RLC Filter for Delta 12 LF

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I use a 25W tube amp with a diy loudspeaker. This loudspeaker is made with a Fostex FE206E + FT17H on open baffle upon a bass enclosure with an Eminence Delta 12 LF. The problem is that the damping factor of this amplifier is not friendly with Delta 12LF and I have not enough dynamic and efficiency on basses. I need a RLC series notch filter before using bi amplification but I'm not sure how to calculate it. Does somebody could help me ? :(


Eminence Delta 12LF :

Re = 6,06 ohms
fs = 45 Hz
Qes = 0,45
Qms = 7,28
Impedance at fs = 93 ohms

Pascal / France
Hi Andrew,

thank you for a so quick response. It's free air, Eminence specifications. I have mounted the driver in a approx. 100 Litres box with 2 vents 7cm x 12cm. The crossover is approx 380Hz. The sound is good, but not good efficiency. I have tried with a 70W solid state and it's better, so I realy think it's a problem of RLC. I don't have time to build an active crossover and money :) for a bi amp at this time, only in the future. For the moment I want try passive. The sound of FE206E+FT17H is fabulous on open baffle. The cross between FE and FT is just made by a 1,8µF cap. I have tried an Eminence Delta 12 LF because if I can make a real good loudspeaker with it, it will be a real good DIY project for all the people who want a great loudspeaker with not spend all their money.

Thank for your help if you can help me.

Also, the values you have quoted are not self consistent. The expression for maximum impedance at resonance is

Re + Re(Qms/Qes)

By your figures this should be 104 ohms, not 93.
Would you please check your measurements or calculations for any errors, as the notch filter cannot be designed correctly otherwise.

Hello Andrew,

excuse me , specs are Eminence specs:

Resonance: 45Hz
Average Sensitivity 1W @ 1m:97 dB
Impedance (RE) 6.06 ohm
Coil Inductance (Le) 1.45mH
Electromagnetic Q (Qes) 0.45
Mechanical Q (Qms) 7.28
Total Q (Qts) 0.42
Compliance Equivalent Volume (Vas)
67.88 liters
2.4 cu.ft
Peak Diaphragm Displacement
Volume (Vd) 241cc
Mechanical Compliance
of suspension (Cms) 0.24mm/N
BL Product (BL) 14.1 T-M
Diaphragm Mass inc. Airload (Mms) 51 grams
Equiv. Resistance of mechanical suspension loss (Rms) 2.27N*sec/M
Efficiency Bandwidth Product (EBP) 101
Voice Coil Overhang (Xmax) 4.8mm
Surface Area of Cone (Sd) 506.7cm2
Impedance at Resonance (Zmax) 93 ohm

Pascal, thanks for the data. These are the raw driver specs, not the ones of the driver plus box. The network cannot be designed without either measuring the box, or calculating the effect of the box. Please supply as much detaila bout the box as you can.

Thanks Andrew,

Box: internal dimensions are w 39 cm x deep 42 cm x high 79 cm. The internal faces are coating ("enduit" in french) with tar. In the box they are 3 renforceiment rectangulars bars (2cmx2cm) and one behind the driver (just under magnet between sides right and left).
Vents: 2 vents 6,5cm diameter 12cm long (positionned 7cm under the driver and at 7cm each of each side, 17 cm between the 2 vents)
On front side,the bottom of Delta 12 is approx at 38/40cm of the bottom of the box.

All external angles of loudspeaker are made with 1/4 of round

Thank you very much for all




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Hello Bill,

I have read on the net that at the resonnance frequency, the power fall down with the peak impedance. For a driver wich have 6 ohms at 200Hz, and wich have 45 ohms at Fs, if you have 100W at 200Hz you have only 13W at Fs. So it certainly my problem with my 25-28W tube amp and the 104 ohms at Fs of Delta 12LF. I have tried my loudspeaker with a solid state amplifier (yamaha of my home theater system) and the problem is not so perceptible/discernible.
I have found a link to calculate series notch filter on the net, but I'm not sure of the quality http://www.mhsoft.nl/SeriesNotchfilter.asp.
and it's why I prefer to ask the question of some experts if they could help me.
The problem for me at this time is that I can't build active filter. For the future, yes, I will build a tube crossover and a mosfet amplifier for basses.
I think that my project could be work very very good if I can find a solution for basses like notch filter. Of course, if someone want to build this project and have a tube amp, it's certainly better tu use a delta 15. Unfortunately when I have start my project, I don't have think of that problem (am i stupid ? :( ) and now I must do with.


Hello boys,

after I have adding a LPad on Fostex drivers (more than 100db) the result is better. And I have added a little bit of "gaze" (in french) in vents. Basses are more present and good. I think it will be better with LRC parallel filter, so if someone have an idea.
On "money" of Pink Floyd, basses are very good but could be better with a little more dynamic, and medium + trebles are precise/definite .


Hi Bill

I assume he is talking about impedance linearisation. But using a reflex enclosure this could get a bit tricky.


From the power that is delivered to your woofer the impedance raise isn't a problem simply because the efficiency of the driver raises at these resonances as long as the voltage the amplifier delivers to it is constant.
Otherwise the amplitude response of any woofer connected to an SS amp would have one "hole" for closed box speakers and two for reflex enclosures.
What probably happens is that your amplifier looses some control over the woofer because it's damping factor isn't very high or some amplifiers are even having troubles achieving a constant output voltage (versus frequency) into complex loads (though this happens usually with short-circuit protected SS amplfiers).

What happens if you try to connect a power resistor of maybe 22 Ohms in parallel with your woofer just as a simple experiment ?

Although your statement that things improve when using an SS amp would lead to the conclusion that it has something to do with the amplifier/speaker interface, it might also be worth checking if the lowpass section of your crossover works correctly
into this driver's impedance.
I.e. connect the speaker to an SS amplifier, connect the amps input to a signal generator and measure if the voltage across the woofers voice-coil behaves as expected (from a guess the resonance frequencies will be well below the crossover frequency of 380 Hz, and not influence the crossover too much, but somewhere you'll have to start). If not, the real culprit will still be the woofer's impedance but not because it is affecting the amps performance but the crossover's working conditions.


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