Zaph's L18 design, notch filter---Do I miss something?

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I consider building Zaph's L18/27TBFCG 2 way (http://www.zaphaudio.com/audio-speaker17.html). Looking at his XO design, I got a question.

He said he used a series notch filter in the woofer XO to reduce most of breakup garbage peaks at 7 kHz and above. I don't understand why he used an LC notch filter in this case instead of just a capacitor only. According to my calculation, a capacitor only reduces the woofer's higher frequency response more greatly than the LC notch filter because of a steeper decreasing slope. Do I miss something here? (perhaps phase issue?)

I would appreciate any help.

Jay
 
Jay_WJ said:
I consider building Zaph's L18/27TBFCG 2 way (http://www.zaphaudio.com/audio-speaker17.html). Looking at his XO design, I got a question.

He said he used a series notch filter in the woofer XO to reduce most of breakup garbage peaks at 7 kHz and above. I don't understand why he used an LC notch filter in this case instead of just a capacitor only. According to my calculation, a capacitor only reduces the woofer's higher frequency response more greatly than the LC notch filter because of a steeper decreasing slope. Do I miss something here? (perhaps phase issue?)

I would appreciate any help.

Jay

The LC notch filter works because the capacitor has an impedance phase of -90 degrees, and the inductor has an impedance phase of +90 degrees. At the specific notch frequency, the capacitor and inductor have the same impedance magnitude, but since the impedance phases of the two are opposite, the net impedance of the two in series is 0.

In Zaph's design, the woofer impedance is in parallel with only the resistor (R4) impedance at the notch frequency (since the inductor and cap impedances cancel). At this frequency the series inductor (L0) must have a much higher impedance than the resistor (R4) for the notch to provide adequate attenuation.

A parallel cap by itself can only attenuate 6 dB per octave.
 
Now I understand how a series notch filter works. Then my question is how an LCR parallel notch works. Can I still take into account the phase characteristics of inductor and capacitor to calculate the total impedance of a parallel notch filter? Or could I simply use inductive/capacitive reactance formulas and calculate a parallel resistance of them (including a resistor)?

Can anyone explain to me?
 
Jay_WJ said:
Now I understand how a series notch filter works. Then my question is how an LCR parallel notch works. Can I still take into account the phase characteristics of inductor and capacitor to calculate the total impedance of a parallel notch filter? Or could I simply use inductive/capacitive reactance formulas and calculate a parallel resistance of them (including a resistor)?

Can anyone explain to me?

The LCR parallel circuit will notch the load impedance when it is placed in series with the load. At the resonance (notch) frequency, the LCR will have an impedance of R, so the value of R relative to the load impedance at the notch frequency determines the attenuation of the notch.

Here's why:
Using the Laplace transform, the cap has an impedance of 1/jwC (magnitude of wC with angle of -90 degrees), and the inductor has an impedance of jwL (magnitude of wL with angle of +90 degrees), where j=sqrt(-1) and w=2*pi*frequency.

The cap and inductor in parallel have an impedance of:
Z = 1 / ((1 / jwL) + (1 / (1/jwC))).

This simplifies to:
Z = jwC / (1-(w*w*L*C))

The notch frequency is the frequency where the term (1-(w*w*L*C)) equals 0. At this frequency, the parallel L and C form an infinite impedance. So at this frequency, you have R in parallel with an infinite impedance, which results in a total parallel impedance of R.

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By the way, the Seas TBFC/G is a fantastic sounding tweeter. I made a pair of the AR.com kit design a few years ago and was never quite happy with the sound. I replaced the Peerless 812687 tweeter with the Seas, coupled with the Peerless 850122 woofer, and the improvement was huge. The crossover adjustments were only minor as well.
 
Thanks a lot again. This is really educational! The reason why I'm asking how to calculate the impedance of different alignment of components is that I think I can easily simulate XO performance using my computer language (Matlab). I know I can use an XO design software like Speaker Workshop, but I just want to do it myself for fun. I'm a cognitive scientist and do some mathematical/computational modeling in my research. So math/statistics is not a problem to me, but not being an EE or physicist, I don't have much knowledge about this stuff.

One thing I still don't understand is how to use phase properties of caps and inductors to compute the final voltage and phase across a driver unit in an XO network. Maybe a simple explanation is not possible. In that case, I think I need patience to learn things slowly.

Also I want to know how to simulate a driver unit (woofer/tweeter) in a circuit.

BTW, in the above equations why did you use j=sqrt(-1)? Apparently this does not affect the impedance calculation of the notch filter itself. In what situation will it be useful?
 
Jay_WJ said:
Thanks a lot again. This is really educational! The reason why I'm asking how to calculate the impedance of different alignment of components is that I think I can easily simulate XO performance using my computer language (Matlab). I know I can use an XO design software like Speaker Workshop, but I just want to do it myself for fun. I'm a cognitive scientist and do some mathematical/computational modeling in my research. So math/statistics is not a problem to me, but not being an EE or physicist, I don't have much knowledge about this stuff.

One thing I still don't understand is how to use phase properties of caps and inductors to compute the final voltage and phase across a driver unit in an XO network. Maybe a simple explanation is not possible. In that case, I think I need patience to learn things slowly.

Also I want to know how to simulate a driver unit (woofer/tweeter) in a circuit.

BTW, in the above equations why did you use j=sqrt(-1)? Apparently this does not affect the impedance calculation of the notch filter itself. In what situation will it be useful?

