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Old 13th January 2016, 02:14 PM   #1
Beez is offline Beez  United States
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Default Vintage University Corner Reflex

Hello there,

I have in my possession 2 University 15" C15W woofers and 2 University 6303 Diffaxial Full Range drivers, and a pair of University N2A crossovers.

My plan is to build cabinets for these in a corner bass reflex design. I am limited to 22" on each side so to get the required minimum amount of cabinet volume (8 cu ft)for the woofer as per University recommendation I will need to make them about 3.75' tall minimum, obviously ported. They will then crossover at 350hz to the full range driver which will cover mid and and up frequencies. According to what I have read, these will have to be 100% separated from the woofer enclosures due to the paper cones with a minimum of 1 cu ft of volume so the plan would be a sealed enclosure above which will bring the speakers to about 5-5.5' tall. Big boys! I can upload my plans today, but does this seem to jive? My concern is whether the design will make them too tall and narrow for the woofers for proper bass response. I would consider it a modified Altec 606 cabinet.

May be easier if I upload some drawings.
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Old 14th January 2016, 03:26 AM   #2
GM is online now GM  United States
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Greets!

A true corner cab's baffle goes from wall to wall, creating a crude conical horn baffle, so acoustically much wider.

Anyway, at this height, best performance is with the driver, vent at some odd harmonic of the cab's TL length, so around 0.33-0.349*L [internal height] with the vent around 0.8-0.86*L. This high aspect ratio cab alignment nowadays is referred to as a MLTL, so plenty of variants can be found on-line and superior performing to a BR, i.e. think TL-like transient response except with more gain.

Note that if you sim it as a BR, then the vent will be too long due to the pipe's 1/4 WL pipe action can add significant damping to vent, so if you start with a baffle thickness reflex vent, then it will need to be larger for a given tuning. Best then to use a TL/horn design program that allows offset driver, vent such as AkAbak or similar or just tune it empirically like we did way back then.

With a 350 Hz point, the driver above ideally needs to be within ~3390/350 = ~9.69" c-t-c, but many folks are fine with up to 1 WL apart [~38.75"] depending on the listening distance [LP], so your call as to what to use, otherwise just mount the woofer at the extreme top and the vent at the extreme bottom.

GM
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Old 14th January 2016, 04:29 PM   #3
Beez is offline Beez  United States
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GM, thanks for the note. I spent a little time this morning looking at the MLTL design, and it looks very interesting. I am not a speaker builder so I'm going to try and carefully absorb your comments and hopefully generate an intelligent response! I'll try and post tonight with some more data on the drivers themselves. Any sites of reference you think would be helpful would be appreciated! I happen to have some raw drivers and happen to have a wood shop so looking to have some fun putting a pair of high efficiency speakers together.

Thanks
Bruce
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Old 14th January 2016, 04:37 PM   #4
GM is online now GM  United States
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You're welcome!

Nearly all 'things' [ML]TL related have been developed/documented by MJK and a real pity his software is no longer available: Quarter Wavelength Loudspeaker Design

GM
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Old 15th January 2016, 03:50 AM   #5
Beez is offline Beez  United States
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Default Driver Info

GM,

You may be familiar with the drivers or maybe not, but let me know if this is helpful at all.

The Full Range Driver is the University 6303 "Diffaxial" which is a 3-way speaker design. It crosses over to the tweeter at 5000Hz. and can handle up to 30W.

http://www.hifilit.com/University/1957-4.jpg

I will be crossing these over at 350Hz, with a pair of original Model N2-A crossovers.

"Models N-2A and N-2B Adjustabla L/C Dividing Networks These networks permit any combination of speakers to be used in a great variety of voice coil impedances and cross-over points. Use either network singly as 6 db/octave 2-way network, as 12 db octave low or high-pass filter, or in pairs as 12 db/octave network. N-2A and N-2B can also be used in combination for 3-way speaker systems. N-2A Crossover: at 350 or 700 cps at 8 or 16 ohms."

University Speakers Circa 1963 | Preservation Sound

The woofers will be the University C15W:

Model C-15W l5" DVC Wooter Maximum efficiency is maintained by a special dual voice coil with exceptional depth and excursion. . . Positive transient control being ensured at all times by the exclusive twin spider arrangement and 6 lbs. of Alnico 5 Gold Dnt magnet. Response from below 25 to 1500 cps. 50 watts+ rating. 4-20 ohms, 10"" deep. Shpg. wt., 26 1bs.

