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Microphone vs. Speaker wire

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#1 rsspencer



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Posted 28 August 2012 - 08:48 PM

Just an observation and question from a noob. Everyone seems to recommend shielded wire for microphones/intercomms. However, speaker wire has no such recommendation. Speaker wire is larger to accomodate the wattage of the amplified signal but other than being amplified, the speaker wire carries the same exact signal as the microphone isn't it? Just going the opposite direction down the wire. So why does microphone wire need to be shielded? Is it just because the speaker signal is amplified so overcomes interference? Or is it something else? Also, what awg wire should I use for microphone wire for a 100'+ run? 18/4? Thanks for any explanations.

#2 DELInstallations



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Posted 28 August 2012 - 08:53 PM

AWG depends on the application, as well as if it should be STP or not, as well as construction of the cable. Also, not all intercoms need shielded, UTP or STP cabling, some require parallel conductors with no twist, so again, it's the application (and manufacturer). Some recommendations are based on where the cabling is run in relation to possible sources of interference.

In the case of a mic, any incoming or stray interference is going to be amplified multiple, if not hundreds (thousands) of times in order to have enough of a signal to drive a speaker via the output section of an amp.

#3 Frederick C. Wilt

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Posted 28 August 2012 - 10:20 PM

Lets say you are pumping 100 watts into 8 ohms. If you do the math you have signal amplitude of about 28 volts - and it's on a low impedance line - 8 ohms.

Now an inexpensive microphone is likely to be putting out fractions of a volt - and its likely on a much higher impedance line.

So if both are in the presence of a source of interference the higher voltage and lower impedance of the speaker line is going to make it much less susceptible to the interference.

#4 easytimhal



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Posted 29 August 2012 - 02:06 AM

You can use speaker wire for the mic if it is sheilded, 16ga 100ft no problem

#5 Mr Spock

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Posted 29 August 2012 - 12:29 PM

I'll add to what the others have stated.

The voltage output from a microphone is very low; typically about 0.001V. It's so low that when you string unshielded cable about 25 feet or more it tends to pick up AC powerline hum, AM radio, and a lot of other noise. The unshielded cable tends to act like an antenna. Once the microphone signal is at your panel (or where ever it's going) there is an amplifier that will boost this low level to about 1V. Problem is it's also boosted all the hum and other noise too.

The two ways around the problem is to either use shielded cable (blocking the antenna effect) or put the microphone amplifier at the microphone. Remember that shielded cable should have the shielding wire connected to ground only on one end, not both ends. It's generally best to do it at the panel end instead of the microphone end, but that can vary.

Regarding the wire gage. It's not important. The thinner stuff (22 through 26) is just fine. Shielded speaker wire will work, but is a waste of money.

Edited by Mr Spock, 29 August 2012 - 12:41 PM.

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