> Yes, there has always been nonsense talk about "impedance matching".
> However, as we don't use dynamic mic's and input transformers so
> much
> any more, it doesn't affect my sleep. It was much worse 30 years
> ago.
I've been sitting on my hands trying not to do a bit on impedance
matching, but I've failed. :-)
Summary: With source and destination impedances between 50ohms and
2000ohms, cables with around a 150ohm impedance work fine up to
several hundred metres.
I go back to when phones had a handle on the side and carbon mics.
When they went fizzy or weak, you had to shake the handset or hit it.
Your handset mic had to generate all the signal energy to go long
distances down the single wires strung along the "telegraph poles" to
the distant earpiece. The impedance of the wires was about 600ohms,
and this impedance is still used in the definitions of various 0dB
levels.
Impedance matching generally has three elements; source, transmission
line and receiver. A mismatch at either of these junctions will
reflect back some of the signal energy instead of passing it on. With
most current audio recording methods, the impedance of a short mic
cable is not important, but long cables produce a reflection at any
point of impedance mismatch which produces a time shifted reflection
which can affect the response. For instance a long cable driven from a
high impedance will look like a capacitor.
As above, with source and destination impedances between 50ohms and
2000ohms, cables with around a 150ohm impedance work fine up to
several hundred metres.
Modern powered mics are often low impedance output but are sometimes
specified to run into 1000ohms or greater. You get a power mismatch
here, and a reflection is sent back from the far end of the cable but
the original mic voltage is hardly affected. In practice any noise
increase is small.
Running a higher impedance mic, say 50Kohms, into a low impedance
input will cut the voltage in proportion and the energy by the voltage
squared.
There is a parameter called "noise impedance". You won't find a simple
definition for this but it is the ideal load impedance to get the
lowest signal to noise ratio. It is not critical, but many mics are
designed to run into a higher impedance than they show and and this
generally gives the optimum noise figure.
Microphone transformers can give an optimum matching for the lowest
noise, but they are out of fashion these days. I use transformers on
"affordable" electret mics which have an output impedance of around
30Kohms and match well into low impedance cables and mixer. They also
give me a floating source which minimises pickup and losses from 100+
metre cables.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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