Rockin some A15s today

Hi everyone, Long time no talk. Been extra crazy at work but getting back to normal crazy so I finally had some time to build stands and hook my A15s up to the long term reference system and thought I?d share my initial impressions. The short version? pretty damn good but they need some massaging. This is my first intimate experience with a mantaray horn and I gotta say I like them better than the sectorals. To my ears they are much smoother though their pass band without losing the immediacy and snap you expect with a horn system. That being said, I think they are running a couple of dB too hot in the A15s. The first thing I?m going to try is tweaking the high pass down a notch. The woofer is pretty damn good too. I don?t think it has quite the speed of a 416 but the LF extension is nice, subjectively about the same as my KEF 104/2s but notably more efficient. I?m gonna give the 104s the edge in the upper bass detail department but that is a high mark to hit. The A15s win hands down in the dynamics department, with a 50 watt class A SS amp they were hitting 113dBc peaks without a hint of strain. They are also considerably more revealing in the midrange than the 104s, Everything I hate about CD was laid bare to the point I could barely listen to one. Switching to the vinyl front end helped, it?s tuned on the sweet side and made for a nice ying -yang combination. All in all a bunch of fun and I am pretty much committed to seeing what they can do since I carried the things down the stairs by myself and dread the thought of making the reverse trip. :-)

After a day of exploration?There appears some easy to get improvements for home use. The box tuning looks to way way too high at right around 40Hz which is probably fine in a huge space but not for me in a typical living room. Blocking 3 of the 4 Ports tunes the box to roughly Fs. That gets a curve that matches better with typical room gain and flattens out the group delay curve.

I?ve posted a link to download the crossover schematic below. It looks pretty easy to fix what I perceive as a too hot HF driver pass band. The 5R0 series resistor in the tweeter circuit is shorted in the ?High Frequency Emphasis? position. I?m no filter guy but I believe increasing the value of that resistor should knock down the high pass section in the ?Equalized Response? switch position.

Looks like a guy could end up with a pretty nice set of speakers with low/mid 90s efficiency

Altec A15-8A crossover (A15-8Axo.pdf) - PDF Archive

And now for your next entertainment.....the plan details. First, some graphs courtesy of WinISD. Frequency response at 1 watt, cone excursion at 100W input and group delay. Red = All vents open , Blue = 3 vents open, White = 2 vents open and Green = 1 vent open. These are relative, I didn?t get down to the gnats *ss with the volume of the cab but the FR curve shape looks close to published M500 info.

And then?.. the proposed changes to the XO. The nice thing is that the switch they used is actually SPDT push button so I can switch the out to the unused ?T? and when I put the switch in ?High frequency emphasis? position it will be stock equalized response sound (I already said I thought the horn was too hot). And? in the equalized response position it should be padded in the HF ( the value of the always in circuit resistor is a Wild*ssGuess at this point) but the goal is to get to two values that result in 5 Ohms +- tolerance when the switch is closed and sound right when the switch is open.

Hopefully the screen captures will work out

I have WinISD, but it seems to be corrupted and I need to re-install it. I have no idea how to use said program, but in messing with box size and port size, no matter what I did, responce was up or down on either side of f3, but f3 was always the same and was the speaker Fb. I notice that yours are wandering all over the place. I might be wrong though.

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Originally Posted by VolvoHeretic

I have WinISD, but it seems to be corrupted and I need to re-install it. I have no idea how to use said program, but in messing with box size and port size, no matter what I did, responce was up or down on either side of f3, but f3 was always the same and was the speaker Fb. I notice that yours are wandering all over the place. I might be wrong though.

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Sounds like something is wrong with your install. I use pro alpha on a few different machines with XP and have never had a problem. I tried zooming in on the attachment for FR which is the SPL window from WinIsd with one watt input. F3 changes depending on tuning. The green trace has the box tuned to about 24 Hz

Okay, the size limits on uploads make it hard for me to read so here are links to photo bucket

http://i142.photobucket.com/albums/r...ps486914a0.png

http://i142.photobucket.com/albums/r...ps354fd2d2.png

http://i142.photobucket.com/albums/r...ps564a862e.png

Well, I?m sold, the A15-8A can be a damn good speaker with a few minor tweaks. Adjusting the Bass tuning paid huge dividends. I recommend that anyone using them experiment with that. I used 3? expandable plumbing plugs to close off two vents in each speaker and the circa max flat alignment is light-years ahead of the stock configuration in a home setting. I have yet to experiment with lower tunings but re-tuning them is the way to go.

I have also started messing around with the HP section of the crossover. I replaced or bypassed the electrolitics and increased the value of the 5 ohm resistor to 7.5 (BTW, there is an error in the schematic I posted earlier. The 8uF shown should read 6uF) Anyway, 7.5 ohms may be a tad too much. The weekend plans are to play with that value to arrive at the best balance.

I?ll start a new thread about how to start fixing them up when I am done with initial tweaking. These speakers deserve more attention than they typically get.

Sounds good - and looking forward to your new thread. Which resistor was 5 ohm?

Hi Mike,
If you are looking at the JPEG in post 3. The 8 Ohm in the red cloud is was 5 Ohm from the factory. The 16 Ohm and switch above it was originally just a switch and wire to short the 5 Ohm.

Here is a link to the M500 lit sheet showing the FR with the switch closed which is the “high frequency emphasis” position.
http://altecpro.com/pdfs/vintage/Spe...r%20System.pdf

Subjectively, the peak and overhang you see in the same graph at 50Hz has been cured by re-tuning the box.

Thank you for sharing this! Looking forward to your new thread as I just happened to pick up a pair of these. Please do post your further impressions and any other tweaks!