The catalog in the link also makes recommendation to sub enclosure sizes for putting these two speakers together:

Use one or two N2-A crossover networks at 350 cycle crossover from new woofer to original coaxial which now functions as an integrated mid hi-range sound source...retains advantage of coaxial design....gives cleaner mid range sound since it no longer must handle the bass at the same time.

The C15W calls for min 8 cu. ft of enclosure space. The other note mentions that since the 6303 (or UC-153) is an open basket speaker it must be in an "air-tight, padded sub-enclosure over the back of the speaker".

In another old document I found it said that port area for 8 cu ft. should be "13 sq in.". Has a table for different cabinet volumes and port sizes but mentions that any cab volume over 12 cu ft will not improve the bass response. Theres some other tips like don't make the cabinet with one dimension more than 3x the smallest and it should have at least 15" of depth.

In any event, as I mentioned in my first post I was thinking about a "pentagon" shaped cabinet drawing on the plans for the Altec 606 corner BR cabinet housing the woofer only and some type of sub-enclosure or separate chamber for the 3-way speaker on top. Perhaps those plans should not be changed as I mentioned I wanted to make the sides shorter and I think they spec at 25.5" and I would like to go to 22" if possible and make up for the volume loss in height. I'll attach those plans.

The MLTL may be a preferred design based on your comments. I wouldn't know where to start and I am not sure the best way to enclose the 3-way speaker. I saw one fellow built them with the 604 speakers, that was the only design I saw with vintage drivers.

Anyhow,

If you have any other thoughts I would love to hear them.

Thanks,

Bruce
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Old 15th January 2016, 04:19 PM   #6
GM is online now GM  United States
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Other than maybe answering specific Qs, not sure I do since all you need to know WRT the basic cab design is already spelled out in my first post, but for a variety of reasons may be some time before I can spend much time on any one thread, so if others don't answer your Qs, then 'bump' the thread as required to send me a reminder as the sheer volume of email I get can/does regularly make some 'disappear'.

For now, all I have is that the C15W's recommended cab is tuned to ~38 Hz, though your signal chain, room acoustics, personal preference normally determines what the best overall cab alignment is and still recommend you make the 606's baffle full width [without its purely decorative 'bumped out' baffle that causes some additional baffle related 'ripple' that can comb filter with the driver's response].

GM
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Old 15th January 2016, 11:22 PM   #7
Beez is offline Beez  United States
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I have a lot of reading to do!

Thanks!
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Old 21st January 2016, 01:24 PM   #8
Beez is offline Beez  United States
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Hello,

After some reading I realized I may have been taking the wrong approach to this potential project. Thanks again to GM for giving me some where to start.

Since I know basically nothing about speaker building and since there are a lot of seemingly complicated factors that go into proper enclosure I wonder if it is worth the exercise or whether the drivers would be better off back on the market.

If worth embarking on enclosure design, what basic elements are key to understand? From the post above you can see I have a pair of 15" woofers and PAIR OF 15" 3-way speakers. The 3-ways are not the venerable 315-Cs so per University's "vintage" recommendation, when integrated with the C15W woofer they can be quite excellent speakers. These are vintage high effeciency drivers.

I cannot find the general T/S parameters for these drivers anywhere online. Would that be something that is key to calculate? There are some resources out there for doing so without the need for substantial test equipment although I do not know the reliability of the results.

In terms of an MLTL type enclosure which was suggested, the most similar design that I saw was for Staephens Altec 604, not sure if that would be a start or not although I could not find the specifics.

Any direction would be appreciated.

Thanks
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Old 21st January 2016, 02:32 PM   #9
GM is online now GM  United States
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Greets!

You're welcome!

Fs, Vas, Qts are the minimum needed and since back then 'competing' drivers to allow other brands to 'drop in' was a marketing 'given', so no surprise the 10-20 ohm C15W's specs are 'close enough' to the then current Altec 803 [nee 416]: https://community.klipsch.com/index....iversity-c15w/

Of course time, use, changes suspensions, so no guarantee these match up to yours, but a simple Fs test is 'telling'.

Without access to my audio 'library' and software programs, I don't recall much about that design beyond it being done 'behind my back' for a published article and it being acoustically too short to be much of a proper MLTL alignment, so basically an optimized ported alignment.

The 604's specs are radically different, so while C15W would probably work it would need a higher ~38 Hz tuning for a corner cab per University as the original is tuned to the 604's much lower Fs IIRC.

Gotta go, another hectic day awaits.

GM
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