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Al Klappenberger

Developing a network for the Forte

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Thanks to one of my local customers I have the lone of a Klipsch Forte. I hope to develop an improved crossover network to replace the stock network. I have been asked many times to design networks for people with nothing more to go on than a list of drivers from all sorts of manufacturers. I almost always have to say no. I hope to use this thread to illustrate what is involved in developing a network so people will understand why I usually decline their request as well as the primary objective to develop a new Forte network. This project is going to take a while and I will need to reserect this thread later after I order parts and build a prototype, if I decide to do that. I have already done a computer design but I don't expect to post it until later.

The Forte model sold for around $1100 to $1200 when new. They seem to go on eBay for about half that figure tops. My experience developing a network for the original Cornwall shows that I must design the new network such that the total cost is not more than the cost of the speaker! This means I have got to cut a few corners. It also means that it might not be a worth-while product for me to offer. It that case I'll just give the design to the DIY community.

The first thing I need to do to develop a network is to evaluate the speaker in its original configuration, then to evaluate the three individual drivers for complex impedance and sensitivity (efficiency).

Here are the plots of efficiency, frequency response and impedance of the stock speaker. The frequency response is very impressive. It goes from below 30 Hz to 20Khz with a nominal sensitivity of 95 db SPL at 1 meter for 2.83V input. That would be 1W into 8 Ohms except that it is NOT 8 Ohms! The impedance is all over the place because of the usual cut-every-corner you can "balancing network" Klipsch put in all the heritage speakers! The impedance comes to a peak of 106 Ohms at 2150 Hz. At 950 Hz it looks to the amp like a 7.19 Ohm resistor in series with a 3 mHy inductor. At 5150 Hz it resembles 5.09 Ohms in series with a 1.68 uFd capacitor. Then at 14.5 Khz it's more like 9.28 Ohms in series with 2.59 uFd. This is all the result of the network. The individual drivers provide much more reasonable loads.

Al K.

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Next, in order to measure individual drivers, I removed the cover from the back of the speaker that has the network attached to it. I made a rig out of 3/16 inch thick masonite to bring the cables to each driver out the back and still maintain an air tight seal so the woofer would work correctly. The square block screwed to it is just to stiffen the masonite.

I will post plots for all three drivers next.

Al K.

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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.

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Here's the squawker.

The frequency response extends up to about 7 Khz which is well about the 6000 Hz crossover. The sensitivity is around 105 dB. To align it with the 95 or 96 dB of the woofer it needs to be attenuated about 9 dB. This is the attenuation provided by the transformer in the stock network. It is the same attenuation as the Klipsch T2a and my 3619 transformer between taps 2 and 0. I expect to reuse the stock transformer.

The impedance cure shows the impedance to be about 16 Ohms at both 600 and 6000 Hz. It's very slightly inductive but not enough to be a problem considering the swamping resistor I will connect across the transformer will make it a moot point anyhow. The huge peaks around 1500 Hz is it's resonance. Modern drivers have their resonance around there rather than below their usable range (like the K55), It's damped by the horn loading and will also be easily overwhelmed by the swamping resistor.

Al K.

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Now here's a pretty tweeter! The response goes from below 2 Khz to 20 Khz with ease. The sensitivity is to low to be used with the big heritage speakers but for this speaker it is just right at 95 dB. No attenuation will be needed. The impedance is a nice 10.3 Ohms at 6000 Hz and drifts up with frequency. The Klipsch network had a 40 Ohm 5W resistor in parallel with it. That combination makes it look like 8 Ohms to the network. I plan to do the same thing.

Al K.

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The hard part of this project is going to be finding a place to put the stuff required to make a proper network. The area where the old network was located is only a few inches deep because of a wood partition directly behind it. There is 4.75 inch areas to the left and right, but there will not be a way to get a network into that area through the hole. I may need to mount it below the speaker inside the hollow kick-base. This will require a cable to run from each driver, through the cabinet and down below through holes. This is not easily done by a customer and may put the finished design into the DIY category. It could also be fastened to the back, but it could easily be damaged if mounted there. I will need to get all the parts and do a lot of head scratching about what to do. I expect to order the parts for the preliminary design in a week or so along with the parts for my next batch of network orders. I'll at least make a bread-board prototype for testing. Until then, that's about all I can contribute to this thread.

Al K.

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On second thought, here a plot of the woofer and the passive radiator port. These were plotted with the mike as close to the dust caps as I could get, about 1/4 inch.

AL K.

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Here's a impedance plot of the extreme low end. This shows the enclosure tuning. Notice the Zo phase is zero right at the impedance dip. I think this indicates correct tuning.

Al K.

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Al. Nice to see this. I like the idea of the network outside the cabinet even though it may seem a bit unorthodox. Anything sizeable put inside could affect the volume and tuning of the cabinet.

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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.

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Forget about the little opening -- they can remove the passive radiator and mount it to the bottom of the cabinet.

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Dean,

Going through the passive radiator is a good thought. I didn't think of that. There's a partition running diagonally right behind the network hole that obstructs the bottom, but it might give me access to some other spot inside. I am going to have to look. You can see that partition inone of the pictures. It's covered with foam padding.

Al K.

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Nice work Al, as you always do.

If I may comment (probably about things Al knows already). I believe the impedance is always purely resistive at a dip, at least at the bottoming out of the overall magnitute of impedance. Here there are inductive and capacitive affects in the circuit, but they are perfectly balanced.

One thing you're showing is the dip in the woofer resonance. The valley between the almost equal twin peaks is the action of the passive radiator. In a ported system you see something very similar. It doesn't indicate a perfect tuning (whatever that might be) because the same zero phase (resistive) point always occurs someplace. It does show the overall tuning in frequency which is a target for desired performance.

The other situation of a dip in woofer electrical impedance occurs about an octave above resonance. The impedance magnitute bottoms out. Then above that the voice coil inductance causes a rise in impedance magnitude. Again, your phase angle plot goes to zero there.

Interesting, this dip is generally the same as the d.c. voice coil resistance. Again this seems to be shown on your diagrams.

It is interesting that the dip at resonance has an impedance higher than the voice coil resistance. I believe this is because the passive (or port) is adding an acousic load.

Incidently, if you measure a system with a peak in electrical impedance, that too will show a zero phase condition at the peak. I think other of your tests show this.

Best, and many thanks for sharing. I don't mean to steal your thread.

Wm McD

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