N9NEO wrote:



Ok on the phones. You had mentioned them in your email to me in
September. You had also sent me a diagram of an op-amp with a pair of
emitter followers that I could use to drive the filter. Like I said,
I'm probably going to design the filter to run into a hundred or so
ohms. I'll probably stick an op-amp on the output of the filter to act
as a buffer - although 2k probably wouldn't move the filter response
much. Then of course I'll have to have a baxandall /bandaxall style
tone control so the project grows some. Yes, active filters would
probably be ok and LT has some very nice filter chips that would work
too - and I haven't ruled them out.

More later.
NEO

Before trying a baxandall /bandaxall tone circuit, you owe it to
yourself to
at least bread board the circuit at
http://members.tripod.com/roymal/ReverbTone.htm.

I like baxandall for "HiFi" audio but in 40+ years of SWL I have never
needed
to boast the highs, lows and mids seperately. I played with a
professional
1/3 octave EQ unit, that had variable shelving etc last winter and
except for
the complexity it had some minor advantages. But all in all the single
Tone-Tilt
won the test. A tuneable AF notch is a nice addition.

There have been times when I found a very sharp 60Hz notch filter to
very usefull with
older tube radios with less then perfect power supply filtering. These
days I mainly
operate radi from a well regulated and very well filtered 12V. [13.67V
which is
also the proper voltage to float charge a Gell Cell/PbSO4 battery] So
60/120/180
Hz hum is seldom an internal issue.

At some point I am going to have to redesign my system to seperate the
audio from the outboard detector. When I built it I didn't plan on
adding radio
that didn't have 455KHZ as the IF. Best laid plans and all of that.

There are paradoxes or logical conflicts in my statements and the
operation
of my audio chain. I want smooth response out to say 15KHz. But I add
filtering to chop out the highs and lows. I found it better to use a
set of JBL
speakers designed for "Home Theater" 5.1 (or 7.1) side speakers that
naturally
roll off at about 6K. By 10KHz they are mute. I found this works better
then
adding a low level AF filter. The Minimus 7's almost worked with the
tweeter
switched off. Then when I get the harmonic distortion during bad fades
with ths
SADs, I kick in Dalla's ELP AF filter. When an unwanted het pops up I
use
a low level tuneable notch.

When I look at my system I sometimes want to scream. This was supposed
to
be simple! I designed and built a true hifi system that is flat, +/1
1dB from
~30Hz to above 20KHz. With the minimuis speakers and an added tweeter
and
subwoofer it is flat +/- 4 or 5dB from 60Hz to above 15KHz. [It is very
hard to come
up with accurate measurements that relfect reality in a room!] When
doing serious,
IE money making, audio work on the PC I really need the full bandwidth
with the
subwoofer. But for SWL I chop it to something like ~100Hz to maybe
7KHz.
Simply nuts. No wonder my wife insisted on her own radio.

Since my "far field"/ ISM/HiFer beacon is toast, lightning, I have to
be content with
running my test audio from a high bit rate MP3 into my system and
mixing real radio background noise and measuring the result. Since I
can't, and never could, really
duplicate any of the effedts sfading and selective fading introduced,
this isn't really
that much different then my prior test. As a general rule, it is better
to control BW
with the correct IF filter, a speaker with natural roll off or lastly
low level or high
level filters. Conversly the more filtering you add, the quicker you
get listening
fatigue. Listening to AM through a 1.8KHZ SSB fitler will tire you out
pretty fast.
More later when my brain untangles.

And this is all for fun.


Terry