Charles Eric LaForest, PhD., 2026
A Bridged-T Filter is a notch (band-stop) filter which is broad, but has the advantage that it is easily tunable with a dual potentiomenter over a range of almost 8 octaves (about 200:1), covering the entire piano keyboard. This makes it useful as a "contour" equalizer and for use in feedback networks as its phase offset at the notch frequency is zero.
I did not write the following design notes, except for additions and edits. I found them hiding in a diystompboxes.com forum thread started by "STM" (Sebastian) on October 07, 2004. The poster is the owner of runoffgroove.com. I wanted to keep these design notes handy and correct some math formatting problems.
And of course, Rod Elliott has good notes on bridged-t notch filters also,
Here is the ASCII circuit diagram of the bridged-t filter. It has the convenient advantage that there is no DC path to ground, so this circuit can operate in a system with a virtual ground (or bias voltage) as-is. While we don't have to worry about its impedance loading the virtual ground, do keep in mind the RC constant of R1 and C2 which determine how long until the output stabilizes relative to the virtual ground.
This circuit must be connected to a low impedance source and a high impedance sink.
Bridged Tee circuit:
C1
||
.-------||--------.
| || |
| |
| ___ ___ |
In o-+--|___|-+-|___|--+-o Out
R1 | R2
|
---
--- C2
|
|
===
GND
(created by Andy's ASCII-Circuit v1.27 beta www.tech-chat.de)
fn : notch frequency in Hz depth : notch depth or attenuation in dB
1
fn = ---------------------- [Hz]
2*PI*sqrt(C1*C2*R1*R2)
( C1*(R1+R2) )
depth = 20*Log(--------------------) [dB]
( C1*(R1+R2) + C2*R1 )
1
fn = ------------------ [Hz]
2*PI*R*sqrt(C1*C2)
( 2*C1 )
depth = 20*Log(-----------) [dB]
( 2*C1 + C2 )
| 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 |
1
R1 = R2 = ------------------ [ohms]
2*PI*fn*sqrt(C1*C2)
where fn is the desired notch frequency in Hz
You can trivially change this process to adjust for a specific frequency given a set resistance, such as the max and min values of a potentiometer in series with a resistor. The range of frequencies is given by the ratio of the total maximum resistance (pot + series resistor) and of the minimum resistance (series resistor alone).
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. The taper must be logarithmic to keep the change in frequency, per octave, constant as you turn the pot, else all the high frequencies get compressed into the low end of the pot range.
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.
You can change the notch depth WITHOUT altering the notch frequency if you change the C1/C2 ratio without altering its 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.
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:
___ ___ / | \ / | \ | ' | | ' | | R 1 | | R 2 | \___/ \___/
___ ___ / /\ /\ \ | ' | | ' | | R 1 | | R 2 | \___/ \___/
___ ___ /\ \ / /\ | ' | | ' | | R 1 | | R 2 | \___/ \___/
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 attenuation of the low frequencies is:
AL = 20*Log[ Rp / (R1 + Rp) ] [dB]
As a simple example, if Rp = R1, then the attenuation of low
frequencies is 6 dB.
The response of the filter now is identical 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:
Place a series resistor with C1 (let's name it Rs), thus
attenuating high frequencies while letting low frequencies pass without
attenuation. The attenuation of the high frequencies is:
AH = 20*Log[ R2 / (R2 + Rs) ] [dB]
Here is a filter meant as a bass guitar contour equalizer and general EQ building block:
C1 = 10 nF C2 = 39 nF R1 = R2 = A100K dual pot + 300R
The notch is -9.5 dB, and the frequency range goes from 78 Hz to 25 kHz.
| R | Attenuation [dB] and Frequency [Hz] | ||||
|---|---|---|---|---|---|
| -1 | -3 | -9.5 | -3 | -1 | |
| 100k | 0 | 19 | 78 | 250 | 473 |
| 8k | 163 | 320 | 1k | 2.9k | 5.5k |
| 1 | 4.75k | 8.95k | 25.4k | 79k | 152k |
The large range means that, if no notch is desired, you can turn the filter all the way up, and the notch now sits above the audio range, with only -1 dB attenuation at 4.75 kHz, which is high enough to not affect the bass guitar sound too much.