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Al Klappenberger

Developing a network for the Forte

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Gil,

What you are actually pointing out is another definition of electrical resonance. that is that when the Xl and Xc are equal they cancell out leaving pure resistance. The point of pure resistance may not be the frequency where Xl = Xc. It will be very close though. This is the case in a parallel resonant qircuit where the Q of L is not equal to the Q of C. It seldom is BTW. How all this relates to the equivalent circuit of a box, a driver and a passive radiator is out of my area of expertice. I remember tuning my den speaker woofer for something like what I see on the Forte woofer plot but I didn't bother to go review it. I suppose I should though.

Dean,

I pulled the passive radiator out of the Forte a while ago. That's the answer to getting the network into the box. I have the choice of severl places to mount it inside.

Al K

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Very informative post here Al. Thanks for the level of detail and clarity. Your explanations of what goes into the measurement of drivers in order to build a proper network illustrates why custom networks require good test equipment and thorough knowledge of what's going on in a loudspeaker cabinet. These are not things to be tinkered with foolishly, that's for certain.

Excellent suggestion on going through the PR to locate the new parts Dean. That would have been my suggestion. That also grants a large access hole without moving any costly or heavy drivers and requires no further cabinet modification.

It'll be interesting to see what you come up with. I'm sure a lot of us are standing by...

Michael

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I did a set of Chorus IIs on site a few years ago, and removing the passive radiator also facilitates getting to the wiring attached to the drivers. One thing I remember is the gasket material for the passive being kind of frayed or dried out. Parts Express sells a great gasket tape for that (part number 260-540).

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Dean,

The gasket thing reminds me of an observation I made about this particular speaker. Our friend Max says the woofer box should be air tight. By that you should be able to push in the passive radiator and see the woofer pop out and STAY OUT! It doesn't. It only takes a fraction of a second to pop back to the center position. The box is NOT air tight. Has anybody ever done that with a new speaker using a passive radiator to see how long it takes for the pressure inside to leak out?

Michael,

Here's a hint: The preliminary design looks a bit like a cross between my Universal and Cornwall netwroks. It's 2nd and 3rd order but with less expensive smaller inductors.

AL K.

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I did a little reading in the books about port tuning. It seems what Gil pointed out is correct. The impedance dip between the two resonance peaks in the deep bass will always go through a zero phase (pure resistive impedance) indicating a resonance somehwere. The trick in tuning is to get the resonance to the proper frequency. The book shows a lot of tables and equations based on the parameters of the driver and the volume of the box to determine where the resonance should be. Anyhow, the moral of the story is that I just have to assume the box is tuned properly. I suppose the deep bass impedance plot just looks typical of any ported enclosure.

Al K.

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Al, I do not have any Forte's to answer your question. But it sounds like you have things worked out with Dean's idea to remove the PR. You can see the speakers I have in my sig.

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mark,

I won't know until I do some layout drawings but I can't see how I can get the new one in that little opening! Under the kick-base may be the only practical option. You don't have a set of Fortes do you?

Al K.

AL,

I don't have Forte's I do have Forte' IIs, maybe this will help I am not sure though.

Jay

post-21005-13819474976298_thumb.jpg

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Jay,

Interesting! The Forte II is DIFFERENT! In the Forte I have here there is a big wood partition right across the area in your picture. I just poped the PR out and took a picture of the bottom area. The network can't go on the bottom. I think it will need to be screwed to the side, internally of course. The new crossover is going to be larger than the one you have shown too.

Al K.

post-2934-13819474983178_thumb.jpg

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Oh well AL just trying to help.

Jay

YOU HAVE HELPED! That sort of info is very usefull when the question of "will the Forte netwrok work in the Forte II" comes up later.

Al K.

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Well, I'm glad I got something right!

Don Keele has a paper which includes info on the alignments. Actually, being a horn loaded bass guy I never studied this very carefully. http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20(1973-05%20AES%20Published)Sensitivity%20of%20Alignments.pdf

My read is that in a B5, the resonance of the box system is exactly at the Fs of the driver. Further, the compliance (spring) of the box is exactly that of the driver and the mass of the air in the port is exactly that of the moving mass of the driver. Given this, the twin peaks should be equal magnitude and equally offset from the dip, which is the same as Fs. Also infered from the table overall is that there is a target resonance which is part of achieving the alignment desired.

