New two-way ALtec horn design, questions

[atilsley]

Hi, I've only spent a little bit of time on this forum...but I appreciate the threads and knowledge of others.

I'm using stone horns from Stereolabs (Germany), 250Hz model crossed at 500Hz, plus a 1,000Hz tweeter horn crossed at 5,000hz. Drivers are two inch selenium and one inch respectively. Xover is Bob Crites unit for Klipsch.

Bass cabinet is 160 liter (5.6 cubic foot) in 25mm MDF, heavily braced and internal angled baffles, plus wool dampening on three sides. Ported to floor. Driver is 416-8C from GPA (should arrive next week).

Running two small tapped horns from 30Hz to about 60Hz. 300-watt O-Audio BASH amp.

Weston Acoustics (Melbourne) 300B "Time Machine" SET amp to drive the horns, plus a separate 100-watt pro-amp to drive the 416's.

Now, this multi way setup does sound good (even without the 416's installed)...but I want to change it to a full Altec "two way" system.

I want to run a GPA 902 one-inch driver into a horn (not yet decided)....and use the new GPA 1200 adjustable crossover. I'll keep the existing bass cabinets, and/or incorporate these into a new taller cabinet in order to house the horns. If I did this, I'd increase the cabinet volume to around 240 liters (8.5 cubic feet), which is better for the 416 to get down to a targeted 40Hz. I would use Euro birch ply (19mm) to build the 'outer' cabinet....and open up the top of the existing black cabinet.

I love the look of the Shindo design...and I could build that sort of cabinet. I previously built a Stonehenge V cabinet using a GPA 604 III, which sounded great.

I'd appreciate any thoughts/ideas on this.

Kind regards.

Andrew

[atilsley]

Altec 1204B comes to mind for what I might end up with, size wise.

With the use of a 511 horn, I guess the Model 19 will be the closest overall sound/presentation....albeit taller/less narrow cabinet.

[Altec Best]

Do you still have the 604 III ?

[atilsley]

Do you still have the 604 III ?

No. A gent in Adelaide, South Australia has them in the Stonehenge cabs.

[GM]

...but I want to change it to a full Altec "two way" system.

I want to run a GPA 902 one-inch driver into a horn (not yet decided)....and use the new GPA 1200 adjustable crossover.

The XO will limit you to either a M19 configuration or A7 with either a 511 or MR994, so if keeping your present cab, then it seems that the 811 with the same vertical and depth orientation of the drivers as the M19 is your only viable option without a larger horn extending proud of the baffle.

GM

[atilsley]

Thanks GM, I was hoping you'd read this thread.

So, to understand, the 811 provides a higher xover advantage than the larger 511?

With cabinet depth, I don't have a real constriction...so I could accommodate the 511 if this was more desirable....!? The main objective for me is to try the two-way concept with a small format comp driver.

For bass, do you have an ideal cabinet size you'd accept for the 416? (remembering I can run the tapped bass horns (2) for the 20-40Hz range).

Also, have you had any experience with the new GPA adjustable xover? It seems like a pretty good investment for what is offered.

Kind regards

Andrew

[GM]

You?re welcome!

Depends on the desired performance parameters. Both horns have the same HF design, only the LF sections are different.

The larger 511 will control directivity to a lower frequency with the trade-off of higher HF damping due to increased acoustic loading, collapsing directivity [beaming] in the process. With a high XO point, it will also have a flatter frequency/polar response due to less compression horn loading, acting more as a wave-guide.

If max efficiency isn?t a goal, then depending on the desired top octave response, the 511 can be the best choice overall for a high XO point.

You miss my point, it?s not cab depth that?s the issue, but the location of the horn?s acoustic center WRT the woofer?s that AFAIK is ?cast in stone? by the XO?s design. For a longer horn with the same vertical horn mouth/woofer physical alignment then, the XO would need to be designed for a different horn/woofer offset; otherwise the 511 must ?overhang? the woofer/cab by however much it?s longer than the 811.

My understanding of ?ideal? point source driver loading is that it wants to ?feel? a matching acoustical impedance on both sides of its vibrating membrane, so either an OB or its folded up compression horn variant as defined by Fs*sysQ/2 are the only two ?ideal? alignments.

Unfortunately, both are normally impractical at best in any confined space smaller than a typical school auditorium/channel, so like most things, you choose your performance goals, then which trade-offs you?re willing to accept to get as close as practical to them.

The ideal? then is to find the alignment that is flattest in room at the listening position all the way down to Fb, but without mapping the room?s response, this usually boils down to making the cab as large as practical and tuned to some low frequency, which is usually Fs for a sysQ < ~0.4 and lower if > ~0.4, then using some form of EQ to individually dial in each speaker.

No experience with any of the Altec/GPA adjustable XOs beyond playing with settings on some Altec consumer model speakers and not liking what I hear when any 15? is used this high with a separate horn, no desire to get any.

GM

[atilsley]

GM, thanks.

I now get the point re location of horn.

So, to simplify our discussion, I want to cross over at 1200. As such, is my project like a M19 albeit slightly taller?

With the 1200 xover point, is there any gain in going for the larger 511?

Oh, one other thing, I asked about ideal cabinet volume for the 416. Any thoughts? At present I have 160 liters, but with a rebuild, this can go to 240.

Kind regards

[rontec]

Oh, one other thing, I asked about ideal cabinet volume for the 416. Any thoughts? At present I have 160 liters, but with a rebuild, this can go to 240.

Kind regards

Another consideration in building new cabs....the 511b will require a minimum of 24". Using the 811b you can narrow the cabs another 5"-6". I prefer the 511b in larger rooms but regardles of which horn I would focus much effort on EQ ing the horns(a tip from GM in the past). IMO this is where the mod.19's crossovers came in to play and were so responsible for that great sound! (love to hear the GPA's too) However, as GM pointed out phase alignment may suffer a little with the 511b. A crossover mod could help though.

[GM]

You?re welcome!

Again, if using the GPA XO, then only if you use an 811 in the same location referenced to the woofer location.

I already answered this and don?t know how to explain it any simpler, so with what few details you?ve posted, I?m guessing not in your app.

Huh?! I gave you my thoughts! Guess I gave you too many, so if 240 L is as big as you can go, then do it and initially tune it to ~32 Hz and fine tune it from there if required.

GM