My A7-500s. A journey

[Mustang Marvin]

Pano,
Thanks for this thread. I have just completed my A7-500 project. I posted some of the restoration in another post. Mine have 416-b, 902-8t, 511B, and 800HZ Eminence XO.

I had the 416-8B's re coned by GPA. They also recommended that I reduce the port size for home use. This tuned the port to the 416. The size now is 26 in. X 3 1/4 in. or 84.5 cu in. This does help the bass, but you really have to crank them up. I also have boosted the bass on my amp by about 4db. Someone also told me the cabs are actually too large volume wise for good musical sound reproduction. (I hope not)

I have heard that "bracing" the cabs also helps the bass although I have not seen any detailed description or pictures. I have also used Redux acoustic insulation in pretty much the same manner as the original Altec design. Could more insulation help? I don't know.

The 511 on my cabs was mounted inside. I moved it back to the top. I used steel angled stock with the holes in it I got at Lowes for mounting. The holes line up perfectly with the bolt holes in the 511. I also used rubber grommets, also from Lowes to mount the 511 to the angel stock. Rubber washers were used between the 902 and the rear stand off bracket. this makes the 511/ 902 assembly completely rubber mounted. I have detected NO ring. Striking the horn only gives a dull thud.

As the others have noted, I am not satisfied that my A7's sound as good as they could. I will pay close attention to your progress. Please don't leave anything out.
Thanks and good luck.
Marvin

[Panomaniac]

Thanks for the info and the encouragement, guys! It very nice.

OK, going to start talking crossovers. This is just the beginning. Crossovers can make or break a speaker. They are a full 1/3 of the speaker - drivers, box, crossover. All equally important. Many speakers do not live up to their full potential because of poor crossover design. There has been a lot of progress in the past decade on computer crossover design, but it only helps, it does not do the whole job - at least not well. It takes a lot of experience, time and effort to design a great crossover. I'm a novice, but learning.

So here we go!

My A7-500s came with the stock N-501-8 crossover. This is the 500Hz crossover for 8 ohm drivers. I have attached a schematic that I drew from the actual part. I measured all values. The inductors measure 2.7mH outside the crossover, but probably rise to ~3mH when mounted in the steel frame. I have called them 3mH. Also included is a photo of the insides. The caps are marked 28uF 125V and measure spot on. L-Pad is 24R & 5.6R Just a basic 2nd order crossover with L-Pad. No Zobel, no traps, no EQ.

The values for a nominal 500Hz, 2nd order Butterworth filter are 28uF + 3.6mH. So the N-501 has slightly smaller inductors than "textbook." The problem is, although the drivers are nominally 8 ohms, they are not 8 ohms at the crossover frequencies. (See impedance graphs in the next post.) So the frequencies are shifted a bit. Simulations show the 416-8A electrically crosses a bit higher, the 511/802 combo pretty close to 500Hz. Electrically speaking. I will try to measure both the electrical filter function and the acoustic points for further reference.

Here is the schematic I drew from the actual crossover and pix of the guts. Nice big L-Pad!

[Panomaniac]

When doing box tuning and designing crossover, driver impedance curves are a great help!
One of the best audio gadgets I ever bought is the WT3 woofer tester from Parts Express.
Just a little USB gadget, it makes measuring drivers very fast. Only a couple of seconds to measure everything but Vas. And that only takes a few seconds more.

Plots show here are from the WT3.

Below we see 2 graphs, 3 plots. One is the woofer in the A7 box. This is a 416-8A. You can see the single peak at resonance (~23Hz) and the rising impedance typical of a woofer. Also obvious are some small bumps that indicate box resonances. I'll work to get rid of those.

Do notice the single peak. Does not look like the classic twin peak bass reflex curve, does it? More like infinite baffle. Maybe that's because of the big port. Currently 206 sq. inches or about 160% of cone area. We can see why Marvin brought his port size down!
I'll do the same and run a sweep.

The other graph is the impedance of the 802 driver on the 511 horn. One is with loading cap, the other without. As the peaks come near the crossover point, this makes things a little tricky. An LCR trap could be used to flatten this peak, might be worth trying.

Just for fun, I tried running the drivers with an active crossover. 3rd order at 610Hz. It works well and certainly sounds better than the stock crossover. Worth investigating further.
FWIW, the way my 511s are mounted, there is only 4 cm of depth offset between the acoustic centers of the woofer and horn. Very close to time aligned. Just a small fraction of a wavelength. Nice!

[Old Guy]

Altering the xover point will alter the sound a lot.

500, what you have, results in a more mid heavy, very "up front' aggressive sound.

This can be very desirable with a straight bass reflex, but the 828's short mid bass horn tends to boost that area too.

Going up to 800 will mellow them a lot. To 1200 even more...but how much is good is personal preference.

An electronic xover can help , play around with crossover points till you get the sound you like, then design your passives for that point...or just stay electronic.

Personally, I will never go back to passives. Never.

And this from someone who insists on never saying never...

Really glad the horn worked out for you, in the old days would have had a spare fram to send, but those days are gone...

[Altec Best]

Remember, these are theatre speakers, not home. But with work and patience, they can be quite nice.

There is a reason M19's were better for home use.

