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Old 4th December 2009, 06:48 PM   #1
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Default Microphone self-noise

Would self-noise and noise picked up a microphone coil be common mode or not?

Want to build a mic pre directly onto a dynamic capsule -- can't figure if a differential front-end would be an advantage in the total absence of any cable.

Thanks.
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Old 4th December 2009, 09:16 PM   #2
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It won't help the self noise; that's a consequence of the physics of the transducer. But since you're eliminating the leads, it will prevent common-mode noise pickup from that source. There still might be an advantage to running the output of the preamp balanced, minimizing common-mode noise pickup further down the chain.
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Old 6th December 2009, 12:06 AM   #3
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The term 'common mode' refers to transmission lines not devices.
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Old 6th December 2009, 12:44 AM   #4
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Common-Mode Signals Defined:
When referenced to the local common or ground, a common-mode signal appears on both lines of a 2-wire cable, in-phase and with equal amplitudes. Clearly, a common-mode signal cannot be present if one of the lines is connected to local common. Technically, a common-mode voltage is one-half the vector sum of the voltages from each conductor of a balanced circuit to local ground or common. Such signals can arise from one or more of the following sources:

* Radiated signals coupled equally to both lines,
* An offset from signal common created in the driver circuit, or
* A ground differential between the transmitting and receiving locations.
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Last edited by MEGA_amp; 6th December 2009 at 12:48 AM.
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Old 8th December 2009, 02:13 PM   #5
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@ toddboyle; Perhaps I've misused the terminology

Quote:
Originally Posted by toddboyle View Post
The term 'common mode' refers to transmission lines not devices.
Although MEGA-amp's statement here appears not to agree:

Quote:
Originally Posted by MEGA-amp View Post
Such signals can arise from...
* An offset from signal common created in the driver circuit,
(my emphasis)

Probably my qustion would have been phrased better as:

"what percantage of the noise eliminated by the common mode rejection of a balanced microphone preamp originates in the microphone capsule rather than being contributed by the cable?"

Anyway, Sy appears to have understood what I was getting at.

Quote:
Originally Posted by SY View Post
It won't help the self noise; that's a consequence of the physics of the transducer.
So an unbalanced single-ended mic pre won't be outperformed by a more complicated balanced design in this instance. That will make my life easier.

Quote:
Originally Posted by SY View Post
There still might be an advantage to running the output of the preamp balanced, minimizing common-mode noise pickup further down the chain.
Yup, since I'm planning to use this thing onstage balanced line output on XLR will be mandatory. SSM2142 or DRV134 look right, are available to me, and should be easy enough to wire up.

Thanks
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