[Boatanchors] Mic's / RED FLAG

[Richard Knoppow]

----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, July 19, 2011 11:19 AM
Subject: Re: [Boatanchors] Mic's / RED FLAG


I have one from the 70s also, purchased new from AES in the 
mid to
late 70s, I used it with a Hallicrafters FPM-300 Mk 2 (a 
real dog of a
rig) but I was under the impression they are around 50K and 
would not
do well with a rig that has hundreds of thousands or megs of
ohms.input Z.
The whole business of what mic works with what rig is kind 
of
mysterious to me.   Except that new rigs all take the new 
mics with a
few hundred ohms Z.


Rob
K5UJ

    I think the difference is mainly gain. Vacuum tube 
voltage amplifiers are inherently high impedance. To operate 
from a low impedance source they require a voltage step-up 
transformer. Transistor amplifiers can be made to operate 
from low impedance sources directly. Most of the older 
microphones were either inherently high impedance such as 
crystal and ceramic, or had a transformer inside the 
microphone. Most moving coil or ribbon mics that provide for 
Hi-Z output have such a transformer.
    One puzzle is what the actual amplifier load is. High 
impedance microphones were made to operate into essentially 
an open grid, in practice a load impedance of tens or 
hundreds of kilo ohms. The source impedance of the mic is 
the key. Hi-Z mics may have source impedances from several 
thousand ohms to the crystal mic which is perhaps a megohm 
or more and looks like a capacitance. Moving coil or 
magnetic mics have essentially constant source impedances 
and can operate into a wide range of load impedances but the 
high impedance ones were also designed to work in to tens of 
kilohmes. Low impedance ones can work into anything down to 
their source impedance although that is not ideal. For 
microphones that show a change of source impedance with 
frequency like RCA ribbon mics a nominally 200 ohm mic wants 
a load of at _least_ about 5K.
    So, the microphone impedance is the _source_ impedance 
and in nearly all cases of passive microphones (no amplifier 
or voltage source) the mic really wants to work into a high 
impedance. The requirement for low impedance microphones is 
that the amplifier has enough gain. A 50 ohm microphone will 
work fine into an open grid but the output level will be 
very low.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com