Re: [Harp-L] harmonica microphone cables - impedance matching

[Greg Heumann <greg@xxxxxxxxxxx>]

John, let me start by saying "don't feel bad, this confuses everyone."

You see an XLR connector and assume that the mic is low impedance, therefore needs an impedance matching transformer. I wish it were that simple. In fact the BluesBlaster is a high impedance mic. Worse yet, there are TWO standards for how high impedance XLR jacks are wired, called "Pin 2 Hot" and "Pin 3 Hot". Both of these are called "unbalanced" wiring.  And neither is wired the same way that common, world standard low impedance XLR is wired, which is "balanced".  The cable you need has XLR at one end and a 1/4" connector at the other - and must be pin 3 hot. Many guitar stores carry these cables and don't have any idea of they're pin 2 hot or pin 3 hot, or why it might matter. And I've seen these cables with no indication on the package of which way it is wired. Just make sure you know you're getting a Pin 3 hot cable. 

Where people get into trouble is usually when they use one of these high impedance unbalanced cables and use it with a low impedance mic (i.e., to plug an SM57 straight into an amp.) Physically they can be connected, but they are not "right". Not only is there an impedance mismatch, but the difference between "balanced" and "unbalanced" wiring means you only get half of that mic's output delivered to the amp, with potentially significantly different frequency response than the mic's specs say it has. The right solution in THAT case (low impedance mic to high impedance input) is to use a low impedance (XLR<-->XLR) cable and an impedance matching transformer at the amp end of the cable.

/Greg

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> From: John Kapetan <kapetanj4696@xxxxxxxxxxx>
> Date: March 23, 2012 9:31:02 AM PDT
> To: harp-l@xxxxxxxxxx
> Subject: [Harp-L] harmonica microphone cables - impedance matching
> 
> 
> 
> Hi all, 
> 
> How much of a 'sin' is it to plug in a Shure blues blaster mic to a Music Man tube amp without using an impedance matching transformer (I realize it wouldn't really be considered a sin as much as it could possibly be considered poor practice). :-) 
> 
> Thanks, 
> 
> John Kapetan