Adding 2405H to my custom JBL system

[Beowulf57]

Then don't change the LX5s and go back to your initial idea of simply adding on the N8000s or 3106s and the 2405s and be done with it. If you pull the LX5s and start messing around it'll be another 20 years before you get it right again...

30 years ago I had a system that was nearly identical to yours and I improved it and improved it until there was no part of it still in my listening room. Pop in the tweeters and enjoy it. It has worked for you this long.


Widget

Yes...good point, though I am still young enough to experiment. As well, to return to the LX5s if I do try the N1200s would be as simple as disconnecting and reconnecting a few wires, since everything is easily accessible at the back of the top cabinet housing the LE85s. Nothing to open up or unscrew. Maybe, I'll just try it for fun and report back with my results.

First step, I agree is to add the 3106s and 2405Hs to my existing setup and listen and tweak.

4313B: thank you very much for the schematic; no change as you said, but nice to have the documentation.

[Beowulf57]

Well...i have the 3106s and the 2405Hs in place! Preliminary listening: an upper midrange hardness/glare seems to have disappeared and the subjective effect is increased presence and level in the bass region. On the top end: clearly more extension, air and detail revealed, e.g., the shimmer and decay of cymbals which was mostly absent before is now clearly present.

I still have to set the levels of the various drivers to taste, but this is a very good beginning. I may try .01 uF bypass caps (relcaps that I have lying around) across the 1.5uF HF cap in the 3106 and see if that will smooth out the sound.

I'm curious if anyone has knowledge concerning the development of the LE85/2420 midrange compression driver: a JBL spec sheet from 1970 shows a response peak of around 4dB centred between 5KHz-15KHz (the 2440 shows roughly a 3dB hump between 5KHz-9KHz). However, on a 1977 2420 spec sheet, this hump is gone, and although the response rolls off sooner, it is more gradual. I wonder if JBL was using a resonance to extend the response on earlier models and later modifications to the design removed this and extended the response in another manner?

This might account for my impression of a distinct upper midrange hardness/glare which the addition of the 2405Hs has removed, since the response of the LE85s has been reduced substantially in the region surrounding the 8KHz crossover point.

[Zilch]

Yes, it's there on the 2307 horn, less so on the longer 2312....

[JBL 4645]

Beowulf57

Heat rises and I would ether reposition the plasma or LCD what ever, and place it a safe place away from the fireplace otherwise the rising heat will sooner or later add additional heat and it will more like shut down as I have read on the AVS boards with one poor sausage who started noticing it with his flat screen.

Or shut the fireplace down and use radiator to heat the room up during the wintertime period.

Other than that make it loud and make it shake the house.

[Beowulf57]

Yes, it's there on the 2307 horn, less so on the longer 2312....

Ah soo...I've picked up a pair of N1200S networks (good price, $80 ea.) as my cabinets wouldn't adapt easily to the longer horn. I'll give them a test run when I get them and post results.

[Beowulf57]

Beowulf57

Heat rises and I would ether reposition the plasma or LCD what ever, and place it a safe place away from the fireplace otherwise the rising heat will sooner or later add additional heat and it will more like shut down as I have read on the AVS boards with one poor sausage who started noticing it with his flat screen.

Or shut the fireplace down and use radiator to heat the room up during the wintertime period.

Other than that make it loud and make it shake the house.

Thanks for the thoughts...however, I checked out this potential problem and there was no significant temperature increase around the Pioneer Elite Pro-110FD when a fire was running. The fireplace is very deep and when fires are kept to the back, the flu is kept open and the chimney is kept clean I have no problem. The cooling fans in the plasma display have never come on, even with a fire going and I get no soot deposit on the screen.

[Beowulf57]

Well, after some preliminary listening I think the LX5 crossovers may win out over the N1200Ss. I'll do more auditioning with high quality LPs and SACDs, however, the midrange is over-emphasized with the N1200S to the detriment of warmth and the bass region. The deepest bass is fine, but the mid/upper bass balance is not as good as with the LX5.

After studying the response curves of the D130/131 and the LE85 (2420), I think this may be due to the rising response of the D130/131 which shows a peak between 1KHz and 3-4KHz. The 1200Hz crossover point brings this peak into play as the response is down 4dB at 1KHz and 13.5dB at 2KHz for the LF section; with the LX5, it is down 20 dB at 900Hz. Further down at 500Hz, the N1200 is 0dB, whereas the LX5 is -9dB. Good clarity with the N1200 (and good for dialogue on films), but not as well balanced for music, which is after all is said and done my primary concern.

The LX5 seems to have been designed to compensate for the drop off in response and power handling of the LE85 with the HL91 horn below 500Hz. So far, to my ears this is a better combination than using the 1200Hz crossover with the D130/131.

[Beowulf57]

Okay, a little more help would be appreciated. I'm considering the suggestion to try an L-pad in the low frequency section of the 3106 (N8000) that feeds the LE85 to see if that will ameliorate the mid-range peak problem introduced by the use of the N1200S (rather than the LX5). Would this be the L-pad of choice:http://www.partsexpress.com/pe/showd...umber=260-252? Or, should I use a higher quality unit? Suggestions?

This is a bit of a gamble, as I rather thought the problem was the higher crossover point of the N1200S bringing in a peak from the D130/131 as mentioned below.

Second issue: the LX5 I have been using is rated for a 16 ohm load (matching the impedance of the D130/131), however the LE85 is an 8 ohm model. On the other hand, the N1200S is rated for an 8 ohm load, matching the LE85, but not the D130/131. How do these mismatches alter the crossover points/response curves?

