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Dirty-South Blues Harp forum:>
High vs low impedance - how it effects the sound?
apskarp
4 posts
May 23, 2009
9:50 AM
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As the amplified amp is becoming relevant in my "harpist career" I wonder what the high/low impedance means in practice. I searched the forum for this but didn't find any answers yet.
So, What does it mean in practise if the impedances of the mic and the amp doesn't match? Does it mean that it cuts some of the high or low end of the voice, or does it affect the volume, or what?
Currently I use SM58 with (home made) 15W tube amplifier (which I got from our guitarist) and it seems to sound pretty nice. Perhaps some more distortion could be nice. I don't have any impedance converters and I'm wondering how it might change the sound. Of course the converter isn't very expensive and I'll get one, but I'm interested to here some technical theory of that too. So I'd appreciate if some of you professors could explain it a bit.. :)
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Greg Heumann
47 posts
May 23, 2009
4:57 PM
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The short answer to your question - impedance mismatches deliver suboptimal performance from your mic/amp combination.
A low impedance mic into a high impedance amp like you describe will work, sort of, but frequently it is done with an XLR-to-1/4" cable that (believe it or not) only connects one of the low-Z mic's 2 signal pins (pins 2 and 3) to the "hot" or "tip" of the 1/4" connector, and then connects pin 1 to ground. (Ground is NOT part of the signal path in low impedance wiring.) In this case you get about half the possible signal from the mic. Even if the cable properly routes the full signal from the mic to the amp, the mic doesn't see the low ohms "load" it expects to see, and so all bets are off as far as the mic performing the way it is supposed to. The proper way IS to use an XLR<-->XLR cable and an impedance matching transformer. Now here's a more complete answer:
What's Impedance?
By definition, impedance is "resistance to an AC signal" - AC means alternating current - in this case the signal from your mic which is an electrical picture of the sound going into it. POWER in electrical terms is all that matters, but you can have the same amount of power with low voltage and high current, or high voltage and low current. Low impedance mics have a signal with more volts and less amps; high impedance mics are the reverse. But you don't need to know that. All you need to know is this:
1) Microphones and amplifiers work best with certain load or drive levels, respectively. For this reason, it is desirable to "match" the impedance of the microphone to the impedance of the device it is connected to - an amp, a foot pedal, or....
2) Impedance matching transformers are simple devices that can "match" a low impedance mic to a high impedance load, or vice versa.
3) Low impedance systems came later than high impedance ones, and were developed to enable much longer cable runs and better reject noise (like hum) that gets injected via the cable.
4) There is no inherent difference in tone or feedback rejection between low and high.
5) Low impedance systems are almost always wired with "XLR" jacks plugs. This is a worldwide industry standard.
6) High impedance systems can be wired any number of ways, including use of XLR jacks. When XLR is used, however, the wiring is not the same.
7) There's nothing wrong with connecting a high impedance mic with an XLR connector to a high impedance load (like an amp) with a cable that has XLR at one end and a 1/4" connector. There is something wrong, however, with using that same cable to connect a low impedance mic to the amp - because there is an impedance mismatch. Instead a proper low impedance (XLR to XLR) cable should be used, together with an impedance matching transformer.
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/Greg
http://www.BlowsMeAway.com
http://www.BlueStateBand.net
Last Edited on 23-May-2009 11:39 PM
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Buddha
485 posts
May 23, 2009
6:06 PM
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That's great info Greg. Thanks!!!
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apskarp
5 posts
May 25, 2009
4:02 AM
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Thank's for the info Greg! This explains why the PA isn't responding the way it should when I use a behringer's V-AMP2 with SM58 without the impedance matching device. It feels to miss some of the tone. Somehow it still sounds pretty good when I use headphones with it..
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Jim Rumbaugh
46 posts
May 26, 2009
1:17 PM
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Greg said it all.
Only one thing I could change/add, and it's an electronics "nerd" thing, low impedance in low voltage high current, high impedance is high voltage low current. He just turned it around. See... a nerd thing. And like he said, "you don't need to know that"
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rabbit
35 posts
May 26, 2009
1:35 PM
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Thank you for this!
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