Styroflex Capacitors - capacitance value options - U87 clone

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haysonics

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Oct 8, 2017
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Some time ago I had a problem with a couple of 'thin' sounding U87ai microphones.
I suggested to increase the value of C5. I also put this question on the forum of Klaus Heyne.
This was his answer:
"You increase the measured low end at the mic's output when you increase the value of C104 (corresponding to C5 in U87) through which the calibration tone is fed. But it affects only calibration audio going through this cap. The mic's audio signal from the capsule is not processed through this cap. "
Does this 'microphone guru' even understand how the circuit works?... (C5 is in the feedback loop, also affecting the sound from the microphone capsule!)
This has been on my mind me for awhile and I finally thought of something. Swapping the cap at C104 on your U87Ai and hearing the results proved C104 is in the audio path yet Klaus was adamant that only calibration audio is going through this cap. It was assumed Klaus made a mistake but what if Neumann made a mistake when they changed the audio path at some revision and inadvertently sent it through C5/C104 and Klaus never noticed ?

There are a few reasons why I suspect this may be the case (I can't read schematics so am unable to compare revisions) :

1 - Neumann's published frequency response graph (knowing its smoothed yet should roughly approximate reality) shows a flat frequency response up to 5K (U87i) or 7K (U87Ai) with a +-2dB variance.

2 - Neumann included a high pass filter switch on the mic. IMO including the switch would have been pointless if they actually decided to have C5/C104 attenuate the low end.

3 - Many users report a full bass response yet others report attenuated bass. Klaus has found many K67/K870 capsules with membrane tension too tight leading to attenuated bass and seems to be applying that finding to all cases of attenuated bass (which is understandable). Yet I wonder if there is another factor at play in many cases - that being audio attenuated by C5/C104.

4 - On the ProSoundWeb forum one user advised that "according to Neumann's specifications (in copi0081.pdf from the Neumann "Infopool"), in order to sound right the U 87A amplifier (via its measurement input) should measure about -3 dB at 40 Hz and about -4 dB at 16 kHz, both relative to the gain at 1 kHz". IMO this info is useful in that it relates to the the old German broadcasting standard. The issue is that not only has C5/C104 been found to be in the audio path, but also, that cap produces significantly more attenuation than -3 dB at 40 Hz and takes the low end of the audio out of spec from the old German broadcasting standard (and I assume also out of spec from the current German broadcasting standard). Klaus could counter and say that the reason the low end of the audio falls out of spec is because a lot of K67/K870s membranes are too tight but this ignores the real effect that C5/C104 has on the low end of the audio path. Klaus believes that this capacitor is simply "used for the calibration input from the mic's amp response" and "has no bearing on the mic's overall frequency response, with the capsule as source." I suspect that this was the case and then a revision changed that.

5 - On ProSoundWeb Klaus often seems bewildered that Neumann don't seem to be listening and correcting the defficiencies in the low end of their capsules yet I suspect that Neumann believe that C5/C104 is the reason why the low end is lackluster and that their mics were originally designed this way and that is how they are meant to be. If this is the case then you might expect Neumann to question the cost of including the high pass switch but I can imagine Neumann believing that (again) their mics were originally designed this way and that is how they are meant to be.
 
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