That's where I find my passive speaker filter shines. It passes no audio
section noise and no IF noise, an few DSP HF artifacts.
A fundamental of receiver design is that selectivity works best as close
to the antenna as possible. Unfortunately that ignores the noise
contributions of all the stages after that. The typical product detector
is double sideband so the IF noise of the image is there along with the
signal and the RF noise that passed through the filter plus the same
sideband noise much wider than the filter that was up front. Receivers
would benefit from having a SSB filter at the product detector, but I
know of only one design that way, called the Hohentweil, a 2m
transverter kit. Then they would benefit from making the audio output
stage, often essentially a power op amp into an active low pass filter.
In tube receivers a simple capacitor from audio output tube plate to
ground combined with the tube and the audio output transformer to make a
rudimentary low pass filter. In the 75S-3B, it was effective enough to
make using 2125/2975 tones for 850 shift RTTY (and for all recorded
history, the standard tones for 850 shift RTTY due to an AT&T standard)
difficult until the capacitor was removed from the circuit.
73, Jerry, K0CQ
On 2/26/2011 11:57 AM, kc9cdt@aol.com wrote:
> I think one of the reasons the Drake R-4B, Hallicrafters SX-117 and
> many others are beter in a noisy condition is simply they do not have
> all the high frequency respnse in the audio, or maybe it is the tube
> amp??. I wish there was a HF cutoff on the OII, full EQ like Bob Heil
> recommended day one to TT way back may have helped.
>
> Interestng...last nght, on 40 I was working a really nice guy in St
> Kitt. There was quite a lot of QRN, He was just above the noise floor
> I found that if I used the old Hallicrafers SX-117 to receive
> him...copy was more clear!!!!
>
> OMG, Maybe we need to go back to the older stuff (I have both) Unless
> of course it is contesting at a high level...where you need lot of speed
>
> I use the Collins S line& KWM-2
> Drake C line (all Sherwood mods)
> Halli SX-117/HT-44
> Halli SX-115/HT-32B
>
> Along with the OII of course.....
> 73,
> Lee
>
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