominal capacitance Cnom).
   
We can calculate the the unknown values for effective capacitance at rest from the known data. This does include the nominal capacitance Cnom of the transducer and how much additional capacitance causes a 10 dB reduction in sensitivity. We shall  cleverly deduct the value of CP by rearranging the sensitivity formula in such a way as to eliminate the unknown delta C. We know the only change is in the additional 10 dB-padding capacitor Cpad, thus:

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

With Cpad =560 pF for the 10 dB pad in the U87 we can calculate the value of the all effective 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 + 560 + CP) = 400 + 8 * CP = 1525 + 2,5 * CP
   5.5 * CP = 1125       and finally CP = 205 pF

For the U87A and its Cpad of 270 pF this results in a value of effective total capacitance for the transducer at rest of 78 pF.

   28 * (50 + CP) = 9 * (50 + 270 + CP) = 1400 + 28 * CP = 1525 + 9 * CP
   19 * CP = 1125       and CP = 78 pF

From the circuit diagrams we learn that the rear transducer is NOT burdening the front of the capsule in the U87A! It is this difference in parasitic capacitances which accounts for most of the difference in sensitivity. Now we have the 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:
 
      20 log (CT of U87 CT of U87A) = 20 log (205/78) = 8.4 dB

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

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

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

A word of caution! Eliminating the 10 pF feedback capacitor in the U87 will NOT result in any benefit, since the inherently higher parasitic capacitance (mainly from the permanent parallel back element) is also responsible for significantly higher THD, which needs to be partially counteracted and compensated for by this feedback mechanism.

Some other interesting calculations from the known data allows deduction of the total capacitance change at the reference SPL (94 dB) and at max SPL, with and without the 10 dB pad.

For the U87 and 94 dBSPL the change in capacitance calculates to be 0.07 pF, 1.74 pF at 122 dBSPL and 5.5 pF at the maximum 132 dBSPL. Even though the capsules are acoustically identical, for the U87A these values are 0.06 pF at 94 dBSPL 0.08 pF at 117 dBSPL and 2.6 pF at the maximum 127 dBSPL.

Here is my lay-langauge interpretation of the changes between U87 and U87A Uwe detailed above:

1. Simplifying capsule connections to just one front, one center, and one rear connection in the U87A eliminated the complicated hook-up scheme of the old U87, which required permanently connecting front and rear capsule sides. The new connection scheme lowers (capacitative) distortion and increases output.

2. Increasing polarization voltage in the U87A increased capsule sensitivity, output, but lowered the mic's headroom (more distortion).

3. Eliminating the negative feedback capacitor in the U87A added output, but increased distortion.

In sum, I am not so sure that the circuit changes of the U87A should be automatically regarded as "improvement", given that the headroom of a FET is quite limited, unless negative feedback between gate and drain is added.  In the case of the U87A, the cumulative design changes decreased the mic's headroom by ca. 6dB in comparison with the old U87.

Leaving the audible effect of diaphragm stiffening due to increased polarization voltage in the U87A aside, I am not convinced that the trade-off between lower noise floor and higher output on one side, and reduced headroom on the other, is necessarily more desirable in all recording circumstances.

[usattler]

I still sense misunderstanding the real impact of the design decisions made regarding the U87A.

Apparently disregarded is the importance of dynamic range. Based on the experiences of the target audience for studio microphones, in the upgrade design the dynamic range was expanded slightly and shifted toward higher sensitivity, by lowering the self noise and increasing the output capability. This was achieved through the combination of marginally higher capsule bias and lower parasitic capacitive loading of the FET gate. This in turn reduces distortion and eliminates the need for the compensating feedback capacitor.

How are less self noise, less distortion, more dynamic range and higher output capability possibly regarded as less desirable features?

Arguably, few real world recording scenarios approach the threshold of pain (115 dBspl) at the microphone position, and the standard 10 dB pad should accommodate most exceptions. Furthermore, the distortion at the specified maximum SPL is less than 0.5% and increases gradually beyond this level, rather than causing hard clipping. For capturing of even louder events perhaps more suitable microphones should be considered, inevitably 'featuring' lower sensitivity and higher self noise, even if such microphone were to have the same dynamic range.

[klaus]

Thanks, Uwe, for further clarification. I don't doubt that U87A design elements we are discussing here were conceived and thought of by Neumann as an improvement over the old model.

(...) How are less self noise, less distortion, more dynamic range and higher output capability possibly regarded as less desirable features?

Less distortion? Exposed to high sound pressure levels, the U87A distorts 6dB sooner than the U87.

I might also add that single-FET mics in general, like the U87 and U87A, are not very forgiving when hit hard. Audible distortion sets in rather suddenly when the U87 is exposed to high SPLs, compared with tube mics of similar overall design, which tend to have a "soft knee" type of distortion on-set, with plenty of warning area before it gets critical. Another way to handle the danger zone of high SPLs is to use a modern TLM-type mic with vastly increased headroom (around +20dB or more).

 I often hear from those who own both models that they perceive the 'A' has harder, more "modern" sounding, and, while certainly still up there with the best, the 'A' is often perceived as less sweet and musical.

It will remain a matter of opinion which design factors may have led to the characterization these two mics have acquired over the years. I restore or upgrade on average one to two U87 per week. Both models. Through continuing experimentation I believe I have figured out what accounts for the somewhat less enthusiastic reputation the U87Ai has garnered in the recording community. But I respect that you may not agree with my conclusions.

