Forte II ALK Network Build Question/Clarification - Help Needed

[tlarwa]

Well, a year after considering this, I'm finally going through with a build of the ALK Forte II crossovers.  I re-found Al's schematic and board layout, which makes things super simple, but I've run into a question that I need clarification on.  I'm atttaching the scehmatic and layout pics that I'll be referring to.  On the schematic, boxed in by the dashed lines, are shown a .66 mH inductor and a 6.8 ohm resistor.  The box is labeled "K25-K", which I realize is the woofer model of the Forte II.  On the board layout, these 2 compnents are not shown. 

 

So, is this inductor and resistor already part of the woofer?  Or am I missing something somewhere?  I want to order parts, and I'm not sure if I need to order these.

 

Thanks!

 

Tom

Forte II ALK Network Board Layout.JPG.GIF.pdf

Edited December 3, 2014 by tlarwa

[tlarwa]

Also, I can't find a 36 mfd Bennic polyester cap, which is called for on the Forte II x-over in place of the 39 mfd used in the Forte x-over.  Anyone know where I can find these?  I looked on Madisound, Solen and P-E.

[Fjd]

Also, I can't find a 36 mfd Bennic polyester cap, which is called for on the Forte II x-over in place of the 39 mfd used in the Forte x-over.  Anyone know where I can find these?  I looked on Madisound, Solen and P-E.

 

If you cannot find 36 mfd capacitor values anywhere, you can solder capacitors in parallel where the sum of the values equal 36 mfd.  For example, two capacitors at 18 mfd equals 36, four at 9 mfd equals 36, or even three 10 mfd and a 6 mfd equals 36, etc.

[mboxler]

The .66 mH inductor and 6.8 ohm resistor represent the impedance of the woofer at the crossover point (650 Hz???).

 

You don't need to include these in your crossover.

 

Mike

[tlarwa]

 

Also, I can't find a 36 mfd Bennic polyester cap, which is called for on the Forte II x-over in place of the 39 mfd used in the Forte x-over.  Anyone know where I can find these?  I looked on Madisound, Solen and P-E.

 

If you cannot find 36 mfd capacitor values anywhere, you can solder capacitors in parallel where the sum of the values equal 36 mfd.  For example, two capacitors at 18 mfd equals 36, four at 9 mfd equals 36, or even three 10 mfd and a 6 mfd equals 36, etc.

 

Yeah, I understand that, but it didn't even cross my mind to do that!  Thanks for the slap in the forehead   But, in looking at the completed Forte II crossover picture on the ALK site, he's actually showing a 33mfd capacitor in that position (see attachment).  Does anyone know if that's the way these were shipped when Al was still selling kits?

[Fjd]

The .66 mH inductor and 6.8 ohm resistor represent the impedance of the woofer at the crossover point (650 Hz???).

 

You don't need to include these in your crossover.

 

Mike

 

 

Given the work that Al does in developing and testing his designs, I would recommend sticking to Al's design if building Al's crossover network.  Here is the thread that gives more insight into the work that resulted in the design for the Forte I that evolved in a few changes in values for the Forte II.

 

https://community.klipsch.com/index.php?/topic/106102-developing-a-network-for-the-forte/page-1

 

Here are the plots on the woofer alone (in the cabinet). The frequency response is good well beyond the 600 Hz crossover. The sensitivity is roughly 95 dB or a bit higher. The woofer sensitivity and impedance is what I use as the basis to design the entire network. Each driver needs to have higher or equal sensitivity. The resistive part of its impedance becomes the nominal impedance of the network. The impedance plots show the polar impedance at the crossover frequency (600 Hz) to be 6.2 Ohms at +26.6 deg. In rectangular form, that's 5.54 +j 2.78 Ohms. The 2.78 Ohms is equivalent to a 0.74 mHy inductor at 600 Hz. As it happens, that is a rather low value of inductance and trying to absorb it by synthesizing it into the output inductor of a 3rd order filter like I usually do results in a Butterworth (zero ripple) filter. As the driver voice coil inductor does not add to the filter slope I decided to use a 2nd order filter with a higher ripple (Chebyshev) and null out the voice coil inductance with an R-C Zobel instead. It works out to be a 5.6 Ohm resistor in series with a 22 uFd cap connected across the woofer. The nominal impedance of the network will become 5.5 Ohms. This would put the speaker into the "4 Ohm" category.

Those two huge impedance peaks down low are the driver resonance points. This can't be helped. The dip between them is a nice resistive impedance which says the box IS "tuned" correctly.

Al K.

[Fjd]

But, in looking at the completed Forte II crossover picture on the ALK site, he's actually showing a 33mfd capacitor in that position (see attachment).  Does anyone know if that's the way these were shipped when Al was still selling kits?

 

You might try sending Al an email message and ask him about the value used in the picture.  I suspect that if you have his latest schematic before he took down the DIY portion of his site that may be his latest 'thinking' on the design during his support of the DIY effort unless the picture is a result of a modification after the DIY portion was taken down. 

 

From another perspective, while 7.5 mfd is the correct value for a capacitor in Al's Universal design, I have used a 6.8 mfd capacitor in the Universal where I didn't want to parallel the caps since 6.8 seems to be a much more common value.  Al has indicated that the impact on the impedance for the Universal crossover using the 6.8 mfd rathar than the 7.5 mfd is minuscule and since it was within 10% of the correct value and located in a non-critical position, he did not strenuously object.

Edited December 3, 2014 by Fjd

[tlarwa]

Al confirmed that the 36uF cap value shown in the schematic and layout should really be 33uF.