Was: Best way to build simply DIY stereo mics.
> I am not sure that is something worth spending too much
> time into researching but what I am sure counts is the matching
> of the output impedance of mike onto the input impedance of the
> mic preamplifier. I think a thumb rule is factor 1 to 10. (?),
There is a lot of confusion about impedance matching so I thought I
would have a go at explaining this. Basically, with mic cables you can
hold in your hand, you can get away with a 10:1 mismatch.
A cable is a "transmission line" and impedance matching was important
in the days of telephone wires strung individually with a dozen or
more going from pole to pole. A transmission line has it own impedance
over a long distance and these telephone lines came out at around
600ohms, which is still a "base" impedance for dBV measurements.
To get the maximum energy transmission the source impedance, cable
impedance, and load impedance should all match, but this is only
critical over long lines. At 10KHz, you run into cancellations at
around 10 miles. at 1/10 of a mile, a mismatch of 100:1 will give
serious problems.
Video and data circuits are 50 ohm or 75 ohm , and need to be
terminated properly even over short distances, but a 10:1 mismatch at
audio frequencies will only show up at cable drum lengths.
Audio screened cable has an impedance of 100ohms to 150ohms, and mics
are designed to feed into this, but powered mics give a higher peak
output if run into a higher impedance, hence the cheat of, say a 25ohm
Sennheiser going into a 100ohm cable into a 2500ohm mixer/recorder. In
practice this mismatch will not give any trouble till you get into
hundreds of yards of cable.
A transmission line is a combination of a long inductance with a long
capacitance. If you short it out, it will look like an inductance and
if you drive it from a high impedance like 50,000ohms, a 100ohm cable
will look like a capacitor. This will cut higher frequencies.
Where you run into trouble is with using 100ohm cable on a 10,000ohm
circuit or higher, when you will lose high frequencies at around 10
yards or more.
The good news is that with audio, almost any bit of screened cable and
any plugs or sockets will do. You can get away with a mismatch of 10:1
up to a few hundred yards, but you need a "balanced line" to avoid
interference like mains hum or digital noise over a few yards.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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