Best,
KH

[Jim Williams]

How are less self noise, less distortion, more dynamic range and higher output capability possibly regarded as less desirable features?

Those are concepts not in vogue at this time. Check back in another 20 odd years when this era fades.

I like option # 4:

Sell the 87AI and buy that old workhorse 87. Seems a bit smarter than trying to re-invent the Neumann wheel?

[usattler]

Klaus, I guess the argument boils down to our longstanding difference in emphasis on perfection VS tradition. Aside from my almost daily exposure to both versions, U87 and U87A, I am also an actual owner and user of both types. I mention this in the sincere hope you are not trying to imply that I may be just a poster or poser! [Just saw this inadvertently mangled sentence in the earlier post and have corrected it. My apologies! KH]
As to the marketplace as one source for information, we could look at the total sales for each version, which are overwhelmingly in favor of the U87A. I further venture the guess that at Neumann|USA we receive more contact and feedback about Neumann microphones than anywhere else, except at Neumann\Berlin. In our Service Department we get numerous requests for 'upgrading U87 to U87A, but rarely, if ever, the other way around...  (BTW, we do neither)
By focusing only on the top end of the dynamic range, you unfortunately overlook or dismiss the bottom end of self-noise. Most reputable microphone manufacturers have made great efforts to match the improvements made in recording media and elsewhere in the recording equipment, which exposed limitations in the self-noise of 'vintage' microphones.
Our product development is driven to please the senses, validated through careful analysis and science, rather than pinned to beliefs and plausible theories.
 

[Jim Williams]

There are very low noise jfets available that will lower the self noise of older mic designs. That will extend the dynamic range by lowering the self noise. Some jfets are as quiet as the lowest noise bipolar transistors, at 0.7 nv/hz/sq. You don't have to use the 2N3819. Those are about 3.0 nv/hz/sq noise.

[klaus]

That is interesting to hear from you. I have experimented with lower noise JFETs* in the past, but always found them wanting in headroom. Please elaborate.

*For those not familiar with the term: these are Field Effect Transistors which perform the equivalent functions of a tube: predominantly to link a super-high impedance stage (capsule) to the much lower impedance of the mic's amp.

[Kai]

...these are Field Effect Transistors which perform the equivalent of a tube...

I wouldn't subscribe that a FET does perform like a tube.
When FETs originally were introduced, this statement was often heard from those who found it was easier (and cheaper) to build a circuit using a FET instead of a tube.

There are some similarities, but there are even great differences.
E.g.:

- Most tubes are much more linear and the transition to the nonlinear range is softer.

- A tube works with and can handle much higher voltages (up to several hundred volts on common types). If used with high supply voltages, they have a greater dynamic range.
Tubes can even stand much higher voltages without breaking - in fact you almost never will find a tube that failed due to overvoltage at the input. This means you don't need to take care for that in a mic.
For a FET it's deadly coming in contact with the polarisation voltage.

- Most important for us: tubes do sound different to FET's, as a result of all the differences in their characteristics plus the necessary differences in mic design.

- Tubes give a nice heating to the mic, which helps to keep the capsule dry.


Regards
Kai

[klaus]

I agree with your distinctions. That is why I wrote "perform the equivalent functions" rather than "are comparable in the outcome of these functions..."

[Jim Williams]

That is interesting to hear from you. I have experimented with lower noise JFETs* in the past, but always found them wanting in headroom. Please elaborate.

One example is the LSK series from
www.linearsystems.com

Those are very low noise jfets, an "upgrade" to the 2SK170 jfets commonly found in modern fet input condenser mics.

As for their headroom, they will output within about a volt or so of the power supply voltage, if set to 10 volts these will do 9. If that is not sufficient, one can increase the voltage the fet sees a few more volts by re-arranging the power feeds.

[klaus]

Thanks, I will look into these and test.

[Sage]

Has anyone here ever attempted to mod a U87Ai back down to the lower capsule polarising voltage of the U87/U87i by disabling the DC to DC converter?

If so, is it a straightforward mod?

And...I know I know...why...right?

Apologies for dredging up an old thread, but I have the exact opposite question.  Is it possible to update a vintage U87 circuit (or a facsimile like this one) to the newer U87ai, in order to achieve the lower noise and higher sensitivity of the later version?  If so, what would it require?  Is it just a matter of adding the DC-DC converter into the circuit somewhere and swapping the capsule, or is there more to it?

[Uwe]

'upgrading' an U87 to U87A is theoretically possible, but would be more expensive than purchasing a new U87A. So what would be the point of retaining only the original basket, chassis rails, bottom assembly with connector, threaded cap and some minor hardware? Upgrade would require minimally replacement of the capsule, capsule mounting base assembly, microphone head socket assembly, amplifier pc-board assembly complete, DC-DC-converter board, output filter board assembly and housing tube. You can see the cost adding up beyond reasonable feasibility... Some afficionados do prefer the more vintage U87 over the U87A, and you may be able to sell the older version and get more than enough for a substantial down payment toward the U87A.

[hasbeen]

If you have an old U87 with the 'Gotham Pads' you can restore it to European output specs. Do you have a red or blue dot embossed next to the XLR connector?

[klaus]

That mod, however, does not turn the U87 into a U87Ai, but, where applicable, increases the output of a Gotham-padded U87 (50Ω, pads in) up to 12dB (pads removed + 200Ω strapping).