From: "Rich Peet" <>
>
> Here is the general scoop.
> The mic actually has two different ground lines.
> Therefore do not ground the mic case to the shield of a cable.
You can safely do this is you are using just one mic. If you use two
mics for stereo the shield will form some kind of feedback loop between
the two mics and you will get a steady, audible oscillation in your
output signal.
> The three connections for the mic are ground, line, and power.
> The line out wire has dc that needs isolation at the very least.
> You have to build using quality rf shielding if you are going to run a
> stereo pair. That means full cages, christmas trees, and short leads.
I've found that you can let the two mics of a stereo pair have floating
(unshielded) housings in nature recording if they are fully insulated
from each other. I do that with my SASS/MKH-110 where they are suspended
in a delrin tube on o-rings. It will not work if you attempt to record
near sources of electomagnetic radiation. You would then need to get
into full cages or whatever. I do shield the cables going to the mics,
just don't connect the shield at the mic connector.
> I will define the heart of the systems I have built and there is
> nothing unique about the rest of the circuit other than what I stated
> above. #3 and #4 are my recommended for 2 or more mics
>
> 1. silicon diode used as a 1/2 volt drop off of a 9 volt battery.
> 2. trim pot used as a 1/2 volt drop off a 9 volt battery.
> 3. LM317T IC off of 2 9 volt batteries.
> 4. 7808 IC off of 2 9 volt batteries.
> 5. dual pnp's used with p48 power.
I use the trim pot method, two mics, one battery, separate electronics.
I started from Sennheiser's wiring diagram for powering MKH-110's, which
I have online linked from here:
http://frogrecordist.home.mindspring.com/docs/my_mod_sass.html
I could not come up with a 8 volt zenar diode locally when I made mine,
so left it out. I did use the filter cap and the resistor is a trim pot.
I adjust the trimpot with the mic connected so that the mic voltage is 9
volts from a fully charged 9.6 volt NIMH rechargeable battery. That
gives the longest run time as the mic will run fine all the way down to
7 volts or lower. I've never really worked out how long a run time I
could get. It's been more than enough on any one trip and then I'd
recharge the battery. And I have spares along. Note with the trimpot you
could adjust for different voltages of battery, Though with the 8.4 volt
NIMH you might not even need the trimpot. The 8.4 volt NIMH battery is
the least common "9volt" rechargeable. I don't have any to try. The 7.2
volt are the more common rechargeables and would be too low a voltage,
the 9.6 used to be available from only one company, though now there are
quite a few putting it out. I don't use alkalines, the voltage of a
alkaline starts about the same as a 9.6 volt NIMH but drops rapidly. It
would pass 7 volts with most of it's capacity unused. In contrast a NIMH
starts at greater than it's rated voltage, the rating is it's voltage
when 80% discharged.
I use mylar caps in the signal output. There is roughly 3 volts DC
across the signal line that must be blocked. Some preamps have blocking
built in, but some don't. Best to not depend on that.
I also ignored the separate 5 volt bias circuit shown in Sennheiser's
diagram. Presumably this is another method of handling the DC on the
signal lines.
Walt
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