|[AMRadio] Speech processing technologies and methods?|
jayw5jay at cox.net
Fri Dec 20 13:30:21 EST 2013
Hi Don and all,
Another way to unload the D-104 crystal element is to use an FET follower.
If you go to W9AC on QRZ.com scroll down about half way and you can see how
to roll your own. http://qrz.com/db/W9AC The FET will multiple the source
gate resistance making a D-104 sound the best it can. I have stripped out
the stock pre-amps boards before and used Paul's design before with very
good results. Most of the time the person on the other end will say they
can't believe it is a D-104. Also notice the extra decoupling along the B+
line. Even though a FET follower is unity gain, I find many times I can
bypass the first stage of audio lowering hum and noise even more.
If you are not into rolling your own, but can solder up 4 wires, Paul's
daughter is putting out these. Sort of a way to gain a few buxs while going
to college. Plus save you from going to Radio Shack and getting all the
answers while shopping for parts that have gone through the roof!
The usual disclaimer here guys, no kickbacks, no interest or involvement.
Just good way to make a D-104 sound better, less noise, and with minimum
73 de w5jay/jay..
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Donald Chester
Sent: Thursday, December 19, 2013 12:00 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Speech processing technologies and methods?
The UNAMPLIFIED D-104 is an excellent choice when using vintage amateur
radio transmitters. But one note of caution: the D-104 or any other crystal
mic should work into a minimum load resistance/impedance of around 5
megohms. This is according to the Astatic company in documentation they
published for decades. Working the mic into less than optimum load
resistance kills the lower frequency range of D-104, which is actually
pretty good, and makes it sound more like a tin-can telephone.
A crystal microphone is approximately equivalent to an ideal a.c. generator
with a 500 pf capacitor in series. Naturally, with 500 pf of series
capacitance, the lower frequencies will appear only when the load
resistance is extremely high. Unfortunately, many of the 1950s and 60s
vintage amateur transmitters used a much lower grid resistance in their
first audio stage. It is not uncommon for the stock grid resistor to be only
one or two megohms or lower. I recall a popular transmitter but can't
remember which one, that uses a grid resistor on the order of 100K!
I would recommend changing the input resistor at least to 4.7 megs, and
preferably up to 10 megohms, although higher values of grid resistance may
result in unstable grid bias and plate current, and some individual tubes
are more sensitive to this than others. The RCA receiving tube manual
recommends a grid resistor of no more than 0.5 megohms for most tubes, but
up to 4.7 megs seems to work OK with the majority of good tubes. Check the
voltage across the cathode resistor if one is used, and see if this changes
with the higher value grid resistor. Another good test, and one that can be
used with circuits that don't use a cathode resistor, is to measure the
voltage right at the plate pin, using a good quality DVM or VTVM that does
not load down the plate resistor.
My own D-104 works into 20 megohms of resistance. This is accomplished by
wiring the mic for balanced output and feeding it into a push-pull pre-amp
using two tubes carefully selected for balance and absence of hum, with a 10
meg grid resistor for each tube. Instead of the two sections of a 12AX7, I
use a pair of 6F5 octal-base tubes, which are electrically identical to one
section of an 'AX7, but separate tubes allows for selection for a matched
pair. I used to run a dynamic microphone mixed in with the D-104, but this
system has worked so well that I haven't used the dual microphones for over
a couple of years.
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