Hey Mike, I think Ron brought up a good point. These were designed for large areas not a small room inside a home.So it will take some tweaking to get them right but you do good work very thorough.I thought your port was too big also for the 416 I would even try reducing it even more to what the M19 uses a 2 1/2" x 11" port you can always enlarge it.Also the 802 symbiotik with loading caps, most people I have talked to don't like the loading caps so they remove them and stick in a thick piece of felt in the end cap instead.And the symbiotiks are more or less for power handling replace with a all aluminum diaghram. Well anyway nice job Mike you are no novice we all can learn more each day for sure.I learn something new every time I come to this forum isn't it great !!! Those A-7's look to be very nice shape though. Regards

[Panomaniac]

Thanks guys, lot's of good feedback. I didn't know this thread would get so much interest. Yep, a get a nice set, thanks to O.G.

Some more playing this afternoon. Just FWIW.

• Putting the A7s in the corner or right against the back wall helps the bass a lot. Certainly seems to be the way to go in a room like mine.
• Adding a wing to one side also helps the bass. The A5s I used had wings in both sides.
• Rolling the crossover point up and down from 450Hz to 1.8KHz does not change the sound nearly as much as I thought it would. The 511 seems to blend well with the woofer all thru this range. Above that and the mids start to drop away (woofer roll-off) and below that the low mids go away (horn roll-off). Things do change, but it's fairly subtle. Good drivers, I suppose!
• Pulling out some mid with EQ does help the horn shout. A dip of about 3-4dB centered at 1.5K is a good starting point. Overall or just on the horn.
• Vibration. The woofer horn vibrates like crazy. It's the worse part of the cabinet. And it's pointed right at you. Mine are not kerf cut in the back, but are a solid bent piece. Damping material and some bracing back there ought to work wonders.

I agree about the symbiotic frams and the loading caps. They can't be helping. Will swap in an 806 driver tomorrow for a listen. That may smooth things out a lot.
OK, back to the music (Carlos Santana).

Note, just a quick swap to a 811/806 made a BIG difference. So much smoother, better balanced and more top end. Gives the 803/291 a run for its money. Eager to hear what the 806 does on the 511.

[GM]

Do notice the single peak. Does not look like the classic twin peak bass reflex curve, does it? More like infinite baffle.

The little bump is the reflex's upper impedance peak which is highly damped due to the cab being tuned so much higher than the driver's Fs, ergo if tuned a similar amount below Fs it would become the ~single peak with the lower one just a small bump and of course equal in amplitude if tuned to the driver's Fs.

Anyway, if you recall my remarks about driving vintage speakers with the type of high output or matching impedance sources they were voiced with, this high tuning relative to Fs is desirable since it yields much better woofer control/higher low distortion SPL while still having extension to slightly below Fs same as if tuned to Fs with the trade-off being it sounding a bit 'loose' due to the amp 'stalling out' against the high impedance.

This same high output impedance source would also flatten the HF horn's 'hot' mids somewhat while adding scharron's 'gentle climb' EQ to flatten it out with increasing frequency due to increasing driver inductance.

GM

[Panomaniac]

Hi GM, I was hoping you would "tune in."

Yep, I will be working on the port tuning, for sure. Will aim for equal peaks to test how that sounds. And for sure, a high output impedance or resistive network on the horns sounds better. Not quite sure about it for the bass, yet.

[Panomaniac]

Sorry to report no real progress over the weekend, too busy with yard work and other chores. Mostly just listening. =)

But I did try to figure out the internal volume of the box. I have read and heard various numbers, but nothing definitive. So I started measuring. I figure this:

Gross internal volume = 15 cubic feet
Horn section + woofer = 3 cubic feet.
Net volume = 12 cubic feet = 340 liters.
(for box with horn flares open in back)

Actually did all my calcs in cubic centimeters then converted. Years ago I had figured 11.5 feet going from published plans. So pretty close to my measured results. I sure looks like the horn and woofer should take up more than 1/5th the volume, but I measured and calculated 3 times. Anyone have an opinion?

Given an Fb of 340L, and the port size I have, my cabs must be "tuned" to about 65Hz. Or about an octave too high. A starting point!

[Panomaniac]

Well did a little tuning and testing today. I pulled off the top board from the port and made 3 others boards of different sizes for various port areas. Easy to swap out the boards and change the port size. Or course all were "flat ports" - just 3/4" deep. And all where full width - about 28.5".

Closing down the port definitely extends the bass, no doubt about that. Easy to hear as the port plates are switched in and out. I was trying to get the box tuned to about 35Hz for max flat response. Never sure what tuning I got. Could not tell be the impedance curves.

But about 57 sq in. seemed to work best to my ear. Nice solid bass, less midbass. Listening with a signal generator says that the boxes now play strongly into the mid 30s. Drop fast under that. Tried both more area and less (down to 17 sq inches) but liked the mid 50s best. Will need more time to fine tune.

One thing I can say for sure. Bracing and damping the woofer horn flare is going to be VERY important! They buzz like crazy. I put one hand on the flare while running the sine waves up and down. Real easy to feel the resonances. And they don't all happen at the same points, that was easy to feel with my hand. All the bad stuff seems to be between about 120-200Hz. In fact just pressing my hand on the flare at 200Hz made the fundamental rise a good 6dB! The horn flare must have been vibrating in opposite phase, canceling the fundamental. Super easy to hear. So you A7 owners, get those flares under control!

Sounding pretty decent now. Good port and the 811/806 combo running at 800Hz, 3rd order. Still the dip at ~2K to pull back the midrange. Getting nice. And can only get better!