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0) Definitions & Circuit
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fn : notch frequency in Hz
depth : notch depth or attenuation in dB
Bridged Tee circuit:
C1
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.-------||--------.
| || |
| |
| ___ ___ |
In o-+--|___|-+-|___|--+-o Out
R1 | R2
|
---
--- C2
|
|
===
GND
(created by Andy´s ASCII-Circuit v1.27 beta www.tech-chat.de)
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1) General Case
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1
fn = ---------------------- [Hz]
2·PI·sqrt(C1·C2·R1·R2)
( C1·(R1+R2) )
depth = 20·Log(--------------------) [dB]
( C1·(R1+R2) + C2·R1 )
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2) Simplified Case: R1=R2=R
----------------------------
1
fn = ------------------ [Hz]
2·PI·R·sqrt(C1·C2)
( 2·C1 )
depth = 20·Log(-----------) [dB]
( 2·C1 + C2 )
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3) Step-by-Step Design Procedure
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a. Choose C1 to your liking (1nF to 10nF are good starting values)
b. Choose C2 for the desired notch depth according to the following table:
C2 depth [dB]
---------------------
1 x C1 3.5
1.5 x C1 4.9
2.2 x C1 6.4
3.3 x C1 8.5
4.7 x C1 10.5
6.8 x C1 12.9
10 x C1 15.6
15 x C1 18.6
22 x C1 21.6
33 x C1 24.9
47 x C1 27.8
68 x C1 30.9
100 x C1 34.2
c. Calculate both resistors as:
1
R1 = R2 = ------------------ [ohms]
2·PI·fn·sqrt(C1·C2)
where fn is the desired notch frequency in Hz
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4) Mods & Tweaks
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a. Frequency tuning
If you move both resistors equally you change notch frequency
without altering the notch depth. A dual ganged pot can be used
for this purpose. Don't forget to add a series resistor on each
pot to limit minimum value.
My recommendation here would be: a dual ganged 100k LOG taper +
1kohm in series with each pot, allowing a frequency tuning range
of 100:1.
b. Notch depth adjustment
You can change the notch depth WITHOUT altering the notch frequency
if you change the C1/C2 ratio without altering their product (i.e. C1·C2).
In plain english, choose a constant K and then divide C1 by K and
multiply C2 by K. This effectively changes the notch depth keeping
its frequency intact. K values greater than one increase original
notch depth, while values smaller than one reduce its depth.
There is no easy way to accomplish the above with tunable elements,
so the best you can do in this respect is have a two or three position
switch to select from some preset depth values. A six position rotary
switch would be ideal. By the way, to implement zero dB notch depth
or all-pass response you just have to leave C2 open.
c. Use of two independent pots for R1 and R2
This offers an interesting option to control both frequency and depth
but in an unusual manner. Assuming identical valued pots and series
resistors the situation is as follows:
* Whenever both pots track each other (on the same value), you will
be changing frequency maintaining the original notch depth.
___ ___
/ | \ / | \
| ' | | ' |
| R 1 | | R 2 |
\___/ \___/
* If R1 pot is greater than R2 pot you will increase notch depth with
respect to the original desing value.
___ ___
/ /\ /\ \
| ' | | ' |
| R 1 | | R 2 |
\___/ \___/
* If R1 pot is less than R2 you will reduce notch depth with respect
to the original design value.
___ ___
/\ \ / /\
| ' | | ' |
| R 1 | | R 2 |
\___/ \___/
d. Low-frequencies with more attenuation than high-frequencies
Place a parallel resistor with C2 (let's name it Rp). This attenuates
low frequencies, producing an overall increase in the high frequency
content. The resulting frequency response of this filter is akin to a
typical Fender or Marshall tonestack in the sense that you have a
notch and low frequencies are more attenuated than high frequencies.
There are two advantages in this:
* The circuit has minimum possible attenuation, since very high
frequencies pass directly through C1 (0 dB). On other tone stacks
you can have an overall insertion loss in the range of 6 to 20 dB!
* You only need two capacitors and three resistors to implement a
fixed-setting typical tone stack response. On the other hand, if you
replace values on a standard tonestack, you may need up to three
capacitors!
The attenuation suffered by the low frequencies now is:
AL = 20·Log[ Rp / (R1 + Rp) ]
For instance, if Rp is nearly half R1, the lows will have approximately
10 dB attenuation with respect to the highs.
e. High-frequencies with more attenuation than low-frequencies
Place a series resistor with C1 (let's name it Rs), thus attenuating
high frequencies while letting low frequencies pass without attenuation.
This could be useful at the output of an overdrive type circuit, where
some high-frequency attenuation might be desirable apart from the
mid scoop or notch.
The attenuation suffered by the high frequencies with respect to the
lows is:
AH = 20·Log[ R2 / (R2 + Rs) ]
Note that when adding Rs or Rp the depth of the notch and its frequency are somewhat
affected. Nevertheless the proposed design method still provides a good starting point.
) ma io sono un oggettista puro! la prima cosa che mi è venuta in mente vedendo il programma, e le seguenti discussioni è stato che hai bisogno di un sistema molto elastico per il calcolo di generiche equazioni. L'idea è questa: filtri, polarizzazione e qualsiasi altra rete elettrica, in realtà necessita di formule matematiche... allora generalizza il problema!
)!
) potrei semplicemente creare l'XML e dire: copiatelo nella cartella tal dei tali! ... non ti alletta neppure un pò? 
