|[AMRadio] Microphone recomendation|
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
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