[Elecraft] D-104, only slightly OT

[Paul Christensen]

> "The Astatic D-104 was designed as a *communications* microphone with a
carefully shaped frequency response for speech and not for extreme high or
low frequency response."

Although the presence rise was a design attribute of the mic, limited 
low-end response was not.  The D-104 was produced in the early 1930s as a 
communications microphone intended for reasonably good, balanced audio 
response in the AM mode -- not SSB.  SSB as used in radio communications is 
a post D-104 phenomenon.  Use of the D-104 pre-dates common SSB use in the 
amateur service by 20 years.

According to Astatic, their 2-stage buffer amp was designed as an active 
impedance transformer as low-Z solid-state devices were being ushered-in 
during the 60s   Astatic never intended for the preamp to be used as a 
"power mic" device.  It was the Citizen's Band operators who made the added 
gain function popular.  That said, Astatic could have designed a much better 
buffer for the D-104.

> "The graph of the audio response published by Astatic, using the built in
amplifier, shows 0 dB at 1 kHz. Below  1 kHz the output drops off smoothly
to -5 dB at about 200 Hz, then more steeply down  to -10 dB at 100 Hz where
the published curve ends."

The typical D-104 with 2-stage preamp is substantially more response limited 
than that shown in Astatic's graph.   The response plot shown in their 
instruction sheet is hardly a scientific measurement and the response can 
vary considerably across cartridges.

> "That roll-off is important since excessive low frequency response robs a
signal of intelligibility and "punch" since the bulk of the energy, but
virtually none of the modulation in the spoken voice is down in those
ranges."

True of weak signal communications.  Not true when the SNR is high - and the 
reason for the disclaimer at the end of my post.  If your theory is correct, 
I doubt Elecraft would have included ESSB as a design feature into the K3, 
The lowest fundamental of the deepest male voice can be measured as low as 
70 Hz -- and is typically 75-85 Hz.  It's not that spoken voice does not 
produce frequencies that low, it's that historically, SSB transmissions have 
been deliberately bandwidth-limited.

> "That rising characteristic to a peak in the roughly middle point of the
speech audio spectrum is what made it so effective in communications and
made it so popular."

The D-104 became popular for a variety of reasons including cost, 
aesthetics, availability in the golden age of AM, reasonably good and 
balanced frequency response, etc.

Paul, W9AC