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Topic: noise on long wires (Read 390 times) previous topic - next topic

MarkT

Those typically use balanced signals, so most noise is cancelled out.
With the very low signals from a microphone you need both a shield and balanced pair to take out enough
noise to be usable.  You also need a special layer inside the shield to reduce cable microphonic effects caused by friction inside the cable as it flexes and moves.

Digital signals are much more robust, typically just twisted pair and differential signalling will do, as in ethernet,
RS485, LVDS etc etc.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

wvmarle

I do seem to remember those microphones have a battery in them.... Must be for some kind of amplifier, to get to the typical +/- 1V signals.
Trying to transmit the signal directly from the microphone is quite hopeless indeed.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

SteveMann

Condenser microphones need a power source either from "phantom power" on the audio wires or a battery.  Dynamic microphones make their own power.

raymw

Grounding shields on long cables at both ends - if you don't, hope yo don't get an HV earth fault at the far grounded end, and you're holding the shield at tother. In many countries there are regulations/legal requirements, about running cables between buildings, for example.

wvmarle

In many countries there are regulations/legal requirements, about running cables between buildings, for example.
Regulations are for mains power cables, not for signal cables.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

krupski

#20
Aug 29, 2019, 05:35 pm Last Edit: Aug 29, 2019, 05:36 pm by krupski
I do seem to remember those microphones have a battery in them.... Must be for some kind of amplifier, to get to the typical +/- 1V signals.
Trying to transmit the signal directly from the microphone is quite hopeless indeed.
A "condenser" microphone is basically a capacitor made from a very thin conductive mylar diaphragm and the can that holds it.

It is charged with a few volts from it's own internal battery or by a bias supply sent down the microphone cable.
When sound hits the diaphragm, it vibrates, causing the capacitance to change in step with the sound.
Since the charge is constant, the changing capacitance results in a voltage change across the capacitor.
This voltage change is a representation of the original sound.

This delicate, high impedance signal is then sent to a little mosfet that acts as an impedance transformer, giving the audio signal a nice low (around 1K or so) source impedance to send to the amplifier.

The very low mass of the diaphragm gives even a cheap condenser mike really good sound quality.

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raymw

Regulations are for mains power cables, not for signal cables.
You need to check on that, I think. UK(Europe similar http://www.legislation.gov.uk/uksi/2013/1387/contents/made
and, as I indicated before, there are regulations on earthing or bonding of cables, pipes, any conductor which could be energised from outside the building. I'm trying to point out that if you're touching a length of wire, wet string, even, that if it is not earthed where you are standing, then there is the possibility of an earth fault near the remote end, that the potential difference between your end and the ground you stand on, can be enough to cause you damage. But, others may think otherwise.

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