Hey gang, I've run across several suggested mods to the M49 that supposedly improve bass/high freq response as well as correcting phase shift issues.
Here they are summarized; note they apply equally to B and C versions:
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Remove R3 or make it much higher – eliminates 40Hz bass rolloff as well as corrects phase shift (Klaus)
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Remove C6 – eliminates 18kHz high end rolloff and phase distortion (Klaus and others)
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Make cap C4 2pf or eliminate - improves output and freq. response (David Bock, Nyqvist here, and others)
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Here are some (but not all) relevant links:
Remove R3 or make it much higher:
http://www.neumann.com/forums/view.php?bn=neumann_archive&key=990963033&v=f (plus search this forum)
"...R3 can be gradually increased in convenient stages: first, leave R3 at 5 meg Ohms, and put a 1 meg resistor in series with it, for a total feeback resistance of 6 meg. Then increase the added resistor's value to 2, 3, and 4 meg. You will notice that at around 7 meg total resistance it becomes increasingly hard to hear a difference in the mic's low end response between this very slight feedback and no feedback at all (which would be resistors disconnected altogether) Kind regards, Klaus Heyne
P.S.: I would always try to run the mic as wide open as possible, i.e. R3 out or only minimally engaged; that's when phase shift is at its lowest."
Remove C6:
Nyquist said:
... Last but not least I removed C6. This opened the mic up added more air and seemed to balance the whole shooting match a bit better.
https://groups.google.com/forum/#!topic/rec.audio.pro/jQC3wLhKWo0
"3. Neumann M49, cap. C6.
In this case cap. C6 together with the output impedance of the amplifying
stage (it can be considered as an an active resistance) establish first
order LP (low pass) filter that reduces signal at high frequencies.
Measuring the circuitry, we got the following results:
2 kHz - 0.04 dB
4 kHz - 0.12 dB
8 kHz - 0.43 dB
10 kHz - 0.65 dB
16 kHz - 1.48 dB
20 kHz -2.14 dB
31.5 kHz -4.08 dB
Capacitance C6 brings the following phase distortion:
2 kHz + 4.1 Deg.
4 kHz + 8.2 Deg.
8 kHz + 15.9 Deg.
10 kHz + 19.5 Deg.
16 kHz + 29.1 Deg.
20 kHz + 34.3 Deg.
31.5 kHz + 44.5 Deg.
It is preferable to remove this capacitance."
Reduce or Remove C4:
http://prorecordingworkshop.lefora.com/reply/46983815/Anybody-have-a-BLUE-M49-schematic#.VKz5hMaoXbg (plus search this forum)
"@ryan the 8pF value is from the m49b, b for broadcast, so there's a filter as well as higher amount of NFB. You could go as low as 2pF or up to 4pF, I think 8 is ridiculous as the AC701 barely has enough gain to start with." (David Bock)
https://groups.google.com/forum/#!topic/rec.audio.pro/jQC3wLhKWo0
"2. Neumann M49, cap. C4. Cap. C4 (8 pF) is placed between tube's AC 701 plate and grid. So it is parallel to the tube's own capacitance Cag. This capacitance according to the tube's manufacturer's literature is 2.2 pF. As a result we get 10.2 pF. At stage's gain (20 dB) - in other words ? 10 times (it is a typical volume) we get 102 pF (10.2 pF x 10 = 102 pF) known as Miller's capacitance that is connected between tube's grid and cathode. This is a parasitical capacitance that causes attenuation of signal in cases of capacitive acoustical transducer (condenser mic capsule). It will reduce the signal dynamic range. So it is preferable to remove this capacitance."
These sound like no-brainers to me; what do you think?
Also, as a true newbie, is it really ok to simply omit these components from the PCB altogether? Or would a jumper be necessary in any case? As far as I can tell, I can simply omit them all with no problem, with the exception of C4 which I'm uncertain what would happen.
What say you?
Many thanks,
Mike
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