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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
2,740 posts
 
Amfibius, thanks for the link to the photos. I can't tell much more about the woofer from the photos, unfortunately - I was hoping it would be identical to something in the Madisound catalor.

Eyeballing the crossover I see two notch filters and a contour filter, along with some impedance equalization. Looks like some high quality parts, by the way. The absence of an unbypassed inductor makes me think the voice coil inductance is on the high side, so I now propose a zobel consisting of 45 microfarads in series with 10 ohms, along with a 10 millihenry series inductor, to roll off the signal going to the inner woofer.

Isobaric doesn't mean both woofers get the same signal across the spectrum - in fact, in my opinion they shouldn't. If the inner woofer is allowed to play up into the midrange, in effect you have twice as much unwanted midrange energy bouncing around inside the box and re-radiating through the woofer cone. Isobaric loading is only beneficial in the deep bass region.

Duke
downunder
1,329 posts
 
Keith

Have you contacted the manufacturer?? I am sure they will have idea's and be able to help.
aaudio_imports2
19 posts
 

Dear Amfibius,

I have dealers driving the Acapella speakers with Einstein, FM Acoustics, Jadis, Audio Note and Lindemann. These are all among some of the finest electronics that can bring out the qualities of Acapella speakers.

As with any high end speakers, you have to find the right synergy with your amplifier. I remember speaking with many Avantgard owners who went though several amplifiers until they found the right one. Many were just to noisy for these high efficiency speakers, and others made these speakers sound hard and forward.

I do suggest that you borrow other amplifiers from your dealer before trying to modify your speakers. If this doesn't take you into the right direction you would be better off selling your speakers rather than modifying them since this will kill the resale value.

I did speaker with Acapella and they did tell me that no upgrades will be offered from the previous Violon model to the latest MK4.

Dear Johnk,

Something that is unique with Acapella Horn speakers is that they have the performance of a horn midrange, without sounding like a horn. This is one of the reasons why they are not super high efficient like other horn speakers.

Best Regards, Brian
semi
649 posts
 
I used to own Dynaudio Confidence 5 which uses isobaric woofer design. Other speakers I can think of are Eggleston Andra and Totem Mani-2. As far as I know, isobaric design needs both woofer, internal and external, to receive the same signal so they can work in tandem and produce that perfect 0.5 Q that is the ideal setting for speakers.

It doesn't hurt to try a different amp, but I don't think that will transform the sound to the degree you are looking for. My vote will be moving on to a different speaker that suits your taste instead of trial and error on modifying the speakers not to mention zero resale value afterward.
amfibius OP
282 posts
 
Duke, the woofers are NOT in isobaric configuration. Isobaric (as I understand it) means: two woofers, fed the same signal and wired in parallel, located very close together. At bass frequencies, the distance between the two woofers is much less than a bass wavelength - meaning no standing waves.

The woofers are about 50cm apart, with the lower woofer firing into an enclosure which is stuffed full of ... umm, stuffing. My friend (who is a speaker engineer) commented that placing them 50cm apart and running up to 450Hz between them would surely cause standing waves. I mean - the wavelength of a 450Hz wave is 76cm. You only need 1/2 a wavelength to set up a standing wave. I calculate that standing waves between 343Hz to the crossover point could potentially be supported.

He also commented that the intention of the stuffing (for want of a better word) may be for acoustic filtering. The High Suboktav is marketed as having an additional woofer allowing for deeper extension. Perhaps this was the intention.

I was unable to find the SEAS CA25ACA by google search, so I can not tell you what the T-S parameters are. However, there are some other SEAS drivers which are almost similar so the T-S parameters are probably very close.

My friend noted that the cone suspension is not very rigid, and it is very easy to make it reach maximum excursion (Xmax).

I am starting to wonder if the non-Suboktav version of the Violon may have cleaner bass. What do you think, Brian? Thank you for your suggestion of trying different amplifiers, but like I said - if the problem has been "set in wood" then it won't go away no matter how many Watts I feed it.

And BTW I am not too worried about resale value. I am a very slow upgrader and I tend to hang on to my goodies until they die. I have NO INTENTION of selling this speaker, because the mids and the top end are sensational. If I could fix the weakness, I would be very happy.
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