[AMRadio] Readable audio


Donald Chester k4kyv at charter.net
Sat Nov 2 20:17:16 EDT 2013


From: Jim Candela <jcandela at prodigy.net>

>>Running low power AM on the low bands, to be successful, needs to be well
modulated, and equalized to fit in the receivers 5-6 Khz bandwidth. This
does not take hundred of dollars worth of audio processors, equalizers, etc.
What it takes is a transmitter that can modulate cleanly up to 100% (or
higher on upward peaks), a scope to monitor the RF envelope, and just a
D-104 loaded into somewhere around 500K to 1.5 meg (depending on your voice,
and preference). Then phase the audio polarity (try swapping wires on the
D-104) such that any asymmetry in your voice is phased such that the
downward modulation gets the lower amplitude. This allows you to boost the
average (sometimes 2X) without negative peak clipping.>>

Jim,

The D-104 will sound much better if you load it into at least 5 megohms (the
recommend load according to the Astatic data sheet). Mine works into a 20
megohm load. 500K to 1.5  megs is WAY too low. A crystal mic is
approximately the equivalent of an ideal a.c.  generator with a 500 pf
capacitor in series with one of the output leads. To get the full bass
response that the mic is capable of producing, with that low an effective
series capacitance in series with the output, it needs to work into an
extremely high load  resistance.

Astatic recommends a minimum of 5 megohms. Better still, use something
higher, up to 10 megohms. One precaution with a tube type pre-amp is that
many tubes become unstable with a grid resistor that high. The bias may
drift around, causing the plate current to become unstable. For most tubes,
you  will notice that the RCA receiving tube manual recommends no more than
0.5 megohms grid resistance. That appears to be overly conservative; up to 5
megohms doesn't usually cause any problems. I understand that some of the
popular FET preamps allow for 10 megohms or more input resistance.

When you go above 5 megs, some tubes will work perfectly while others may
become flaky. The  secret is to acquire a handful of tubes and select the
one that works best. I still wouldn't recommend going above 10 megohms,
however. My D-104 is wired for balanced output, by using a two-conductor
shielded mic cable, with the two conductors connected to the output
terminals of the crystal element, and the shield grounded to the case of the
mic. The pre-amp uses two tubes in push-pull, with a 10 meg grid resistor
for each. The two conductors of the mic cable are connected directly to the
grids of the tubes. No blocking capacitors, by-pass capacitors or anything
else, other than the grid resistors to ground. 

The tubes I use are type 6F5 metal octals with the grid cap coming out the
top. The 6F5 is identical to one section of a 12AX7. The problem with using
the two sections of a 12AX7 for the push-pull pre-amp is that with the 10
meg grid resistors, the tubes must be carefully selected for the best match
and stability, and minimum hum. Separate tubes allows for a better selection
of characteristics for each tube. The type 6SF5 is identical to the 6F5,
except for pin configuration and grid pin coming out at the base of the tube
instead of the cap on top.

With the 20 megohm load, I found I could get good enough low frequency
response out of the D-104 that I no longer needed to mix in the dynamic mic
to get enough bass response to balance the rising high frequency
characteristic of the mic. The D-104 has a resonant peak in the upper mid
range that gives it a certain amount of "presence rise". That's what makes
the mic sound crispy and so effective with the typical ham radio quality
audio from a transmitter such as the DX-100, Viking I/II, etc. I found that
with good bass response to balance the highs end, the built in presence rise
isn't quite enough, so I added an additional 8 dB or so of presence rise
into the pre-amp. 

For this, I use a very simple treble boost circuit, which consists of a
deliberately chosen low value of cathode resistor by-pass capacitance in
each of the pre-amp stages. The capacitance is selected to give the proper
R-C time constant that results in the desired presence rise. The way it
works is that at lower audio frequencies the cathode resistor is effectively
un-bypassed, which reduces the gain of the stage due to the inherent inverse
feedback, but at higher audio frequencies the capacitance becomes sufficient
to effectively by-pass the cathode resistor, allowing the  stage to operate
at full gain. I use two stages in tandem with this built-in pre-emphasis to
give a total of about 8 dB of boost. Thus, the frequency response of the
amplifier starts off flat from below 40~ and runs flat up to about 800~,
where the presence rise begins to take effect. From there is a steady rise
in gain as the frequency is increased, up to 8 dB boost at a little less
than 2000~, whereupon the response levels off and remains flat from there
all the way to the upper limit of the audio transformers. The actual limit
to the high frequency response is chosen depending on band conditions, using
selectable low-pass filters in a subsequent audio stage, with 3400~
extremely sharp cut-off, 5500~  with more gradual cut-off or no cut-off at
all.

The problem with many of the 50s-60s era ham transmitters is that they use
far too little grid resistance for a crystal mic  like the D-104.  500K to
1.5 meg makes the audio sound tinny because too much of the low end is lost
through the effectively minuscule series capacitance inherent to the
microphone. This partly explains the space shuttle sound of many of the
older Hammy Hambone AM transmitters from the 50s when they are run with a
D-104 and stock audio. 

The original Astatic D-104 element lends itself very well to balanced output
because the package uses a Bakelite or plastic case with two floating
terminals, and the element is non-polarised. Normally, one of these
terminals is grounded, but when wired for balanced output, the  split phase
feeding the push-pull grids is accomplished by the two grid resistors which
the mic element sees as wired in series, with the common  connection tied to
ground, so that the grids are fed 180 degrees out of phase with each other,
with respect to ground. One note of caution; many of the popular substitute
crystal elements for the discontinued stock D-104 element may not work for
balanced output, because most of these have a metal case with one terminal
internally grounded.

The balanced output was not my invention. The circuit was originally
published in the data sheet that came with the pre-WWII versions of the
D-104. Astatic recommended the balanced push-pull circuit when the mic cord
was more than 6' or so long up to as much as 90' or so, to reduce a.c. hum
pick-up. Even with a 6' or shorter mic cord, I find that both the a.c. hum
and rf pickup are negligible  with absolutely no additional rf filtering at
the grids of the first stage. With my old single-ended pre-amp I had to use
an R-C decoupling  network at the  grid of the tube to get rid of the rf
pickup, and run the tube filament on DC to get rid of the hum. My push-pull
stage  runs with normal 6.3 volts a.c. to the filaments, with the midtap of
the transformer filament winding grounded and the filament  leads to the
tubes twisted together.

Don k4kyv



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