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...

Well, the simplest (first order) filter is one resistor and one
capacitor. The resistor is in series, the capacitor is shunted to
ground. Like this (you'll need to have a fixed-width font to see this
properly):

o--------/\/\/\/\-------+------o
R |
|
input ----- output
-----
C |
|
o-----------------------+------o

The value of R should be at least an order of magnitude lower than the
impedance at the output (in other words, in parallel with C). For most
tube amps this will be quite high, 100k to 1M or so, usually the volume
control pot. So, for example, if it's 500k, you'd want to choose R to be
around 50k. Let's choose 47k.

Then choose your corner frequency. Let's say you want the rolloff to
start at 50 Hz. Multiply the frequency F by the resistance R, and divide
that into 159,000 for the value of C in microfarads. In our example,
47,000 * 50 = 2,350,000. Divide that into 159,000 and we get about 0.068 uF.

The formula is derived from the formula for capacitive reactance. The 3
dB corner will be that frequency at which the reactance equals the
resistance R. It's given by the equation:

F=1/(2*Pi*R*C) with R in ohms, C in farads, F in Hertz, Pi ~ 3.1416.

Cheers,
Fred
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