[Harp-L] Re: "never hook an ohmmeter to a crystal mic element" Why, please?

[BluzeHarp@xxxxxxx]

Inducing voltage into a crystal via an ohm meter  test will usually result 
in damage.  Excess heat, moisture, or  impact are other things that a 
crystal element does not  generally tolerate very well.
 
All ohm meters are not the same, some deliver very low current  and 
voltage, some deliver a lot more... this can be true even in cases  where different 
meters use the same battery type.  The higher  current models will often 
kill a crystal right away, lower currents &  voltages may only weaken the 
crystals output but leave it operable. The same  holds true for the crystal 
pickups that used to be used in  phonograph cartridges.
 
In any case, the resistance of a crystal element is extremely high,  
differences in ohm meter readings of them wouldn't be particularly useful,  even 
if they didn't have this potential to cause damage.
 
The higher voltages used with phantom power will instantly kill a crystal  
element.  If you have any pedals, PA gear, or other devices  which provide 
phantom power, then be sure the phantom is  switched off before connecting a 
crystal element.  Many mixers have phantom  that can be switched on or off, 
some are controllable by individual channels,  some turn the phantom power 
for ALL the channels either on or  off.  Beware, some ribbon mic's can also 
be damaged by applying  phantom power.  Would be a real bummer to melt down a 
vintage RCA  77.
 
You can safely test the output of a crystal with an  oscilloscope.  This 
however won't tell you much about the  tone.  In my opinion, the only 
meaningful way is  to hook up a test cord with alligator clips and plug it into a 
good  amplifier, and then play through it. Better still if you have a test 
shell set  up for this purpose.  This will give a non-scientific but reasonable 
 measure of output, and also let you know about the tone, which can  only 
be measured 'by ear'.
 
Ohm meter testing of a CM, CR, or dynamic element is  safe, but all it does 
is provide the DC resistance.  It's a useful number,  but by itself does 
not guarantee that the element will operate properly.   Generally speaking, 
the higher the resistance, the louder the output, BUT, I've  heard 900+ ohm 
elements that were as loud as 1,100+ ohm examples.  And just  as with a 
crystal, no machine can tell you much about the tonal  characteristics.
 
There's a more detailed discussion of this topic in my free Harp  Mic 
Buyers Guide.  You can ask for it via my Harmonica Planet website, just  mouse 
over the 'Harp Mic's' tab and click the appropriate drop down box:
 www. harmonicaplanet. com
 
Christopher Richards
Soul' Proprietor - Twin Tone Harmonica Microphones
Harmonica & Vocals - The Well Diggers
Co Producer - Greeley Blues Jam