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

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

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/U87Sensitivity (cardioid) -42 dBv 31.1 dBv 10.9 dBSensitivity (cardioid) S 8 mV/Pa 28 mV equivalent self noise N weighted per CCIR 468-2) 18 dBA 12 dBA -6 dBunweighted 25 dB 23 dB -2 dBS/N (CCIR) 76 dB 82 dB 6 dBmax. SPL (0.5% THD) 122 dBspl 117 dBspl -5 dB(with 10 dB pad) 132 dBspl 127 dBspl -5 dBdyn. range of amplifier 104 dB 105 dB 1 dBmax. output (<0.5% THD) 200 mV 390 mV -14 dBv -8.2 dBv 5.8 dBmax. input (test port) 320 mV 390 mV 1.7 dBamplifier gain (1kHz) Av 0.625 1 -4 dB 0 4 dBTransducer 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 dBfeedback 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 dBAdding 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.

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

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

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

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.

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?