Hiyee,
WOW~! Thanks for the nice replies...and yeah...I did try the first
order filter...last time...I parelled a 50k pot with a 10K resistor to
adjust the cut-off...and a 68nF capacitor...sound wasn't that bad...but I DO
need to improve on it...any ideas?? I got the calculations and stuff from
this site...
http://www.t-linespeakers.org/tech/f...ssiveHLxo.html
the 10K value was what I have...should I use another value??
Regards,
li_gangyi
P.S this was/is the kinda discussion that I was hoping for in the first
place...

"Patrick Turner" wrote in message
...


li_gangyi wrote:

Hey hey~! I'm not a Mr. yet~! at least in a "lawful" manner coz I'm
14...and
yeah...I have messed with some filters...but I guess it's my component
values that need changing...coz they dun cut at the frequency that I
need...and I dun use transistors...only passive...meaning I've got no
boost...right?? Can you point me in the right direction to build a
simple...filter...
"Patrick Turner" wrote in message

Master Gangyi, I was only suggesting you TRY to work a
bit more on your own on this one, because you might teach yourself
about the fundementals.
The first thing to remember about RC filters is that a low drive impedance
works
best.
So, build one. It can be emitter follower, or a cathode follower, such as
at the
last stage of the preamp at
http://www.turneraudio.com.au/htmlwe...tubepreamp.htm

The input impedance is high, output impedance is low, and to REALLY
understand
this,
try connecting different value loads to the output and measuring the
voltage
drop
and change to output current.
Then from ohm's law, the value of output resistance can be calculated.

A cathode follower can have an LPF with R, then a C to ground, as the
input
circuit, and then a two stage RC LPF at the output, and this will give
a sharp cut off characteristic, rather than have 3 cascaded RC section
with no
tube.
The cut off is too gradual at first, and to get a high slope of
attenuation
by the time we get to 150 Hz, and above, the initial -3dB pole would have
to be
at
30 Hz.

The only way you are going to know what you got is to measure the response
accurately.
This means you need a sig gene, with a flat response output down to 2 Hz,
and preferably a CRO, but at least signal voltmeter, most digital types
will do,

a pencil, and exercise book, where you can get into the habit of drawing
vertical lines
across the page to indicate where 10Hz,100Hz,1kHz,10kHz,100kHz are,
so that intermediate frequencies can be located.
Try to get some logarithmic graph paper for this, but its a lazy lads way
of
approach.
I just draw up a page with a ruler with approximate positions of
the main F, at 10, 20, 32, 50, and 100 for each decade, and its
easy to guess where 15, 25, 40, 60, 70 will appear, once you see how a log
scale

appears.

Then I use each line down the page in the exercise book to represent
a -3dB drop
in signal level,
so two lines means -6dB which is signal /2, and 4 lines means -12dB,
and 6 lines means -18dB, and so on.

Try plotting the response of an LPF with your sig gene
as the low impedance signal source.
This is really basic, but in my school science studies after age 14, this
sort
of approach in the science lab was taught to me all those years ago.
It occurred to me as a teenager, that it seemed perfectly sensible
to try to describe accurately what I was doing in neat notes in a book.
Once you actually do this sort of thing, you get a clear mental picture
about
filters.

Transistors frequency capability are entirely adequate for
all audio frequencies. Once understood, without fear,
its easy to use them. Same goes for tubes, but its more work to set one
up.
Transistors are boring little black things, but nevertheless,
they do a lot, so no need to ignore them. One can dislike them easily,
and prefer something larger sized, which glows in the dark, but
still we should understand both forms of technology,
to understand strength and weakness.

Opamps are also conglomerations of many transistors within, about which it
sometimes is
extremely difficult to comprehend in one single simple thought.
But the overall fundemental purpose of a chip can be understood,
and its all we need to know for its use.
Same could be said about an old radio, or TV, which uses tubes,
and after time and study, each part becomes understandable.
But not without learning the basics.

OK, so try to think of little things seperately at first,
and then later you will have no trouble joining them together.
Your brain won't fall out of your head trying, I assure you.
Knowledge can only be learned by you, it isn't something given
to you, although the path is laid out for you to learn, if you want to.

Patrick Turner.