Third issue: I had previously replaced the switches in the LX5s with sealed Grayhill units and bypassed the HF caps with 0.1uF MIT multicaps...would this be a good idea with the N1200S if I can get the midrange problem sorted out?

[Mr. Widget]

Re-read post# 13.

That said, these crossover networks are not designed for generic use. Each network was designed for a specific set of drivers. If you read the crossover chart here: http://www.lansingheritage.org/image...comp/page7.jpg

You will see you were already going about it all wrong, but with years of messing around you came up with something you liked.

Anyway, all of that aside, there are no 8 ohm LE85s. Some are printed with 8 ohms and some with 16 ohms. Typically they are ALL more like 10-12 ohms. If you were designing a network, it would make a real difference, but going this route, I wouldn't sweat it. Keep screwing around until you find something you like or you blow something up. With care the risk of driver failure is low... you might create an impedance that your amp isn't thrilled with though.


Widget

[Beowulf57]

Re-read post# 13.

That said, these crossover networks are not designed for generic use. Each network was designed for a specific set of drivers. If you read the crossover chart here: http://www.lansingheritage.org/image...comp/page7.jpg

You will see you were already going about it all wrong, but with years of messing around you came up with something you liked.

Anyway, all of that aside, there are no 8 ohm LE85s. Some are printed with 8 ohms and some with 16 ohms. Typically they are ALL more like 10-12 ohms. If you were designing a network, it would make a real difference, but going this route, I wouldn't sweat it. Keep screwing around until you find something you like or you blow something up. With care the risk of driver failure is low... you might create an impedance that your amp isn't thrilled with though.


Widget

Thanks Widget! The info on the LE85 impedance is useful. It is an interesting experiment...and to find out that so far using the recommended N1200S + 3106 for the D130/131/LE85/2405H does not yet sound as good as the use of the LX5 which was designed for the LE15A/H is intriguing. I'm going back to that combination for awhile to see if i notice anything the "properly matched" arrangement is doing better.

I won't have problems with my tube amps as they are built like the proverbial brick ****house, with industrial grade components. They put out around 30 watts, but I rarely listen to more than 1 watt output...10 watts drives everyone out of the house and it ain't distortion. I've had real fun with the corner loaded horns by cracking the windows of the listening room open about three inches and listening to pipe organ music from outside...the whole neighbourhood has fun with this "horn-loaded resonant chamber" effect..

[speakerdave]

Can you point me to a place where there are details about those horns?

Thanks,

David

[Beowulf57]

Can you point me to a place where there are details about those horns?

Thanks,

David

David,

I've never found specific detailed plans of the design. My father, John Ross Cann (president of Raytheon Canada) and a friend (Henry Northcote, who was the head of the machine shop at Raytheon Canada) built these cabinets back in the late 50's, early 60's.

I may have a rough plan of the cabinet configuration somewhere and if I can locate it, I'll post it.

The horn is exponential. Behind the front firing bass driver is a ^ chamber (looking from above) and the throat of the horn feeds from the front edge of the chamber on both sides (roughly a 1.5" gap). Then the exponential flair passes to the rear, turns at the back of the cabinet (which has a wide "^" shape) and returns to the front flair which is a 90 degree angle to fit into the corner of the room. From the throat of the horn backloading chamber to the mouth, the cabinet has two braces continuously on the horizontal plane, so you have three exponential horn chambers on each side as you look at the cabinet from the front. The wood is 3/4" plywood with mahoghany veneer for the finish surface.

I know this is difficult to describe using words alone, so, I'll try to provide a pictorial view of the design.

It loads the bass driver to 40 Hz and drops off at a rate of 12dB per octave after that point. i have usable response down to 28Hz.

[Beowulf57]

Here are a few more pics of the Brociner modified folded corner horn. The cabinet is about 29.5" high, with a 24" front section across (the baffle is 18" wide by 27" high); the ports are roughly 9.5" across and the side sections are 19.5", with a 9.5" section at the rear. These are exterior dimensions. The cabinet is 20.25" deep. Internally, the ports are 7.875" wide by 27.125" high at the mouth of the horn and the horizontal braces are 8.25" from the top and bottom and 9.25" apart in the centre. These dimensions are for the D130 15" bass driver; the dimensions for the D131 12" bass driver are similar, but the height of the cabinet is a few inches less.

I prefer the 15" cabinet designed by my father as the baffle is fixed by 4 large bolts to an internal steel framework; the 12" cabinet (built by Henry Northcote) uses a large brass mounting plate which is bolted to the front face of the cabinet (the D131 is bolted to this plate).

The diagram give s a rough layout of the interior from the top. This design requires considerable woodworking skill (my father was a skilled craftsman in the use of wood, metal, etc., and was a fine furniture maker as well).

[johnaec]

My biggest concern is what looks to be about a 4 foot delay of the sound coming out the horn sections vs. the direct radiator. It seems like certain frequencies would really be additive while others would cancel.

John

[Beowulf57]

My biggest concern is what looks to be about a 4 foot delay of the sound coming out the horn sections vs. the direct radiator. It seems like certain frequencies would really be additive while others would cancel.

John

Yah sure...about 3.25' path difference (you can calculate the effect at various bass frequencies). There is very little horn output above 150-180Hz. To my ears the effect is not that noticeable (yes a few spots with emphasis that can be heard with cello for example), and less of an impact than the room resonant nodes which result from corner loading in the bass region. I could live with the path difference between the D130/131 and the LE85 (in fact the LX5 crossover has the polarity reversed between the LF and HF sections in an attempt to compensate for the path difference, as per JBL's instructions), so this is not a big issue.

Nice bass transient performance with very little overhang...in the end it works quite nicely.