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Khorns with Altec 311-90/290-16K and JBL2404H - help with crossover points and networks


Kudret

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After Q-Man and John Warren's positive comments about this upgrade I decided to do the same and upgrade my Khorns with Altec 311-90/290-16K and JBL 2404H.

I already had the JBL 2404H (one from Shawn and the other from eBay), Altec 290-16K came from George and finally I found a pair of 311-90 horns which should arrive in about two weeks.

I have Type A networks with v-caps in them and I want to continue using my Type As. Now I need to decide on the crossover points so that I can modify the networks before the horns arrive. I was thinking of ~325 HZ and 6500 Hz to make the most use of 311-90/290-16K. Would these be good crossover points? I am hoping that Q-Man will chime in here [:)]. Of course my next question would be how to modify Type As for these crossover points.

Kudret

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I pulled up an old post for you to look at. Go to John Warren's link to his wed site, and you will find a picture of the networks. He called it the Super Klipschorn project. I can mail you the parts list if you want. I still want him to make me another pair, if you want he can order parts for two pairs. I made the mounting boards for him, and I can do that again. The network uses two boards. The woofer choke and input terminal sits outside the speaker.

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Thank you, Q.

So, the crossover points are 450Hz and 6000Hz (please correct me if I am wrong).

I am curious to see the schematic of this network. I sent an email to John to get the schematic as the link ("http://home.comcast.net/~wooferboy/311-90.doc") to the schematic on his website is dead. I believe this is a Type AA modified with a new tweeter filter for JBL 2404H (Here is the schematic of the Add-On filter).

In the past I compared Type A with AA and prferred Type A. Then, I built Type As with expensive V-caps and I would prefer to use them instead of AAs. Here they are:

TypeA.jpg

The question is, can I eliminate the tweeter section (2 uF capacitor) on Type A and directly use John's Add-on filter? If I can, I will just have to modify the squawker section to lower the crossover point to 450Hz. Any ideas about the cap size I should use to lower the crossover point to 450Hz?

Kudret



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10dB down is on a terminated tube. A CD horn will follow this closely, a radial horn will not.

http://alteclansingunofficial.nlenet.net/publications/techletters/TL_231.pdf

http://www.voiceofthetheatre.com/329A.th.htm

The 311-90 is similar to the 329A (used in the graph in TL231), note the Q goes from 5 at 500hz to 40 at 6Khz, a 9dB change in Di (gain).

http://alteclansingunofficial.nlenet.net/proloudspeakers/sectoralhorns/311-Series-Data.pdf

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Yes, that's the graph. You might try 7Khz, you want the driver to be a bit down at the crossover point.

If you want to use a bigger cap you will need to put a resistor in parallel with the driver to raise the crossover point, it's no longer 16 ohms after going through the autoformer.

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Kudret:

I used to also like the type 'A.' However, since you are making the effort to essentially change the entire upper end of the loudspeaker, it may be worth it to consider alterations of the crossover design to better suit the new drivers.

Extracting the autoformer from the equation is often questioned, but for some reason completely changing the horns and drivers is not. However, you can build another network, using an L-pad designed for the impedance of the driver you're using, where the impedance variations imposed by the autoformer become a non-issue. If you like the lower order network, one possibility would be a 6dB/octave with a true bandpass for the midrange, which could be also built using the autoformer (if that's what you would prefer). The network I made has an 18dB/octave slope on the tweeter, and it's fine even with SE triode amps.

My point is just that it may be worth considering something beyond the type 'A' for what you're doing. Regardless of the chosen capacitor, an entirely personal choice, the type/brand of cap is largely insignificant IMO compared to the design of the circuit.

You recently mentioned to me some interest in single driver designs. Have you looked into that any more?

I hope you are doing well.

Erik

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Hi Erik,

These are the same components Q-Man is using and John Warren has a modified Type AA that works very well with them (see John's webpage). My plan was to make the same mods on Type As (this way I would continue making use of my expensive Type As) but this proved to be more difficult than I anticipated.

I also started looking into digital networks which have many advantages. On the other hand, I realized that I won't be able to use my tube preamp and amps anymore.

At this point, I am not sure which way to go. If I can get John to modify my Type As probably that would be the simplest solution.

Single drivers, yes I have them, but that's another story [;)].

Thank you for your concern, I am doing well.


Kudret

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Kudret:

After reading over Johns article on the tweeter upgrade, the changes to either the A or AA are pretty straightforward. You simply need to remove the original tweeter branch of the network, which is everything associated with the tweeter after the autoformer, and then install the crossover for the new tweeter to the amplifier input, NOT after the input capacitor at the #5 autoformer connection. The midbranch remains as it was, except for any changes in attenuation and value adjustments of the capacitor (depending on the changes of crossover point and impedance variations). In other words, I know you can build this network yourself. Also, as far as the tweeter L-pad is concerned, you can also rebalance the level of attenuation to your own liking simply by altering the values of the series and parallel resistors. Everything else will remain the same. Alternatively, you can incorporate a variable L-pad which achieves the same thing by maintaining the original impedance of the driver regardless of level of reduction or gain. John uses two fixed resistors, but a variable L-pad will work just as well. Once you find the level you like, you can always replace the variable control with a fixed L-pad.

I have to say that JBL looks nice. They are expensive, too.

Understood on the single, full-range driver designs. Describing them as 'full-range' is generous and hopeful, but as unhappy as our Lowthers are with orchestral and metal, they are still amazing for classical guitar, woodwinds, and most acoustic jazz.

Good luck with this project; I look forward to your thoughts and impressions when you have a chance to hear the results of the changes.

Erik

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I'm sure this may be a case of not exactly understanding what DJK was referring to concerning the larger capacitor and impedance change through the autoformer, and I absolutely stand corrected if that's the case, but if the tap used on the autoformer remains the same in the presence of a change in value of capacitance, what will change is not the reflected impedance of the driver/autoformer combination, but rather the crossover point. However, if one uses a different tap to attenuate the driver, the impedance in that case WILL be altered, and will subsequently require a change in capacitance (if a swamping resistor isn't used) to offset the impedance difference while maintaining the same crossover point.

Again, this may be a misunderstanding on my part over what was indicated.

ERik

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I received the pdf file you sent for the tweeter l-pad table. You simply choose the values for R1 and R2 based on the amount of attenuation you want/need. The ratio of the different combinations of the two are intended to provide the decrease needed while maintaining the impedance of the driver.

You can also, as mentioned before, use a variable 8-ohm l-pad in place of the two fixed resistors.

Erik

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Thank you, Erik.

I am just thinking of using Type A networks with some attenuation of the mid and tweeter sections for now.

Does anyone know how much I should attenuate these sections? I can use the autotransfomer on Type A to attenuate the mid section, but how much? Do I use another autotransformer for the tweeter?


Kudret

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