[AMRadio] Microphone recomendation


D. Chester k4kyv at charter.net
Mon Sep 21 21:35:00 EDT 2009


The D-104 or any other crystal mic needs to work into PLENTY of megohms of 
load resistance.  Astatic recommends 5 megohms, but I run mine at 10 megs - 
probably about as high a grid leak resistance that you can use in a tube 
type preamp and maintain stable operation.  I have plans in the works to 
replace the single conductor shielded mic cord with a two-conductor one, and 
connect the mic up  in a balanced circuit for pushpull operation, and use a 
pair of high-mu triodes in a push-pull mic preamp.  That way, each tube can 
have its own 10 meg grid leak to allow for a 20 megohm  load on the xtal.

You can think of a crystal mic as an ideal a.c. generator with about a 500 
pf capacitor wired in series.  In order to get the best low-end response, 
the load resistance needs to be HIGH because of the high capacitive 
reactance in series.  But there are inherent limitations to how high an 
input impedance you can achieve with a tube or FET input device. With the 
tube, if the grid leak resistance is too high, the grid will begin to hold a 
static charge that is not being drained off fast enough, and the bias 
voltage and plate current will drift around, resulting in distorted, flaky 
unstable output.  10 megs is about the limit.

The instruction sheet that came with the earlier versions of the D-104 (the 
ones without the CB "power mic" feature) gave details on how to connect the 
mic up for a push-pull input stage.  The stock xtal element is built as a 
balanced device.  The unshielded bakelite case has two terminals, and either 
one can be used as "hot" or "ground" in the unbalanced configuration. For 
balanced output, connect the two wires from a two-conductor mic cord to the 
two terminals, and connect the shield to a ground point inside the mic head. 
Use a 3-connector mic plug, and at the mic preamp, let each conductor go to 
the grid (or gate) of one of the push-pull  input devices (tube or FET), and 
ground the shield to chassis.  The pushpull output is established by the two 
load resistors (grid leaks in the case of a tube pre-amp) of each amplifying 
device.  They must be very close to equal in resistance.  One end of each 
goes to ground.  The two load resistors in series makes up the load resistor 
for the xtal element, and they act as a voltage divider to provide the 
balanced push-pull audio output for the amplifier.  For a perfect match, I 
would recommend a pair of 10 megohm resistors, or two 5 megohm resistors in 
series for each, and using a DVM, match up the total resistances by adding 
smaller resistors series with the one with the lower resistance, until both 
give identical resistance readings.

Besides improving the low frequency response of the mic, the balanced 
configuration makes it more immune to 60~ hum and rf pickup.  Astatic 
recommends the balanced line for long mic cords more than a few feet in 
length.

Don k4kyv
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