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Message-ID: <200108302059.f7UKxOf21575@galahad.joust.net>
Date: Thu, 30 Aug 2001 16:59:20 -0400
To: rsgb_lf_group@blacksheep.org
From: "Steve Dove" <dsp@hifidelity.com>
Subject: LF: Receivers
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Greetings, 

A couple of months ago the "which receiver?" thing came
up over here, too.  The attached below is a message I sent
one of the guys (he was toying with getting a TS-440S, with
which he is reasonably pleased);  I'd just got a TS-140S as
an LF-specific radio (really cheap) and measured what I
had to hand.  The following evening I measured an IC-735
(a radio I really like  -  probably optimum radio-per-dollar)
which is excellent 160 - 10, but laughably deaf  -  even with
the BCB attenuator removed  -  at 136.  AGC threshold of
about 60uV, if I remember.  A friend's 746 was similar. 

If anyone's interested, I'll get hold of a 756 (which has the
same front-end as the PRO, too) and 'measure' that. 

One issue that hasn't come up in the discussion so far is
that of stability and the means of improving same.  Old
radios which otherwise rock (such as the IC-751a) have
too many (i.e. more than one) crystal oscillators that affect
stability;  mine has been 'retired' for that reason.  By way
of contrast, even the bottom-of-the-line TS-140S has a
single reference, which cooks up nicely.         

        Cheers,                 

                Steve 

Forwarded:

-Hi Phil,

Well, I got to play with the TS-140 this evening.  Thought you
might like to see these numbers:  (Hope they format OK):

Freq.    IC-781   IC-751a   TS-140S

reqd. signal for 's9'
3.6M    56uV     45uV        50uV
200k    56uV     80uV       160uV

agc threshold
3.6M    2.5uV    2.7uV      1.7uV
400k    0.9uV    1.1uV      1.1uV
200k    1.1uV    1uV         1.8uV
135k    1uV       1uV          5uV
100k    1.25uV  1.3uV      15uV

The oscillator I was using leaks too much to do a proper
Minimum Discernable Signal measurement, so I thought
that agc threshold would tell a reasonable story.

The '140 holds it's own with the 'big boys' very well;  the
very rapid drop-off at the low-end is a mystery  -  it's too
rapid for just general giving-up-the-ghost;  it's almost as
though it's the edge of a notch there or something.  I'll 
track it down eventually.

Yes, the '781 is best, but it had better be!

The '140 chucks in an almighty attenuator between 500
and 1600kHz (for good reason) and then very nicely
removes it below 500kHz.  Good news.  The Bad News
is that the (not especially strong) front-end makes a 
pig's ear of all the now un-attenuated broadcasters  -
intermod everywhere.  A low-pass filter or some 
other selectivity is essential to use it on LF.  I threw
in a sloppy thing-in-an-Altoids-tin filter and it breathed
a sigh of relief.

Then, just for good measure, I opened it up and chucked
in the IRC 125Hz filter out of the 751a (such as the one 
you had).  Life is looking good . . .

        Cheers,

                Steve