Hi Steve, 

3dB is about right. 

For voltage gain dB = 20 log
(G1/G2) 
20 log (52/36) = 3.2dB. 

The original unit was the Bel (sp ?)
which is the log of a power ratio. 
1 Bel = log (P1/P2) 

The Bel is an inconveniently large
unit so the decibel is normally
used. 
1 dB = 10 log (P1/P2) 

Power is proportional to voltage
squared so 
1 dB = 10 log ((V1/V2)^2) or 
1 dB = 20 log (V1/V2) 

Dave


    Rather than just come up with raw numbers
    which we might not know how to interpret
    (like the 3Hz vs. 8Hz figures you came up
    with).

Steve,

You can interpret the raw numbers like this -

The -3dB frequency of the filter is given by
F=1/(2*PI*RxC) ie. F is inversely proportional to both
the resistor and capacitor so if you double the value of
either the R or C the -3dB frequency is halved. If you
halve the R or C then the -3dB frequency is doubled.
What happens at frequencies other than the -3dB point
depends on whether the filter is high pass or low pass.

For Mook’s high pass filter the -3dB point is at 8Hz.
This is a HIGH PASS filter so frequencies ABOVE 8Hz
are not attenuated. Frequencies below 8Hz will be
attenuated at a rate of 6dB/octave (=20dB/decade). If
you double the capacitor the -3dB point will be 4Hz. All
the attenuation is below the -3dB point and both 4 and 8
Hz are already way below anything you can hear or get
out of a guitar so changing the cap should not make an
audible difference. 

For a LOW PASS filter frequencies BELOW the -3dB
point are not attenuated. Frequencies above the -3dB
point will be attenuated at a rate of 6dB/octave.

A bit of (not too serious) theory for you -

The reason the filter ultimately has a slope of -6dB per
octave is because the capacitor impedance is inversely
proportional to frequency. A low pass filter is like a 2
resistor potential divider with the bottom resistor
replaced by a cap. When the frequency doubles (an
octave) the capacitor impedance is halved so the output
of the potential divider is halved which is -6dB (20log0.5
= -6). When the frequency increases by a decade the
cap impedance is divided by 10 so the output of the
filter is divided by 10 which is -20dB (20log0.1 = -20).

Dave 


Doc
    hornerwr@dayzim.com
                   7/30/98
                     7:41
                     AM

As you probably know,
the decibel isn't an
absolute quantity, but a
ratio of two quantities.
When you're speaking of
power, the expression is
decibels equals ten times
the log of the power ratio.
The ratio is the reultant
power divided by the
initial power. When
dealing with sound
pressure level, the initial
power reference is 1
milliwatt. Sounds louder
than that will have a
positive db level and
sounds softer than that
will have a negative db
level. 

It's easy to construct a
table for your particular
application using your
trusty pocket calculator.
Some books on speaker
system design, or theater
audio systems should have
decibel tables.

Hi Steve,

Some comments...

1) A change of 20 dB/decade and a
change of 6 dB/octave are exactly the
same.

Here's how it works: If we have a
change of 20 dB in one decade, how
much of a change will there be in one
octave ? Look at a logarithmic plot,
and you will see that 2 Hz is about a
third of the way between 1Hz and
10Hz.

The relationship between decades and
octaves is given by:

log(2)/log(10) = 0.30103

This means that one octave is 'worth'
0.30103 decades.

Suppose we have a 20 dB magnitude
change for every decade of frequency
change. One octave is worth 0.30103
decades, so the amount of change will
be:

20 dB/decade * 0.30103
decades/octave =
6.02 dB/octave

It seems that audio people talk about 6
dB/octave, because people can hear a
frequency change of one octave.
Control systems people (like me) use
20 dB/decade. Both terms refer to the
same slope, and both are correct and
interchangable.

2) A change from a .022 to .047
capacitor is a doubling in capacitance.
This changes the low-pass cutoff by
only one octave, not two.

Steve, you might want to go to the
MicroSim site and download a demo
version of PSpice. You can run your
own simulations and see for yourself
how these circuits work. Trust me, if
you think Dave Cigna's tone control
simulator is interesting, you'll love
PSpice.

Also, I just got a copy of "Valve
Amplifiers", by Morgan Jones. It has
really, really good explainations of the
various tube amp circuits. It also
describes what those funny numbers
and curvey lines in the spec sheets are
all about.

At this point, you seem to know
enough about tweaking amps. It's time
to get serious about theory. 

(moocow)