Modification of the A15-8A, M500

[martyh45]

Modification of the A15-8A, M500
I?m starting this thread to collect info that is in a couple of other threads and hopefully put together something useful on working with these speakers. The A15 and M500 share the same woofer, driver, box and horn. Not sure about the XO but if I had to guess I would say no, maybe the same board but different components. If anyone has a picture or schematic of the m500 xo please post it.

I?ll begin with a couple of disclaimers, First: I am not yet setup to do usable measurements. What follows is a tuned by ear approach. Second: I am by no means an expert in cabinet tuning or crossover design. I?ve tried to take a scientific approach to these modifications but I am no engineer, not even close. What I do have is listening experience and a good long term reference system to gauge against. And.. I am like a crazy cat lady only with audio equipment so I have plenty of stuff around for very long term listening comparisons. I normally follow in the footsteps of giants when modifying equipment but in the case of these speakers there is not much info available and none that I can find on modifications. So hopefully if anyone has advice or better ideas they can post it in this thread but I?ll start it off with a few simple changes that yield (IMO) a damn fine domestic speaker in a package that while huge by today?s standards at roughly 6 cubic feet is small compared to most of the VOTT range.

There is a lot to like about them beside the relatively small size. The cabinets are higher quality plywood than any of the older Altec cabinets I have seen. The horn mouth is large compared to a 511 and appears to be smoother near cutoff than the sectorals. The 909 drivers are pretty damn good. I switched in a set of light phrams and they lasted all of 20 min. before it was back in with the pascalites. It may be that pascalites just work better with loading caps but the drivers won?t fit into the cabinets with a cover installed and I am not going to cut up these boxes for an experiment that may or may not work.

Starting off I noticed a few problems. First: The height of the horns was low compared to seated ear height. Based on my past experience I didn?t even bother listening to them before I built some 7? high stands with a slight vertical tilt to aim the horn axis at my ears. Second: The bass was just not right, it had the one note sound of a peaky tuning, plenty of slam but no detail. I prefer the sound of sealed low Q boxes so take what I say with a grain of salt if you like the bottom end of a stock M19. Third and last, the HP section seemed hot particularly in the voice range. I could not stand to listen to a CD through these speakers stock for any length of time. Everything I don?t like about the CD sound was there in spades and then some. While I am at it another disclaimer, I?m not necessarily a luddite. My CD stuff is mid-fi at best, assemblage dac fed by an older Dennon player as a transport. I?ve certainly heard way better digital front ends. My vinyl front end is where I spend the money and time. It may not be the highest of high end but it is at least respectable in audiophile circles. I have hundreds of LPs and maybe 20 CDs so I don?t really care to invest any more money in digital.

I?ll follow up shortly with what I found as fixes for the last two problems I listed and how I arrived at what I did with a little backup for the process. The investment was minimal and the results are well worth the effort.

- - - Updated - - -

Bass tuning:

The Bass was easiest to fix. The 4 port layout leaves some flexibility. I modeled the cabinet in WinIsd and found that plugging 2 of the 4 vents yielded a quasi max flat alignment. The ports are just under 3? diameter so I purchased some Oatey Model # 33402 plumbing test plugs available at Home Depot.

Oatey 3 in. Plastic Mechanical Test Plug-33402 at The Home Depot

These form an airtight seal and are easily reversible. Plugging two vents fixed the bass to the point I moved on but I still intend to try plugging 3 which should yield a curve that looks more like an SBB4 alignment. The bass alignment will come down to room and personal preference so adjust to taste. Once you arrive at the desired tuning the plugs can be installed on the interior side of the vents by removing the horn and reaching through the cutout.

For reference here are some plots of the of the bass tuning options available by closing off vents in all cases the Red trace is stock, Blue = 3 vents open, White = 2 vents open and Green = 1 vent open

Cone excursion at 100 watts input
http://i142.photobucket.com/albums/r...ps486914a0.png

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

FR

http://i142.photobucket.com/albums/r...ps564a862e.png
up next the crossover....

The more difficult thing to work out for me was the crossover. First, a link to the stock highpass schematic.

https://www.circuitlab.com/circuit/2kkt7h/a15hppublic/
R1 and R6 are the copper resistance of the coils not separate components and R7 is the driver. When SW1 is closed the crossover is in the high ?frequency emphasis? position.

