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Old 26th September 2011, 08:58 PM   #1
StevenZ is offline StevenZ  United States
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Default Alpair 7.3 FAST help needed!

Okay guys, I've been tossing around a couple ideas, from small sealed, Dave's CGR, Pensil, etc. I think I've settled on the idea that I want to do a FAST setup using my Mark Audio 7.3's along with Eminence Alpha 15A's. The ideal situation would be open baffle, passive XO network, mated on top of an H-frame Alpha 15A. So here's where I get lost.

Problem 1. I have no idea how to do this. I just can't seem to wrap my mind around MJK's worksheets for some reason. I can build just about anything I have detailed plans to, but can't get into the groove to figure out the exact size of the H-frame, the size of the baffle and placement of driver, as well as the layout/design of the XO network needed.

Problem 2. I only have 1 amplifier, so bi-amping these isn't possible. I'd like to have an XO that can handle both the Alpair, as well as the Alpha 15A with a final impedance load range between 4 and 8 ohms, no less, no higher.

If someone could please help me walk through this project and let me know exactly what needs to be done, or possibly just let me know what sizes everything needs to be, that would be extremely helpful. I can barely read schematics so, a detailed schematic and parts count would be helpful. Any advise you guys can provide would be more than appreciated. I'm struggling to figure this out and could use a really big boost.


Thanks,
Steven Z.
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Old 28th September 2011, 10:46 AM   #2
StevenZ is offline StevenZ  United States
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Never mind, I talked with MJK last night about this topic and he stated that the Alpair 7.3 wouldn't be a good candidate for FAST using the Alpha 15A's due to it's low sensitivity.
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Old 28th September 2011, 06:42 PM   #3
chrisb is online now chrisb  Canada
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Originally Posted by StevenZ View Post
Never mind, I talked with MJK last night about this topic and he stated that the Alpair 7.3 wouldn't be a good candidate for FAST using the Alpha 15A's due to it's low sensitivity.

well if you can active/ bi-amp the sensitivity issue is moot - and the cost of small plate amp etc for the woofers can be surprising low
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Old 28th September 2011, 06:53 PM   #4
StevenZ is offline StevenZ  United States
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Without shelling out more money for separates and a mini DSP, there's no way this is possible with my current amplifier (Pioneer SA-7800). I've asked around for help, but many are hesitant to voice their opinion or lend a hand. So I've fired up cad, drew the program for the Pensil's and will begin the process tomorrow some time, aiming for a Saturday morning completion.
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Old 28th September 2011, 07:21 PM   #5
chrisb is online now chrisb  Canada
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Originally Posted by StevenZ View Post
Without shelling out more money for separates and a mini DSP, there's no way this is possible with my current amplifier (Pioneer SA-7800). I've asked around for help, but many are hesitant to voice their opinion or lend a hand. So I've fired up cad, drew the program for the Pensil's and will begin the process tomorrow some time, aiming for a Saturday morning completion.

Well I'd have thought the risk might be of getting too much helpful advice - of course even after you think the project's completed, it's never too late for that..

Might it be possible to access / modify the circuitry of the SA7800 to (temporarily) re-purpose at least one of the tape loops as pre-out / main in? That could give you a signal to drive an outboard amp for woofers, as well as a connection node for series HP filter to the internal amp section to drive the 7s. (or the other way around)


Re the Pensils - great design for the 7, and a relatively simple build. At the risk of repeating the obvious - at the very least fabricate for removable back panel to adjust damping fill levels, rebate the driver for flush mounting, and chamfer the rear side of driver opening.
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Old 28th September 2011, 07:26 PM   #6
StevenZ is offline StevenZ  United States
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Thanks for the input Chris. I've not the know how to create a pre-out in the amp's circuitry. I'm planning on doing the removable back, flush mounted driver and chamfer on the inside lip of the mounting hole. I don't think I'll use any internal bracing, though. Now that you have me thinking, my options are really opened up when using the mini DSP. I could actually create a huge system from basically nothing with that. Oh the ideas are flowing!
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Old 28th September 2011, 07:46 PM   #7
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Originally Posted by chrisb View Post
Might it be possible to access / modify the circuitry of the SA7800 to (temporarily) re-purpose at least one of the tape loops as pre-out / main in?
Looks like Pioneer dropped the pre-out jumpers on the 7800, which would have been easy peezy for adding extra pre-outs.

jeff
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Old 28th September 2011, 08:59 PM   #8
chrisb is online now chrisb  Canada
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Originally Posted by StevenZ View Post
Thanks for the input Chris. I've not the know how to create a pre-out in the amp's circuitry. I'm planning on doing the removable back, flush mounted driver and chamfer on the inside lip of the mounting hole. I don't think I'll use any internal bracing, though. Now that you have me thinking, my options are really opened up when using the mini DSP. I could actually create a huge system from basically nothing with that. Oh the ideas are flowing!


I'd highly recommend at least a minimal amount of longitudinally oriented bracing struts ( i.e. not horizontal window pane/etc) between the long panels.

And don't forget that along with rudimentary XO controls, most dedicated "plate / sub amps" have high impedance speaker level inputs that run in parallel with your main speakers. While their common ground inputs can cause havoc with some chip or T-maps, that shouldn't be a problem with the 30+ yr old Pioneer. This wouldn't eliminate the need for a speaker level HP filter on the Alpairs in a FAST or OB application, but could very much streamline the set up.

Plenty of candidates here, from less than $100 in a plate style to whatever you wanna spend - the Dayton SA230 looks like a very interesting option


Dayton Audio SA230 230W Subwoofer Amplifier
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Last edited by chrisb; 28th September 2011 at 09:16 PM.
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Old 28th September 2011, 09:16 PM   #9
StevenZ is offline StevenZ  United States
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Roger that.
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Old 28th September 2011, 09:19 PM   #10
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(Pioneer SA-7800).
Pretty easy to do. Sacrifice a tape loop for jacks, and then just reroute the wires that connect the tone amp to power amp boards.

And for cheap 2nd amps keep an eye out at boot/garage/rummage sales, local 2nd hand shops, swap-n-shop for a cheap used integrated or receiver. With eyes open it is quite amazing what can be found for $5-$50.

dave
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