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The Anatomy of the Acapella Violon - shocking find

For quite a while now I have been having trouble with the bass on my Acapella High Violon Suboktav 2001. With the help of a friend, we have dismantled this speaker and studied the internal construction and measured the crossover points. I am hoping that my findings will be helpful to all of you Acapella owners.

The bass problem is this: bass can not keep up with the speed of the midrange and top end. On some recordings, the bottom end becomes disconnected - you can hear music from the midrange and the top, followed by the bass response a microsecond later. Furthermore, the bass is poorly controlled and flabby. From my other Audiogon threads, you can see that I have been wondering whether the damping factor of my Cary CAD-211AE amps is sufficient to control the wild bottom end, and whether a solid state amp will cure this problem.

The current iteration of the Violon is Mk. IV. I am not sure what a "Violon 2001" is, I am guessing either Mk. I or Mk. II. Acapella's own website does not reveal any secrets, all it says is that the High version of the Violon has an additional driver inside.

Anyway, this is what we found.

PLASMA TWEETER: 4th order high pass crossover (24dB/oct) at 5000Hz. Measures very flat all the way to the limit of measuring equipment. Incidentally, Acapella marks the recommended tweeter level with a pencil mark on the tweeter volume pot. At the minimum recommended range, the tweeter comes in 12dB ABOVE the reference SPL. I had to wind the tweeter almost all the way down to get a flat response.

MIDRANGE HORN: First order high pass crossover (6dB/oct) at 450Hz, with a very gentle taper between 3dB/oct - 6dB/oct from 5000Hz and up. Goes all the way up to 10,000Hz. The shallowness of the low pass section of the band pass crossover makes me suspect that it is relying driver rolloff.

This is a surprisingly wide band of frequencies (4 1/2 octaves) to ask a horn to handle. As you know, horns are tuned to work over a fairly narrow frequency range and the response drops off at either extreme of this range. Wavelengths which are too long for the horn do not couple with the horn. Wavelengths which are too short will bounce around chaotically. That is only the theory however, because the horn measures very flat between 450Hz - 5kHz.

The integration between the tweeter and midrange horn is very good.

BASS UNIT. As advertised, there are two 10" drivers in the unit, and both appear to be SEAS drivers. The external driver is run through a passive crossover from the binding posts, with a low-pass first order crossover at 450Hz.

And now, the surprise. The internal driver is run directly from the binding post with no crossover in between. In other words, it is run full range, relying on driver rolloff only.

I can think of no advantages for a setup like this, only disadvantages.

Firstly, the drivers are wired in parallel. This will drop the impedance, making it difficult to drive them with valve amps (Acapella supposedly voice the speaker with the Einstein OTL).

Secondly, a configuration like this will result in destructive interference between the two drivers, ESPECIALLY if the crossover introduces phase problems in one of the woofers. Given the other woofer is crossover-less, any difference in phase will definitely cause interference.

Thirdly, running a woofer full range will cause cone breakup at the top, which will muddy the lower midrange (exactly what I have been hearing).

Possible solutions:
- wire both drivers to the crossover, maybe in serial configuration to increase the input impedance (Zin),
- disconnect and remove the internal driver, i.e. convert the speaker from the "High Suboktav" version to normal Violon,
- remove the passive crossover entirely and use a preamp-level crossover (active crossover).

Now, I am no speaker designer. I am just an enthusiast struggling to understand these things. But this just makes no sense to me. I am hoping that someone with more experience will be able to explain why Acapella made these design choices, and what you think of the possible solutions.
amfibius
audiokinesis
2,737 posts
 
Keith,

Thanks for the information. I think the woofer system you have is what would be called a "sealed isobaric".

It has been twentysomething years since I worked on an isobaric system. Basically, the inner woofer dances in sync with the outer woofer in an attempt to keep the pressure constant in that chamber between the two. "Isobaric" means "constant pressure". I did some measurements of an isobaric system years ago and my findings contradicted the conventional wisdom of how to model an isobaric system, but I won't bore you with the details.

Okay, in my opinion the inner woofer of an isobaric system should be rolled or else it mucks up the midrange. I do not know what order the rolloff should be, but either first order or fourth order make the most intuitive sense to me. In my homebrews, I used a first order rolloff on the inner woofer because that was much simpler to do.

There's a good chance that the discontinuity you hear is due to the inner woofer blaring away into the back of the main woofer. In my opinion, it would be a good idea to roll off the inner woofer above 80 Hz or so, and we'll try a first order network because that's probably the safest bet.