Here is a photo of the stock crossover
http://i142.photobucket.com/albums/r...s88cbd72e.jpeg

Here is drawing of the components laid over the circuit traces as viewed from above. The component labels in the HF section match the schematic above
http://i142.photobucket.com/albums/r...psb5a6f244.jpg

My initial thought was to increase the value of R5. I replaced the electrolytics in the crossovers and installed a set of clip wires so I could switch in different values of Resistor for R5 easily. This did not work out as well as I expected. Increasing the value did have an impact on the voice range but knocked down the treble too. The speakers were easier to listen to but not quite right. Encouraged by my success with the cabinet tuning I decided to dig further into the crossover. I thought I remembered Zilch discussing XO transfer functions so I decided to try to build a simulation model and see what I could learn. The model is the schematic I linked to above. Using a simple resistor for the driver and an ideal source to drive the network is probably not the most accurate way to model but I was looking for basic knowledge so it seemed to work for me. Beyond the basic CL filter at the input I could not look at the schematic and guess what the rest of the parts were doing when I started. I varied the values one by one of the various parts and plotted the resulting response curves trying to get a handle on what they were doing and correlate it to what I was hearing. This was a bit tedious but not nearly as tedious as the reading I would need to do to be able to predict what was going to happen when I changed values. After about a week of playing around, digging through the junk box, and scrounging suitable parts I ended up building this.

https://www.circuitlab.com/circuit/s...5-junkboxmods/
I did not use any fancy audiophile parts, mostly mylar caps and regular ol? wirewound resistors. I ended up with enough of each value to match the between channels though.

Here is a link to the resulting voltage transfer function and phase plotted with SW1 in both positions. against the stock version both with SW1 open. SW1 open is the stock ?equalized response? setting.
http://i142.photobucket.com/albums/r...ps216770d1.jpg

I prefer the sound of the new version with SW1 closed. You can see above that It change the response above 5K resulting in a good deal of boost (or more accurately less attenuation) at 20KHz.

Here is a plot I found of the 994 horn driven by the 909 driver.
http://i142.photobucket.com/albums/r...psb42e348e.png

I think the new EQ matches what you see in this plot better than the stock version but I am not sure about the measurement conditions for the horn/driver combo and have not measured the horn with the new crossover

I have said before that my hearing sucks above 15K so I recruited my daughter to help analyze the changes. She has bat ears compared to mine and a love of music so she made for a good test. She preferred the sound with the switched closed too and didn?t report any aggravating treble nasty?s. So FWIW, if your HF hearing is better than mine this should still work for you.

Finally, I guess I should write a few words about the sound of the modified speakers.....
The first thing that grabbed me was the imaging. Perhaps I was distracted by what I perceived as the speakers flaws when I first auditioned them but I now perceive a nice layered and deep soundstage that presents a convincing image of any size group. The voice range boost I had trouble with originally made the speakers hard for me to enjoy for long periods. Now, the presentation is more relaxed and I can spend many hours spinning LPs. Even my sorta crappy sounding CD rig is listenable. The detail is still there but not in your face like it was.
Dynamics are what you would expect from a 15? woofer and a big horn, kick *ss! These bad boys jump. If you want to rock out with a smaller amp these will do nicely. I?m still using a 50 watt class A SS amp to drive them and even during too loud sessions there is plenty of headroom for peaks. I measured 110 dBc peaks during one post modification session and the speakers sounded like they were just loafing along. Based on what I hear these should do well with the classic circa 30 watt tube amps. W5Ms, 1568s and Stereo 70s should have no problem driving these speakers to more than sane listening levels. 20~25 watts should be plenty.
For Altec Fans, you all know you can?t get the upper bass/ lower mid speed and detail of a 416 or 515 without a 416 or 515. The 3154 seems trades some of that off for extension and power handling. Think of the difference in that department between a 515 and 416, 416 to 3154 is about the same distance in the same direction if that makes sense. On the top end, I?m just gonna say it? the big mantaray is flat out better than the 511 or 811 the sound is more relaxed and not as lumpy near cutoff but they still do everything you probably like about horns.

[GM]

'Sounds' like it needs a MLTL cab alignment and CD horn EQ.

GM

[martyh45]

'Sounds' like it needs a MLTL cab alignment and CD horn EQ.

GM

The MLTL would be cool to try, the xo did have some CD eq which I ended up tweaking. I've just finished the post so you should be able to see the HF schematic now.

[GM]

Driver specs, stock cab's basic dims?

GM

[martyh45]

Driver specs, stock cab's basic dims?

GM

Here is a link to the spec sheet for the 3154 woofer
http://www.greatplainsaudio.com/vint...spec_sheet.pdf
The ID of the cabinet is 25” W x 31-5/8” H x 16” D. 4 vents 2-15/16”D x 3-1/8” long. One large shelf brace under the horn. I get roughly 6 feet^3 net volume

[GM]

Thanks, c-t-c driver spacing? Don't want to 'upset' your XO, EQ design.

So you trust GPA's specs?

GM

[martyh45]

Thanks, c-t-c driver spacing? Don't want to 'upset' your XO, EQ design.

So you trust GPA's specs?

GM

The drivers are roughly 16? apart vertically. The loading cap compresses the 1-1/2? insulation on the back side of the cabinet when installed so horizontally they are as far apart as they can be considering the depth. As far as the published specifications go, they seemed to work for my purposes in that what I was hearing roughly matched what I was seeing in the sims. I listen in a space with solid block walls of about 8k^3 feet so I think the bottom end sound is a pretty honest representation of tuning. I couldn?t swear that the peak was centered in the mid 50s but there was a pronounced sub 100Hz peak that went away when I lowered the tuning.