M147 question

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Gus

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Jun 3, 2004
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I do no have a M147 and I have not looked at one. I like to look at schematics and figure out what is going on like people do puzzles

Looking at a schematic you can find on the web for the M147 I noticed it uses a what looks like it might be an impedance balanced output.
If you look the + side output resistance before C11 is 95.3 ohms / 2.
I do like the parallel opamp output and use of 32VDC. I like the 40Hz HPF filter selection. Clever use of a resistive voltage divider for the pad in the follower emitter leg.

The - is connected to ground unlike other impedance balanced microphones  that would have a another matching resistance( 95.3 ohms / 2 in this case) with a cap to ground
The output does appear across R17 and then a filter network.

Is there anything in the power supply enclosure that connects to the + and/or -output except a connection to  the 3 pin XLR? I have not found a picture of the inside of the power supply on the web.

I am a puzzled by this output stage with the - to ground unless this is going into a high impedance input preamp(transformer or a high  impedance semiconductor circuit )
The CMRR will be degraded by the impedance not be low and the same, unless I am misunderstanding  CMRR.

The 22uF is about 1/2 of 47uf. Two 47uf in series
 
I never understood the idea behind the M147.
A combination of a tube, semiconductors and an integrated circuit.
In the past it has been proven that it is possible to build excellent microphones with a single tube, or a single FET.
And not to speak about the Chinese(?) plastic switched power supply inside Neumanns power supply unit.
(Not to forget the 6111 tube, that is glued in place and is soldered on the PCB, with very small tracks that make it almost impossible to change the tube. And if there is something that should be servicable, it would be the tube after so many years... But fortunately you can replace the whole PCB, for ~ $350.)
Most people I know who have a M147 say that they don't use it very much, because they don't like it.
Certainly not for the price they paid for it!
It sounds completely different than a U47. (As is advertised.)
I have heard about hum problems and ground loops, because of the unbalanced output.
(Although Neumann/Sennheiser still keep saying that the output IS balanced...)

More here: https://www.gearslutz.com/board/geekslutz-forum/1161445-neumann-m149a-power-supply-faulty.html
 
I had one, never liked this microphone... Lack  of hi end frequencies and doesn't sound like a tube mic at all. I sold it to buy an Audio Tecnica AT 4060. beautiful mic for the price.
 
I am more interested in what Neumann did for the output.
Is it like the schematic you can find on the web for the microphone OR is there more circuity inside the power supply between the microphone and the 3 pin XLR?
 
As far as I know the power supply for the M147 (and M149) only has a standard 12 Volts switched power supply (in a plastic enclosure) and some circuitry to produce +5.2V, +33V, +70V and - 70V.
If I remember correctly, the audio comes in and goes out...
 
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