If you can by any chance come up with the model number of the woofer, that would be very valuable. Even just a physical description of the woofer would help.

If not, then we can "wing it" and use a 16 millihenry iron core inductor in series with the woofer (like a Madisound Sledgehammer), stabilizing the impedance with a SWAG zobel consisting of a 30 microfarad cap and a high-power 10 ohm resistor.

If you can come up with any identifying information on the woofer, or even just a physical description (cone material, phase plug, surround description, magnet description, frame description) I can hopefully make a more educated guess on the component values for the inner woofer's crossover.

Duke
timrhu
2,779 posts
 
Given that I now know that the woofers are wired in parallel, it would make sense that it is an 8 Ohm woofer.
I'm not a speaker designer but doesn't this mean the internal woofer is operating at the same frequency as the other and not full range?
aaudio_imports2
19 posts
 

Dear Amfibius,

Before you start modifying your speakers I would suggest trying other amplifiers.

Consider this, Acapella has been building speakers for over 30 years, and the Violon's have been around for at least 10 years. Many pair of Violon's have been sold around the world, and I don't know of any owners that have thought about modifying their speakers. Unless you are an expert speaker manufacturer and can completely understand the design theory then you are probably going to make things worse.

As with many of the higher end speakers on the market, the designer of Acapella speakers has many proprietary things inside these speakers that can't be understood by you or me. They have tuned the speaker to perform a certain way. If we try to change this then we end up with something else that the designer didn't intend.

I used to own Cary amplifiers and they don't drive all speakers well. My experience with Cary amps is that they are better suited to 2 way designs with simple crossovers, which are an easy load. And even then they still have that Cary sound. Which on the right speaker's is very pleasing, but on the wrong speaker is a disaster.

Something that's important to know about Acapella speakers in general is that they need very good to excellent electronics, otherwise you won't get their true performance. Remember that you own one of the few true reference speakers in the world, so now you need to use equal electronics because these speakers will basically tell you what your system is made of.

Think of your High Violon's as a perfect measuring device for testing electronics, cables ect... At this point the problem is not the speaker, but the system you are driving the speaker with. The speaker is simply showing you what your system sounds like. Many other speakers may sound great with your current system, but that's because they are not capable of this level of fidelity.

Best Regards, Brian
amfibius OP
282 posts
 
Duke, my understanding of an isobaric woofer is that BOTH woofers receive the same signal. In this case, the outer woofer is receiving a signal from the crossover, and the inner woofer is receiving a full range signal from the amplifier. All crossovers introduce phase issues, particularly a crossover as complex as this one. If there were ANY phase differences between the two drivers (in the same enclosure!) they would interact and produce peaks and dips in the frequency response.

I think that Acapella themselves have realized this, which is why there are notch filters built-in in the crossover.

If you want to see pictures of the dissected speaker, take a look here:

http://www.pbase.com/amfibius/things

One of the pictures shows the back of the woofer (notice the flimsy looking spider?) with what appears to be a model number - SEAS CA25ACA. There is also a picture of the front of the woofer. A google search fails to bring up the driver. A pity, I was hoping to find the T-S parameters.

Brian, thank you for your suggestion however I do not believe a replacement amplifier will solve the problem. A solid state amp will only improve the damping factor, and it looks as if Acapella have "set in wood" the problem with the bass.

Having said that, I can easily borrow some amps from my dealer and see whether 600W of solid state Class A/B would be enough to drive the speaker. I realize that an amp like this will probably ruin the top end, but it should immediately answer the question of whether it can make the woofers move.

In another thread, I asked you if the updates to the Mk.IV version of the Violon can be retrofitted to mine (Violon 2001). The answer you gave was "no". My option at present is to sell the speaker or to modify it.
johnk's avatar
johnk
1,975 posts
 
So many horn designers wimp out on the bass for only bass horns integrate proper and do justice to front horn systems. These are large costly why you dont see them. And who would want a 91db horn:) One of the benifits of horns is hi-eff and massive dynamic range seems this design would lose much of this. Its all about the tweeter thats how I feel about Acapella. The isobaric driver in cabinet wouldnt need a crossover sure most frequincy range gets filtered by cabinet not hard to set it up phase correct dont see a problem with that just the whole bass system, cool tweeter though;) I needed 8-15in hi-eff woofers mounted 4 in ea. in front horns to match my comp mid horn and comp tweeter the amazing fostex t500amk2. Now seemless integration all 1st order all levels match. Can run on one small amp no biamping. Like a seemless whole and once one hears proper horn bass nothing else is close. So much bass detail is lost to conventional designs.
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