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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
2,716 posts
 
Keith aka Amfibius, "isobaric" means constant-pressure. It can be accomplished by many different internal geometries, and does not require that the internal woofer be fed the same signal as the outer woofer. I used to build 'em. The very-close-coupled format you describe sounds like what we used to call a "compound woofer" (often configured with the woofers mounted face-to-face), which was only used in the bass region. For operation up into the midrange, a larger inter-driver spacing was used to allow the use of stuffing between the drivers.

Ralph aka Atmasphere, one of the issues with running isobaric woofers in series is that the two woofers will have dissimilar impedance curves because they will have significantly different resonant frequencies. The inner woofer may well have a resonance an octave higher than the outer woofer. Since it's the outer woofer that we're listening to, in most cases it would be detrimental to have in effect a very high resistance in series with it in the octave above its resonant frequency. The impedance peak of the inner woofer would approximate a series notch filter.

Duke
amfibius OP
282 posts
 
Ralph - sounds like a great idea! I'll ask him what he thinks of it. Would the caps and inductors need to be changed as well, or only the resistors on the notch filter? Could be a good temporary solution.

Duke, very interesting and helpful comments, thank you. It appears that my understanding of isobaric enclosures is incomplete and there is a different type of isobaric as you have just described. I will have to do more reading on this - there may be a method in the madness after all.
atmasphere's avatar
atmasphere
11,590 posts
 
Amfibius, Duke's comment sounds like one to be paid attention to. So if you proceed, keep his comments in mind!

The crossover parts would change as the impedance would be different and you would have to set up for the same notch frequencies, and if Duke is right, probably a notch for the unwanted secondary impedance peak.

I'm glad I make amps and not speakers!! :)
gawdbless's avatar
gawdbless
1,235 posts
 
Now If that would have been on expensive speakers, I would have been seriously worried!
rlawry
477 posts
 
I thought I would weigh in here. I have a 3 year-old pair of High Violins with the single woofer and drive them with a very powerful Essence solid-state amp, actually monoblocks. Each one has 230K of filter capacitance and a 2 KVA transformer. I tried a lot of SS and tube amps on Wilson speakers I had previously and the Essence amps produced the most prodigious bass I found on the Wilsons, who are known for bass. It has been my experience that the Violins with the single woofer have a tighter, faster, more tuneful bass than the Wilsons I had. The bass is not as prominent nor as dynamic, but better integrated with the rest of the sonic spectrum. And this is with them sitting on a carpet with no spikes. I just acquired a set of spikes and will be spiking the speakers to my carpeted cement floor. I have experienced none of the discontinuity problems you describe with the twin woofers, as each instrument appears coherently in space. So whether your issues are the isobaric woofer arrangement or low damping factor of your tube amp, I am not sure. My suggestion would be to try a boat-anchor sized SS amp before I would resort to changing crossovers and woofer hookups.
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