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.

Showing 4 responses by rlawry

Hi, Ralph, thanks, yes, I would imagine my single woofer version to be an easier load, even for my big SS amp. It will be interesting to hear what Amfibus finds when he tries driving his suboktav version with a SS amp. I must admit the indicated woofer alignment seems a little strange, but I would be hesitant to alter the driver arrangement prior to trying a possible easier fix such as an amp with a much higher damping factor.
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.
I have had BAT SS amps, the VK500 and VK600, and the bass was pretty darned good. The only thing I didn't like was the lack of soundstage size, but for bass, they are great. You might be able to pick up a used VK500 cheap. I would think just about any SS amp with a big power supply would work pretty well for bass and lower midrange, like one of the big Conrad-Johnsons, Krell, Mark Levinson, Classe, etc. The Cary would probably work, although I am unfamiliar with their new SS amps. I have had a couple of their tube jobs and they are great.
I agree with you, Oshag. The midrange and treble on this speaker are stunning. Detail in these areas is the best I have heard, bar none. Soundstaging, dynamics are state of the art. Unfortunately the bass is just ordinary. Extension is OK, speed and pitch definition are good, but the bass doesn't have the weight or dynamics of other speakers, especially the Wilson Watt Pupp 7's and Sophias I had. I spent a really long time getting my JL Audio subwoofer to integrate with the High Violins, and now the bass is liveable. BTW, I just assume my speakers have the single bass driver. On the back there is an emblem stating "High Violin 2001 Bass." Maybe someone knows which bass driver version they are. Thanks.