[Hallicrafters] DD-1 Audio amplifier


Richard Knoppow 1oldlens1 at ix.netcom.com
Fri Feb 24 14:08:15 EST 2012


----- Original Message ----- 
From: "Duane Fischer, W8DBF" <dfischer at usol.com>
To: "Waldo Magnuson" <magnuson at mac.com>;
<Hallicrafters at mailman.qth.net>
Cc: "Waldo Magnuson" <magnuson at mac.com>
Sent: Friday, February 24, 2012 9:37 AM
Subject: Re: [Hallicrafters] DD-1 Audio amplifier


> Was it a "Hi Fi" amplifier or a Dual Diversity RX?
>
    The DD-1 was an elaborate dual-diversity receiver. It
was an amazing product for a company which seems to have
been dedicated to building equipment of good value rather
than exotic, high-end stuff. If I ever had a handbook for it
its lost due to my archive hard disc dying. I don't know if
it had a variable IF; Hallicrafters knew how to make them
and used a variable IF in the SX-28 and other receivers. The
fidelity would depend on the IF bandwidth. However, the
amplifier was intended to drive a large loudspeaker with a
good quality signal. I am suspicious that the performance
might seem poor by hi-fi standards because it had too many
transformers in it and no feedback. Most pre-1945 audio
transformers were not very good, the best of the bunch was
the Linear Standard series made by UTC but they were also
about the most expensive. Western Electric also knew how to
make decent transformers but they were found mostly in WE
equipment. Probably this amplifier had a frequency response
of no more than perhaps 50hz to 10Khz, perhaps less, and THD
of perhaps a few percent. Similar amplifier stages can be
found in a number of "deluxe" radio sets of the time.
     The main advantage of diversity is to reduce the
effects of fading. This includes selective fading which can
cause severe distortion. A more modern approach would be to
use a synchronous detector which does not rely on the
carrier being present and combines the sidebands in the
detected output. The result being a substantial reduction in
the effects of selective fading. A diversity receiver with a
synchronous detector would deliver good audio from quite
poor signals.
    The effectiveness of a space diversity system depends on
the antennas. In commercial applications the antennas were
spaced a few wavelengths apart. Dual and triple diversity
systems were common in commercial long-haul radio circuits.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com



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