It is interesting that PWKs paper on developing the CW starts with an electical analogy of both the box/port and the driver. IIRC he started with the component values being equal . . . and then went on to caution that it is only a starting point.

Wm McD

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Gil,

I think there is a direct relation between box tuning and a basic bandpass filter. All the "sections" of a bandpass filter are tuned to the same frequency. The coupling between them, the termination applied to the load and source end determine the bandwidth and passband ripple. The tuning of the box and port seem to be the same thing. I also see and analogy with the stopband "slope" in that the low frequency limit goes down faster in a tuned box than in an infinite baffle, but the tuned box goes down lower before it drops down. Just like a filter, if it isn't tuned right it comes to a peak. The analogy is that the infinite baffle is a first order "network" and the tuned box is a second order! This is an example of one of the things that fascinate me about science in general. So may seemingly unrelated things have similar ways of looking at them!

Al k.

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The new network prototype is built. Now I need to get the lead out of my butt to start the testing. It means dragging a lot of equipment from one room to the other and my wife always gets upset when I do it!

Anyhow, this is a bare bones design. To keep costs down I have not even painted the board. It is going to have to be mounted inside the cabinet where it can't be seen, so why paint it?

I have also reused the autoformer from the stock network. If it happens that the attenuation is not correct I can drop the 3619 transformer in with no other changes than an adding extra terminal to move the other connection to the squawker off ground.

I have done without the barrier block connection to the drivers. I have simply added a #6 flat washer under the screw to the "L-Shaped" terminals. These should be able to hold the wires to the drivers easily.

All of the resistors are the wire-wound cement types. The tiny inductance of these is insignificant. They can actually be used in any of my network designs where I usually use the film types.

All of the capacitors are Solen FasCaps except for the yellow Bennic mylar in the woofer filter and one non-polar electrolytic in the Zobel network.

I have also used inductors of #16 Litz wire in the tweeter filter rather then #14. The squawker to tweeter crossover section is the same design as in my Universal network otherwise. The 40 Ohm resistor at the output of the tweeter filter was reused from the stock network only because I forgot to order a 39 Ohm resistor when I ordered the parts! I had to splice one of the leads.

The network was quite easy to build. At this point my thinking is that I will offer it as a kit of parts and release the design as a DIY project for anyone who might want to build it.

I did some investigation into what parts Klipsch used in similar speakers (Forte II, Chorus and Chorus II). It turns out that they are all different enough that this design will ONLY work properly with the original Forte.

Stay tuned. I will be starting the testing soon.

Al K.

post-2934-13819475763616_thumb.jpg

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The instrument tests are done.

Here's the impedance plot looking into the speaker as seen by the amp. It's virtually resistive impednace between 4.5 and 6 Ohms.

Al K.

post-2934-1381947577678_thumb.jpg

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Here's the full frequency responseplot. It's still a 95 dB sensitivity. Lower loss in the tweeter filter combined with the energy wasted in the squawker by the old netwrok has made the tweeter a bit hoter. My customer will need tell me if it seems to hot.

post-2934-1381947577776_thumb.jpg

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Here's a blow-up of the 6000 Hz crossover. In "theory" a 2nd order filter inverts the phase. I did this plot to see that the squawker was phased properly. The black plot is with the squawker connected normally. The green plot is with the squawker inverted. The "theory" does not include propogation time difference between the drivers.Clearly the NON-inverted connection is best.

Al K.

post-2934-1381947577832_thumb.jpg

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Southern,

I would like to look at those pictures but "Grumpy" kicked me off that forum claiming I was using it as a sales tool. I would have to reregister under an assumed name to view them. That forum isn't worth the effort! Could you post them again here where some Klipsch people might actually see them! I am particulary interested to see them now becasue I am just about to actually mount the new network inside the speaker.

Al K.

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