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Author Topic: Neumann U87ai sounding 'thin'?  (Read 971 times)

RuudNL

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Neumann U87ai sounding 'thin'?
« on: April 16, 2017, 04:29:32 pm »

I am the owner of a couple of (recent) Neumann U87ai microphones.
They sound identical, but I have the impression that they sound a bit 'thin', compared to other condenser microphones.
(Of course the LF filter is switched off!)
I measured the frequency response through the "mess eingang" and also found a considerable LF rolloff.
Now I am playing with the idea to increase the value of the 47 nF capacitor in the filter (C 104), to 100 nF or even maybe even 150 nF.
I did some measurements on a similar circuit (outside the microphone).
As far as I remember, the 'old' U87 had a much better LF response.
Would there be any drawbacks increasing the value of C 104?
What are your thoughts about this?
(Please see the graph attached with 47 nF and 150 nF)
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klaus

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Re: Neumann U87ai sounding 'thin'?
« Reply #1 on: April 16, 2017, 11:46:39 pm »

The "thin" sound of your U87ai more than likely is due to your specific K870 capsule (tight membrane tension), not the capacitor which also serves as input point for calibration tones.

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.

In mics with no calibration input, Neumann and others usually use an even smaller DC filter cap (typically 0.01mfd or lower) for this function. These values do not attenuate the mic's low end output.
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Klaus Heyne
German Masterworks®
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David Satz

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Re: Neumann U87ai sounding 'thin'?
« Reply #2 on: April 18, 2017, 11:39:47 pm »

The frequency response of a U 87 amplifier is tailored for the expected uses of the microphone, and 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.

I'm sorry that I don't have full information handy about the exact way in which this measurement should be carried out, but if I recall correctly, the source impedance of the test signals can be critical, and a substitute for the capsule's capacitance must be in place. I suggest making extra sure that you're measuring the amplifier's response in the correct way before you base any circuit modifications on your results.

--best regards
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RuudNL

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Re: Neumann U87ai sounding 'thin'?
« Reply #3 on: May 22, 2017, 12:58:54 pm »

The frequency response was measured using a signal generator with an output impedance of 50 Ohms and the microphone capsule in place.
(The input of the "Messeingang" is a 560 ohms resistor, coupled to the rest of the circuit through a 6800 ohms resistor.)
The simulated circuit used a capacitor of 56 pF to simulate the microphone capsule.
In both cases, the LF rolloff was much more than the specified -3 dB @ 40 Hz.
My ears confirm this...
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klaus

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Re: Neumann U87ai sounding 'thin'?
« Reply #4 on: June 05, 2017, 01:29:00 pm »

Your ears hear the complete mic, including the capsule. Your "Mess-Eingang" frequency response does not incorporate the capsule's contribution to the sound of the mic. As I mentioned earlier, you need to disassociate the capacitor value used for the calibration input from the mic's amp response. Changing that value has no bearing on the mic's overall frequency response, with the capsule as source.

If anything, in my experience, Neumann's mic amps (not capsules!) are uniformly tight in tolerance, measuring rarely more than 1/2 dB deviation from mic to mic.
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Klaus Heyne
German Masterworks®
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