Your in luck. I do all my simulations using Matlab as well. I'm an EE doing mostly DSP work so I am naturally most comfortable with Matlab. I have M files for a few crossover designs. PM me and I will send you one on Monday (they're on my work PC) to use as a reference. I do individual driver frequency response measurements in baffle using a Behringer DEQ2496 RTA with an ECM8000 mic, then use the crossover simulation to get a desired response. It's not the ideal method since room reflections impact the response, but I've gotten very good results.

The j=sqrt(-1) term is the imaginary component of a complex number. Impedance is represented as a vector on a two dimensional plane, with the x-axis the real component, and the y-axis the imaginary component. This vector will have a magnitude and phase.
 
You have some m-files for XO designs! That's great. I'll give you a reminder to send me those.

I don't have a measurement setup and won't bother to have one for my first DIY speakers. That's why I chose Zaph's design and wanted to tweak from there if I need something a bit different.

Actually I want to swap its L18 Seas woofer with Dayton RS180---more cost-effective and better distortion performance than the L18 according to people's measurement. I know the two woofers are not interchangeable without XO redesign but actually they are not far different from each other, either, in many aspects. So I'll try.
 
Dave Bullet said:
Hi jimangie,

would you mind posting the crossover details of the 27TBFCG with the Peerless 850122? I also have a pair of AR.com DIYs and wouldn't mind an improvement or two. I believe it is the tweeter that lets the system down driver wise in the original.

Thanks,
DAvid.

Hi Dave,

I was fully expecting to have to redesign the whole crossover. For an initial listen, I replaced the Peerless with the Seas. I didn't use any resistor in series with the tweeter. It was way too bright but with some eq it sounded great. I then measured the drivers and simulated the combined response starting with the AR.COM crossover, using a 2.3" driver acoustical offset.

All I had to do to get a flat response was replace the 4.7 uF cap in the woofer circuit with an 8.2 uF. Then add a 4 Ohm resistor in series with the tweeter terminals. This provides a X-over frequency of 2.5 kHz, and flat response with a reverse null about 15-20 dB down. This is what I'm using now and it sounds very good.

Another option I would like to try to lower the x-over frequency is changing to an 18.0 uF cap in the woofer circuit, changing to an 8.2 uF cap in the tweeter circuit, and using the same 4 Ohm series resistor on the tweeter. This lowers the x-over frequency to 1.8 kHz, gives a flat response, and provides a reverse null of greater than 20 dB. I need to order the 18uF caps before trying this one. I'm not sure the Seas will like the x-over below 2K with only 2nd order though.

Jim
 
augerpro said:
I have a question concerning notches. When does one use the parallel LCR versus the series LCR shunt?

I don't think there's really a rule on this one. It depends on the circuit and what you're trying to accomplish. With an inductive/capacitive load (speaker circuit), the parallel and series notch circuits will attenuate differently around the notch. You really need a simulation to see what each one does in the circuit and choose the one which comes closer to the desired result.

It is advantageous to use the series notch parallel with the woofer for woofer circuits if possible. This is so you don't add a second series inductor in series with the woofer which will raise the DCR. You can get away with the parallel notch in series with the woofer if you use a very low DCR inductor, or you need the extra DCR.
 
Hi Jim,

Thanks for providing the details on the Ar.Com changes. Do you use the updated version of the woofer crossover with the 1ohm resistor and 0.4 mH inductor paralleled - then paralleled with the woofer? or the original which I think just has the 4.7 uF capacitor paralleled with the woofer.

Thanks,
David.
 
Dave Bullet said:


Do you use the updated version of the woofer crossover with the 1ohm resistor and 0.4 mH inductor paralleled - then paralleled with the woofer? or the original which I think just has the 4.7 uF capacitor paralleled with the woofer.


Yes, I started with the updated version of the crossover.

Also, I have the tweeter flush mounted and I'm using a 1 cu/ft ported enclosure for an extended bass response. I believe the kit design has the tweeter surface mounted with a much smaller enclosure.

Jim
 
augerpro said:
I have a question concerning notches.
When does one use the parallel LCR versus the series LCR shunt?

Hi,

It depends where you want the notch, above or below the c/o point.
Below the c/o point you have to use a series (parallel LCR).
Above the c/o point a shunt (series LCR) is far more convenient.

Mulling over the above it should become self evident.

:)/sreten.
 
Jay_WJ said:


Also I want to know how to simulate a driver unit (woofer/tweeter) in a circuit.



Jay_WJ said:


I don't have a measurement setup and won't bother to have one for my first DIY speakers. That's why I chose Zaph's design and wanted to tweak from there if I need something a bit different.

Actually I want to swap its L18 Seas woofer with Dayton RS180---more cost-effective and better distortion performance than the L18 according to people's measurement. I know the two woofers are not interchangeable without XO redesign but actually they are not far different from each other, either, in many aspects.

Hi,

For a) download the free trial version and manual for Basta!.

b) yawn.. yet again.... I'm building this .... but changing the drivers....
You cannot swap the drivers. Every reasonable tweaking option
is described by Zaph so either build it or build something else.

:)/sreten.
 
Jay,

Zaph has the BAMTM design which uses the RS180 with the same tweeter. Suggest you use his BAMTM design rather than be disappointed by the results of driver swapping. Of course the extra woofer will mean the same price as an L18...... If you want to cost save - find an existing RS180 TM design.

Driver swapping is like swapping a small for a large 4 cylinder engine in a car. Yes they both take petrol (gas), yes they both have 4 cylinders - but using the same fuel lines, mounting points, gear box etc... you won't get a decent car. the driver is an integral part.

Cheers,
David.
 
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