ALTEC A7-8G special

[alhuds]

If any of you have interest.
I have created a design variation of the ALTEC A7-8G that i would like to build for home use. I would like to hear your opinions. Infact, i would like to find out if i did anything wrong. I have already purchased the 902-8a's and 515-8g drivers, so i am committed to doing a project.

You may download a drawing from my ftp site, it is a small tif file (154 KB).

ftp://parr:[email protected]

it will be the only file in a directory : "als"

Thanks,
a.l.

[marcuswilson]

Very interesting. Looks great.
I guess I would make it taller with the port in the front. Depends how low your seating is I guess.

[jmarkwart]

A.L.,
Looks good to me. What crossover type, frequency, and slope do you have in mind?
Jeff

[alhuds]

I not sure yet.
Cross-Over: 500-800, 12 db / oct.
I really wish to bi-amp when all is said and done.
Kurt Chang of the Chicago Horn Club had some great sounding little 8 watt SS amp that he used in a demo with another members A7s.
I hope when done with building these monsters i can start down that road.
I'm also thinking about ALTEC vintage electronics.
But in the beginning i will pick up some ALTEC N 1285 Cross-Overs and use my (don't laugh) Bang&Olufsen 5000 system.

[jmarkwart]

A.L.,
For a 12db/oct slope you probably want to have an octave of headroom above horn cutoff to stay as linear as possible - that suggests 770Hz for your xover point. With your voice coils in vertical alignment you can easily acheive time-domain alignment and not suffer any polar tilt by using a Linkwitz-Reiley function. Q is .5.
Jeff

[airboss]

Very nice work A.l.:
Reminds me of a 125HP axial flow fan I installed a few years ago. Hehehehehehe....low WAF I fear.
Anyway, keep us posted.

Dan
Fairbanks

[alhuds]

Jeff,

You wrote:

>>>For a 12db/oct slope you probably want to have an octave of headroom above horn cutoff to stay as linear as possible - that suggests 770Hz for your xover point. With your voice coils in vertical alignment you can easily acheive time-domain alignment and not suffer any polar tilt by using a Linkwitz-Reiley function. Q is .5.

Please forgive me, i'm really a virgin here. My hi-frequency horn is designed with a tractrix flare / 380 hz cut-off. i thought (without any logic) i should be able to cross-over at 500 hz?
If i must i will re-model this horn to a new cut-off. Help, what should this new cut-off be???

For the Time alignment i have made a little rib on top of the bass enclosure that the hi horn can slide on.

[jmarkwart]

The cutoff of your 902 horn is fine at 385Hz. My comment was to just to suggest you cross over high enough above cutoff (+ 1 octave for 12db/oct)to avoid throat loading non-linearities that increase as frequencies drops to near horn cutoff. The tractrix curve doesn't load as well near horn cutoff as some others. Bruce Edgar wrote some interesting articles in Audio Amateur/Speaker Builder a few years ago about Tractrix horn loading you might want to review.

[Fitts]

Very attractive design, I like it.

I think I recall reading that the A7 LF horn had a cut off of just below 100 hz. Your plans say that the LF cut off is 140 hz. I think you have enough cabinet depth behind the woofer to make the horn a bit deeper and maybe using that extra room you could lower that cutoff frequency a bit. You might want to make up the lost volume by increasing the overall size but that should be easy.

Keep up the good work !

[rolandr]

A.L:

Great design:

Jeff M remarks about Tractrix horn loading at higher freq than similar sized exponential is accurate. See Hi efficiency speaker asylum posting title Re Eq compensation by Wayne Parham 2/12/03. More details about horn Eq compensation posted there too in the bottom. It makes sense since the last 20% of the traxtrix is basically (wasted) aligned same as part of cabinet. In other words tractrix flare rate is about one octave above similar size exponential. Therefore if tractrix and exponential horns have similar size mouths the exponential will go lower also be longer. Dr Edgar feels no particular benefit in using tractrix in LF horns. Jeff M experience may be different, his comments will be appreciated.

Additionally no need to limit yourself to A7 like performance. If you can have larger/longer LF horn why not go for it!!. The lower the horn cutoff in the LF enclosure the better.

Also why not have the 10 cubic foot rear space venting below close to the floor. The additional 1/2 space =3 dB Bass reinforcement from the floor is helpful in reducing the SPL mismatch between horn and ported output.

Dr Olson's studies of cabinet diffraction in the 50's showed that the best cabinet is a bagel shape with the speaker in the middle. Therefore if you could round out the cabinet edges even better. The 1-2 msec diffraction caused scatter affects imaging primarily.

Sorry to complicate your life further but since you are starting from scratch you might as well go for it!!!

Have no idea how you plan to built it. Composite, fiberglass???