[Kenwood] With a Turner 454X mic work with a TS120S?

[Richard Knoppow]

----- Original Message ----- 
From: "Michel Bartolone" <MBartolone at wi.rr.com>
To: <Kenwood at mailman.qth.net>
Sent: Sunday, March 20, 2011 11:38 AM
Subject: [Kenwood] With a Turner 454X mic work with a 
TS120S?


>
> I've picked up an old Turner 454X mic for a couple of 
> bucks at a swapfest.
> I will need to change the connector, but the wiring 
> diagram is on the
> bottom,
> so that's not an issue. The issue is that this is a high Z 
> crystal mic.
> Will this
> work or can I build a simple impedance match to get it to 
> work with my
> old Kenwood TS-120S?
>
> best 73s
> Mike (NX9A)

    I'm not familiar with the TS 120S but in general crystal 
microphones like to work into very high impedances, 
essentially an open circuit. That is because they look like 
a capacitance. The capacitance of the mic forms an RC high 
pass filter with the resistance of the load so if the load 
is too low the low frequencies are attenuated. Sometimes one 
can simply put a high value resistor in series with the mic. 
Because the output level of most crystal mics is so high the 
loss of overall level from the resistor will not be 
significant.
    Matching a crystal mic to a low impedance input is best 
done with a transistor, for instance a emitter follower or 
source follower FET. No gain is necessary, just the 
impedance change. Because crystal mics are also sensitive to 
the capacitance of the cable, which acts like a low pass 
filter, putting the matching circuit close to the mic is 
desirable. This is the arrangement in the amplified version 
of the D-104, the amplifier is really mostly an impedance 
matching circuit.
     The easiest thing to do is just try the microphone. If 
it sounds thin or tinny you need the impedance matcher but 
you find that it works fine as is.


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